Pale Blue Dot: A Vision of the Human Future in Space

parallel, long, dark streaks that seem to have been blown by the wind and then deposited on the icy surface despite how sparse Triton’s atmosphere is (about 1/10,000 the thickness of the Earth’s).

All the craters on Triton are pristine—as if stamped out by some vast milling device. There are no slumped walls or muted relief. Even with the periodic falling and evaporation of snow, it seems that nothing has eroded the surface of Triton in billions of years. So the craters that were gouged out during the formation of Triton must have all been filled in and covered over by some early global resurfacing event. Triton orbits Neptune in the opposite direction to Neptune’s rotation—unlike the situation with the Earth and its moon, and with most of the large moons in the Solar System. If Triton had formed out of the same spinning disk that made Neptune, it ought to be going around Neptune in the same direction that Neptune rotates. So Triton was not made from the original local nebula around Neptune, but arose somewhere else—perhaps far beyond Pluto—and was by chance gravitationally captured when it passed too close to Neptune. This event should have raised enormous solid-body tides in Triton, melting the surface and sweeping away all the past topography.

In some places the surface is as bright and white as freshly fallen Antarctic snows (and may offer a skiing experience unrivaled in all the Solar System). Elsewhere there’s a tint, ranging from pink to brown. One possible explanation: Freshly fallen snows of nitrogen, methane, and other hydrocarbons are irradiated by solar ultraviolet light and by electrons trapped in the magnetic field of Neptune, through which Triton plows. We know that such irradiation will convert the snows (like the corresponding gases) to complex, dark, reddish organic sediments, ice tholins—nothing alive, but here too composed of some of the molecules implicated in the origin of life on Earth four billion years ago.

In local winter, layers of ice and snow build up on the surface. (Our winters, mercifully, are only 4 percent as long.) Through the spring, they are slowly transformed, more and more reddish organic molecules accumulating. By summertime, the ice and snow have evaporated; the gases so released migrate halfway across the planet to the winter hemisphere and there cover the surface with ice and snow again. But the reddish organic molecules do not vaporize and are not transported—a lag deposit, they are next winter covered over by new snows, which are in turn irradiated, and by the following summer the accumulation is thicker. As time goes on, substantial amounts of organic matter are built up on the surface of Triton, which may account for its delicate color markings.

The streaks begin in small, dark source regions, perhaps when the warmth of spring and summer heats subsurface volatile snows. As they vaporize, gas comes gushing out as in a geyser, blowing off less-volatile surface snows and dark organics. Prevailing low-speed winds carry away the dark organics, which slowly sediment out of the thin air, are deposited on the ground, and generate the appearance of the streaks. This, at least, is one reconstruction of recent Tritonian history.

Triton may have large, seasonal polar caps of smooth nitrogen ice underlying layers of dark organic materials. Nitrogen snows seem recently to have fallen at the equator. Snowfalls, geysers, windblown organic dust, and high-altitude hazes were entirely unexpected on a world with so thin an atmosphere.

Why is the air so thin? Because Triton is so far from the Sun. Were you somehow to pick this world up and move it into orbit around Saturn, the nitrogen and methane ices would quickly evaporate, a much denser atmosphere of gaseous nitrogen and methane would form, and radiation would generate an opaque tholin haze. It would become a world very like Titan. Conversely, if you moved Titan into orbit about Neptune, almost all its atmosphere would freeze out as snows and ices, the tholin would fall out and not be replaced, the air would clear, and the surface would become visible in ordinary light. It would become a world very like Triton.

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.