Astronomers Discover "10th Planet"

By David Tytell and Greg Bryant

Saturday July 30: After 75 years of false leads and speculation, it finally seems to have happened. A team of astronomers using the 48-inch Samuel Oschin Telescope at Palomar Observatory and the 8-metre Gemini North telescope on Mauna Kea, Hawaii, has discovered the largest Kuiper Belt object (KBO) ever.

It is bigger than Pluto, the 9th planet.

The object, designated 2003 UB313, is currently 97 astronomical units (Earth-Sun distances) away — more than twice Pluto's average distance from the Sun. This makes it the farthest object ever seen in the Solar System. It is a scattered-disk object, meaning that at some point in its history Neptune likely flung it into its highly inclined (44°) orbit. It's currently glowing at magnitude 18.9 in the constellation Cetus. Its high inclination is the only reason it wasn't discovered years ago; no one was looking for planets so far from the plane of the Solar System.

Discoverers Michael E. Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) first imaged the object on October 21, 2003, but didn't see it move in the sky until reimaging the same area 15 months later on January 8, 2005.

"We tried looking at it with the Spitzer Space Telescope and didn't detect it. So we have an upper limit on the size. It can't be any more than 3,000 kilometres across," says Brown. But the lower limit derived from its brightness — even by assuming its surface is 100 percent reflective — still makes it larger than Pluto, which is 2,250 km across.

"It's a big sucker!" says Brown.

To try to pin down its size, observations with the Hubble Space Telescope are scheduled to take place shortly.

Preliminary near-infrared spectra, obtained by team member Chad Trujillo at the 8-metre Gemini North telescope, show methane ice on the object's surface — much like the spectrum of Pluto, which is also dominated by methane ice in the near infrared.

Trujillo says in a Gemini press release, "It is clear that [2003 UB313] is very similar to Pluto in both size and composition, at least upon first glance." Pluto is about 30 percent reflective. Methane ice is unusual and suggests a primitive surface that has never been warmed much since the formation of the solar system. "To date," notes Trujillo, "no one has seen methane on any other Kuiper Belt object, only on Pluto and Neptune's moon Triton."

Ambitious CCD imagers with setups capable of seeing to 19th magnitude can shoot for it at right ascension 1h 39.3m, declination –5° 21' (2000.0 coordinates) through the first half of August 2005. Its position in central Cetus is currently fairly well up in the northeastern sky (for observers at temperate southern latitudes) before the first light of dawn.

Two More Big KBOs

A second big Kuiper Belt discovery also made news: 2003 EL61. That body, located about 52 a.u. away, was discovered by astronomers at the Sierra Nevada Observatory in Spain, and independently by Brown and his team some time earlier. It appears to be the third-largest Kuiper Belt object known to date, with about 70 percent of Pluto's diameter — bested only by Pluto itself and 2003 UB313.

The Sierra Nevada Observatory astronomers were the first to announce the discovery, and Brown (on his website) is quick to congratulate them.

"There is no question that the Spanish group is rightly credited with discovery...We could have announced the object earlier, but we took a chance that no one else would find it while we were awaiting our observations from the Spitzer Space Telescope. We were wrong! And we congratulate our colleagues on a very nice discovery".

Moreover, by a great stroke of luck, 2003 EL61 has a tiny satellite revolving around it, at an apparent distance of about 1.5 arcseconds. According to Brown's group, the satellite completes an orbit every 49 days in a nearly circular orbit some 49,500 km from the main body. The satellite's orbit has allowed the team to determine the mass of 2003 EL61: about a quarter that of Pluto.

Brown and his team have also looked at 2003 EL61 with the Spitzer Space Telescope. Those observations are still being analysed, but, Brown notes, "The spectra are dominated by water ice. It looks much like [Pluto's moon] Charon."

A third large KBO, 2005 FY9, almost overlooked in the attention given to the above two, was also announced at the same time as 2003 UB313. Spitzer was used in an attempt to image this KBO in late July. Results are still being analysed, but Brown told Australian Sky & Telescope that they were "pretty sure it is smaller than Pluto."

Australian Sky & Telescope continues to follow these breaking stories. Check back for more details.

We'll be covering these discoveries in our upcoming September issue, available to subscribers next week, and in newsagencies on Wednesday 17th August.