 One view of comets as destroyers of worlds. |
 King Harold and nation cowers in fear at the close passage of Halley's Comet (picture from part of the Bayeaux Tapestry). |
The passage of the comet Hale-Bopp through our part of the solar system created spectacular displays in the spring of 1997. Every newspaper, television and radio station carried some report with photos of the comet. We had a foretaste of the Hale-Bopp show when Comet Hyakutake passed close to the Earth in the spring of 1996. Hale-Bopp was one of the brightest comets to grace our skies this century, coming close to the displays put on by Comet West in 1976 and Halley's Comet in 1910. Many people were justifiably interested in Hale-Bopp---it was a gorgeous site! Many people also tuned into the news and astronomy web sites in the summer of 1994 when the comet Shoemaker-Levy 9 smashed into the planet Jupiter. Predictions of Jupiter's demise were, of course, greatly exaggerated---Jupiter took the hits in stride.
One view of comets as destroyers of worlds. |
King Harold and nation cowers in fear at the close passage of Halley's Comet (picture from part of the Bayeaux Tapestry). |
Our favorable view of comets is a big change from the dread and fear people held of comets even less than a century ago. Comets were usually thought to be omens of bad events to occur on the Earth. King Harold of England took the passage of Halley's Comet to be a sign of his defeat in 1066. However, William the Conqueror took it as a good sign and led the Normans to victory over King Harold's army. As recently as 1910 people thought the end of the world was near when it was discovered the Earth would pass through the tail of Halley's Comet. Astronomers had discovered the presence of cyanogen molecules in the tail, so the popular media spread tales of cyanide poisoning of the Earth. Even with great effort the astronomers were not able to convince many people that we faced no danger---a comet's tail is extremely diffuse so the minute amounts making it through the atmosphere and being breathed by helpless human beings was much, much less than the noxious stuff they breathed everyday from industrial pollution. The tragedy of the Heaven's Gate cult shows that despite our current knowledge of comets, there are still those who view comets with great superstition or as something much more than the icy bodies they are from the outer limits of the solar system.
Comets are small ``potato-shaped'' objects a few hundred meters to about 20 kilometers across. They are made of dust grains embedded in frozen volatiles like water, methane, ammonia, and carbon dioxide (they are like ``dirty icebergs''). They are primitive objects which means they are unchanged since they first solidified from the solar nebula about 4.6 billion years ago. Comets are frozen relics of the early solar system holding valuable information about the formation of the planets.
When a comet gets close enough to the Sun, it changes into something more spectacular. The picture above shows the parts of a comet that form when the cold ``dirty iceberg'' is warmed up by the Sun. This picture is courtesy of David Doody at JPL and is part of the Basics of Space Flight manual for all operations personnel.
When a comet nears the Sun around the Jupiter-Saturn distance, it warms up. The ices sublime---they change from solid to gas without going through a liquid phase (like the white mist coming from a block of frozen carbon dioxide, ``dry ice''). Jets of material will shoot out from the nucleus. These jets can alter the comet's orbit (remember Newton's third law of motion?)
Since the orbit of Halley's Comet is known so well, spacecraft were sent to it when it passed through our part of the solar system in 1986. Here is a close-up of Halley's Comet. The spacecraft Giotto launched by the European Space Agency on July 2, 1985 reached Halley's Comet on March 13, 1986 and snapped this photo from 25,660 kilometers (15,950 miles) away. It got to within 596 kilometers (370 miles) of the nucleus, passing by at 68 kilometers/second (42 miles/second). The nucleus of Halley's Comet has dimensions of 8×8×16 kilometers (5×5×10 miles). The nucleus has a density of only 0.1 to 0.25 times the density of water and is very dark---it reflects only 4% of the sunlight (coal reflects about 6%). The density is about that of a loosely-compacted snowball and is quite fragile---you could break a piece of the nucleus in two with your bare hands!
Clicking on the picture will bring up a JPEG version of this picture. The bright white jets at the top of the picture are pointed in the direction of the Sun. Comet Hale-Bopp's nucleus is large---10 to 40 kilometers in size (about twice the size of Halley's Comet's nucleus) and is dust-rich. It began ejecting material when still at the distance of the outer planets, so it was discovered while still a couple of years from its perihelion passage in March of 1997. Comet Hyakutake (bright comet of spring 1996 that passed within 0.1 A.U. of the Earth) has a nucleus 1 to 3 kilometers in size.
The Sun is constantly spewing out charged particles, called the solar wind, into the solar system. The solar wind travels along solar magnetic field lines extending radially outward from the Sun. Ultraviolet light from the Sun ionizes some of the gases in the coma. These charged particles (ions) are forced along magnetic field lines to form the ion tail millions of kilometers long. The blue ion tail acts like a ``solar'' wind sock. The ion tail always points directly away from the Sun, so when the comet is moving away from the Sun, its ion tail will be almost in front of it! The blue color is mostly from the light emitted by carbon monoxide ions but other types of ions also contribute to the light. Since the gas is so diffuse, the observed spectrum is an emission-line spectrum.
The dust tails forms from the solar photons colliding with the dust in the coma. The dust forms a long, curved tail that lies slightly farther our from the Sun than the nucleus' orbit. The dust tail has a yellow-white color from reflected sunlight. Both of the tails will stretch for millions of kilometers. Because of the large amount of dust, Hale-Bopp's tail was much brighter and whitish-yellow from reflected sunlight. Hyakutake's tail was dimmer and blue-green in appearance because of the low amount of dust and relatively more ions.
At the great distances of the Oort Cloud, comets can be affected by the gentle gravitational tugs of nearby passing stars. The passing stars tug on the comets, ``perturbing'' their orbits, sending some of them into the inner solar system. The comets passing close to a jovian planet are deflected by the planet's gravity into an orbit with a shorter period, only decades long. Jupiter and Saturn tend to deflect long period comets completely out of the solar system (or gobble them up as Jupiter did with Shoemaker Levy-9). Uranus and Neptune tend to deflect the long period comets into orbits that stay within the solar system. Halley's Comet may be an example of a deflected comet. Unlike other short period comets, Halley's Comet's orbit is retrograde.
The Oort cloud comets probably formed at the about the same distance as Uranus and Neptune from the Sun 4.6 billion years ago and were then deflected outward when they passed to close to the two planets. Comets forming at the distance of Jupiter and Saturn were either ejected from the solar system by these massive planets in a ``gravitationally slingshot'' or gobbled up. Comets forming further out than Neptune never coalesced to form a planet and now make up the Kuiper Belt.
The comets observed in the Kuiper Belt have more circular orbits and do not stray close to Uranus or Neptune. The Kuiper belt comets observed from the ground are 100 to 300 kilometers in size and orbit between 30 and 60 A.U. from the Sun. Ground-based telescopes have observed at least 28 and the Hubble Space Telescope has detected at least 29 smaller objects (10 to 20 kilometers in diameter). Another group of objects between Saturn (9.5 A.U.) and Uranus (19.2 A.U.) may be an extension of the Kuiper Belt. These objects include Chiron (170 kilometers in diameter) and 5 others orbiting between Saturn and Uranus.
Because of its small size and low density, some astronomers view the planet Pluto (2300 kilometers in diameter) as just a large comet. Pluto and its moon, Charon (1200 kilometers in diameter), may be members of the Kuiper Belt. The currect list of objects of the Kuiper Belt is at the Minor Planets Center. They keep a list of the tran-Neptunian objects and a list of the Centaurs which are small bodies orbiting between Jupiter and Neptune (like Chiron and 5145 Pholus). Select here to bring up a plot of the positions of the observed Kuiper Belt objects.
Regardless of where it is in the solar system, the Sun's gravity is always pulling on the comet. When the comet is close to the Sun, it moves quickly because of the great force of gravity it feels from the Sun. It has enough angular momentum to avoid crashing into the Sun. Angular momentum is a measure of the amount of spin or orbital motion an object has---see appendix A for more on angular momentum. As the comet moves away from the Sun, the Sun's gravity continually slows it down. Eventually, the comet slows down to the aphelion point and the Sun's gravity pulls it back.
The comet's motion around the Sun is sort of like a swing on the Earth. When the swing is closest to the ground, it moves quickly. As the swing moves up, the Earth's gravity is continually pulling on it, slowing it down. Eventually, the swing is slowed down so much that it stops and the Earth's gravity pulls it back down. The swing has enough angular momentum to avoid crashing to the ground.
Short period comets make hundreds to thousands of passes around the Sun spewing out gas and dust. Over time a comet will leave bits of dust along its orbit, each piece of dust has an orbit close to the comet's orbit. The dust grains are the size of a grain of sand or smaller. If the Earth passes through the comet's orbit, the dust grains can hit the Earth's atmosphere to make the spectacular displays called meteor showers. After many passages around the Sun, the nucleus has no more volatile material and it becomes ``dead.''
The famous Perseid meteor shower in mid-August is due to Earth passing through the orbit of Comet Swift-Tuttle and the Leonid meteor shower in mid-November is due to Comet Tempel-Tuttle. The meteor showers appear to be coming from a particular direction in the sky so the meteor showers are named after the constellation they appear to be coming from. The Perseids appear to diverge from the Perseus constellation and the Leonids diverge from Leo. When the parent comet passes through the inner solar system, the meteor shower display is particularly impressive---several hundred meteors can be seen in one hour. Such events are called meteor storms. A meteor storm is predicted for the Leonids in November 1997, 1998, and 1999. Unfortunately, the 1997 storm was partially washed out by the waning gibbous moon. The 1998 storm took place on a moonless night, so observers were able to count up to several hundred meteors per hour.
The meteors not associated with a meteor shower are bits of rock from asteroids. The meteors that ARE associated with a meteor shower are much too fragile to survive their trip through our atmosphere. Some of the comet dust intercepts the Earth at much slower speeds than those making the meteors and can make its way to the surface gently. Don Brownlee, an astronomer at the University of Washington has pioneered the collection of this comet dust in the stratosphere. More information about the comet dust samples is available at the Stratospheric Dust web site at the Planetary Materials Curation office of NASA.
| angular momentum | dust tail | ion tail |
|---|---|---|
| Kuiper Belt | long period comet | meteor shower |
| Oort Cloud | short period comet | solar wind |
| sublime |
When the solar nebula stopped collapsing it began cooling, though the core forming the Sun remained hot. This meant that the outer parts of the solar nebula cooled off more than the inner parts closer to the hot proto-Sun. Only metal and rock materials could condense (solidify) at the high temperatures close to the proto-Sun. Volatile materials (like water, methane and ammonia) could only condense in the outer parts of the solar nebula. This explains item (e) of the observables above.
Around Jupiter's distance from the proto-Sun the temperature was cool enough to freeze water (the so-called ``snow line''). Further out from the proto-Sun, ammonia and methane were able to condense. There was a significant amount of water in the solar nebula. Because the density of the solar nebula material increased inward, there was more water at Jupiter's distance than at the distances of Saturn, Uranus, or Neptune. The greater amount of water ice at Jupiter's distance from the proto-Sun helped it grow larger than the other planets.
Material with the highest freezing temperatures condensed to form the chondrules that were then incorporated in lower freezing temperature material. Any material that later became part of a planet underwent further heating and processing when the planet differentiated so the heavy metals sunk to the planet's core and lighter metals floated up to nearer the surface. Observables item (f) is explained.
The coalescing particles tended to form bodies rotating in the same direction as the disk revolved. The forming planet eddies had similar rotation rates. This explains items (g) and (h) above. The gravity of the planetesimals tended to divide the solar nebula into ring-shaped zones. This process explains item (i) above.
Astronomers have detected disks of dust and gas around young stars using sensitive infrared detectors on the largest telescopes in the world. An equivalent amount of material locked up into a single object will have a smaller total surface area than if it was broken up into many tiny particles. The disks have a lot of surface area and, therefore, can emit a lot of infrared energy. Some bright stars in our sky have dust around them: Vega, Beta Pictoris, and Fomalhaut. These are systems possibly in the beginning stages of forming planets. One disk around the star HR 4796A appears to be in between the dust disk stage and a fully-fledged planet system. The inner part of the disk has been cleared away. Presumably, the dust material has now coalesced into larger things like planets. The planets would have a smaller surface area than if the material was still in tiny particles form, so the planets will be much fainter. The Hubble Space Telescope has also detected disks of gas and dust around 50% of the stars still forming in the Orion Nebula. It appears that the formation of planet systems is a common process in the universe.
The easiest way to look for planets around other stars is to notice their gravitational effect on the stars they orbit. One signature of a planet would be that the star would appear to wobble about as the star and the planet orbit a point situated between them, proportionally closer to the more massive star, called the center of mass. Our Sun wobbles because of the gravity of the planets orbiting it. Most of the wobble is due to Jupiter which contains more mass than all of the other planets combined. However, the wobble is tiny! Because the Sun is over a thousand times more massive than Jupiter, the center of mass is over a thousand times closer to the Sun, or about 47,000 kilometers above the surface of the Sun (this distance is less than 7% the radius of the Sun). Despite the tiny wobble, astronomers on planets orbiting nearby stars could detect this wobble using the same technology we have here on Earth if they observed the Sun's motion very carefully over a couple of decades.
Another signature of a planet would be doppler shifts in the star's spectral lines as they orbit their common center of mass. This technique has been used to find planets around at least 17 other stars (as of the time this was written). The searches have so far focussed on stars similar to the Sun, though one system, found three years before the rest, has planets orbiting a pulsar (a type of ultra-compact, dead star discussed in the stellar evolution chapter). The number of systems discovered and the details about them changes so rapidly that the best place to find up-to-date information on extrasolar planets in on the internet. Some web sites are given at the end of this chapter.
The orbital motion of the planets can be derived from the shifting spectal lines and the information about the orbits can be used to derive the masses of the planets. However, the doppler effect tells you about the motion along the line of sight only. The planet orbits are undoubtedly inclined, or tipped, to our line of sight and the amount of inclination is uncertain. This introduces an uncertainty in the derived masses of the planets. Usually, astronomers will quote the masses as ``mass×sin(orbit inclination angle)'', so the actual planet masses could be higher. The figure below compares some of the other planetary systems with our solar system. Above each planet is the lowerbound estimate of their mass given in units relative to the mass of Jupiter (remember the orbit inclination uncertainty!).
Two things to notice are how close the large planets are to their stars and the large eccentricities of some of the planet orbits. The concensation model outlined in the previous section predicts that large planets will only form far from the young star. Giant planets start from a core of rock and ice that was able to solidify far from the intense heat of the young star. The rock-ice cores then pull in surrounding gas by their gravity. Near the star, the temperature is too high to form the rock-ice cores.
Over a decade before the discovery of the first extrasolar planets, astronomers predicted as part of the condensation model that large gas/rock clumps would form far from a young star and spiral inward toward the star because of friction with the gas remaining in the disk around the forming star. The gas/rock clumps can also interact with each other sending one into a small orbit while the other is ejected out of the system. Such interactions may explain the elliptical orbits we see. Some astronomers working on planet formation models are looking for ways to halt the inward spiral of the gas giant planets near the star through tidal interactions between the planet and star. Perhaps the gas giant planets we see are simply the ones that did not have time to spiral completely into the stars before the gas disk was cleared away by the strong T-Tauri winds that accompany the start of nuclear fusion. Perhaps in our solar system other giant planets had formed but did not survive.
Astronomers cannot yet determine the diameters of the giant extrasolar planets so their densities, and, therefore, their composition is still unknown. In the next few years, ground-based interferometers will be completed that can image large extrasolar planets. What about Earth-like planets? It is unlikely that life could arise on a gas giant planet. NASA's proposed Terrestrial Planet Finder, a large interferometer using four telescopes arranged in a straight line about the length of a football field that would orbit the Sun at about the distance of Jupiter's orbit, should be able to obtain infrared pictures of life-bearing planets. With the Terrestrial Planet Finder, astronomers would also be able to analyze the spectrum of the planets to determine the composition of their atmospheres. Spectral lines from water would say that a planet has a vital ingredient for life. If oxygen is found in the atmosphere, then it would be very likely that life was indeed on the planet. Recall from the previous chapter that molecular oxygen would quickly disappear if it was not continually replenished by the photosynthesis process of plants and algae. Current plans are to have the Terrestrial Planet Finder operational by 2010.
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last update: 19 February 1999
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