a much denser atmosphere of gaseous nitrogen and methane would form； and radiation would generate an opaque tholin haze。 It would bee 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 bee visible in ordinary light。 It would bee 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 pounds 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 ets （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 ets 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 et 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 ets—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 etary bodies （with names like 1992QB and 1993FW） are not planets in this sense。 If our detection threshold has just enpassed 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 et 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 et 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 ets 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 ets 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 panion。 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 pensate 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 paratively 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 paratively easy and more distant Jovian planets paratively 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 bee known in ing 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