DISCOVER Vol. 22 No. 10 (October 2001)
Table of Contents

One of Our Planets is Missing
Maverick astronomers insist there's something way the heck out there that's a lot larger than Jupiter— and it orbits our sun
By Kathy A. Svitil

Late one night last November, astronomers at Australia's Royal Observatory stole an hour from their regular work to assist John Murray in the astronomical equivalent of the hunt for the White Whale. Murray believes our solar system contains an unseen tenth planet, a giant world orbiting in near darkness 3 trillion miles from the sun, a thousand times more distant than Pluto. Deep in the Oort cloud— a vast sphere of dormant comets at the edge of the solar system as we know it— this phantom world circles the sun every 5 million years. It may be as much as 10 times more massive than Jupiter, making it by far the largest planet in the solar system. If it exists.

Planet X, a hypothetical tenth member of the solar system, may be stirring up comets far, far beyond Pluto. It could be a giant world, many times the mass of Jupiter (above), or even a brown dwarf— a failed star. Photograph from Ciclops/University of Arizona

Like Moby Dick, a tenth planet has long been more inspired storytelling than fact (see "Good and Bad Guesses," below). But Murray, a planetary scientist at the Open University in Milton Keynes, England, believes he has good reason to think he's closing in on this legendary quarry. He first grew excited when he was studying comets and suddenly noticed that an improbably large number of them top out at about the same distance from the sun, roughly 3 trillion to 4.5 trillion miles away. When he zeroed in on the 13 comets with the most accurately established orbits, Murray found their motions to be even more peculiar. "They all aligned along a band, as would be expected if they had been perturbed by some large body. The odds of this happening by chance are 1,700 to one," he says.

Working backward from these 13 orbits, Murray tracked Planet X to a region of the sky centered on the tiny constellation Delphinus, located near Cygnus in the northern summer sky. Now comes the hard part: pinpointing the location of a planet that scarcely moves through the sky and receives so little sunlight that it has virtually none to reflect. Finding a single whale in the ocean would be easy by comparison. Still, Royal Observatory scientists forged ahead and trained the powerful UK Schmidt Telescope on Delphinus. In a single snapshot, the telescope took in 40 square degrees of sky— an area about the size of the bowl of the Big Dipper— and at least 100,000 stars. One of them could be Planet X. By comparing the image with an identical one taken six months earlier, Murray hopes to spot it by its back-and-forth movement, a perspective effect that occurs as Earth swings around the sun.

So far, the search has revealed nothing more than a few dozen unexplained dim smudges, most likely asteroids or defects on the photographic plate. In all likelihood, Planet X is far too faint to find even with the UK Schmidt Telescope. But Murray remains undaunted. He keeps busy with his full-time work as a planetary geologist and his full-time hobby as an amateur lepidopterist. If he can add astronomy's White Whale to his collection, all the better. Next month, astronomers at the Royal Observatory will snap another series of pictures of the targeted zone, and they will do so again next spring. "If the entire area gets searched and all suspect objects are accounted for, this does not mean there is no distant planet," Murray says. It simply means he will need a more sensitive telescope.

Not everyone shares Murray's easygoing optimism. "I think it's a silly idea. If you tinker with his comet data, the effect just goes away," says astronomer Harold Levison of the Southwest Research Institute in Boulder, Colorado, an expert on the Oort cloud. He admits that Earth-sized planets could be floating undetected in the Oort cloud, but he doubts anything as big as Jupiter is hiding out there. "I can't prove it's not true, but incredible claims like this require incredible proofs," he says.

All this comes as scientists are starting to reevaluate the results of an earlier planet hunt. From 1908 until his death eight years later, the eccentric aristocrat Percival Lowell engaged in a futile search for a body, which he also dubbed Planet X, whose gravitational tug could account for discrepancies in the orbits of Uranus and Neptune. Astronomer Clyde Tombaugh, who continued the project at Lowell's observatory in Flagstaff, Arizona, had better luck. He discovered Pluto in 1930 but then concluded he hadn't found the right planet. Pluto seemed far too small to explain the orbital blips. In the early 1990s, new data on planetary masses and orbits from Pioneer 10 and 11 and Voyager 1 and 2 eliminated the apparent orbital anomalies. Many astronomers now doubt that tiny Pluto— just 1,400 miles wide— deserves its historical status as a planet.

Yet the solar system continues to cough up intriguing new objects. Astronomers have recently discovered that the region beyond Neptune abounds with icy bodies, possibly billions of them, located in a broad band called the Kuiper belt. This is the source of short-period comets— those that, like Halley, take less than 200 years to circle the sun. (Long-period comets, such as the ones studied by Murray, come from the more distant Oort cloud.) In the revisionist theory of our solar system, Pluto and its moon, Charon, are considered merely the biggest of the Kuiper belt objects. Just five months ago, researchers found another object, Varuna, which is nearly half the diameter of Pluto. Larger objects could lie undiscovered in the distant belt. So why not giant unseen planets farther out?

That nagging question, which lured Murray into this hunt, has also captivated astrophysicists Daniel Whitmire and John Matese of the University of Louisiana at Lafayette. Like Murray, they believe a gigantic world slowly blunders through the Oort cloud, occasionally knocking a comet toward the sun. They published evidence for their Planet X in October 1999, the same month that Murray presented his initial data.

In fact, Whitmire and Matese have been playing Captain Ahab even longer than Murray, following the trail of the planetary beast since the mid-1980s. Back then, Whitmire made headlines when he and a colleague proposed that our sun has a potentially world-obliterating companion star. This dim second sun— also predicted by a second team of researchers at the University of California at Berkeley and Princeton, who named it Nemesis— was supposed to explain swarms of comets that seemed to hit the Earth at near-clocklike 30-million-year intervals, causing mass extinctions. Whitmire and the others argued that Nemesis periodically plowed through the Oort cloud during its 30-million-year orbit around the sun, sending deadly bunches of comets tumbling earthward.

It was a dramatic, headline-grabbing story. Whitmire soon teamed up with his colleague Matese and developed a related theory that blamed the killer comets on a rogue planet, not a star. The key evidence for periodic mass extinctions wound up on shaky ground, however, and Nemesis fell out of vogue. "There is so much uncertainty in the dates now that people have become a little suspect about whether the 30-million-year period is real. If the period isn't real, you don't have anything you need to explain," Whitmire says. But by this time he and Matese had been bitten by the Planet X bug.

These days they are looking for patterns in space, not in time. Most comets spend their lives unnoticed in the deep freeze of the Oort cloud. Occasionally they break free and travel through the inner solar system, disturbed by passing stars or by slight undulations of our galaxy as it rotates. Only then do they develop gaseous tails that brilliantly reflect the sun's light. Gravitational nudges from stars and from the galaxy occur at random, so the comets entering the inner solar system should have an equally haphazard pattern.

Yet when Whitmire and Matese, along with their colleague Patrick Whitman, charted the paths of 89 comets, they— like Murray— found an odd sense of organization. One quarter of them clumped into a swath of just 10 percent of the sky. These associated comets also had some similarities in their orbits. The astronomers concluded that a large something— a body three to five times as massive as Jupiter— was bouncing the comets our way. "A bound companion to our sun is so far the best explanation for the comet distribution," Matese says.

The UK Schmidt Telescope in New South Wales, Australia, is aiding John Murray's search for Planet X. The telescope has a huge field of view, more than a dozen times as wide as the full moon, which is essential because Murray doesn't know exactly where to look. Photograph by E.W. Hadley Aiip, Royal Observatory, Edinburgh

Whitmire and Matese calculate that their Planet X is about 2 trillion miles away, lying somewhere along a "great circle" running across the sky perpendicular to the band of the Milky Way. The object is impossibly dark, illuminated only by a distant sun that would appear hardly brighter than Venus as seen from Earth, and bitterly cold. Temperatures would hover around 100 degrees below zero Fahrenheit. It could be a Jupiter-like planet or, more likely, a brown dwarf, an undersized star that never grew massive enough to shine. "We don't think this thing could have formed like a planet, because there's no way for it to have gotten from the planetary region all the way out to where it is," says Whitmire. More likely it formed like a star, out of the same cloud of gas and dust as the sun. "Most stars are part of binary systems, so it is no big deal for the sun to have a companion. In fact, it's unusual for it not to," Whitmire says.

All of these attributes are very similar to those of Murray's putative world, although he favors a different type of origin: a Jupiter-type body that was ejected from another solar system and then captured by our sun. Otherwise, his Planet X seems a lot like the Whitmire and Matese Planet X, except for one glaring difference: The two are in totally different parts of the sky. Weirder still, Murray doesn't report any evidence of Whitmire and Matese's planet, and vice versa.

For astronomer Brian Marsden of the Harvard-Smithsonian Center for Astrophysics, that discrepancy makes the whole Planet X story look as make-believe as Melville's whale. "My question is: Why didn't each of them see the other one's object? That makes no sense at all," says Marsden, who thinks that the clumping patterns seen by the researchers could easily be the product of passing stars. "I'm trying to be open to the idea, but I don't think either Whitmire and Matese or Murray have proven their points. If they did someday see something, it would take quite a bit of time— decades or even centuries— to tell for sure if it was a planet or a passing star."

Harold Levison isn't buying any of it either. He suggests that Whitmire and Matese, like Murray, are chasing random statistical fluctuations. Or they may be seeing what astronomers call observational selection effects. Comet observers routinely seek their prey more often in some parts of the heavens (say, in the skies of the Southwest) than in others (above the bright lights of Paris), leading to skewed results. "And they're assuming the comets in the Oort cloud must be evenly distributed. We have no idea if that is the case. There could be an excess of comets coming in from one direction with no object out there at all," Levison says. Matese, in turn, has doubts about the inferences Murray makes from the data supporting his Planet X. "I would be inclined to be skeptical about its existence," he says.

Murray, sanguine as ever, isn't deflated by the nay-saying. "Even I won't be convinced until it is found," he says. A moment later, he avers that he is cautiously optimistic the new photos he's collecting next month and in the spring will produce the clinching evidence. Nor does he worry about the lack of consensus between the two groups. Whitmire and Matese looked at different comets in different orbits, he says, "so it is unlikely that the alignment of the 13 comets would have shown up."

In contrast, Whitmire and Matese have no plans for a targeted search for Planet X; the number of places to check is too huge. Still, they are doing their best to pin down its location. Matese proposes that the best way to find it is by looking not for its light but for its limited thermal energy. That means searching with a sensitive infrared telescope, preferably one in space. NASA's Space Infrared Telescope Facility, scheduled for launch next July, could settle the issue of Planet X for good. But "without our being able to tell them where to look, finding the object would be a matter of serendipity," Matese says.

An orbiting infrared survey telescope, able to scan the entire sky, would be the best bet for finding a giant distant planet. "There is nothing like that right now, or even being considered for funding. But eventually one will go up. If the object is there, and if I'm lucky, it will be found in my lifetime," Whitmire says. If not— well, hunting for a White Whale is at least romantic.

Counter-Earth (5th century B.C.): Philolaus, a student of Pythagoras's, introduced a model of the solar system with a Counter-Earth, also known as the Antichton. The Counter-Earth orbited between Earth and an unseen "central fire," whose light was visible only when reflected off the disks of the sun and moon.

Titius-Bode's Law planet (1772): German astronomer Johann Bode, drawing on earlier work by his countryman Johann Titius, noticed a geometric regularity in the spacing of the planets. This "law" predicted that a planet should exist in the yawning, seemingly empty gap between Mars and Jupiter. Starting with the discovery of Ceres in 1801, scientists realized the empty zone contains not one large planet but thousands of small asteroids.

Uranus (1781): While carrying out a thorough survey of the sky, the German-British astronomer William Herschel spotted a fuzzy star that moved from night to night. He quickly realized it was a new planet— the first one discovered since ancient times. He proposed naming it Georgium Sidus after King George III; Johann Bode proposed the classical, less controversial name Uranus.

Neptune (1846): Noticing irregularities in the motion of Uranus, two mathematicians— John Couch Adams in England and Urbain-Jean-Joseph LeVerrier in France— deduced the existence of a new planet whose gravity was causing the disturbance. Johann Galle and Heinrich d'Arrest soon spotted the planet, later named Neptune, in exactly the anticipated location.

Vulcan (1860): LeVerrier, heartened by his experience with Neptune, proposed a small planet between Mercury and the sun to account for an observed drift in Mercury's orbit. Despite decades of searching, astronomers never spotted the fiery world. In 1916, Albert Einstein's general theory of relativity explained that the anomaly in Mercury's motion is actually caused by the curvature of space around the sun.

Planet X (1908): For decades, astronomers noted that Uranus and Neptune still did not follow their expected paths. Percival Lowell spent fruitless years looking for a "Planet X" to explain the anomaly. Astronomer Clyde Tombaugh, hired to carry on Lowell's effort, discovered Pluto in 1930— but only by luck, as Pluto is far too small to be Lowell's Planet X.

Mercury's moon (1974): Days before the Mariner 10 spacecraft flew past Mercury, it picked up unusual ultraviolet emissions that seemed to come from a moon orbiting the planet. After a buzz of excitement, NASA researchers realized they had been fooled by an ordinary star, 31 Crateris.

Nemesis (1984): Daniel Whitmire and his colleagues, along with a group led by Richard Muller of the University of California at Berkeley, independently suggested our sun might have a low-mass companion star, which they called Nemesis, that sends swarms of comets earthward every 30 million years. The evidence for now looks suspect, but observations have still not conclusively ruled out the existence of Nemesis.

Matese, John, P. G. Whitman, and D. P. Whitmire. 1999. "Cometary Evidence of a Massive Body in the Outer Oort Cloud." Icarus vol. 141: 354-366. Find additional information on the object at Matese's Web site: www.louisiana.edu/~jjm9638.

The Hypothetical Planets Web site describes searches for several proposed planets and moons: www.seds.org/billa/tnp/hypo.html.