TPS: The Image of a Distant Planet

LARGER THAN PLUTO!
10th Planet Discovered?

July 29, 2005

The object awkwardly known as 2003UB313 orbits the Sun in the Kuiper Belt, at the farthest edge of the Solar System. Its orbit is eccentric, and tilted from the mean orbital plane of the planets. It is extremely distant - three times further away from the Sun than Pluto, the 9th and outermost planet. But here’s the big news: 2003UB313 is bigger than Pluto.

Has a 10th planet been added to our Solar System?

Michael Brown of the California Institute of Technology, who leads the team that discovered 2003UB313 certainly thinks so: “It’s time to rewrite the textbooks” he suggested at a hastily arranged press conference at the Jet Propulsion Laboratory in Pasadena on Friday afternoon. He seems to have a point: if little Pluto is large enough to be considered a planet, then certainly any larger object orbiting the Sun has a claim to the same rank. It is the first time since the discovery of Pluto by Clyde Tombaugh 75 years ago that a new object has been discovered in the Solar System with such a strong claim to be designated a planet.

The new “planet” was discovered on January 8 of this year as part of a systematic survey of the outer edges of the Solar System conducted by Brown and his colleagues, Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale. Using the robotic Samuel Oschin 48 Inch telescope in Mount Palomar Observatory near San Diego, they have in the past few years found some of the largest and most intriguing objects ever discovered in the Solar System. Quaoar, discovered in 2002, is 1250 kilometers in diameter; 2004DW, discovered two years later is around 1600 kilometers; and Sedna – that most mysterious of objects – is also around 1600 kilometers in diameter. Pluto, by comparison, is 2300 kilometers end to end.

Brown is not sure exactly how large 2003UB313 is. The brightness of such an object in the sky is a function of its size and its reflectivity. A small reflective object would appear to as bright as a large body with an unreflective surface. In order to arrive at a minimum possible size of 2003UB313, Brown and his team assumed that it was extremely reflective. Even so, its diameter came out as 2700 kilometers, substantially larger than Pluto.

At the same time, 2003UB313 cannot be larger than twice the diameter of Pluto. This is because Brown and his colleagues tried and failed to detect it in the infrared range using the Spitzer Space Telescope. Cold though 2003UB313 is, currently oribiting 97 AU from the Sun, (AU, short for "astronomical Unit," being the average distance of the Earth from the Sun) an object larger than two Pluto’s would necessarily emit enough heat to be detected by Spitzer.

At a distance of 97 AU, 2003UB313 is now at the farthest end of its orbit. In 240 years it will reach its closest point to the Sun, at 36 AU. Together, the two extremes define the boudaries of a highly eccentric orbit. Unlike the 8 inner planets, whose orbital planes coincide very closely, the new object's orbital plane is tilted 45 degrees to that of the planets. Pluto's orbitalplane, by comparison, ist tilted 20 degrees. But while these characteristics seem to make 2003UB313 an odd planet, they are not at all unusual in its own neighborhood. "Such orbits are typical in the Kuiper Belt" said Brown, adding that the shape of the new planet's orbit was likely shaped by repeated close encounters with Neptune over billions of years.

At the press conference Brown was apologetic about the rough estimate of the new “planet’s” size. “We usually like to have more information about an object before we announce a discovery,” he explained. In this case, however, he had no choice. Earlier in the day Brown discovered that the data base containing his team’s observations of 2003UB313 and two other Kuiper Belt Objects had been hacked into, raising the possibility that someone was preparing to preempt their announcement of the discovery of the 10th planet. This forced the issue: on very short notice Brown called an unusual Friday afternoon press conference and made his announcement.

As a potential 10th planet in the Solar System, 2003UB313 will not retain its awkward name for long. The International Astronomical Union (IAU) is the only body with the authority to name celestial objects, and it does so in consultation with the object’s discoverers. At the press conference Brown said he had a very good suggestion for the new “planet’s” name, but he declined to reveal it before presenting it to the IAU. Brown has previously been criticized for the public naming of Sedna before the name was officially approved by the IAU. He seemed determined to avoid a similar controversy in this case.

And so, we do not yet know the name of the newest planet in the Solar System. But for the first time in 75 years it appears that a new member is about to join a very exclusive family.

Read about the discovery of Pluto, the last to join the exclusive family of planets
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