Astronomy fanatica
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Extrasolar planet - Cont'd
Methods of detection
There are currently six methods of detecting extrasolar planets which are too faint to be directly detected by present conventional optical means.
The planned Space Interferometry Mission, Terrestrial Planet Finder and Darwin would all try to examine planets in a more direct fashion.
All extrasolar planets discovered by radial velocity (blue dots), transit (red) and microlensing (yellow) to 31 August 2004. Also shows detection limits of forthcoming space- and ground-based instruments.
Pulsar timing
The first method used to discover extra-solar planets was to observe anomalies in the regularity of pulses from a pulsar. This led to the 'discovery' of the first planet with the orbital period of one year. That was later retracted as it was actually the failure to account for the motion of the Earth through its orbit. However, this method did lead to the discovery of the first planets, and first stellar system outside of our own, by Aleksander Wolszczan. It also led to the discovery of the oldest known planet, by Steinn Sigurdsson's team, around PSR B1620-26's binary stellar core. This planet is the only known planet to orbit two stars. The pulsar timing method involves precise measurements of the signal of a pulsar in order to determine if there are any timing anomalies in the period of the pulses. Subsequent calculations are used to determine what could cause the anomalies. This method is commonly used to detect pulsar companions but is not used to specifically find planets.
Astrometry
Astrometry is the oldest method used in the search for extrasolar planets, used as early as 1943. A number of candidates have been found since but none of them are confirmed and most astronomers have given up on this method for more successful ones. The method involves measuring the proper motion of a star in the search for an influence caused by its planets, but unfortunately changes in proper motion are so small that the best current equipment cannot produce reliable enough measurements. This method requires that the planets' orbits should be nearly perpendicular to our line of sight, and so planets detected by it could not be confirmed by other methods.
Radial velocity
The radial velocity method measures variations in the speed with which the star moves away from us or towards us, i.e. the component along the line of sight, of the relative velocity of the star with respect to us. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. Its variations are induced by the planet orbiting the star, because both orbit their mutual barycenter. The velocity of the star around the barycenter is much smaller than that of the planet (the radii of the orbits and hence also the velocities are inversely proportional to the masses). Velocities down to 1 m/s can be detected.
This is the first and by far most successful technique used by planet hunters. It is also known as the "Doppler method" or "Wobble method". But it works only for relatively nearby stars out to about 160 light-years. It easily finds planets that are close to stars, but struggles to detect those orbiting at great distances. The Doppler method can be used to reaffirm findings made by using the transit method.
Gravitational microlensing
The Gravitational microlensing effect occurs when the gravitational field of a planet and its parent star act to magnify the light of a distant background star. For the effect to work the planet and star must pass almost directly between the observer and the distant star; since such events are rare, a very large number of distant stars must be continuously monitored in order to detect planets at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars.
Gravitational microlensing has a checkered past. In 1986, Bohdan Paczynski of Princeton University first proposed using it to look for mysterious dark matter, the unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets. Successes with the gravity lensing method date back to 2002, when a group of Polish astronomers (Professors Andrzej Udalski and Marcin Kubiak and Dr. Michal Szymanski from Warsaw, and Polish-American Professor Bohdan Paczynski from Princeton) during project OGLE (the Optical Gravitational Lensing Experiment) perfected a workable method. During one month they claimed to find 46 objects, many of which could be planets.
Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than 1,000 stars have been detected in microlensing relationships over the past ten years.
The key advantage of gravitational microlensing is that it allows low mass (i.e. earth-mass) planets to be detected using available technology. A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations would not be possible. However, if enough background stars can be observed with enough accuracy then the method can be used to determine how common earth-like planets are in the galaxy.
In addition to the NASA/National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics (MOA) group is working to perfect this technique. Astronomers expect that it may be possible to detect an earth-sized world within five years.
Issue #45 -
Extrasolar planet - Cont'd
Methods of detection - Cont'd
Transit method
The most recently developed method detects a planet's shadow when it transits in front of its host star. This "transit method" works only for the small percentage of planets whose orbits happen to be perfectly aligned from our vantage point, but can be used on very distant stars. It is expected to lead to the first detection of an Earth-size planet when employed by NASA's forthcoming space-based Kepler observatory.
Most of these extrasolar planets found are of relatively high mass (at least 40 times that of the Earth); however, a couple seem to be approximately the size of the Earth. This reflects the current telescope technology, which is not able to detect smaller planets. The mass distribution should not be taken as a reference for a general estimate, since it is likely that many more planets with smaller mass, even in nearby planetary systems, are still undetected.
The Kepler Space Mission is a space-based telescope set to launch in 2007. It designed specifically to search large numbers of stars for Earth-sized terrestrial planets using the transit method.
Circumstellar disks
An even newer approach is that of studying dust clouds. Many solar systems contain a significant amount of space dust that is present due to frequent dust generation activity such as comets, asteroid and planetary collisions. This dust forms as a disc around a star and absorbs regular star light and re-emits it as infrared radiation. These dust clouds can provide invaluable information through studies of their density and distortion, caused either by an orbiting planet "catching" the dust, or distortion due to gravitational influences of orbiting planets.
Unfortunately this method can only be employed by space based observations because our atmosphere absorbs most infrared radiation making ground based observation impossible. Our own solar system contains enough dust to make up about 1/10th the mass of our moon. Despite this mass being negligible its surface area is so great that at a distance, its infrared emissions would outshine all our planets by a factor of 100.
The Hubble Space Telescope is capable of these observations using its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument, but was unable to do so due to a cooling unit malfunction that left NICMOS inoperative between 1999 and 2002. Even better images were then taken by its sister instrument, the Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility), in 2003. The Spitzer Telescope was designed specifically for use in the infrared range and probes far deeper into the spectrum than the Hubble Space Telescope is capable of.
Direct Observation
In March 2005 it was announced that scientists using the Spitzer Space Telescope were able to detect infrared radiation emitted from two extrasolar planets. The two teams, from the Harvard-Smithsonian Center for Astrophysics, led by Dr. David Charbonneau and the Goddard Space Flight Center, led by Dr. L. Drake Deming studied the planets HD 209458b and TrES-1.
They were able to measure the temperatures of the planets: 1,060 kelvins (1,450°F) for TrES-1 and about 1,130 kelvins (1,570°F) for HD 209458b.
In April 2005 astronomers using the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile announced they had taken the first photograph of an extrasolar planet orbiting the star GQ Lupi.
The planet's mass is estimated at one to two times that of Jupiter and it orbits GQ Lupi three times farther than Neptune orbits our Sun. A more precise determination of the planet's mass is required to rule out the possibility it could be a brown dwarf. Its surface temperature has been measured at 2,000 kelvins (3,140°F).
The work was led by Ralph Neuhaeuser of the Astrophysical Institute & University Observatory Jena (AIU). Images were also supplied by the Hubble Space Telescope and the Japanese Subaru Telescope.
GQ Lupi is part of a star-forming region about 400 light-years away. It is a T Tauri star with a mass approximately 70 percent of the mass of the Sun and is only 1 million years old.
Issue #45 -
Astronomy Picture of the Day
2005 April 8
Sideways Galaxy NGC 3628
Explanation: Dark dust lanes cutting across the middle of this gorgeous island universe strongly hint that NGC 3628 is a spiral galaxy seen sideways. About 35 million light-years away in the northern springtime constellation Leo, NGC 3628 also bears the distinction of being the only member of the well known Leo triplet of galaxies not in Charles Messier's famous catalog. Otherwise similar in size to our Milky Way galaxy, the disk of NGC 3628 is clearly seen to fan out near the galaxy's edge. A faint arm of material also extends up and to the left in this deep view of the region. The distorted shape and tidal tail suggest that NGC 3628 is interacting gravitationally with the other spiral galaxies in the Leo triplet, M66 and M65.
Credit & Copyright: Russell Croman -
Extrasolar planet - Cont'd
Solar system formation processes
One question raised by the detection of extrasolar planets is why so many of the detected planets are gas giants which, in comparison to Earth's solar system, are unexpectedly close to the orbited star. For example, Tau Boötis has a planet 4.1 times Jupiter's mass, which is less than a quarter of an astronomical unit (AU) from the orbited star, that is closer to the star than Mercury orbits the sun. HD 114762 has a planet 11 times Jupiter's mass, which is less than half an AU from the orbited star. One possible answer to these unexpected planetary orbits is that since astrometrics detects the extrasolar planets due to their gravitational influences and partially-ecliptic interference, perhaps current technology only permits the detection of systems where a large planet is close to the orbited star, rather than such systems being the norm.
The frequency of extrasolar planets is one of the parameters in the Drake equation, which attempts to estimate the probability of communications with extraterrestrial intelligence.
Notable extrasolar planets
- In 1992, Wolszczan and Frail published results indicating that pulsar planets existed around PSR B1257+12 in Nature, volume 355, 145-147. Wolszczan had discovered the millisecond pulsar in question in 1990 at the Arecibo radio observatory. These were the first exoplanets ever verified, all the much more rare, that they orbit a pulsar.
- The first verified discovery of an exoplanet (51 Pegasi B) orbiting a main sequence star (51 Pegasi) was announced on October 6, 1995 by Michel Mayor and Didier Queloz in Nature, volume 378, page 355.
- A microlensing event in 1996 of the gravitationally lensed quasar Q0957+561, observed by R. E. Schild in the A lobe of the double imaged quasar, has led to a controversial, and unconfirmable speculation that a 3 Earth mass planet is possibly in the unknown lensing galaxy, between Earth and the quasar. This would be the most distant planet, if it could be confirmed, and is assumed to reside at redshift 0.39; 2.4 Gpc away (7.8 billion light years or 74 Ym), where the lensing galaxy is. The double-image quasar itself, called The Twin Quasar or Old Faithful, Q0957+561 A/B, resides at redshift z=1.41.
- In 1999, HD 209458b was the first exoplanet seen transitting its parent star, conclusively proving that the radial velocity measurements that were planets actually were planets.
- On November 27, 2001, astronomers using the Hubble Space Telescope announced that they had detected the atmosphere of the planet orbiting HD 209458 (known as HD 209458b and provisionally dubbed "Osiris"). Also during that year, a star was located which had the remnants of one or more planets within the stellar atmosphere — apparently the planet was mostly vaporized. It has been suggested that there may be planets that orbit so closely to their suns that most of their mass has been stripped away by the heat, provisionally referred to as Chthonian planets.
- On July 10, 2003, using information obtained from the Hubble Space Telescope, scientists discovered the oldest extrasolar planet yet. Dubbed Methuselah after the biblical figure, the planet is about 5,600 light years from Earth, has a mass twice that of Jupiter, and is estimated to be 13 billion years old. It is located in the globular star cluster M4, in the constellation Scorpius.
- On April 15, 2004, separate teams announced the discoveries of three planets outside our solar system.
-- One of which is 17,000 light years away, more than three times farther away than the previous record holder. The background star that was used in the gravitational lensing is 24,000 light-years away. The newly-discovered exoplanet is estimated to be about 1.5 times the mass of Jupiter and presumed to be similarly gaseous. It orbits the star about 3 astronomical units (AU). Jupiter is 5.2 AU from the Sun.
-- The same day, a European team of planet hunters based at the Geneva Observatory found two giant planets using the transit method. Both planets are called "hot Jupiters," close to one Jupiter-mass but orbiting its star so closely that it completes an orbit in less than two earth days.
- In August 2004, a planet orbiting mu Arae with a mass of approximately 14 times that of the Earth (see link) was discovered with the ESO HARPS spectrograph. It is the lightest extrasolar planet orbiting a main sequence star to be discovered to date, and could be the first terrestrial planet around a main sequence star found outside the solar system.
- In August 2004, a planet was discovered using the transit method with the smallest aperture telescope to date, 4 inches. The planet was discovered by the TrES survey, and provisionally named TrES-1, orbits the star GSC 02652-01324. The finding was confirmed by the Keck Observatory, where planetary specifics were uncovered.
Candidate planets
- On September 10, 2004, a team of European and U.S. astronomers announced (see link) what may be the first direct optical observation of an extrasolar planet, orbiting the brown dwarf 2M1207, 230 light-years from Earth. Its spectrum, which shows the presence of water, and other characteristics make the object almost certainly a planet, but further observations will be required to confirm that it does in fact orbit the star next to which it was photographed. The planet is believed to be a young gas giant approximately 5 times as massive as Jupiter, and to have an orbital radius of approximately 55 AU, which would also make it farther from its host star than any exoplanet previously detected by a factor of nine.
- In April 2005, a team of European astronomers released the first direct image of an extrasolar planet, GQ Lupi b, orbiting the main sequence star GQ Lupi, 400 light-years from Earth. Its orbital radius is approximately 100 AU.
Issue #46 -
Astronomy Picture of the Day
2005 April 9
Inside the Elephant's Trunk
Explanation: In December of 2003, the world saw spectacular first images from the Spitzer Space Telescope, including this penetrating interior view of an otherwise opaque dark globule known as the Elephant's Trunk Nebula. Seen in a composite of infrared image data recorded by Spitzer's instruments, the intriguing region is embedded within the glowing emission nebula IC 1396 at a distance of 2,450 light-years toward the constellation Cepheus. Previously undiscovered protostars hidden by dust at optical wavelengths appear as bright reddish objects within the globule. Shown in false-color, winding filaments of infrared emission span about 12 light-years and are due to dust, molecular hydrogen gas, and complex molecules called polycyclic aromatic hydrocarbons or PAHs. The Spitzer Space Telescope was formerly known as the Space Infrared Telescope Facility (SIRTF) and is presently exploring the Universe at infrared wavelengths. Spitzer follows the Hubble Space Telescope, the Compton Gamma-ray Observatory, and the Chandra X-ray Observatory as the final element in NASA's space-borne Great Observatories Program.
Credit: W. Reach (SSC/Caltech) et al., JPL, Caltech, NASA -
Gas giant
A gas giant is a large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a rocky or metallic core—in fact, it is expected that such a core is probably required for a gas giant to form—but the majority of its mass is in the form of gas (or gas compressed into a liquid state). Unlike rocky planets, gas giants do not have a well-defined surface. Terms like diameter, surface area, volume, surface temperature and surface density may refer to the outermost layer visible from outside, e.g. from the Earth.
There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. These are also known as the Jovian planets.
The four gas giants in our solar system: (from top) Neptune, Uranus, Saturn, and Jupiter.
Uranus and Neptune have been referred by scientists in the past as a separate subclass of giant planets, ice giants or Uranian planets due to their structure made mostly of ice and rock and gas, which differs from the "traditional" gas giant such as Jupiter or Saturn. This is because their proportion in hydrogen and helium is much lower than the latter's, mostly because of their greater distance from the Sun.
Common structure
The four solar system gas giants share a number of features in common. All have atmospheres that are mostly hydrogen and helium, and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body. They have very hot interiors, ranging from about 5000 K for Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their atmospheres, the planets are most likely entirely liquid. When discussions refer to a "rocky core", one should not picture a ball of solid granite, or even, at 20,000 K, liquid granite. Rather, what is meant is a region in which the concentration of heavier elements such as iron and silicon is greater than that in the rest of the planet.
All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent in Jupiter, muted in Saturn and Neptune, and barely detectable at all in Uranus.
Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and the only ones known before the 1970s. As of 2004, Jupiter was thought to have the most moons, with more than 60 found.
Jupiter and Saturn
These consist almost entirely of hydrogen and helium, and they are so large that one can still say this even though both are thought to have several Earth masses of heavier elements. Their deep interiors consist of liquid metallic hydrogen, a form of hydrogen distinguished by the fact that it conducts electricity.
Both have magnetic fields oriented fairly close to their axes of rotation.
Uranus and Neptune
Uranus and Neptune have distinctly different interior compositions, with the bulk of their interiors thought to consist of a mixture (or layered assortment) of rock, water, methane, and ammonia. Both have magnetic fields that are sharply inclined to their axes of rotation.
Terminology
The term was coined by the science fiction writer James Blish. Arguably it is a misnomer, since all of these planets are primarily liquid and not gaseous. In fact, for Neptune and Uranus, the gaseous atmospheres are quite thin compared to the planetary radii -- only extending perhaps one percent of the way to the center. However, at least for Jupiter and Saturn, the name is defensible because their compositions are dominated by hydrogen and helium, which are gases in the outer solar system when not under pressure.
Planetary scientists often use 'rock', 'gas', and 'ice' as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase they appear in. In the outer solar system, hydrogen and helium are "gases"; water, methane, and ammonia are "ices"; and silicates are rock. When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they mean hydrogen and helium.
With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants", to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.
Extrasolar gas giants
Because of the techniques currently available to detect extrasolar planets, all of those found to date have been of a scale associated in the Solar system to gas giants. The smallest found as of September 2004 is comparable in mass to Neptune, and many have masses several times that of Jupiter. Many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in the solar system. Considering the relative abundances of the elements in the universe (approximately 90% hydrogen), it would be surprising to find a predominantly rocky planet more massive than Jupiter. On the other hand, previous models of planetary system formation suggested that gas giants would be inhibited from forming as close to their stars as have many of the new planets that have been observed.
The gas giants against the Sun's limb. The diameters are to scale. The limb of the Sun is in the background. From left to right, Jupiter, Saturn, Uranus and Neptune.
The upper mass limit of a gas giant planet is approximately 70 times that of Jupiter (around 0.08 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begins to induce nuclear fusion and the planet ignites to become a red dwarf. Interestingly there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs. This has led to suggestions that the formation process for planets and binary stars may be fundamentally different.
Issue #47 -
Astronomy Picture of the Day
2005 April 10
Venus' Once Molten Surface
Explanation: If you could look at Venus with radar eyes - this is what you might see. This computer reconstruction of the surface of Venus was created from data from the Magellan spacecraft. Magellan orbited Venus and used radar to map our neighboring planet's surface between 1990 and 1994. Magellan found many interesting surface features, including the large circular domes, typically 25-kilometers across, that are depicted above. Volcanism is thought to have created the domes, although the precise mechanism remains unknown. Venus' surface is so hot and hostile that no surface probe has lasted more than a few minutes.
Credit: E. De Jong et al. (JPL), MIPL, Magellan Team, NASA -
Planetary differentiation
In planetary science, planetary differentiation is a process by which the denser portions of a planet will sink to the center; while less dense materials rise to the surface. Such a process tends to create a core, crust, and mantle.
Heating
When the Sun ignited in solar nebula, hydrogen, helium and other volatile materials were evaporated in the area near the Sun. The solar wind and light pressure forced such material of low density away from the Sun. Rocks, and material trapped in them, accumulated in protoplanets.
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center; while lighter elements rose to the surface. Composition of some meteorites show that differentiation took place in some asteroids.
When protoplanets accrete more material, the energy at impact causes local heating. In addition to this temporary heating, when a body is large enough then the gravitational force upon a new lump on the surface will create pressures and temperatures which are sufficient to melt some of the materials. This allows chemical reactions and density differences to mix and separate materials, and soft materials to spread out over the surface.
On Earth, a large piece of molten iron is sufficiently more dense than continental crust material that it can force its way down through the crust to the mantle. In the outer solar system similar effects may take place but the materials may be hydrocarbons such as methane, water ice, or frozen carbon dioxide.
Chemical differentiation
Note that some materials may differentiate to regions due to their chemical affinities rather than their densities, "carried along" by other materials that they're associated with; the uranium in Earth's crust is an example.
Physical differentiation
Gravitational separation
Materials with a high density tend to sink through lighter materials. This tendency is affected by the relative structural strengths, but such strength is reduced at temperatures where both materials are plastic or molten.
Lighter materials try to rise through material with a higher density. On Earth, salt domes are salt deposits in the crust which rise through surrounding rock. Diapirs of other materials exist, and sometimes appear on the surface as mud volcanos.
Moon's KREEP
On the Moon, a material formed on the surface which is believed to have formed due to its components being incompatible with the cooling molten material. The material is high in potassium (periodic table symbol K), rare earth elements, and phosphorus and is often referred to with the abbreviation KREEP. It also is high in uranium and thorium.
Fractional crystallization
On Earth, when magma rises above a certain depth the dissolved materials may crystallize at certain pressures and temperatures. The resulting solids remove various elements from the melt, and melt which returns to the mantle is thus depleted of those elements.
Thermal diffusion
The Soret effect is displayed when material is unevenly heated. Lighter material migrates toward hotter zones and heavier material migrates toward colder areas.
Differentiation through collision
The Earth's Moon seems to have been formed out of material splashed into orbit by the impact of a large body into the early Earth. Differentiantion on Earth had probably separated many lighter materials toward the surface, so the splash probably removed many light materials from the planet. The Moon's density is substantially less than that of Earth. The Earth's crust is probably much thinner than it otherwise would have been, and plate tectonics functions on this planet differently than it could on Venus or Mars.
Density differences on Earth
On Earth, such processes created a surface density of 3000kg/m³, while the average density of the planet is 5515kg/m³.
Issue #48 -
Astronomy Picture of the Day
2005 April 11
Clouds, Plane, Sun, Eclipse
Explanation: How can part of the Sun just disappear? When that part is really hiding behind the Moon. Last Friday, the first partial solar eclipse of 2005 and the last total eclipse of the Sun until March 2006 was visible. During a solar eclipse, the Sun, Moon and Earth are aligned. The total solar eclipse was primarily visible from the Southern Pacific Ocean, while a partial solar eclipse was discoverable across South America and lower North America. The above image composite was taken with a handheld digital camera last Friday. After a day of rain in Mt. Holly, North Carolina, USA, a partially eclipsed Sun momentarily peaked through a cloudy sky. After taking a sequence of images, the best eclipse shot was digitally combined with a less good eclipse shot that featured a passing airplane.
Credit & Copyright: Dorothy Verdin -
Precession
Precession (also called gyroscopic precession) is the phenomenon by which the axis of a spinning object (e.g. a part of a gyroscope) "wobbles" when a torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession. As a spinning object precesses, the tilt of its axis goes around in a circle in the opposite direction that the object is spinning. If the speed of the rotation and the magnitude of the torque are constant the axis will describe a cone, its movement at any instant being at right angles to the direction of the torque. In the case of a toy top, if the axis is not perfectly vertical the torque is applied by the force of gravity trying to tip it over. A rolling wheel will tend to remain upright due to precession. When the wheel tilts to one side, the particles at the top are pushed to one side and the particles at the bottom are pushed the other way. However, since the wheel is rotating, these particles eventually switch places and cancel one another out. Precession or gyroscopic considerations may have a minor effect on bicycle performance at high speed. Precession is also the mechanism behind gyrocompasses.
The physics of precession
Precession is due to the fact that the resultant of the angular velocity of rotation and the angular velocity produced by the torque is an angular velocity about a line which makes an angle with the permanent rotation axis, and this angle lies in a plane at right angles to the plane of the couple producing the torque. The permanent axis must turn towards this line, since the body cannot continue to rotate about any line which is not a principal axis of maximum moment of inertia; that is, the permanent axis turns in a direction at right angles to that in which the torque might be expected to turn it. If the rotating body is symmetrical and its motion unconstrained, and if the torque on the spin axis is at right angles to that axis, the axis of precession will be perpendicular to both the spin axis and torque axis. Under these circumstances the period of precession is given by:
In which Is is the moment of inertia, Ts is the period of spin about the spin axis, and Q is the torque. In general the problem is more complicated than this, however.
Issue #49 -
Astronomy Picture of the Day
2005 April 12
Earth or Mars?
Explanation: Which image is Earth, and which is Mars? One of the above images was taken by the robot Spirit rover currently climbing Husband Hill on Mars. The other image was taken by a human across the desert south of Morocco on Earth. Both images show vast plains covered with rocks and sand. Neither shows water or obvious signs of life. Each planet has a surface so complex that any one image does not do that planet justice. Understanding either one, it turns out, helps understand the other. Does the one on the left look like home? Possibly not, but it is Earth.
Earth Image Credit & Copyright: Filipe Alves;
Mars Image Credit: Mars Exploration Rover Mission, JPL, NASA -
Precession - Cont'd
Precession of the equinoxes
The axis of the Earth undergoes precession due to a combination of the Earth's nonspherical shape (it is an oblate spheroid, bulging outward at the equator) and the gravitational tidal forces of the Moon and Sun applying torque as they attempt to pull the equatorial bulge into the plane of the ecliptic. The Earth goes through one complete precession cycle in a period of approximately 25,800 years, during which the positions of stars as measured in the equatorial coordinate system will slowly change; the change is actually due to the change of the coordinates. Over this cycle the Earth's north axial pole moves from where it is now, within 1° of Polaris, in a circle around the ecliptic pole, with an angular radius of about 23.5 degrees. The shift is 1 degree in 180 years (the angle is taken from the observer, not from the center of the circle).
Polaris is not particularly well-suited for marking the north celestial pole, as its visual magnitude, which is variable, hovers around 2.1, fairly far down the list of brightest stars in the sky. On the other hand, in 3000 BC the faint star Thuban in the constellation Draco was the pole star; at magnitude 3.67 it is five times fainter than Polaris; today it is all but invisible in light-polluted urban skies. The brightest star known to have been North Star or to be predictable as taking that role in the future is the brilliant Vega in the constellation Lyra, which will be the pole star around the year 14,000 AD. When viewed looking down onto the Earth from the north, the direction of precession is clockwise. When standing on Earth looking outward, the axis appears to move counter-clockwise across the sky. This sense of precession, against the sense of Earth's own axial rotation, is opposite to the precession of a top on a table. The reason is that the torques imposed on the Earth by the Sun and Moon act in the sense of trying to align its axis normal to the ecliptic, i.e. to stand up more vertically in regards to the ecliptic plane, while the torque on a top spinning on a hard surface acts in the sense of trying to make the top fall over, rather than to stand up straighter.
Polaris is not exactly at the pole; any long-exposure unguided photo will show it having a short trail. It is close enough, though. The south celestial pole precesses too, always remaining exactly opposite the north pole. The south pole is in a particularly bland portion of the sky, and the nominal south pole star is Sigma Octantis, which, while fairly close to the pole, is even weaker than Thuban -- magnitude 5.5, which is barely visible even under a properly dark sky. The precession of the Earth is not entirely regular due to the fact that the Sun and Moon are not in the same plane and move relative to each other, causing the torque they apply to Earth to vary. This varying torque produces a slight irregular motion in the poles called nutation.
Precession of the Earth's axis is a very slow effect, but at the level of accuracy at which astronomers work, it does need to be taken into account. Note that precession has no effect on the inclination ("tilt") of the plane of the Earth's equator (and thus its axis of rotation) on its orbital plane. It is 23.5 degrees and precession does not change that. The inclination of the equator on the ecliptic does change due to gravitational torque, but its period is different (main period about 41000 years).
The following figure illustrates the effects of axial precession on the seasons, relative to perihelion and aphelion. The precession of the equinoxes can cause periodic climate change (see Milankovitch cycles), because the hemisphere that experiences summer at perihelion and winter at aphelion (as the southern hemisphere does presently) is in principle prone to more severe seasons than the opposite hemisphere.
Hipparchus first estimated Earth's precession around 130 BC, adding his own observations to those of Babylonian and Chaldean astronomers in the preceding centuries. In particular they measured the distance of the stars like Spica to the Moon and Sun at the time of lunar eclipses, and because he could compute the distance of the Moon and Sun from the equinox at these moments, he noticed that Spica and other stars appeared to have moved over the centuries.
Precession causes the cycle of seasons (tropical year) to be about 20.4 minutes less than the period for the earth to return to the same position with respect to the stars as one year previously (sidereal year). This results in a slow change (one day per 58 calendar years) in the position of the sun with respect to the stars at an equinox. It is significant for calendars and their leap year rules.
Precession of planetary orbits
The revolution of a planet in its orbit around the Sun is also a form of rotary motion. (In this case, the combined system of Earth and Sun is rotating.) So the axis of a planet's orbital plane will also precess over time.
The major axis of each planet's elliptical orbit also precesses within its orbital plane, in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called "perihelion precession." Discrepancies between the observed perihelion precession rate of the planet Mercury and that predicted by classical mechanics were prominent among the forms of experimental evidence leading to the acceptance of Einstein's Theory of Relativity, which predicted the anomalies accurately.
The precession of the orbit of the Earth is an important part of the astronomical theory of ice ages. It is generally understood that the gravitational pulls of the sun and the moon cause the precession of the equinoxes; these operate on cycles of 23,000 and 19,000 years.
Precession is also an important consideration in the dynamics of atoms and molecules.
Issue #50 -
Astronomy Picture of the Day
2005 April 13
A Window to the Once Secret Sky
Explanation: If there was a window nearby to the distant universe -- would you look through it? Quite possibly, there is, in the form of a small telescope. A local skykeeper could be a relative or a stranger and is frequently proud to show off the sky free of charge. Through a window called an eyepiece, on a dark cloudless night, you can see clusters of stars, rings around Saturn, glowing nebulas of gas, craters on the Moon, and galaxies across the universe. The technology to create this window -- and the secret sky it reveals -- was unknown only 400 years ago. Modern sky opportunities may occur this Saturday, Astronomy Day, at local amateur astronomy clubs, universities, science centers, or planetariums. Pictured above is a small telescope being deployed at picturesque Hohe Wand, about 50 kilometers south of Vienna, Austria. The spin of the Earth is visible in the above photo as the long star trails.
Credit & Copyright: Peter Wienerroither (U. Wien) -
Geosynchronous orbit
A geosynchronous orbit is a geocentric orbit that has the same orbital period as the sidereal rotation period of the Earth. It has a semi-major axis of 42,164 km.
Synchronous orbits exist around all moons, planets, stars and black holes —unless they rotate so slowly that the orbit would be outside their Hill sphere. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing Lagrange points. Objects with chaotic rotations (such as Hyperion) are also problematic, as their synchronous orbits keep changing unpredictably.
If a geosynchronous orbit is circular and equatorial then it is also a geostationary orbit, and will maintain the same position relative to the Earth's surface. If one could see a satellite in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not exhibit diurnal motion, while one would see the Sun, Moon, and stars traverse the heavens behind it.
A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the centre of the Earth) or approximately 35,790 km (22,240 statute miles) above mean sea level.
This can be demonstrated analytically by application of the Law of Gravity and the physics of centripetal acceleration. Drawing the free body diagram and using the analysis methods of engineering dynamics and physics allows the determination of the distance from Earth's centre of mass which will satisfy this specified operating condition.
Circular geosynchronous orbits
Circular geosynchronous orbits at the equator are known as geostationary orbits. A perfect stable geostationary orbit is an ideal that can only be approximated. In practice the satellite will drift out of this orbit (because of perturbations such as the solar wind, radiation pressure, and the gravitational effect of the Moon), and thrusters are used to maintain the orbit.
Other geosynchronous orbits
Elliptical orbits can be and are designed for communications satellites that keep the satellite within view of its assigned ground stations or receivers. A satellite in an elliptical geosynchronous orbit will appear to oscillate in the sky from the viewpoint of a ground station, tracing an analemma in the sky. Satellites in highly elliptical orbits must be tracked by steerable ground stations.
Theoretically an active geosynchronous orbit can be maintained if forces other than gravity are also used to maintain the orbit, such as a solar sail. Such a statite can be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the conic section orbit formed by a gravitational body. Such devices are still theoretical.
A further form of geosynchronous orbit is obtained by the theoretical space elevator in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone if under tension.
History
Author Arthur C. Clarke is credited with popularizing the notion of using a geostationary orbit for communications satellites. The orbit is also known as the Clarke Orbit.
The first communications satellite placed in a geosynchronous orbit was Syncom 2, launched in 1963. Geosynchronous orbits have been in common use ever since, including satellite television.
Initially, geostationary satellites also carried telephone calls but are no longer used so predominantly for voice communication, partly due to the inherent disconcerting delay in getting information to the satellite and back (it takes light or radio about a quarter of a second to make the round trip).
Nearly all locations on the planet now have terrestrial communications facilities (microwave, fibre-optics), even undersea, with more than sufficient capacity. Satellite telephony is now mainly limited to small, isolated locations that have no terrestrial facilities, such as Canada's arctic islands, Antarctica, and the far reaches of Alaska and Greenland. These locations still use satellite telephony, and innovations have made it possible for Internet transmissions to cope with the time delay.
Issue #51 -
Astronomy Picture of the Day
2005 April 14
April's Moon and the Pleiades
Explanation: After parting with the Sun late last week, April's moon graced the early evening sky. Its slender, three-day-old crescent shares this lovely telescopic skyview with the nearby Pleiades star cluster. Here, the Moon's sunlit crescent is overexposed while the lunar terminator, or boundary between lunar night and day, is jagged with craters and mountains. Lunar surface features can also be seen in the dim lunar night illuminated by earthshine - light from sunlit planet Earth. The sister stars of the Pleiades are grouped at the right, but their alluring blue reflection nebulae, usually highlighted in telescopic images of the cluster, are washed-out in the much brighter moonlight.
Credit & Copyright: Jerry Lodriguss (Catching the Light) -
Geosynchronous satellite
A geosynchronous satellite is a satellite whose orbital speed equals the Earth's rotational speed. If such a satellite's orbit lies over the equator, it is called a geostationary satellite. The orbits are known as geosynchronous orbit and geostationary orbit.
Definition
According to Kepler's Third Law, the orbital period of a satellite in a circular orbit increases with increasing altitude. Space stations and shuttles in Low Earth Orbit (LEO), typically two or four hundred miles above the Earth's surface make between fifteen and sixteen revolutions per day. The Moon, at an altitude of about 240,000 miles (385,000km), takes thirty days to make a complete rotation. Between those extremes lies the "magic" altitude of 22,300 miles (35,786km) at which a satellite's orbital speed exactly matches the rate at which the earth rotates: once every sidereal day (23 hours 56 minutes). In that case, the satellite is said to be geosynchronous.
If a geosynchronous satellite's orbit is not exactly aligned with the equator, known as an inclined orbit, it will appear (when viewed by someone on the ground) to oscillate daily around a fixed point in the sky. As the angle between the orbit and the equator decreases, the magnitude of this oscillation becomes smaller; when the orbit lies entirely over the equator, the satellite remains stationary relative to the Earth's surface – it is said to be geostationary.
Application
There are about 200 geosynchronous satellites.
Geostationary satellites appear to hover over one spot above the equator. Receiving and transmitting antennae on the earth do not need to track such a satellite. These antennae can be fixed in place and are much less expensive than tracking antennae. These satellites have revolutionized global communications, television broadcasting and weather forecasting, and have a number of important defense and intelligence applications.
One disadvantage of geosynchronous satellites is a result of their high altitude: radio signals take approximately 1/4 of a second to reach and return from the satellite, resulting in a small but significant signal delay. This delay increases the difficulty of telephone conversation and reduces the performance of common network protocols such as TCP/IP, but does not present a problem with non-interactive systems such as television broadcasts. There are a number of proprietary satellite data protocols that are designed to proxy TCP/IP connections over long-delay satellite links -- these are marketed as being a partial solution to the poor performance of native TCP over satellite links.
History
The concept was first proposed by the science fiction author Arthur C. Clarke around 1945, based on Herman Potocnik's previous work. Working prior to the advent of solid-state electronics, Clarke envisioned a trio of large, manned space stations arranged in a triangle around the planet. Modern satellites are numerous, unmanned, and often no larger than an automobile.
The first geosynchronous satellite was Syncom 2, launched on a Delta rocket B booster from Cape Canaveral 26 July 1963. It was used a few months later for the world's first satellite relayed telephone call, between U.S. President John F. Kennedy and Nigerian Prime minister Abubaker Balewa.
Issue #52 -
Astronomy Picture of the Day
2005 April 15
RCW 79: Stars in a Bubble
Explanation: A cosmic bubble of gas and dust, RCW 79 has grown to about 70 light-years in diameter, blown by the winds and radiation from hot young stars. Infrared light from the dust embedded in the nebula is tinted red in this gorgeous false-color view from the Spitzer Space Telescope. A good 17 thousand light-years away in the grand southern constellation Centaurus, the expanding nebula itself has triggered star formation as it plows into the gas and dust surrounding it. In fact, this penetrating infrared picture reveals groups of new stars as yellowish points scattered along the bubble's edge. One remarkable group still lies within its own natal bubble at about 7 o'clock (lower left), while another can be seen near the upper gap at about 3 o'clock (right) from the bubble's center.
Credit: E. Churchwell (Univ. Wisconsin-Madison) et al., JPL-Caltech, NASA -
Terrestrial planet
A terrestrial planet or telluric planet is a planet which is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which may not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes.
Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed. Only one terrestrial planet, Earth, is known to have an active hydrosphere.
Portrait of the 29 small bodies of the solar system of 400+ km diameter, at the scale 1 px = 100 km.
The gas giants and the numerous trans-neptunian objects (other than Pluto and Charon) of sizable dimension have been excluded.
From left to right and top to bottom:
- 1st row: Mercury, Venus, the Earth and the Moon, Mars, the four main belt asteroids of 400+ km size (1 Ceres, 2 Pallas, 4 Vesta and 10 Hygiea)
- 2nd row: Io, Europa, Ganymede, Callisto, Mimas
- 3rd row: Enceladus, Tethys, Dione, Rhea, Titan
- 4th row: Iapetus, Miranda, Ariel, Umbriel, Titania
- 5th row: Oberon, Proteus, Triton, Pluto and Charon
By coincidence, the spacing between the objects matches closely the Pluto-Charon distance. The curve of the Sun's limb is in the background.
NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets outside of our solar system (orbiting other stars). Three possible terrestrial planets have been discovered orbiting the stars Mu Arae, 55 Cancri and GJ 436. All other known extrasolar planets are extremely large, and are most likely to be gas giants.
Issue #53 -
Astronomy Picture of the Day
2005 April 16
Celebrating Hubble With NGC 6751
Explanation: Planetary nebulae can look simple, round, and planet-like in small telescopes. But images from the orbiting Hubble Space Telescope have become well known for showing these fluorescent gas shrouds of dying Sun-like stars to possess a staggering variety of detailed symmetries and shapes. This composite color Hubble image of NGC 6751 is a beautiful example of a classic planetary nebula with complex features. It was selected in April of 2000, to commemorate the tenth anniversary of Hubble in orbit. The colors were chosen to represent the relative temperature of the gas - blue, orange, and red indicating the hottest to coolest gas. Winds and radiation from the intensely hot central star (140,000 degrees Celsius) have apparently created the nebula's streamer-like features. The nebula's actual diameter is approximately 0.8 light-years or about 600 times the size of our solar system. NGC 6751 is 6,500 light-years distant in the high-flying constellation Aquila.
Credit: A. Hajian (USNO) et al., Hubble Heritage Team (STScI/ AURA), NASA -
Astrobiology
Astrobiology (in Greek astron = star, bios = life and logos = word/science), also known as exobiology (Greek: exo = out) or xenobiology (Greek: xenos = foreign) is a speculative field within biology which considers the possible variety of extraterrestrial life. It also includes the concept of artificial life, since any life form that might naturally evolve elsewhere could conceivably be created in a laboratory using a future technology. It might be difficult to tell whether a truly strange life form had in fact arisen in space, or was designed much nearer to home.
The term is also used in science fiction, with the difference that is not speculative, but an established science. A xenobiologist is usually a human doctor or biologist who is expert on the physiology of alien organisms and life forms.
Overview
Although this is currently a speculative field, the presence of life in the rest of the Universe is a verifiable hypothesis (though it has yet to be verified), making xenobiology a valid field for scientific enquiry. Likewise, computer simulations of basic life processes have made it possible to do exploratory engineering of alternate life forms (like left-handed DNA) to determine their characteristics.
Research primarily involves studying data from telescopes and planetary space probes, comparing the conditions on other planets in the solar system with extreme environments on Earth. Microbes, classed as extremophiles, have been found in apparently inhospitable environments such as frozen lakes, within rocks, at extreme depths in the oceans, at deep-sea hydrothermal vents and in geysers, among others. Several planets in the Solar System have similar conditions and could well harbour similar life forms. Such research is conducted at many universities around the world.
Missions that have specifically searched for life include the Viking and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to reach the surface of Mars. A future mission with a strong exobiology role will be JIMO, designed to study the frozen moons of Jupiter, some of which may have liquid water.
For these reasons the search for extraterrestrial life is of great relevance to xenobiologists. Some contend that the number of planets with intelligent extraterrestrial life can be estimated from the Drake equation if and when we ascertain the values of its variables. However uncertainties in the term of the equation make it impossible to predict whether life is rare or common. Another associated topic in xenobiology is the Fermi paradox, which suggests that if intelligent life is common in the universe then there should be obvious signs of it.
Xenobiology also figures in much science fiction as the fictional science of the biology of alien organisms. This use of the term demonstrates the speculative generation of possible models of such life, e.g. silicon-based.
Search for extraterrestrial life
As of 2005, there is no definite evidence of extraterrestrial life. However examination of meteors from Antarctica which are presumed to have originated from the planet Mars have provided what some scientists believe to be microfossils of extraterrestrial life, although that interpretation of the evidence is still controversial. In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Methane has a relatively short half-life in the Martian atmosphere, so there must be a recent source of it. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life.
Missions to other planets (such as Beagle 2: Evolution to Mars, Cassini to Saturn's moon Titan, and the future JIMO mission to Jupiter's icy moons) hope to further explore the possibilities of life on other planets in our solar system.
Issue #54 -
Astrosociobiology
Astrosociobiology (also referred to as exosociobiology and xenosociology) is the speculative scientific study of extraterrestrial civilizations and their possible social characteristics and developmental tendencies. The field involves the convergence of astrobiology, sociobiology and evolutionary biology. Hypothesized comparisons between human civilizations and those of extraterrestrials are frequently posited, placing the human situation in the same context as other extraterrestrial intelligences. Whenever possible, astrosociobiologists describe only those social characteristics that are thought to be common (or highly probable) to all civilizations. Thus far, it is entirely theoretical.
Methodologies
Evolutionary Psychology (or Sociobiology) attempts to explain animal behavior, group behavior and social structure in terms of evolutionary advantage or strategy and using techniques from ethology, evolution and population genetics. Sociobiologists are especially interested in comparative analyses, especially in studying human social institutions and culture.
Astrobiology is the speculative field within biology that considers the possible varieties and characteristics of extraterrestrial life. Astrobiologists speculate about the possible ways that organic life could come into being in the universe and the potential for artificial and postbiological life.
Astrosociobiologists, like evolutionary biologists and sociobiologists, are concerned with the phenomenon of convergent evolution--the evolutionary process in which organisms not closely related independently acquire some characteristic or characteristics in common, usually (but not necessarily) a reflection of similar responses to similar environmental conditions. Examples include physical traits that have evolved independently (e.g. the eye), ecological niches (e.g. pack predators), and even technological innovations (e.g. language, writing, the domestication of plants and animals, and basic tools and weapons). Astrosociobiologists take the potential for convergent evolution off-planet and speculate that certain ecological and sociological niches may not be Earth-specific or human-specific and are archetypal throughout the universe.
However, there may be limits to this kind of speculation--particularly if there is a dearth of comparable habitats to our own across the galaxy. Some thinkers, while acknowledging that biological and social evolution may follow similar patterns across the universe, also note the problem of evidence and the absence of extraterrestrial contact. Simon C. Morris, in his book, Inevitable Humans in a Lonely Universe, notes life's "eerie" ability to repeatedly navigate to a single solution. "Eyes, brains, tools, even culture: all are very much on the cards," he writes. "So if these are all evolutionary inevitabilities, where are our counterparts across the galaxy? The tape of life can only run on a suitable planet, and it seems that such Earth-like planets may be much rarer than hoped. Inevitable humans, yes, but in a lonely Universe."
Assumptions
In order for astrosociobiologists to embark on speculations about the condition and characteristics of extraterrestrial civilizations, a number of assumptions are necessarily invoked:
1. Extraterrestrial civilizations exist
2. Extraterrestrial civilizations operate in agreement with the known laws of physics
3. Extraterrestrial civilizations must in some part resemble our own, both in terms of:
3.1. morphological and psychological characteristics, and
3.2. civilizational traits and tendencies
In other words, astrosociobiologists assume that intelligent life arises from similar environmental conditions and similar evolutionary processes as humanity.
It is currently difficult to tell if these are valid assumptions. For example, the Rare Earth hypothesis and the Fermi Paradox suggests that we might be alone in the galaxy. It's also conceivable that aliens and their civilizations may scarcely resemble our own. Astrosociobiology also involves a fair degree of environmental determinism. Astrosociobiologists counterargue that all of these points can be countered by the Copernican principle and the self-sampling assumption (a variant of the anthropic principle). We shouldn't assume, they argue, that we're unique and we should start from the premise that we are very typical.
Issue #55 -
Astrosociobiology - Cont'd
Possible extraterrestrial characteristics
Given these assumptions, astrosociobiologists attempt to make predictions about those characteristics that may be common to all extraterrestrial societies. For example, based on human experience, astrosociobiologists conclude very broadly that all civilizations go through similar developmental stages, including agrarian culture, industrialization, democratization, globalization, and an information age (this being said, it's still not an absolute certainty that democracy is here to stay, nor that ETIs are able to maintain it either; totalitarian and quasi-authoritarian tendencies may in fact be the norm). Similar assumptions are made about the development of technological innovations (universal technological archetypes) and scientific breakthroughs (including the rough chronological order in which these advancements are developed). The possibility also exists for the existence of common cultural and meta-ethical characteristics of advanced societies (i.e. the notion that advanced societies will independently reach the same conclusions about ethics, morality and social imperatives).
Astrosociobiologists also theorize about the existence of developmental mechanisms that constrain and give directionality to the evolution of organisms and society itself. One such guiding evolutionary force is the notion of the megatrajectory. Posited by A. H. Knoll and R. K. Bambach in their 2000 collaboration, "Directionality in the History of Life," Knoll and Bamback argue that, in consideration of the problem of progress in evolutionary history, a middle road that encompasses both contingent and convergent features of biological evolution may be attainable through the idea of the megatrajectory:
We believe that six broad megatrajectories capture the essence of vectoral change in the history of life. The megatrajectories for a logical sequence dictated by the necessity for complexity level N to exist before N+1 can evolve...In the view offered here, each megatragectory adds new and qualitatively distinct dimensions to the way life utilizes ecospace.
According to Knoll and Bambach, the six megatrajectories outlined by biological evolution thus far are:
1. the origin of life to the "Last Common Ancestor"
2. prokaryote diversification
3. unicellular eukaryote diversification
4. multicellular organisms
5. land organisms
6. appearance of intelligence and technology
Some astrosociobiologists, such as Milan Cirkovic and Robert J. Bradbury, have taken the megatrajectory concept one step further by theorizing that a seventh megatrajectory exists: postbiological evolution triggered by the emergence of artificial intelligence at least equivalent to the biologically-evolved one, as well as the invention of several key technologies of the similar level of complexity and environmental impact, such as molecular nanoassembling or stellar uplifting.
Along similar lines, historian of science Steven J. Dick, in his 2003 paper "Cultural Evolution, the Postbiological Universe and SETI," posited a central concept of cultural evolution he called the Intelligence Principle:
The maintenance, improvement and perpetuation of knowledge and intelligence is the central driving force of cultural evolution, and that to the extent intelligence can be improved, it will be improved.
It is through the application of this principle, argues Dick, that speculations about the developmental tendencies of advanced civilizations can be made.
The difficulty of engaging in such speculation, however, is that it is highly theoretical; there is very little empirical evidence. Moreover, humanity hasn't progressed through these later developmental stages. Astrosociobiologists currently have no data to support the idea that human civilization will continue on into the foreseeable future. Indeed, in considering the Fermi Paradox, scientists may actually have a data point suggesting a limitation to how far advanced civilizations can develop.
However, with each advancing step that the human species takes, astrosociobiologists will assume that extraterrestrials--both past and present--will have gone through similar stages.
Civilization types
A method for classifying civilization types was introduced by Russian astronomer Nikolai Kardashev in 1964. Known as the Kardashev scale, classifications are assigned based on the amount of usable energy a civilization has at its disposal and increasing logarithmically:
- Type I - A civilization that is able to harness all of the power available on a single planet, approximately 1E16W.
- Type II - A civilization that is able to harness all of the power available from a single star, approximately 1E26W.
- Type III - A civilization that is able to harness all of the power available from a single galaxy, approximately 1E36W.
Human civilization has yet to achieve full Type I status, as it is able to harness only a portion of the energy that is available on Earth. Carl Sagan speculated that humanity's current civilization type is around 0.7.
A major criticism directed at the Kardashev scale is that the difference between a Type II and Type III civilization is ten orders of magnitude and that significant civilization types likely reside within that range. Moreover, given the seemingly extreme energy sources available to Type II and III civilizations, the question as to why we haven't seen evidence of these advanced societies remains unanswered, a possible indication that no such civilizations exist.
Issue #55 -
Astronomy Picture of the Day
2005 April 17
Asteroids in the Distance
Explanation: Rocks from space hit Earth every day. The larger the rock, though, the less often Earth is struck. Many kilograms of space dust pitter to Earth daily. Larger bits appear initially as a bright meteor. Baseball-sized rocks and ice-balls streak through our atmosphere daily, most evaporating quickly to nothing. Significant threats do exist for rocks near 100 meters in diameter, which strike the Earth roughly every 1000 years. An object this size could cause significant tidal waves were it to strike an ocean, potentially devastating even distant shores. A collision with a Massive asteroid, over 1 km across, is more rare, occurring typically millions of years apart, but could have truly global consequences. Many asteroids remain undiscovered. In fact, one was discovered in 1998 as the long blue streak in the above archival image taken by the Hubble Space Telescope. In 2002 June, the small 100-meter asteroid 2002 MN was discovered only after it whizzed by the Earth, passing well within the orbit of the Moon. 2002 MN passed closer than any asteroid since 1994 XM1, but not as close as 2004 MN4 will pass in 2029. A collision with a large asteroid would not affect Earth's orbit so much as raise dust that would affect Earth's climate. One likely result is a global extinction of many species of life, possibly dwarfing the ongoing extinction occurring now.
Credit: R. Evans & K. Stapelfeldt (JPL), WFPC2, HST, NASA -
Astronomy Picture of the Day
2005 April 18
Saturnian Moon and Rings
Explanation: When can a robot produce art? When it glides past the rings of Saturn, for one. As the robot spacecraft Cassini orbiting Saturn crossed outside the famous photogenic ring plane of the expansive planet, the rings were imaged from the outside, nearly edge on, and in the shadow of Saturn. From the upper left, ring features include the A ring, the Cassini gap, the B ring, and the darker C ring that includes the Titan gap and a gap yet unnamed. Last month when the above image was taken, the gliding spacecraft was about one million kilometers from foreground Enceladus, a small Saturnian moon only about 500 kilometers across. Cassini is scheduled to continue its 70 orbit tour of Saturn over the next three years, sending back images of the gas giant, its rings, and its moons that will be studied for decades to come.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
Fermi paradox
The Fermi paradox is a paradox proposed by physicist Enrico Fermi that questions the probability of finding intelligent extraterrestrial life. More specifically, it deals with attempts to answer one of the most profound questions of all time: "Are we (human beings) the only technologically advanced civilization in the Universe?". The paradox was formulated in response to the Drake equation for estimating the number of extraterrestrial civilizations with which we might come in contact. Subject to the values inserted into this formula, the Drake equation seems to imply that we should not expect such contact to be extremely rare.
Radio telescope observations play a role in researching the Fermi paradox.
The Parkes radio telescope. The collecting area of the telescope is a 64-m diameter paraboloid (the dish). The surface is high precision aluminium millimetre wavepanels to a diameter of 17-m (for operation to 43 GHz), then perforated aluminium plate out to 45-m, and rectangular galvanised steel 5/16-inch mesh for the remainder of the surface. The aerial cabin, which houses feeds and receiver equipment, is supported by a tripod.
The Parkes Observatory is situated near Alectown, 25 kilometres north of the town of Parkes which is approximately 365 kilometres west of Sydney. The Observatory is part of the Australia Telescope National Facility (ATNF) a division of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The Parkes site contains a 64-metre RadioTelescope, an administration building with offices, workshops and library, an Observer's Quarters and Visitors Centre.
Fermi questioned this conclusion. He asked that if there were a multitude of advanced extraterrestrial civilizations in our galaxy, the Milky Way, then, "Where are they? Why haven't we seen any traces of intelligent extraterrestrial life, such as probes, spacecraft or transmissions?". Those who adhere to the premise behind the Fermi paradox often refer to that premise as the Fermi principle.
The paradox can therefore be summed up as follows: The commonly held belief that the universe has many technologically advanced civilizations, combined with our observations that suggest otherwise, is paradoxical, suggesting that either our understanding or our observations are flawed or incomplete.
The Rare Earth hypothesis
An emerging line of thought, dubbed the Rare Earth hypothesis, argues that multicellular life may be exceedingly rare in the universe because of a possible rarity of Earth-like planets. The argument is that many improbable coincidences converged to make complex life on Earth possible. A few examples of such conditions follow.
Spiral arms have many novae, and the radiation from them is believed inimical to higher life. The solar system is in a very special orbit within the Milky Way (our galaxy). It is a nearly perfect circular orbit, at a distance in which the solar system moves at the same speed as the shock waves forming the spiral arms. The Earth has been between spiral arms for hundreds of millions of years, more than thirty galactic orbits, almost all of the time there has been higher life on Earth.
Another crucial item is the Moon. The popular Giant impact theory asserts that it was formed by a rare collision between the young Earth and a Mars-sized body 4.45 billion years ago. The collision had to occur at a precise angle, as a direct hit would have destroyed the Earth, and a shallow hit would have deflected the Mars-sized body.
The moon is important because its gravitational pull creates tides which stabilize Earth's axis. Without this stability, its variation, known as precession of the equinoxes, could cause the weather to vary so dramatically that it could potentially suppress life. The impact heating from the moon's formation, as well as subsequent Lunar tides, may have also significantly contributed to the total heat budget of the Earth's interior, thereby both prolonging the life of and strengthening the dynamos that generate Earth's magnetic field (Note: These dynamos can only exist in circulating, charge-carrying, fluid portions of a spinning body. For the present-day Earth, this includes only the outer core. However, shortly after lunar formation, a significant portion of the Earth's mantle was also likely molten and probably contributed dramatically to the planet's total magnetic field). This field protects the conditions conducive to Earth life by preventing the solar wind from stripping away too much of the planet's atmosphere as well as by deflecting much of the gene-damaging high-energy radiation directed towards the planet's surface off into space.
Although the Rare Earth hypothesis is considered compelling by many, others disagree because complex life simply may not exclusively require Earth-like conditions for complex life in order to evolve. Furthermore, in response to the Great impact theory, regardless of whether or not the impact occurred, the premise that the creation of the moon was a singular event cannot be proved.
Issue #56
