Note: This is a “classic” Interesting Thing of the Day article from over 10 years ago. It has not been edited recently, so it may contain broken links, outdated information, or other infelicities. We plan to eventually update or retire most classic articles, as time permits.

Author’s note: Much of the information in this article (originally published in June 2006) is now outdated. In September 2006, Xena was deemed to be a “dwarf planet” and given the name Eris. You can read more about it on Mike Brown’s Eris Page. At some point in the future, I’ll update this article to reflect the current facts.

Most people, I think, have a pretty good idea of what a continent is, and can name all such areas that appear on Earth. And yet, if pressed to supply a definition (such as “a large contiguous landmass”), one may run into a problem when it comes to, for example, Europe and Asia. People in the U.S. have traditionally considered Europe and Asia to be two separate continents, but even a casual glance at a map reveals that the division is quite arbitrary. Unlike all the rest of the continents, they don’t have an obvious geographic feature, such as a large body of water, separating them from each other. Thus, in some parts of the world, Eurasia is considered a single continent, whereas in other parts of the world, North America and South America are considered a single continent. In fact, there are at least 10 different ways of dividing up the globe into anywhere from three to seven continents, and the way you choose to define “continent” will be conditioned by such factors as culture, education, language, and politics.

But the point is, once you’ve made up your mind about what a continent is and how many there are, you tend to regard that number as fixed and immutable, a fact of nature. If someone suddenly suggests that there may be one fewer, or three more continents than you’d thought, you might have a rather negative emotional response. Continents aren’t the only landmasses that cause this sort of trouble. Astronomical bodies discovered in the farthest reaches of the solar system are posing challenges to our concept of what a planet is. The latest such troublemaker is a big rock called 2003 UB313, affectionately known for the time being by the nickname Xena. Numerous news articles have breathlessly declared Xena to be the “tenth planet” in our solar system, but at the moment, it’s anyone’s guess whether it will turn out to be a planet at all, or what it will be called officially.

Fueling a Controversy

An earlier article here discussed whether or not Pluto should be considered a planet. The arguments against planethood are that it’s too small, has the wrong sort of orbit, and is really just one of a cluster of objects; the argument in favor mainly boils down to the fact that we’ve been calling it a planet for decades. The International Astronomical Union (IAU) has taken up the question numerous times but has so far failed to come up with a definitive answer; their latest claim is that the matter will be decided once and for all in September, 2006. One of the thorny issues is that any good definition of “planet” that includes Pluto will also include several other objects with similar characteristics, so in one fell swoop the solar system could gain a number of new members—and every possibility exists that still more would be added in the future.

Xena is the most recently discovered object that this list may or may not contain. Although news of the discovery wasn’t made public until July 2005, Xena was located in early 2003 by Mike Brown from the California Institute of Technology, Chad Trujillo from the Gemini Observatory, and David Rabinowitz from Yale—the very same three astronomers who were to receive a lot of press later in 2003 for their discovery of a planetoid called Sedna. Brown’s team initially reported that Sedna must have a moon due to its unusually slow rotation, but when the Hubble Space Telescope later photographed it and found no moon, other astronomers took a closer look and discovered that the original interpretation of the data had been flawed—Sedna rotated at an ordinary speed after all. Interestingly, a similar situation arose with Xena. Brown and his colleagues at first estimated that Xena was 30% larger than Pluto, but later measurements by the Hubble showed it to be a mere 5% larger.

Rocky X

Like other objects in that region of the solar system called the Kuiper Belt, Xena follows an elliptical path that, at a certain point, crosses over Neptune’s orbit; hence it’s also known as a Trans-Neptunian Object. But Xena’s orbit is outrageously big: at its farthest distance from the sun, which is roughly where it is now, it’s three times as distant as Pluto, making it the most remote known object in the solar system. At that distance, Xena takes nearly 560 years to orbit the sun just once. In addition to being elliptical, Xena’s orbit is tilted about 44° off the plane of the inner planets’ orbits, a highly unusual feature.

Yet another surprising attribute is Xena’s brightness. With a color almost as bright as snow, Xena is among the solar system’s most reflective objects. Brown believes this is due to a coating of frozen methane on the surface. Whether that methane once formed an atmosphere or whether it originated inside the planet in gaseous form and then solidified is unknown.

Beyond Jupiter and Jason

Brown’s team had apparently chosen the nickname Xena (after the eponymous heroine of Xena: Warrior Princess) long before they actually discovered 2003 UB313. The X is apt in that it’s the Roman numeral for 10 (if indeed this is eventually considered the tenth planet); it’s also a nod to “Planet X,” the nickname astronomer Percival Lowell had given to the as-yet-undiscovered ninth planet that was later named Pluto. Xena has a moon, by the way—nicknamed, logically enough, Gabrielle (after Xena’s sidekick on the TV show).

If the IAU deems 2003 UB313 to be a planet, they might ultimately choose a Greek or Roman god to name it after, as was done with most of the other planets. But since most names of Greek and Roman gods have already been used, they might have to look in other mythological traditions, as they did for Sedna (the name of an Inuit goddess). If they consider Xena a mere planetoid, on the other hand, it won’t rate such a heavy moniker. In any case, we can be certain that Xena will not be the final name.

In all likelihood, even more astronomical bodies like Xena will be discovered in the future, and I wouldn’t be surprised if the tricky question of how to define a planet is revisited numerous times. By the time we have the matter settled, we may be facing a new challenge: how to count continents on other planets. —Joe Kissell

More Information

This article was featured in The 40th Skeptics’ Circle.

For a good overview of the varying ways of defining “continent,” see the Wikipedia article Continent.

New Scientist magazine has followed this story closely since the beginning. A few noteworthy articles are these:

Mike Brown’s New Planet page has tons of information about Xena, though with a few apparent biases since it comes from one of the discoverers. (Note that Brown also mentions the analogy to the ill-defined term “continent.”) Other sources of information include: