A bright, slowly moving object in the outer Solar System may be a world larger than Pluto. A team of astronomers led by Jose-Luis Ortiz at the Sierra Nevada Observatory in Baja, California found the object, called 2003 EL61, using observations made in 2003. It is some 51 AU from the Sun (one AU, or Astronomical Unit, is the distance from Earth to the Sun), and evidently comes as close as 35 AU, inside Pluto’s average distance of 39 AU. An analysis of older observations shows the object in images dating back to 1995. [Note: the Sierra Nevada Observatory was mistakenly identified as being in Spain in an earlier version of this post].

Is 2003 EL61 a new planet? And for that matter, how do we define what a planet is? That debate is sure to be reignited as we weigh the possibilities here, for a world larger than Pluto surely has to be considered a planet. But size measurements this far out from the Sun are tricky, and rely on an object’s albedo, a measure of how much light the object reflects. If 2003 EL61 is similar to other distant objects like Sedna and Quaoar, then the large size estimate makes sense. If, on the other hand, it is similar to Pluto in having a bright albedo, it would be smaller, though still significant at some 1500 kilometers in diameter (Pluto’s diameter is 2300 kilometers).

Image: The bright moving object now known as 2003 EL61 in a photograph made by Jose-Luis Ortiz and colleagues. Credit: Instituto de Astrofísica de Andalucia.

New Scientist has a write-up of the discovery here. The Sierra Nevada team has asked amateur astronomers to study the object, and its observations have been verified by the International Astronomical Union’s Minor Planet Center (MPC) in Cambridge, Massachusetts. Interestingly, 2003 EL61 could only be found using observations taken a full day apart, an indication of how slowly it is moving. And like Pluto, the new object is tilted substantially in relation to the orbital plane of the other planets, some 28° as compared to Pluto’s 17°.

Centauri Dreams‘ take: Given the widening hunt for planetoids in the Kuiper Belt, it comes as no surprise that a major object like this one should appear, although it does seem unusual that something this large has eluded detection until now. But before we draw too many conclusions, it will be necessary to get some hard data about the object’s true reflectivity and other characteristics that will give us some clue as to its composition.

A second thought is to note the heartening collaboration that is once again taking place between professional and amateur astronomers. With time on the big telescopes severely allocated, and with amateur equipment constantly increasing in sophistication, this is a synergy we can exploit to map the location of numerous deep space objects in the Kuiper Belt and beyond. We may indeed find that the Solar System is embedded in a veritable halo of Kuiper Belt planetoids, some of them large enough to qualify as planets in their own right.

The Ortiz team evidently intends to publish an animation of 2003 EL61 showing its movement against background stars, but the link is not yet operational; keep an eye on the site. A separate page gives background (in Spanish) on the equipment deployed at Sierra Nevada Observatory.