Nine Planets After All?

You’ll have to fogrive me; tonight was supposed to be the night I finally got around to blogging about what I learned at last week’s colloquium. But then I stumbled upon a really interesting paper that I just had to spend some time reading, and that wound up taking a large portion of the evening. Large enough that now I just want to go to bed. The colloquium post will be along tomorrow afternoon (it’s already half finished).

But for now, let me at least tell you about this paper. John Matese and Daniel Whitmire, both astrophysicists at U-Louisiana Lafayette, are claiming that, even without Pluto, the solar system may have nine planets after all.

How could this be, you ask? Surely we’d have noticed another planet lurking in the solar system. Well that’s exactly what I asked myself when I first encountered this poor-quality Yahoo article earlier in the evening. After all, as far as places in outer space go, the solar system has been pretty well mapped out, both with telescopes and with actual robotic explorers.

Well as you might have guessed, the truth about Matese and Whitmire’s prediction is slightly less dramatic than as it’s being presented in the popular press, but then again, it’s hardly nothing, and in fact is a lot more bold and convincing than I expected at first (popular articles tend to be full of such exaggerations I am always a bit skeptical). But you can judge for yourself. What they’re claiming is that, when one observes the orbits of so-called “long period comets,” the comets whose orbits take the longest to complete and (in general) travel furthest away from the sun, some strange clustering behavior is observed. That is, these comets all seem to have orbits with several properties in common, even though our current understanding of where comets come from suggest that these properties should all be essentially random. And according to their calculations, a large planet orbiting well, well beyond the  edge of what mot people tend to think of as the “solar system” might have precisely the effect of creating these bunches of similar comets.

They’ve actually been claiming this for a while, having first put forward the theory in 1999, but it was only just in the last few days that they’ve published the results of a 10-year long study of data gathered by NASA’s Wide-field Infrared Survey Explorer telescope. And whereas in 1999 they were only able to claim that the existence of another planet would be one possible explanation for the comets behavior, now they’ve been able to statistically analyze the probability that the comets have bunched that way by chance alone (or as a result of a collection of otherwise well-understood phenomena) compared to the chance that it is caused by a planet. Their final conclusion? No worse than even money for the new-planet hypothesis. That may not sound like much, but as I said, it’s more than you often hear claimed in papers like these.

So just what would the details be on this new planet, if it exists? According to the paper, it would be somewhere between one and four times as large as Jupiter, making it the most massive planet in the solar system. Like Jupiter, it would be an enormous gas giant, but it would orbit much, much further away from the sun than Jupiter does, and even much farther away than Neptune and the hapless Pluto. Indeed, the new planet would be nearly 500 times further away from the sun than our current outermost planets, and while it takes slowpoke Pluto nearly three hundred years to complete an orbit of the sun, the dwarf planet seems like Usain Bolt compared to the gas giant Matese and Whitmire propose, which might take a whopping 13 million years to complete a revolution around the sun.

So this new planet, if it exists, would not exactly be a sudden, surprise addition to the region of space most people consider the solar system. It would be a bit like your parents telling you that you were going to get a new set of next-door neighbors, only to discover that they meant someone was building a new house several miles behind yours. But, if there’s nothing but open space between you and the new house, they’re still technically next-door. And this new gas giant, if it exists, would certainly meet the current definition of “planet.” And even if it’s much further away than the headline writers at Yahoo might like you to believe, it would still be pretty special in my book to discover that there was another world lurking out there much, much closer to home than the nearest star, slowly making it’s way through the rubble of the Oort cloud, occasionally knocking loose a new comet or two. And now, it falls on the Astronomers to make some observations, to start looking and to keep combing the old data sets for evidence of the planet. Because of its extraordinary distance and its slow, slow rate of travel, it would be hard to spot, but hardly impossible.

Matese and Whitmire, by the way, have named this hypothetical discovery of theirs “Tyche,” as a bit of an astrophysicist in-joke. Tyche was in some versions of Greek mythology the sister of the goddess Nemesis, and “Nemesis” was the name given to a hypothetical star that some scientists once believed might exist out in the region where Tyche is now predicted to lie. Nemesis, of course, turned out to be a bad guess as part of an attempt to explain a perceived pattern in the Oort cloud, the source of comets in our solar system. Is this Matese and Whitmire’s subtle way of hinting that they know they’re going out on a limb? Or is it just the opposite, a name designed to contrast with the Nemesis debacle if they’re someday proven right? I suppose that, unlike the existence of Tyche itself, that is something we might never know.


About Colin West
Colin West is a graduate student in quantum information theory, working at the Yang Institute for Theoretical Physics at Stony Brook University. Originally from Colorado (where he attended college), his interests outside of physics include politics, paper-folding, puzzles, playing-cards, and apparently, plosives.

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