A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is there a Planet X after all?

April 29, 2011

Dear Cecil:

Your 1996 column on the mysterious giant Planet X some astronomers thought might lurk beyond Pluto is in need of an update. In particular, you should tell the story of the dwarf planet Eris, whose discovery was directly responsible for the demotion of Pluto from planet to dwarf planet.

Cecil replies:

Thanks for reminding us of our sacred mission, Voron. Ordinary bloggers and journalists are fine for staying current on nuclear meltdowns, Middle East upheavals, and other passing phenomena. But it's up to the Straight Dope to keep the planets straight.

Here's where things stood as of 1996. Planet X was the name astronomer Percival Lowell gave in the early 20th century to the hypothetical celestial body that would account for apparent irregularities in the orbit of Neptune, the outermost planet then known. In 1930, what we now call Pluto was discovered in the approximate location Lowell had predicted for Planet X. But this discovery was coincidence, made possible by sheer doggedness on the part of Clyde Tombaugh, the astronomer who found the new planet — Pluto, it turned out, was far too small to budge Neptune.

The search for Planet X continued. In 1972 astronomer Joseph Brady predicted it'd be a massive body with an orbit at a steep angle to those of most other planets that took 464 years to revolve around the sun. Brady's conjecture was never widely accepted, and more precise observations during the 1989 Voyager 2 flyby established that the supposed anomalies in Neptune's orbit were, in fact, nonexistent. In short, there was no Planet X.

End of story? Naturally not. In 2005 astronomers poring over telescopic images discovered another largish object out past Neptune. Initially called 2003 UB313 (the 2003 part identifies the year the crucial photos were taken), it bore several distinctive traits: First, it was the most distant object known to orbit the sun other than a few comets. Second, it was pretty big, with a diameter roughly a fifth of earth’s, although it was only a quarter of one percent as massive. Third, and here we get to the interesting part, it takes 557 years to circle the sun, and its orbit is at a steep angle to those of the other planets, much as Joseph Brady in 1972 had predicted for Planet X.

Again, however, it was just coincidence. Eris was nowhere near as massive as Brady's hypothetical giant. Nonetheless, 2003 UB313's discoverers had Planet X on their minds and nicknamed their find Xena, after TV's warrior princess. Wiser heads prevailed, and ultimately the thing was named Eris.

The discovery of Eris proved to be the downfall of Pluto. The two objects were about the same size; if Pluto was a planet, so was Eris. Astronomers were now faced with the prospect of a planetary roster consisting of eight large to really large planets plus two relatively dinky ones. Pluto's status as a planet had always struck some people as dubious, not only because of its size but also its funky orbit, which is itself sharply angled and at times brings Pluto closer to the sun than Neptune. The arrival of Eris on the scene revived the argument.

The problem wasn't just Eris. Pluto is located in a remote section of the solar system beyond Neptune's orbit known as the Kuiper Belt. (Eris is in an even more distant zone called the scattered disk.) For a long time astronomers thought there wasn't much in the Kuiper Belt other than Pluto and its moons, dust, and the occasional comet. Then starting in 1992 they discovered the first of what are now more than 1,000 known Kuiper Belt objects, or KBOs. Some think the KBO count could eventually reach 70,000. The Kuiper Belt, in other words, was like the asteroid belt between Mars and Jupiter — that is, a region of crud, little if any of it worthy of planetary designation.

But there was no way to define planet so that Pluto stayed in the club while the largest of the crud was kept out. One attempt to preserve Pluto’s planethood wound up elevating several other miscellaneous objects to planetary status as well, including the asteroid Ceres and Pluto's moon Charon. This will never do, the International Astronomical Union decided. In 2006 it demoted Pluto to dwarf planet.

You'd think that would mean the end of the Planet X saga, too. Not so. “Planet X” has now become the default term for any large orbiting item of interest on the solar system’s fringe. To learn more about what's out there, NASA has launched the New Horizons space probe, which will fly past Pluto in 2014 and later, it's hoped, past some KBOs. The space agency doesn't expect to spot any new mystery objects, project scientist Harold Weaver told my assistant Una. But at least speculation about future Planet Xs (in the Kuiper Belt, anyway) won’t be so wildly off the mark.

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References

Akwagyiram, Alexis "Farewell Pluto?" BBC News 02 August, 2005. 

Delsanti, Audrey and Jewitt, David. "The Solar System Beyond The Planets" Institute for Astronomy, University of Hawaii NASA Astrobiology Institute, Cooperative Agreement No. NNA04CC08A. Office of Space Science.  

Jewitt, David and Luu, Jane "Discovery of the candidate Kuiper belt object 1992 QB1" Nature 362 (1993): 730-732. 

Stern, S. Alan and Colwell, Joshua E. "Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30-50 AU Kuiper Gap" The Astrophysical Journal 490 (1997): 879-883.  



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