t took more than seventy years to find a new world in our solar system that was bigger than Pluto, but it shouldn't take anywhere near that long to find the next "Planet X." Statistically speaking, the chances are good that somewhere on the solar system's edge, something bigger than Mercury or perhaps even Mars is lurking.
After all, if the oceans of space beyond Neptune contain a respectable number of objects around Pluto's size, there should be at least one or two that are substantially bigger. Our telescopes have yet to plumb the full depths of the Kuiper Belt—let alone the vast Oort Cloud beyond, which serves as the source of the solar system's most distant comets. Astronomers can't predict exactly what might be found, or exactly where, but most are confident that there's something big out there.
This fascination with planets yet unseen has spawned a modern-day doomsday myth dressed up in ancient lore. According to the doomsayers, a giant planet referenced in Sumerian texts, dubbed Nibiru, is making its way through the solar system and will set off a planetary catastrophe when it passes by Earth.1 The Nibiru tale has been rolled up with yet another myth tied to the end of a Maya calendar cycle in 2012, resulting in a double dose of doomsday.
"It's disheartening, because people are really frightened," NASA astronomer David Morrison said. As senior scientist for the NASA Astrobiology Institute at Ames Research Center, Morrison has had to cope with hundreds of e-mailed questions about Nibiru, the talk about 2012, and claims of a government cover- up.
Despite Morrison's repeated assurances that there's nothing to fear, the rumblings about a malevolent Planet X continue—fed in part by the real science surrounding the search for worlds beyond Pluto. Some point to an infrared sky survey back in the 1980s that turned up anomalous readings (which were later traced to distant galaxies).2 Others cite theoretical studies that suggest how distant worlds could divert comets toward the sun (even though the studies make clear that Earth's orbit would not be disrupted). Still others wonder if the dwarf planets are the messengers of doom foretold by the Sumerians (a worry that was sparked by all the news reports about Eris, initially nicknamed Xena or Planet X).
"Maybe we should be asking about Eris and not Nibiru," one questioner told Morrison in an e-mail. "Thank you for your time, as I am scared to death!"3
The reality is that scientists see no signs that any planet is coming to get us. But that's not to say all the mysteries have been resolved. As astronomers learn more about the solar system's icy frontier, they are asking deeper questions as well: How did today's planetary zones develop? Are there objects out in the Oort Cloud that send storms of comets our way? How do you explain Sedna, an icy world that takes twelve thousand years to complete just one orbit and comes nowhere near the Kuiper Belt?4
Today, Sedna and its kind, commonly known as distant detached objects, are the solar system's oddest oddballs. One of the giant planets—say, Neptune—could have kicked Sedna outward into a crazily eccentric orbit. But changing the orbit again so that it never comes back would require another gravitational kick from something big on Sedna's far side. Could there be planets that big yet to be discovered, hundreds or thousands of AU from the sun?
Several teams of researchers have concluded that such worlds are plausible, even though no one can yet say whether they actually exist. Astronomers in Japan, for instance, used computer modeling to determine that a world somewhere between the mass of Mars and Venus could explain all the weird orbits of the objects beyond Neptune—that is, if it were 80 AU
or farther from the sun. That would be roughly twice Pluto's distance from the sun, but the mystery world could still be detected from Earth during the closest part of its orbit.5
"I would like the public to understand that the research on distant hypothetical planets is still active (including my own) and that several questions remain open yet," said one of the astronomers, Patryk Sofia Lykawka. "In addition, it is important to understand that such theories in planetary sciences have absolutely no relation with Nibiru, 2012 or other hoaxes that claim for the existence of 'apocalyptic' or 'mystic' celestial bodies."
Another research team has proposed the existence of an object as big as Neptune or even Jupiter, placed fifty to one hundred times farther out from the sun than Pluto.6
In all these cases, the suggested location of a Planet X is based not on any actual observations, but on what it would take to get the right results in the computer simulations— essentially, a twenty-first-century mathematical exercise akin to the reverse calculations made by Le Verrier and Lowell in the nineteenth and twentieth centuries.
Some astronomers are looking at an even bigger picture: Could the weird goings-on at the edge of the solar system have resulted from a close encounter between our sun and another star? At the Carnegie Institution of Washington, planetary scientist Alan Boss suggests that Uranus and Neptune could have been sculpted into their current forms by ultraviolet light coming from a hot, young star passing through the cosmic neighborhood.7
Such a star also could have diverted some of the Oort Cloud's icy worlds into the weird orbits we see today. But because we don't see any stellar hotties in our cosmic neighborhood today, this scenario supposes that the hot star left the scene, leaving behind the mysterious planetary rearrangement as its calling card. Some even worry that a stellar intruder has made periodic trips through the solar system, stirring up storms of comets and causing mass extinctions on Earth. That's sparked yet another set of doomsday worries about a supposed "Death Star" that's been nicknamed Nemesis.8
John Matese, an astrophysicist at the University of Louisiana at Lafayette, has been looking into the Nemesis scenario for more than two decades. In collaboration with a colleague at Lafayette, Daniel Whitmire, and NASA researcher Jack Lissauer, Matese has traced the ebb and flow of come-tary impacts over billions of years. He agrees with the widely accepted view that the main factor behind the ebb and flow is the solar system's movement through the galactic tide, an up-and-down bobbing motion that takes millions of years to play out. As far as he knows, there's no sign of a Nemesis. But he's not willing to rule out a giant Planet X just yet.
"If there is anything in the Oort Cloud that is a cause of this suggestive data, this companion would have to be relatively massive—something on the order of three to ten Jupiter masses, with its mean position out at a distance of 10,000 AU," he said. "But the data just isn't good enough at the moment to go beyond saying it's suggestive."
Matese said that if such a Planet X does exist, astronomers should be able to detect it once a satellite known as the Wide-Field Infrared Survey Explorer (WISE) makes its full scan of the sky. "If it doesn't discover it, then the whole discussion should be concluded," he added.
WISE is just one of several space projects that could bring about fresh breakthroughs in the search for planets, from A to Z:
• Discovery Channel Telescope: Lowell Observatory, the place that made Pluto famous, has partnered with the Discovery Channel to build a $40 million telescope in Arizona that will extend the search for Kuiper Belt objects, as well as extrasolar planets and near-Earth asteroids.
• Giant Magellan Telescope: The $700 million GMT, due to be built in Chile by 2018, will combine the power of seven 27.6-foot-wide mirrors to produce images sharper than those of the Hubble Space Telescope. The instrument should be able to see the disks of Sedna and more of its faraway kin, piecing together the evidence for or against the existence of a giant Planet X—or even a Nemesis-type star that may have passed by during the solar system's infancy.9
• Large Synoptic Survey Telescope (LSST): The $400 million LSST is expected to become fully operational in Chile in 2016. "In the first week, we will see more data from this telescope than all the telescopes in humanity up to that point," billionaire backer Charles Simonyi says. The LSST is expected to spot up to 100,000 orbiting objects beyond Neptune, including ice dwarfs as big as Pluto out to a distance of 200 AU. Among the researchers involved in the LSST project is dwarf-planet discoverer Mike Brown.10
• Panoramic Survey Telescope and Rapid Response System (Pan-STARRS): The $100 million Pan-STARRS is an array of four telescopes being set up in Hawaii primarily to track fast-moving asteroids, some of which might threaten Earth. However, Pan-STARRS is expected to spot about 20,000 Kuiper Belt objects and should be able to find objects as small as Pluto well beyond the belt. In fact, a Planet X like Jupiter could be seen at a distance of 2,140 AU—more than fifty times farther away than Pluto. One of the leaders of the Pan-STARRS effort is the University of Hawaii's David Jewitt, who co-discovered the first Kuiper Belt object beyond Pluto back in 1992.11
Brown estimates that as many as 200 dwarf planets could be found in the Kuiper Belt, plus another 2,000 or so when the Oort Cloud is surveyed.12 But if a world the size of Mercury or Mars is found, as Brown and most other astronomers expect, that planet would be a "dwarf" in name only.
The American Museum of Natural History's Steven Soter thinks such objects would require a new label. "We might well find Mars-sized or larger objects in the outer Kuiper Belt or the more distant Oort Cloud," he said. "If so we would probably conclude that such objects were formed in the inner solar system before the gravity of the giant planets tossed them into the outer regions. Such bodies would not be dynamically dominant and, to distinguish them from the regular planets, we might call them'scattered planets.'"
Brian Marsden, the consummate planet classifier, said the status of far-off objects the size of Earth would depend on the shape of their orbit. If they stayed well beyond the Kuiper Belt, in well-behaved, cleared-out orbits, they could be regarded as planets under the International Astronomical Union's definition.
"If we do find something like that, I think that's much more likely that we would consider them another set of planets, just as Jupiter through Neptune are actually different from Mercury through Mars," he said.
But if a distant world's orbit was so eccentric that at its closest point it dipped closer to the sun than Neptune, it would be disqualified—at least in Marsden's book. "We would have to say that that Earth-sized object is not a planet," he said.13
The hypothetical case of a dark Earth orbiting far beyond Neptune sheds a different light on the exercise of defining a planet on the basis of not only where it is, but where it eventually goes. For critics of the IAU definition, this is a fatal flaw. "The one thing that dynamicists realize would topple the IAU apple cart altogether would be to discover a trans-Neptunian object bigger than Mars," Mark Sykes said. "And that is quite possible!"
As if that's not mind-boggling enough, there's the question of what to call planet-sized objects that are outside the orbital influence of a star, either because they were kicked out of their planetary nest or because they formed in isolation. Some of these objects are even thought to have moons (or are they subplanets?).14 If you're an astronomer who's a stickler about terminology, you'd deny these objects the planet label and call them sub-brown dwarfs or "planemos" (short for planetary-mass objects). But for most people, even for most astronomers, the term "rogue planet" will do just fine.
The oddball planets demonstrate how handy it is to have adjectives at the ready, for dwarf planets and dominant planets as well as scattered planets and rogue planets. "The word 'planet' by itself doesn't give us enough information to think critically about what someone is telling us," Vanderbilt University astronomer David Weintraub said. "Jovian . . . Neptune-sized . . . Earth-sized. . . . You almost have to have those adjectives in order to make the word 'planet' useful anymore."
Why is quibbling over one noun and a few adjectives so important? One answer is that the concept of planethood plays such a key role in the deepest questions we have about the universe, questions that range far beyond our own solar system. Are we alone? Could alien planets harbor life? Could they become future homes for our descendants, even if it takes millions of years for them to get there?
Scientists have detected more than three hundred planets beyond our own solar system, and the pace of planet-hunting is accelerating. The number of known extrasolar worlds is almost certain to rise to thousands in the next few years. And if you think the planetary menagerie is crowded now, just wait until you hear about the oddballs that scientists are finding out in our galaxy's depths—including alien Plutos.
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