By Mike Brown
Caltech University

IN 1846, MORE THAN 50 years after the discovery of Uranus, both John Couch Adams in England and Urbain LeVerrier in France independently realized that Uranus was not precisely following its expected path around the Sun, but rather was being perturbed by some unseen force. Using the recently developed methods of physics and calculus, they both calculated that everything could be explained if there was another planet beyond Uranus slightly tugging Uranus from its expected path. Moreover they knew right where to look. LeVerrier contacted astronomer Johann Gottfried Galle in Berlin and gave him precise coordinates at which to point his telescope. On the very first night of his search Galle found Neptune gleaming in his eyepiece. Adams and LeVerrier were correct! And, moreover, their chains of reasoning were correct. Neptune was indeed responsible for giving Uranus a tug, and that tug pointed right at the new planet. The discovery was a spectacular triumph of the new physics; the universe itself was now within the grasp of the mind.

If one new planet could be found this way, why not more? Very soon other astronomers were looking very closely at the orbit of Neptune to see if it, too, was perturbed. It was, or so it appeared at the time. Several astronomers immediately tried to use the same method as Adams and LeVerrier and predict precisely where to find this Planet X (X for unknown and X for 10, though at least one astronomer called his hypothesized body Planet O, a less catchy name which rarely gets mentioned these days).

Percival Lowell, an astronomer from a wealthy family in Boston, even decided to construct an entire telescope (at what is now the Lowell Observatory in Flagstaff, Arizona) with the main purpose being to find the planet. Lowell died before the search began in earnest, but in 1930 Clyde Tombaugh, carrying out Lowell's program, discovered just what he thought he was looking for: Pluto, in orbit beyond Neptune. Planet X discovered! Except that it wasn't. Pluto is far too small to even be noticed by Neptune, much less tug it away from its orbit. Its discovery, though inspired by Lowell, had nothing to do with his prediction. In fact, these days we know that the early measurements of the position of Neptune were simply in error and, as far as anyone can tell, Neptune goes precisely where it is supposed to go. Pluto was not the predicted Planet X, nor does Neptune do anything unusual that would require such a planet.

The idea of finding something large outside of Neptune still remains appealing, though. Over the years different astronomers have revisited the idea to explain a variety of things including the periodic extinction of the dinosaurs, the orbits of comets, and many more.

The latest attempt at Planet X predicting comes in a paper in the April issue of the Astronomical Journal by Patryk Lykawka and Tadashi Mukai at Kobe University in Japan. The essential idea is unchanged from 1845: look for the gravitational affect of an unseen planet. But Lykawka and Mukai have a great advantage over the early 20th century Planet X seekers: they have much much more than simply Uranus or Neptune to go on. We now know of more than 1000 tiny objects in the region beyond Neptune known as the Kuiper Belt, and each one of these is affected by whatever planets are out there.

In the outer solar system, these tiny bodies in the Kuiper Belt function like the debris left over after a flood: while the water from the flood may be long gone, you can still use the location of the debris to trace the rivers and currents of the flood and the highs and lows of the water. The Kuiper Belt objects similarly trace catastrophic events that we can no longer see but can now infer. In the outer solar system these catastrophic events were mainly the migrations of the orbital positions of the giant planets and the subsequent gravitational rivers and currents that swept through the tiny bodies of the Kuiper Belt as the giant planets swirled around them. Today we see only where the small bodies ended up, but these locations are an intimate product of what the giant planets were doing.

For a while now we have known, however that some aspects of the orbits of the objects in the Kuiper Belt simply cannot be explained by the giant planets. The most dramatic of these unexplainable orbits is that of the extremely distant extremely eccentric object known as Sedna. Sedna has an orbit so distant from the Sun that it takes 12,000 years to complete a single orbit. During the course of this orbit it moves from about 80 times the distance from the Earth to the Sun to about 1000 times the distance from the Earth to the Sun. Such a peculiar orbit is an almost certain sign that sometime in its past Sedna got a large gravitational kick by something big, knocking it far away. But Sedna is currently so far away from any giant planet that there is nothing that it ever comes close to that could have kicked it around. Sedna on the outskirts of the solar system was like finding an overturned truck 20 feet above the high water line of the flood. Something put the truck there, but it certainly wasn't the flood. While Sedna is the most extreme case of strange orbits in the outer solar system, there are many others.

So what helped rearrange the Kuiper Belt besides the giant planets? Several ideas have been proposed, each of which can explain some, but not all of the anomalies seen. The ideas fall into three main categories: something from outside the solar system, something from inside the solar system that has escaped and is now gone, and something from inside the solar system that is still here but has yet to be found.

The Lykawka and Mukai proposal falls into the latter category. They demonstrate that if an approximately Mars-sized body once existed in the region of the Kuiper Belt, and if all of the conditions were just right, it could not only provide just enough gravitational kicking-around to sculpt the Kuiper Belt into something that closely resembles what we see today, but the Mars-sized body could also conveniently find itself on an orbit sufficiently distant from the Sun to have escaped detection. It’s a tidy proposal which explains much about the outer solar system all in one shot.

Planet X discovered? Well, not yet. Unlike the calculations of Adams or LeVerrier, those of Lykawka and Mukai are unable to provide any prediction at all of where in the sky their object would be (the technical reason for this difference is that the main effects of the Lykawka and Mukai planet were in the distant past, so its current precise position has little effect, and thus can't be determined).

But the good news is that, in the past few years, searches of the outer solar system (including my own) have already begun to specifically target the more distant regions where the predicted object would reside. If any such large object is out there it could be discovered anytime, perhaps even tonight (by my survey, if the object is on the bigger or closer side) or, more likely, by one of the next generation all sky surveys that will be operational over the next decade.

So is the Lykawka and Mukai prediction right? Maybe. Their paper is a thorough and exhaustive demonstration of just how this Mars-sized planet could explain many of the oddities of the outer solar system. But just because such a planet could explain the oddities doesn’t mean that it does explain the oddities. To make everything work out, Lykawka and Mukai need their planet to do some very specific things. It has to be neither too large nor too small, neither too distant nor too close. It needs to follow a rather precise history from its formation on the inner edge of the Kuiper Belt to its eventual ejection by Neptune to a distant orbit. The number of coincidences that must have happened is enormous. The probability that such a chain would have occurred seems slim.

And yet the probability of almost anything specific is slim. The Moon, going around the Earth, was only created because of an impact that happened to hit the Earth at the right angle and the right speed. If the impact had occurred slightly differently: no Moon. The whole solar system itself might be a coincidence caused by a precisely timed supernova that went off nearby just as a cloud of dust and gas was beginning to coalesce. I am married to my wife only because of an improbable chance encounter in a basement seven years ago. So coincidences happen. Coincidences are what take general cases and make them specific. Still, scientists shy away from specific explanations, because while they may work once, they won’t ever work again. They are less satisfying. But that doesn’t mean they are not true.

Nonetheless, I suspect that most astronomers will file this new Planet X prediction away with the rest of the possible explanations for the oddities in the outer solar system. They will consider it plausible, but not very likely. Other explanations will continue to be sought. Other observations will continue to be made. But those of us who have been scanning the skies for a long time looking for whatever might be out on the edge of the solar system cannot help but to secretly grin and hope that maybe – just maybe -- Lykawka and Mukai are on to something. Some night, perhaps, someone somewhere will point a telescope at the sky and see a bright distant object slowly wandering across the path of the stars and get to say, in the words of Galle, sending a telegraph to LeVerrier after he found Neptune just where it was supposed to be: "the planet whose place you have [predicted] really exists."