PASADENA, Calif.—The International Astronomical Union (IAU) today downgraded the status of Pluto to that of a "dwarf planet," a designation that will also be applied to the spherical body discovered last year by California Institute of Technology planetary scientist Mike Brown and his colleagues. The decision means that only the rocky worlds of the inner solar system and the gas giants of the outer system will hereafter be designated as planets.
The ruling effectively settles a year-long controversy about whether the spherical body announced last year and informally named "Xena" would rise to planetary status. Somewhat larger than Pluto, the body has been informally known as Xena since the formal announcement of its discovery on July 29, 2005, by Brown and his co-discoverers, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University. Xena will now be known as the largest dwarf planet.
"I'm of course disappointed that Xena will not be the tenth planet, but I definitely support the IAU in this difficult and courageous decision," said Brown. "It is scientifically the right thing to do, and is a great step forward in astronomy.
"Pluto would never be considered a planet if it were discovered today, and I think the fact that we've now found one Kuiper-belt object bigger than Pluto underscores its shaky status."
Pluto was discovered in 1930. Because of its size and distance from Earth, astronomers had no idea of its composition or other characteristics at the time. But having no reason to think that many other similar bodies would eventually be found in the outer reaches of the solar system—or that a new type of body even existed in the region—they assumed that designating the new discover as the ninth planet was a scientifically accurate decision.
However, about two decades later, the famed astronomer Gerard Kuiper postulated that a region in the outer solar system could house a gigantic number of comet-like objects too faint to be seen with the telescopes of the day. The Kuiper belt, as it came to be called, was demonstrated to exist in the 1990s, and astronomers have been finding objects of varying size in the region ever since.
Few if any astronomers had previously called for the Kuiper-belt objects to be called planets, because most were significantly smaller than Pluto. But the announcement of Xena's discovery raised a new need for a more precise definition of which objects are planets and which are not.
According to Brown, the decision will pose a difficulty for a public that has been accustomed to thinking for the last 75 years that the solar system has nine planets.
"It's going to be a difficult thing to accept at first, but we will accept it eventually, and that's the right scientific and cultural thing to do," Brown says.
In fact, the public has had some experience with the demotion of a planet in the past, although not in living memory. Astronomers discovered the asteroid Ceres on January 1, 1801—literally at the turn of the 19th century. Having no reason to suspect that a new class of celestial object had been found, scientists designated it the eighth planet (Uranus having been discovered some 20 years earlier).
Soon several other asteroids were discovered, and these, too, were summarily designated as newly found planets. But when astronomers continued finding numerous other asteroids in the region (there are thought to be hundreds of thousands), the astronomical community in the early 1850s demoted Ceres and the others and coined the new term "minor planet."
Xena was discovered on January 8, 2005, at Palomar Observatory with the NASA-funded 48-inch Samuel Oschin Telescope. Xena is about 2,400 kilometers in diameter. A Kuiper-belt object like Pluto, but slightly less reddish-yellow, Xena is currently visible in the constellation Cetus to anyone with a top-quality amateur telescope.
Brown and his colleagues in late September announced that Xena has at least one moon. This body has been nicknamed Gabrielle, after Xena's sidekick on the television series.
Xena is currently about 97 astronomical units from the sun (an astronomical unit is the distance between the sun and Earth), which means that it is some nine billion miles away at present. Xena is on a highly elliptical 560-year orbit, sweeping in as close to the sun as 38 astronomical units. Currently, however, it is nearly as far away as it ever gets.
Pluto's own elliptical orbit takes it as far away as 50 astronomical units from the sun during its 250-year revolution. This means that Xena is sometimes much closer to Earth than Pluto—although never closer than Neptune.
Gabrielle is about 250 kilometers in diameter and reflects only about 1 percent of the sunlight that its parent reflects. Because of its small size, Gabrielle could be oddly shaped.
Brown says that the study of Gabrielle's orbit around Xena hasn't yet been fully completed. But once it is, the researchers will be able to derive the mass of Xena itself from Gabrielle's orbit. This information will lead to new insights on Xena's composition.
Based on spectral data, the researchers think Xena is covered with a layer of methane that has seeped from the interior and frozen on the surface. As in the case of Pluto, the methane has undergone chemical transformations, probably due to the faint solar radiation, that have caused the methane layer to redden. But the methane surface on Xena is somewhat more yellowish than the reddish-yellow surface of Pluto, perhaps because Xena is farther from the sun.
Brown and Trujillo first photographed Xena with the 48-inch Samuel Oschin Telescope on October 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of 2005.
The search for new planets and other bodies in the Kuiper belt is funded by NASA. For more information on the program, see the Samuel Oschin Telescope's website at http://www.astro.caltech.edu/palomarnew/sot.html.
For more information on Mike Brown's research, see http://www.gps.caltech.edu/~mbrown.
