Should we pity poor Pluto, lonely and cold at the outer edge of the solar system, cruelly demoted from its previous planethood? I say no: if we must anthropomorphize our neighborhood icy bodies (and I'm not recommending that we do), far better to celebrate lucky Pluto, a family man (with three bouncing baby moons, two newly discovered in the last year), and a home in a very popular part of town. The more closely astronomers examine the outer end of the solar system, the more of a happening place it seems to be. There are now nearly a thousand objects known in trans-Neptunian space, with more being added to the list almost daily. Most are fairly small asteroid-sized bodies with diameters of a few hundred kilometers, but there are quite a few big ones tipping the scales at the top of the mass distribution. The trans-Neptunian object Quaoar is by itself more massive than all the asteroids put together, and distant Sedna (three times further from the sun than Pluto) is probably larger still. But the prize for most massive goes to Eris: at 97 AU (astronomical units) from the Sun and 2400 km across, it is both the most distant known solar system object and more massive than Pluto. As the largest new object discovered in the solar system in over 75 years, it was Eris's discovery in 2005 which began the contentious debate about the definition of "planet" that ultimately led to Pluto's demotion this past summer by the International Astronomical Union.

While the debate over the meaning of "planet" may be settled (at least for now), the search for further members of the solar system goes on unabated. There is little reason to suppose that Eris was the only large body remaining undiscovered, and no reason at all to think that it represents the true outer edge of the solar system. Its neighbor Sedna is currently located some 90 AU from the Sun, but its highly elliptical orbit carries it out to almost a thousand AU away from the Sun and back again every twelve thousand years. Sedna and Eris both strain the detection abilities of our telescopes right now. An object like Sedna, but a few times further away, would easily slip past our most sensitive searches. Right now we can only wonder: what else lurks out there in the distance, in the cold, in the dark? How big and how massive an object could be out there undiscovered in the night?

Hunting for Planet X

It turns out that particular question has quite a long and respectable pedigree. Uranus, the first planet to be found in historical times, was discovered in 1781 almost by accident during the course of routine observations by the British astronomer William Herschel. This sensational discovery received the 18th-century equivalent of wild press coverage, turning Herschel into a minor celebrity and earning him employment for life as the Astronomer Royal. (His attempt to name the newfound world after the king probably didn't hurt, either. We came disturbingly close to having Mars, Jupiter, Saturn, and George.) But observations of Uranus over the following years revealed a problem: its orbital motion wasn't quite as expected, and over time it drifted slightly away from its predicted course. In a mathematical tour de force, the astronomers John Couch Adams and Urbain Le Verrier independently calculated how the gravity of an unseen eighth planet could tug Uranus off course, and by 1846 both predicted a location in the sky for that object. Observations quickly revealed the world we now know as Neptune, and touched off a fierce battle between the British and French over credit for the discovery. (See Tom Standage's book The Neptune File for more on this tale of science, nationalism, and just plain ugly academic brawling.)

Neptune's discovery explained Uranus's wayward motion, but by the end of the 19th century, it appeared that Neptune, too, was wandering awry. Modern observations (in particular the Voyager-measured accurate masses for the outer planets) have shown that this discrepancy was just an illusion arising from the limited precision of earlier measurements, and that Neptune indeed moves precisely as predicted. But there was no way to know that eighty years ago, and so the logical explanation was Yet Another Outer Planet. The wealthy American astronomer Percival Lowell—widely known for his claims of canals and alien civilizations on Mars—devoted the last decade of his life to an extensive search for the unseen world, which he called Planet X. This project continued on and off after Lowell's death, leading ultimately to the 1930 discovery of good old Pluto. (Which happened just in time for an aspiring animator to name a mouse's pet dog after the newfound planet, thereby ensuring the screams of millions of sentimental future schoolchildren when the namesake world was demoted to dwarf planet status.) Pluto of course had nothing to do with any discrepancy in Neptune's orbit—the anomaly wasn't real in the first place, and even if it had been, Pluto is far too small to have caused it—so its discovery was essentially by coincidence. After Pluto, nothing else was found in the outer solar system for over six decades. In the 1990s, advances in telescope and detector technology at last enabled observations sensitive enough to reveal fainter bodies than Pluto. I've already mentioned several of these—Quaoar, Sedna, and Eris—but those are just a few of the hundreds that orbit with Pluto in a wide belt around the Sun, now known as the Kuiper belt. With its true nature exposed as just one body of many in a large family of similar objects, astronomers last summer downgraded Pluto from full planet to dwarf planet. And that's the end of Planet X, right? Right?

Nemesis: Hidden Doom?

Well, no. Planetary motions may provide no evidence for the gravitational influence of an unseen body, but there are other kinds of evidence, some of which can be found surprisingly close to home. In the mid-1980s, some paleontologists found that there appeared to be a pattern to extinctions in the fossil record, with waves of mass extinctions coming every 26 million years or so. The passing of the dinosaurs 65 million years ago seemed to be only one event in a long chain of extinctions, albeit one of the more major events. At least two of those mass extinctions appeared to be due to impacts of an asteroid on the Earth (including the dinosaur-ending one). A team of astronomers from Berkeley and Princeton, led by Berkeley's Marc Davis, hypothesized that the periodic extinctions could be caused by waves of comets that swept through the inner solar system, triggered every 26 million years by the orbital passage of an unseen but massive stellar companion to the Sun. In order to have an appropriately long orbital period, the orbit of this body would have to carry it out to almost two and a half light years from the Sun, before swinging back towards the inner solar system to wreak gravitational havoc and send comets sliding around, some on collision courses with the Earth. Davis and co. provided a name for this doomsday star: "If and when this companion is found, we suggest that it be named Nemesis, after the Greek goddess who relentlessly persecuted the excessively rich, proud and powerful," they wrote in their 1984 Nature paper, before adding, "We worry that if the companion is not found, this paper will be our own Nemesis." It is actually fairly plausible that the Sun could have a faint binary companion: most stars are binaries, and most binaries (particularly those where the stars have a large separation) have elliptical orbits which bring them alternatively together and apart. The 26-million-year orbit required to explain the fossil record would be a surprisingly long period, true, but not inconceivably long.

"But wait!" you cry, "If we can see tiny and faint rocks like Pluto and Eris out there, how could we possibly miss a star?" It turns out that's not as hard to do as you might think, provided the star is small and dim enough. The brightness of stars is a very steep function of their mass, so that the smallest stars (around a tenth the mass of the Sun) are less than one thousandth as bright. Making them even harder to see, most of their light is emitted in the infrared, so that in visible light they are very dim indeed. Furthermore, determining distances is a notoriously hard problem in astronomy: Is a faint point of light a nearby, faint object, or instead a much more distant but brighter objects? Most nearby faint stars have been discovered because of their rapid motion across the sky. As the Sun and its neighbors orbit around the galactic center, each has a slightly different course, and thus their positions appear to shift with time. The effect is far smaller than can be seen with the naked eye, but compared to the sensitivity of modern telescopes, it's actually quite a large motion and can be easily measured. The closest stars usually have the largest apparent motion (just as objects closest to a moving car appear to move far faster than distant objects by the horizon), allowing nearby stars to be identified by their large motion. However, since the binary companion Nemesis would move through space together with the Sun, from our perspective its motion would be very slow. The motion from its excruciatingly slow 26-million-year-long orbit would be undetectable, and Nemesis would appear to be just another distant, slowly moving star. The situation has been further complicated in recent years by the discovery of brown dwarfs, objects termed "failed stars" which are intermediate in mass between stars and planets. A brown dwarf version of Nemesis, perhaps a few dozen times more massive than Jupiter, would be hundreds of times fainter still, rendering it nearly impossible to detect by present observations.

The original paper proposing the idea of Nemesis in 1984 attracted a flurry of initial attention, and a few research groups, including the Berkeley group, attempted systematic searches for it. However, limited resources were available for such a speculative project, nothing was found by the initial surveys, and interest gradually faded. In the late 1990s, another group of astronomers noticed that about 25% of newly-discovered comets appeared to come from the same part of the sky, and suggested that if it existed, Nemesis might presently lie in that direction. They calculated that an object as small as 3-5 times larger than Jupiter might be sufficient to explain their observations.

Technology marches onward, and the same improvements that allowed the discovery of the Kuiper belt also provide an ever-greater chance of detecting Nemesis, if it exists. In 2005, a team of astronomers from Bombay, India (showing how global an enterprise astronomy has become today) presented an updated estimate of Nemesis's brightness. Based on current knowledge of brown dwarf brightnesses, those authors estimated that if Nemesis does exist, then it would have been seen by existing sky surveys unless it is no larger than 45 times bigger than Jupiter (that is, unless it is smaller than five percent of the mass of the Sun). Based on that limit, a small brown-dwarf-sized or planet-mass object could remain undiscovered out there, but there no longer seems to be any chance that a large brown dwarf or small star could be a companion to our sun.

But if Nemesis does not in fact exist, what could explain the 26-million-year period from the fossil record? Well, for starters 20 additional years of paleontology has muddied the situation, with some scientists arguing based on improved dating methods that the 26-million year cycle doesn't really exist, or is purely coincidental. On the other hand, an alternate hypothesis for starting comet showers has been proposed, based on the Sun's oscillating orbit crossing the galactic plane. That hypothesis is in some ways even more shaky than Nemesis itself, since the period of solar galactic plane crossings seems to be about ten millions years off from the needed 26 megayears. With the fossil record in dispute, perhaps the most conservative course is to assume there is no need for Nemesis, and thus no firm evidence yet for any large bodies in the outer solar system. The question will be settled one way or another within the coming decade: a planned NASA mission called the Wide-field Infrared Sky Explorer (WISE) will survey the entire sky in infrared light starting in late 2009. One of its prime missions is to hunt for brown dwarfs near the Sun. WISE's cryogenically cooled optics and deep-space vantage will allow it to spot the faint infrared glow from any brown dwarfs lurking within a few light years of the Sun. We will soon know for certain whether the outer solar system has hidden in its darkness anything of brown dwarf size or larger.

An Extrasolar Nemesis?

Thus it seems fairly unlikely—though not impossible—that the Sun truly has a distant companion. But that does not mean that the story of Nemesis is destined to be only a minor footnote on the history of astronomy—at least, not if we're willing to look beyond the confines of our own solar system. HD 3651 is a nearby K0V-type star, slightly smaller than the Sun. A planet about the mass of Saturn orbits it at about the distance of Mercury from our own sun, and there's a good chance that it possesses more, smaller planets still remaining to be discovered. But HD 3651 also has a brown dwarf companion, about 33 Jupiters in mass and orbiting far outside the inner planets at a distance of about 500 AU, as reported this past November by MIT astronomer Adam Burgasser. He speculated that this brown dwarf companion, HD 3651B, may play the role of Nemesis in that solar system, instigating cometary impacts upon the inner worlds or perhaps directly affecting the orbital dynamics of those planets through its own gravity. The discovery of HD 3651B as a distant brown dwarf companion orbiting around a extrasolar planetary system suggests that Nemesis-like situations may in fact be common occurrences in the galaxy. Our own solar system may not have ended up with a brown dwarf lurking in its own outer reaches. But will that be much consolation to the green-skinned, tentacled inhabitants of the planet Zarblon, when HD 3651B rains down comets upon them?