Washington

Two astronomers at the California Institute of Technology have found a 1,200-kilometre-wide rock in the Kuiper Belt that circles in the region of Neptune's orbit.

Big issue: the large size of Quaoar raises the question of whether Pluto is a Kuiper-belt object. Credit: C. TRUJILLO & M. BROWN/CALTECH

The rock — named Quaoar after the creation force of the tribe that once inhabited the Los Angeles area, where the institute is based — is the largest of the 600-plus Kuiper-belt objects (KBOs) so far identified. And with a diameter half as big as Pluto's and roughly the same size as that of its moon Charon, Quaoar bolsters the argument that Pluto can also be considered a KBO.

Chad Trujillo and Mike Brown found Quaoar on a digital image taken on 4 June with the Palomar Observatory's 1.2-metre Oschin Telescope, which they set up to search for objects moving against the background star field. The pace of KBO discovery has accelerated in recent years, thanks to this and other Kuiper-belt searches that can spot even fainter objects.

Since the discovery, Trujillo and Brown have used the Hubble Space Telescope, the Keck telescope in Hawaii, and the IRAM millimetre-wave telescope in Spain to collect further details about Quaoar. With the benefit of hindsight, they have identified Quaoar in astronomical images taken as far back as 1982. It appears to be spherical, or nearly so — unlike another large KBO, the 900-kilometre Varuna, which is elongated.

Quaoar is also relatively bright, reflecting 10% of the light that hits it. This adds to evidence that the 4% reflectivity assumed for KBOs may be wrong. And because the size of such objects is often inferred from their brightness, Trujillo says that many KBOs may be smaller than originally believed. Two other KBOs — Ixion and 2002 AW197 — were once thought to be as big as Quaoar, assuming 4% reflectivity. But Trujillo says that Quaoar's size has been confirmed by measurements from Hubble, whereas the others' haven't.

Alan Stern, a planetary scientist at Southwest Research Institute in Boulder, Colorado, and leader of the New Horizons spacecraft mission to reach Pluto by 2015, called Quaoar “a wonderful discovery”. But its size record may turn out to be short-lived. Stern won't be surprised if someone eventually finds an Earth-sized object in the Kuiper belt. In fact, he adds, “I'd be surprised if we don't”.