Borderlands of Science Page 19
The Voyager 2 encounter revealed Neptune's equatorial radius to be 24,700 kms (since Neptune does not have a solid surface, this is taken as the radius where the pressure equals one Earth atmosphere). Neptune has a mass 17 times that of the Earth, and an average density of 1.64 grams/cc. The Neptunian day was revised to 16.11 hours, based on the rotation of the planet's magnetic field. That magnetic field is substantial, and its axis is offset 47 degrees from the planet's axis of rotation. In addition, the center of the magnetic field does not coincide with the planet's center of mass. As a result the field at the surface ranges from less than 0.1 gauss in the northern hemisphere to more than 1 gauss in the southern.
The appearance of the planet itself is striking. Unlike bland Uranus, Neptune shows atmospheric detail more like Jupiter and Saturn. There is a Great Dark Spot of midnight blue, calling to mind the Great Red Spot of Jupiter, and around the spot are bright, cirrus-like clouds that move along lines of latitude. This atmospheric activity may be a consequence of a net heat outflow, for like Saturn and Jupiter but unlike Uranus, Neptune gives off more energy than it receives from the Sun; in this case about 2.7 times as much. The minimum observed temperature on Neptune is a frigid 50 Kelvin, up near the top of the atmosphere.
Earth-based observations of Neptune, plus theoretical arguments, had suggested that its atmosphere would be hydrogen and helium with some methane. That has been confirmed. The helium is about 15 percent of the total, and small amounts of both methane and acetylene were found.
In the mid-1980s evidence had been found of rings around Neptune based on ground observations; or rather, there seemed to be evidence of partial rings. The way to find rings is to look for a star dimming and then brightening again, just before the planet passes in front of it. If there is a ring, then the same thing should happen again when the star reappears on the other side of the planet. This stellar occultation method was used for Neptune, just as was done in the case of Uranus.
However, although applying the technique to Neptune sometimes gave a dimming of the star for a couple of seconds, and a brightening before it vanished from sight behind the planet, there was no dimming when it reappeared!
In any event, full rings were found during the Voyager 2 encounter. There are three complete rings, and an outermost ring containing three bright, dusty arcs within it. These ring arcs caused the peculiar occultation results found in the earlier ground-based measurements.
Before Voyager 2, Neptune had two known satellites. The larger, Triton, was found in 1846 by that remarkable observer and discoverer of Uranus's Ariel and Umbriel, William Lassell, just ten days after the discovery of Neptune itself. Triton is big, with a radius of 1,350 kms, and has about a third of our Moon's mass. It travels in a retrograde orbit, opposite to the direction of planetary rotation. It has a period of 5.9 days, inclined at 23 degrees to the Neptune equator.
Nereid, the second satellite, is much smaller. It was discovered by Gerald Kuiper in 1949, and it travels in a very elliptical orbit, far out from the planet, with a period of 360 days. It and Triton are almost certainly captured bodies, caught in Neptune's gravitational net.
The Voyager encounter added half a dozen to the count of Neptune's moons. I have a personal fondness for Proteus, the biggest of these moons. Proteus is shaped like a knobby apple, and it may be the largest highly asymmetrical body in the solar system. Not much is known about it. Proteus orbits close to Neptune, where its own reflected light is overpowered by the light of its primary.
As for Triton, it is bright, and it is cold. The surface temperature of 38 Kelvin is the lowest measured for any body in the solar system. Nitrogen is solid at this temperature, and so is methane. The atmosphere is very thin, surface pressure between 10 and 20 millionths of an Earth atmosphere, and it is mainly nitrogen vapor with a little methane.
Any disappointment at Triton's cold, thin atmosphere is more than made up for by the satellite's astonishing surface. It possesses active geysers, "cryovolcanoes" that blow icy plumes of particles tens of kilometers high. The surface is fantastically cracked and complex, much of it showing meteorite impact craters crisscrossed by ridges of viscous material in a pattern that the Voyager team termed "cantaloupe terrain."
The score card for Neptune's moons is given in TABLE 7.4 (p. 189).
7.12 Pluto and the limits of the solar system. This planet has never been visited by any probe, so it is still wide open for science fictional conjecture. Discovered by Clyde Tombaugh in 1930, Pluto is described in most astronomy textbooks as "the most distant planet from the Sun." Actually, from 1979 to 1999, Neptune was the most distant known planet. For part of its eccentric orbit, Pluto moves within the orbit of Neptune.
Pluto's best images have been gained by the Hubble telescope. The planet has a mean radius of 1,140 kilometers. Its average surface temperature is about 43 Kelvin. There is some evidence that the surface is partly covered with methane ice, and it is conjectured that, like Triton, which it resembles in size and distance from the Sun, Pluto may have a coat of solid nitrogen.
Pluto, smaller than some satellites of Jupiter and Saturn, surprisingly has a moon of its own. Discovered in 1978 from ground-based observations, it is named Charon. It is about 590 kms in radius. Since Pluto itself is only 1,140 kms in radius, relative to the size of its planet Charon is the largest moon in the solar system. Pluto and Charon orbit each other in 6.4 days, and are 19,400 kilometers apart. The discovery of Charon allowed a good estimate of the mass of Pluto itself. That mass turns out to be small indeed, about one five-hundredth of Earth's mass. Charon's mass is still less, only one-seventh that of Pluto.
Might there be a "tenth planet," out beyond Neptune and Pluto? The search for such an object has been proposed, because one reason for seeking Pluto was a slight discrepancy between Neptune's observed and computed positions. However, after Pluto was discovered its faintness indicated that it could not be massive enough to cause the observed differences. Hence the search for "Planet X."
No such single planet has been found, but more than thirty small bodies—planetoids, minor planets, large comets, or whatever we choose to call them—have recently been discovered beyond the orbit of Neptune. They range in size from a hundred to four hundred kilometers in diameter, and are believed to be members of the Edgeworth-Kuiper Belt. This is often called the Kuiper Belt, but its existence was first suggested by the Irish astronomer K.E. Edgeworth in 1943. The EK Belt is believed to extend to at least twice the distance of Neptune from the Sun, but detection of its more remote members is extremely difficult because of the distance and low illumination levels there. The EK Belt is believed to be the source of many of the short-period comets that from time to time visit the inner solar system.
Even with the Edgeworth-Kuiper Belt, we are not at the "edge" of the solar system. In 1950, the Dutch astronomer Jan Oort suggested a source for the long-period comets. Oort proposed that there must be a vast "cometary reservoir," somewhere far out in space.
The roughly spherical Oort Cloud of comets drifts around the Sun, weakly bound by solar gravitational attraction. Sometimes a comet will be perturbed by another star, or perhaps by a close encounter with another Cloud member. Then its orbit will change, and it may fall in toward the Sun and become visible to us. Clearly, if comets are fairly common occurrences, there must be a lot of them in the cloud. Estimates put the number in the Oort Cloud as somewhere between a hundred billion and a trillion. Each comet is thought to be a loose aggregate of water, gravel, and other volatile substances such as ammonia and hydrocarbons—the "dirty snowball" theory introduced by Fred Whipple in 1950. The Oort Cloud is a great setting for stories. I put my novel Proteus Unbound out there, and had lots of fun with it.
The Oort Cloud is believed to extend as far as fifteen trillion kilometers from the Sun. Fifteen trillion kilometers takes us more than a third of the way to the nearest star. Are we finally at the "edge" of the solar system?
Well, there is still Nemesis. This highly hypothetical "dark comp
anion" to the Sun is supposed to return every 26 million years, to disturb the solar system and shower us with species-extinguishing comets that fall in from the Oort Cloud.
The existence of Nemesis is highly controversial, and I find the arguments for it unpersuasive. However, few explanations are available for periodic large-scale species extinctions. At its most distant point from the Sun, Nemesis would be almost three light-years away. At that distance, it would hardly be gravitationally bound to our Sun at all. Should it be discovered (it may be very faint, because if its mass is small enough it will not sustain its own fusion reactions), then the size of the Solar System has expanded in two hundred years from the orbit of Saturn, one and a half billion kilometers from the Sun, to the thirty trillion kilometers limit of Nemesis's orbit.
If all the natural bodies of the solar system are not enough as possible homes, there remains the possibility of making more in open space. One approach to the construction of such space colonies is discussed in Chapter 8.
7.13 Planets around other stars. Although humans can live in space and will do so in increasing numbers, planets are likely to remain our preferred home. In our own solar system, Mars is the most tempting new prospect. If Europa's water ocean exists, then that moon of Jupiter will be an equally attractive goal.
But what about more distant planets, around other stars? Do they exist? And if so, are they likely to be suitable for the development of life?
Science fiction writers have always assumed that the answer to all these questions was a definite and unambiguous Yes! In half the stories you will ever read, or movies and TV shows you will ever watch, it is assumed that planets exist around other stars, that they are suitable for life, and that they nurture intelligent life. Many of the intelligent life-forms are human-like to the point of ludicrous implausibility. Yet, up to 1996, there was no firm evidence at all that even one planet existed around any star other than Sol.
Certain properties of any such planets could be inferred, even if none had been observed. For example, no matter what shape a planet starts out at the time of its formation, gravitational forces will tend to make it spherical over time. When a planet happens to be rotating fast, like Jupiter or Saturn, centrifugal forces will give it a bulge at the equator. This oblateness, as it is called, is greater for Saturn than for any other planet in the solar system, but our eyes still see the disk of Saturn as circular. Anything big enough to be called a planet must be roughly spherical in shape.
For a spherical planet, the escape velocity at the surface (the speed of an object needed to escape from the planet completely) depends on only two things: the mass and the radius. Although internal composition—the way matter is distributed inside—will have a small effect, the escape velocity, V, will be close to 2GM/r, where M is the mass in kilograms, r the radius in meters, and G is the universal gravitational constant, equal anywhere in the universe to 6.672x10-11. Here V is given in meters/sec. For example, in the case of Earth, M=5.979x1024, r=6,378,000 and we find V=11,180; i.e., 11.18 kms/sec.
Escape velocity is important, and not only because it tells us what speed a rocket needs to get clear of Earth's gravity. It is also one of two key variables that decide whether or not a planet can hold on to an atmosphere. The other variable is the planet's temperature. If a planet is too hot, or too small, some of the molecules of atmospheric gases will always be moving faster than escape velocity. Unless they have a scattering collision with some other, slower, molecule, they will escape the planet completely. And unless they are replaced, from the interior or in some other way, the planet will at last lose its atmosphere.
A body as cool, big, and far from a star as Jupiter (escape velocity 60 kms/sec) or Saturn (escape velocity 36 kms/sec) is from the Sun will hold onto its atmosphere indefinitely. A body as small and hot as Mercury (escape velocity 4 kms/sec) or as small as Ceres (escape velocity 0.46 kms/sec) has no chance. Any atmosphere will vanish over time.
The surface gravity of a planet, g (or gee), a quantity with which we are more personally familiar, depends on exactly the same variables. We have g=GM/r2, where M, r, and G are the same as before. For the case of Earth, we find g=9.80 m/sec2.
In the past few years, the existence of planets around other stars has changed from optimistic guess to fairly confident reality. TABLE 7.5 (p. 190) gives a list of some of them, all admittedly based on evidence that is, if not weak, at least indirect. The list is representative rather than complete, because the number is growing fast. A new planet is added every month or two. We have not yet actually seen a planet around another star, even though every planet on the list is big, Jupiter's size or more.
That should not be taken to mean that most planets in the universe are massive. It merely shows that our detection methods can find only big planets. Possibly there are other, smaller planets in every system where a Jupiter-sized giant has been discovered.
Two planets in TABLE 7.5 are more than five times the mass of Jupiter. They are so big that these worlds are candidate "brown dwarf" stars, glowing dimly with their own heat. It is also disconcerting to see massive planets orbiting so close to their primary stars. In the case of 51 Pegasi and 55 Cancri, we have planets at least half the size of Jupiter, and perhaps a good deal bigger, orbiting only seven and sixteen million kilometers out from their sun. A planet of that size and in that position in our own solar system would have profound effects on Earth and the other inner planets.
If we cannot actually see a planet, how can we possibly know that they exist? There are two methods. First, it is not accurate to say that a planet orbits a star. The two bodies orbit around their common center of mass. That means, if the planet's orbit lies at right angles to the direction of the star as seen from Earth, the star's apparent position in the sky will show a variation over the period of the planetary year. That change will be tiny, but if the planet is large, the movement of the star may be big enough to measure.
The other, and so far more successful, method of detection also relies on the fact that the star and planet orbit around their common center of gravity, but in this case we look for a periodic shift in the wavelength of the light that we receive. When the star is approaching us because the planet is moving away from us, the light will be shifted toward the blue. When the star is moving away from us because the planet is approaching us, the star's light will be shifted toward the red. The tiny difference between these two cases allows us, from the wavelength changes in the star's light, to infer the existence of a planet in orbit around it.
Since both methods of detection depend for their success on the planet's mass being an appreciable fraction of the star's mass, it is no surprise that we are able to detect only the existence of massive planets, Jupiter-sized or bigger. The size distribution of planets around other stars remains an open question. Will we ultimately find a continuum, everything from small, Mercury-sized planets on up to planets able to sustain their own fusion reactions and thus to multiple star systems? Or are there major gaps in sizes, as we find in our own solar system between the inner and outer planets?
Are all stars candidates for planets that might support life? They are not, and we can narrow the search process. First, as noted in Chapter 3, massive stars burn their nuclear fuel much faster than small ones. A star ten times the mass of the sun will consume its substance several thousand times as rapidly. As a result, instead of continuing to shine as Sol will, more or less unchanged for over five billion years, our massive star will find its fuel exhausted in just a few million years. Its end, as we saw in Chapter 4, is cataclysmic. No planet could survive the explosion of its primary as a supernova.
The chance that native life, still less intelligence, might be wiped out in such a stellar conflagration is negligible. It would not have had time to develop. We do not know how long life took to establish itself on Earth, but it was surely longer than a few million years. The solar system was a turbulent place four and a half billion years ago, and Earth did not have a surface suitable to support life f
or at least the first few hundred million years. A planet orbiting a massive star would be gone before its crust had solidified.
Recall our horrible example from Chapter 1. The home world of the aliens orbited Rigel. But Rigel is a super-giant star, with a mass as much as 50 solar masses. It runs through its stable phase so fast that alien intelligence would have no time to develop. Add that to the list of story problems that need fixing.
We must deal with one other obstacle to the formation of planets suitable for life. The Sun is a star, and when we speak of, for example, Sirius or Rigel, we tend to think of them as single stars also. However, double and triple star systems are very common. Alpha Centauri, the nearest star to us, is actually three stars, labeled Alpha Centauri A, Alpha Centauri B, and Proxima Centauri ("proxima," meaning "close," refers to the star's distance from us, not from its companions; it is a tenth of a light-year away from the A and B components, and has an orbital period of at least half a million years). Since Proxima is small and dim, as seen from a planet circling Alpha Centauri A or B it would not be among their top thirty bright stars.
In the same way, Sirius is two stars, Sirius A and Sirius B. The second is sometimes called the "dark companion," not because it is really dark, but because it is small and condensed. Its existence was deduced by Bessel in 1844, from observations he had made of the perturbation of the brighter Sirius A. However, no one saw the companion until Alvan Clark observed it in 1862. That only added to the mystery, because although calculations showed that Sirius B had to be about as massive as the Sun, it shone only one four-hundredth as bright. Sirius B is a white dwarf star, the first one discovered, and its average density is several tons per cubic inch. Finally, Rigel also has a companion—and the companion itself seems to be a binary star.