These two worlds are not identical. The interior of Titan seems to contain much more ice than that of Triton, and much less rock. Titan's diameter is almost twice that of Triton. Still, if placed at the same distance from the Sun they would look like sisters. Alan Stern of the Southwest Research Institute suggests that they are two members of a vast collection of small worlds rich in nitrogen and methane that formed in the early Solar System. Pluto, yet to be visited by a spacecraft, appears to be another member of this group. Many more may await discovery beyond Pluto. The thin atmospheres and icy surfaces of all these worlds are being irradiated—by cosmic rays, if nothing else and nitrogen—rich organic compounds are being formed. It looks as if the stuff of life is sitting not just on Titan, but throughout the cold, dimly lit outer reaches of our planetary system.
Another class of small objects has recently been discovered, whose orbits take them—at least part of the time—beyond Neptune and Pluto. Sometimes called minor planets or asteroids, they are more likely to be inactive comets (with no tails, of course; so far from the Sun, their ices cannot readily vaporize). But they are much bigger than the run-of-the-mill comets we know. They may be the vanguard of a vast array of small worlds that extends from the orbit of Pluto halfway to the nearest star. The innermost province of the Oort Comet Cloud, of which these new objects may be members, is called the Kuiper Belt, after my mentor Gerard Kuiper, who first suggested that it should exist. Short-period comets—like Halley's—arise in the Kuiper Belt, respond to gravitational tugs, sweep into the inner part of the Solar System, grow their tails, and grace our skies.
Back in the late nineteenth century, these building blocks of worlds—then mere hypotheses—were called "planetesimals." The flavor of the word is, I suppose, something like that of "infinitesimals": You need an infinite number of them to make anything. It's not quite that extreme with planetesimals, although a very large number of them would be required to make a planet. For example, trillions of bodies each a kilometer in size would be needed to coalesce to make a planet with the mass of the Earth. Once there were much larger numbers of worldlets in the planetary part of the Solar System. Most of them are now gone—ejected into interstellar space, fallen into the Sun, or sacrificed in the great enterprise of building moons and planets. But out beyond Neptune and Pluto the discards, the leftovers that were never aggregated into worlds, may be waiting—a few largish ones in the 100-kilometer range, and stupefying numbers of kilometer-sized and smaller bodies peppering the outer Solar System all the way out to the Oort Cloud.
In this sense there are planets beyond Neptune and Pluto—but they are not nearly as big as the Jovian planets, or even Pluto. Larger worlds may, for all we know, also be hiding in the dark beyond Pluto, worlds that can properly be called planets. The farther away they are, the less likely it is that we would have detected them. They cannot lie just beyond Neptune, though; their gravitational tugs would have perceptibly altered the orbits of Neptune and Pluto, and the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft.
The newly discovered cometary bodies (with names like 1992QB and 1993FW) are not planets in this sense. If our detection threshold has just encompassed them, many more of them probably remain to be discovered in the outer Solar System—so far away that they're hard to see from Earth, so distant that it's a long journey to get to them. But small, quick ships to Pluto and beyond are within our ability. It would make good sense to dispatch one by Pluto and its moon Charon, and then, if we can, to make a close pass by one of the denizens of the Kuiper Comet Belt.
The rocky Earthlike cores of Uranus and Neptune seem to have accreted first, and then gravitationally attracted massive amounts of hydrogen and helium gas from the ancient nebula out of which the planets formed. Originally, they lived in a hailstorm. Their gravities were just sufficient to eject icy worldlets, when they came too close, far out beyond the realm of the planets, to populate the Oort Comet Cloud. Jupiter and Saturn became gas giants by the same process. But their gravities were too strong to populate the Oort Cloud: Ice worlds that came close to them were gravitationally pitched out of the Solar System entirely—destined to wander forever in the great dark between the stars.
So the lovely comets that on occasion rouse us humans to wonder and to awe, that crater the surfaces of inner planets and outer moons, and that now and then endanger life on Earth would be unknown and unthreatening had Uranus and Neptune not grown to be giant worlds four and a half billion years ago.
THIS IS THE PLACE for a brief interlude on planets far beyond Neptune and Pluto, planets of other stars.
Many nearby stars are surrounded by thin disks of orbiting gas and dust, often extending to hundreds of astronomical units (AU) from the local star (the outermost planets, Neptune and Pluto, are about 40 AU from our Sun). Younger Sun-like stars are much more likely to have disks than older ones. In some cases, there's a hole in the center of the disk as in a phonograph record. The hole extends out from the star to perhaps 30 or 40 AU. This is true, for example, for the disks surrounding the stars Vega and Epsilon Eridam. The hole in the disk surrounding Beta Pictoris extends to only 15 AU from the star. There is a real possibility that these inner, dust-free zones have been cleaned up by planets that recently formed there. Indeed, this sweeping-out process is predicted for the early history of our planetary system. As observations improve, perhaps we will see telltale details in the configuration of dust and dust-free zones that will indicate the presence of planets too small and dark to be seen directly. Spectroscopic data suggest that these disks are churning and that matter is falling in on the central stars—perhaps from comets formed in the disk, deflected by the unseen planets, and evaporating as they approach too close to the local sun.
Because planets are small and shine by reflected light, they tend to be washed out in the glare of the local sun. Nevertheless, many efforts are now under way to find fully formed planets around nearby stars—by detecting a faint brief dimming of starlight as a dark planet interposes itself between the star and the observer on Earth; or by sensing a faint wobble in the motion of the star as it's tugged first one way and then another by an otherwise invisible orbiting companion. Spaceborne techniques will be much more sensitive. A Jovian planet going around a nearby star is about a billion times fainter than its sun; nevertheless, a new generation of ground-based telescopes that can compensate for the twinkling in the Earth's atmosphere may soon be able to detect such planets in only a few hours' observing time. A terrestrial planet of a neighboring star is a hundred times fainter still; but it now seems that comparatively inexpensive spacecraft, above the Earth's atmosphere, might be able to detect other Earths. None of these searches has succeeded yet, but we are clearly on the verge of being able to detect at least Jupiter-sized planets around the nearest stars—if there are any to be found.
A most important and serendipitous recent discovery is of a bona fide planetary system around an unlikely star, some 1,300 light-years away, found by a most unexpected technique: The pulsar designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense sun, the remnant of a massive star that suffered a supernova explosion. It spins, at a rate measured to impressive precision, once every 0.0062185319388187 seconds. This pulsar is pushing 10,000 rpm.
Charged particles trapped in its intense magnetic field generate radio waves that are cast across the Earth, about 160 flickers a second. Small but discernible changes in the flash rate were tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State University, in 1991—as a tiny reflex motion of the pulsar in response to the presence of planets. In 1994 the predicted mutual gravitational interactions of these planets were confirmed by Wolszczan from a study of timing residuals at the microsecond level over the intervening years. The evidence that these are truly new planets and not starquakes on the neutron star surface (or something) is now overwhelming—or, as Wolszczan put it, "irrefutable"; a new solar system is "unambiguously identified." Unlike all the other techniques, the pulsar timing method makes close-in terrestrial planets comparatively easy and more distant Jovian planets comparatively difficult to detect.
Planet C, some 2.8 times more massive than the Earth, orbits the pulsar every 98 days at a distance of 0.47 astronomical units* (AU); Planet B, with about 3.4 Earth masses, has a 67-Earth-day year at 0.36 AU. A smaller world, Planet A, still closer to the star, with about 0.015 Earth masses, is at 0.19 AU. Crudely speaking, Planet B is roughly at the distance of Mercury from our Sun; Planet C is midway between the distances of Mercury and Venus; and interior to both of them is Planet A, roughly the mass of the Moon at about half Mercury's distance from our Sun. Whether these planets are the remnants of an earlier planetary system that somehow survived the supernova explosion that produced the pulsar, or whether they formed from the resulting circumstellar accretion disk subsequent to the supernova explosion, we do not know. But in either case, we have now learned that there are other Earths.
* The Earth, by definition, is 1 AU from its star, the Sun.
The energy put out by B1257+12 is about 4.7 times that of gun. But, unlike the Sun, most of this is not in visible light, but in a fierce hurricane of electrically charged particles. Suppose that these particles impinge on the planets and heat them. Then, even a planet at 1 AU would have a surface around 280 Celsius degrees above the normal boiling point of water, greater than the temperature of Venus.
These dark and broiling planets do not seem hospitable for life. But there may be others, farther from B1257+12, that are. (Hints of at least one cooler, outer world in the B1257+12 system exist.) Of course, we don't even know that such worlds would retain their atmospheres; perhaps any atmospheres were stripped away in the supernova explosion, if they date back that far. But we do seem to be detecting a recognizable planetary system. Many more are likely to become known in coming decades, around ordinary Sun-like stars as well as white dwarfs, pulsars, and other end states of stellar evolution.
Eventually, we will have a list of planetary systems—each perhaps with terrestrials and Jovians and maybe new classes of planets. We will examine these worlds, spectroscopically and in other ways. We will be searching for new Earths and other life.
ON NONE OF THE WORLDS In the outer Solar System did Voyager find signs of life, much less intelligence. There was organic Matter galore—the stuff of life, the premonitions of life, perhaps but as far as we could see, no life. There was no oxygen in their atmospheres, and no gases profoundly out of chemical equilibrium, as methane is in the Earth's oxygen. Many of the worlds were painted with subtle colors, but none with such distinctive, sharp absorption features as chlorophyll provides over much of the Earth's surface. On very few worlds was Voyager able to resolve details as small as a kilometer across. By this standard, it would not have detected even our own technical civilization had it been transplanted to the outer Solar System. But for what it's worth, we found no regular patterning, no geometrization, no passion for small circles, triangles, squares, or rectangles. There were no constellations of steady points of light on the night hemispheres. There were no signs of a technical civilization reworking the surface of any of these worlds.
The Jovian planets are prolific broadcasters of radio waves—generated in part by the abundant trapped and beamed charged particles in their magnetic fields, in part by lightning, and in part by their hot interiors. But none of this emission has the character of intelligent life—or so it seems to the experts in the field.
Of course our thinking may be too narrow. We may be missing something. For example, there is a little carbon dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere out of chemical equilibrium. I think the CO2 is provided by the steady pitter-patter of comets falling into Titan's atmosphere—but maybe not. Maybe there's something on the surface unaccountably generating CO2 in the face of all that methane.
The surfaces of Miranda and Triton are unlike anything else we know. There are vast chevron-shaped landforms and crisscrossing straight lines that even sober planetary geologists once mischievously described as "highways." We think we (barely) understand these landforms in terms of faults and collisions, but of course we might be wrong.
The surface stains of organic matter—sometimes, as on Triton, delicately hued—are attributed to charged particles producing chemical reactions in simple hydrocarbon ices, generating more complex organic materials, and all this having nothing to do with the intermediation of life. But of course we might be wrong.
The complex pattern of radio static, bursts, and whistles that we receive from all four Jovian planets seems, in a general way, explicable by plasma physics and thermal emission. (Much of the detail is not yet well understood.) But of course we might be wrong.
We have found nothing on dozens of worlds so clear and striking as the signs of life found by the Galileo spacecraft in its passages by the Earth. Life is a hypothesis of last resort. You invoke it only when there's no other way to explain what you see. If I had to judge, I would say that there's no life on any of the worlds we've studied, except of course our own. But I might be wrong, and, right or wrong, my judgment is necessarily confined to this Solar System. Perhaps on some new mission we'll find something different, something striking, something wholly inexplicable with the ordinary tools of planetary science—and tremulously, cautiously, we will inch toward a biological explanation. However, for now nothing requires that we go down such a path. So far, the only life in the Solar System is that which comes from Earth. In the Uranus and Neptune systems, the only sign of life has been Voyager itself.
As we identify the planets of other stars, as we find other worlds of roughly the size and mass of the Earth, we will scrutinize them for life. A dense oxygen atmosphere may be detectable even on a world we've never imaged. As for the Earth, that may by itself be a sign of life. An oxygen atmosphere with appreciable quantities of methane would almost certainly be a sign of life, as would modulated radio emission. Someday, from observations of our planetary system or another, the news of life elsewhere may be announced over the morning coffee.
