Tenth Planet Has A Moon.

In a sweep through the Sky and Telescope web site, just after completing my Supreme Court Downgrade/DeLay Troubles Part Deux, I came across this story that seems to indicate the discovery of a moon in orbit around the recently discovered tenth planet. Excerpt:

October 2, 2005 The solar system's largest known Kuiper Belt object (KBO), the recently discovered body known as 2003 UB313, isn't wandering through space alone.

Michael E. Brown (Caltech) and his colleagues have discovered that it has a small companion, by using the newly commissioned Laser Guide Star Adaptive Optics system on the Keck II telescope atop Mauna Kea, Hawaii, on September 10th.

The team doesn't yet know the moon's orbit, since they have only a single night of images. But observations with the Hubble Space Telescope planned for November and December should help determine the moon's orbital period and distance from 2003 UB313, and thus that object's mass. Due to scheduling constraints the team can't observe the system with Keck again for several months. Still, Brown says, "by January we should know the orbit."

The satellite is about 100 times (5 magnitudes) fainter than 19th-magnitude 2003 UB313. So if its surface has about the same reflectivity, its diameter is about a tenth that of the main body. That would make the satellite about 270 kilometers across.

Having a moon may not help 2003 UB313 gain official status as a "planet," but certainly it can't hurt. Right now an International Astronomical Union (IAU) committee is deciding whether 2003 UB313 should be officially classified as the tenth planet in the solar system.

In the meantime, recent new observations with the infrared Spitzer Space Telescope have allowed Brown and his team to confirm that the object is indeed larger than the previous KBO record holder, Pluto, by about 20 percent. Their current value for its diameter is 2,700 km, compared to Pluto's 2,274 km. The size measurement will be finalized in the coming weeks, but "if it's not larger than Pluto, then I'll eat my telescope," says Brown.

Full Story

Not long ago, correspondent and fellow space enthusiast Em Jeigh and I were discussing this concept of what constitutes a planet. This discussion was triggered by the discovery of the KBO Quaoar a couple of years ago. Quaoar was nearly the size of Pluto, and some astronomers became vocal about downgrading Pluto's status from that of planet to "King of the KBOs". That got us thinking about things.

The mention in the excerpt that the evidence of a moon might play a factor means little because there are several asteroids that seem to have moons, and nobody is clamoring for them to be called planets. Also, Mercury and Venus do not have moons, and that does not disqualify them as planets.

My humble idea was that from now on, any newly discovered object must have at least 5% of the Earth's mass to qualify as a planet--moons or no moons. The object also had to travel in an orbit that made it independent of the influence of any other bodies in the region. That is an important distinction because Ganymede, the largest moon of Jupiter, and Titan, Saturn's largest moon, are both larger than Mercury, and are almost as large as Mars, but they clearly do not qualify as planets because they orbit larger bodies. Feel free to comment if I've overlooked anything. In the meantime, celebrate the fact that the Solar System's available real estate market is growing!