Astronomy Now Online

Large planetoid found beyond Pluto
Posted: February 20, 2004

A new object located by the Near Earth Asteroid Tracking (NEAT) survey on February 17 and found in images by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale), may be the largest object discovered in the solar system since Pluto in 1930.

Image credit: Caltech    
2004 DW, to give its official designation, lies 2.4 billion km (1.5 billion miles) farther away than Pluto, placing it some 46 astronomical units (a.u.) from the Sun.

It belongs to a group known as the Kuiper Belt objects (KBOs), thousands of which are believed to have diameters larger than 100 km and lie in orbits that extend from Neptune (30 a.u.) out to 50 a.u. KBOs represent the primitive remnants of the early accretion phase of the solar system and are composed of ice and rock.

Given its distance, and assuming an albedo of about 9%, the diameter of 2004 DW is reckoned to be about 1,600 km (1,000 miles). If confirmed by subsequent measurements, it is considerably larger than a similar object named Quaoar (1,250 km) discovered in the summer of 2002. By comparison, Pluto is about 2,300 km in diameter.

Image credit: Chad Trujillo    
Further observations are needed to refine the orbit, but it has already been located on images taken in 2002. Initial indications are that it is a 'Plutino' — a Kuiper Belt object with an orbit similar to that of Pluto. Curiously, the longitude of 2004 DW's perihelion is also close to that of Pluto.

Observers wishing to view the new object in backyard telescopes against the constellation backdrop of Hydra may be disappointed: its visual magnitude is currently +19.2.