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Planetary News: Trans-Neptunian Objects (2005)

"10th Planet" Has a Moon!

By Amir Alexander
October 3, 2005

It took thousands of years of planetary observations before the first moons orbiting a planet other than the Earth were discovered by Galileo in 1609. It took 48 years after the discovery of Pluto in 1930, before its moon Charon was detected in 1978. But it took barely two months following the discovery last July of 2005 UB313, also known as the “10th planet,” before astronomers found that it had a companion. Clearly, the science of moon detection is hitting its stride.

In a paper submitted to the Astrophysical Journal on October 3, Mike Brown of Caltech, Marcos Van Dam of the W. M. Keck Observatory, and several co-authors, announced the discovery of a moon orbiting trans-Neptunian object 2003 UB313, known to some as the “10th Planet.” 2003 UB313, it will be recalled, was discovered orbiting the Sun in the far off Kuiper Belt, well beyond the orbit of Pluto. Since the best estimates of its diameter put it at around 2700 kilometers (1700 miles), significantly larger than Pluto’s 2274 kilometers (1400 miles), some astronomers think it should qualify as a proper planet – the 10th orbiting the Sun. While the International Astronomical Union ponders the object’s status, it has not as yet been assigned an official name. Unofficially, 2005 UB313 is known as “Xena,” the nickname given to it by its discoverers Mike Brown of Caltech, David Rabinowitz of Yale, and Chad Trujillo of the Gemini Observatory.

Xena’s moon was discovered on the night of September 10, when astronomers at the Keck II telescope at Mauna Kea in Hawaii turned their sites on Xena, and observed a faint point of light nearby. Throughout the night the telescope operators observed as the point kept pace with the “planet,” passing in front of background stars and keeping station with respect to Xena itself. This meant that the point of light was not in the background, nor was it an artifact of the telescope’s optics. It was, they concluded, a moon. Naturally, the discoverers named the newly discovered moon “Gabrielle,” after the warrior princess’s sidekick.

“Since the day we discovered Xena,” said Brown, “the big question has been whether or not it has a moon. Having a moon is just inherently cool – and it is something that most self-respecting planets have, so it is good to see that this one does too.”

The cool moon Gabrielle is small, probably around 250 kilometers across, and about 60 times fainter than Xena itself. It is so faint, in fact, that in previous years it probably would not have been seen at all. But recently the Keck II telescope has been equipped with a state of the arts Laser Guide Star Adaptive Optics system (LGS AO). Like Adaptive Optics (AO) systems used in most major telescopes these days, the LGS AO measures atmospheric disturbances and corrects for them, producing an image of the sky far superior to the uncorrected optical view. Most AO systems rely on the presence of a known bright star near their object, which enables the system to continually measure the atmospheric effect and correct for it. Unfortunately, in Xena’s case, no bright background stars are present in the “planet’s” vicinity. To compensate for this, the LGS AO system sends out a powerful laser beam, which scatters off sodium atoms in the atmosphere at an altitude of around 90 kilometers (55 miles). This creates an artificial “star,” enabling the AO system to operate as if it were monitoring a true star. With the aid of LGS AO, astronomers were able to observe to observe Xena and Gabrielle at a clarity and resolution previously possible only with the Hubble Space Telescope.

Apart from making Xena a more “self-respecting” planet, the discovery of Gabrielle is significant in other ways as well. For one thing, it will make it possible for astronomers to calculate Xena’s mass. For while Xena’s diameter is now known with reasonable precision, its mass, and consequently its density, are not. Once a moon’s distance from its home planet and its orbital period are given, however, calculating the planet’s precise mass is a straightforward exercise in Newtonian mechanics. Although Brown and his colleagues know that Gabrielle completes its orbit every two weeks or so, their observations are not precise enough as yet to calculate Xena’s mass. This may well change soon, however, when astronomers will observe Xena and its companion with the Hubble Space Telescope in November and December.

The discovery of Gabrielle is also significant because it provides researchers with a better understanding of the nature and history of large Kuiper Belt Objects (KBOs). Around the time Xena was discovered, Brown and his colleagues also announced the detection of two other giants – 2003 EL61, nicknamed “Santa,” and 2005 FY9, nicknamed “Easterbunny.” These two, together with Xena and Pluto, represent the four largest known objects in the Kuiper belt, and three of them – Xena, Pluto, and Santa – possess moons.

Moons are not new in the Kuiper Belt. Astronomers have known for some years that some KBOs, much like asteroids in the Asteroid Belt, have moons. Most of these, however, appear to be “captured” moons – small space rocks that strayed close to a larger object and were captured by its gravitational field. Gabrielle, Pluto’s moon Charon, and Santa’s moon nicknamed “Rudolph,” are different. Their size and orbit indicate that they were, most likely formed in the same way our own moon did – through collision. At some point in their history, it seems, Xena, Pluto, and Santa, it seems, collided with a large space rock, which launched a substantial amount of rock and debris into orbit, where it congealed into a moon.

The fact that three of the four largest known objects in the Kuiper Belt have moons borne of major collisions is significant for scientists trying to understand the history and composition of the Kuiper Belt. Pluto, until recently the largest KBO and the only one with a moon borne of a collision, no longer appears to be the exception. “While once Pluto appeared unique in the outer solar system in terms of size and satellite formation mechanism, it now appears to be one of a family of similar-sized objects with perhaps similar collisional histories” write Brown, Van Dam, and their co-authors in their paper. Given the steady stream of discoveries of similar unknown giants lurking in the outer reaches of our Solar System, there is little doubt that the family of Xena, Gabrielle, Santa, and Rudolph will only keep growing.