Artist’s depiction of Eris and its moon, Dysnomia
Credit: Robert Hurt (IPAC)
In July 2005, astronomers found a large object deep in the Kuiper belt, with a slow orbit some three times further from the Sun than Pluto. It’s relative brightness was so high that it had to be either extremely reflective, or very large – larger than Pluto. It was named Eris and stirred up controversy – it was so far away, and in such a crowded area of space (the Kuiper Belt is a vast area populated with the clumps of ice and dirt that never formed into planets, and occasionally fall inwards to become comets) that many felt it should not be classified as a planet. But if Eris couldn’t be a planet, then neither could smaller Pluto. This reawakened the controversy about Pluto’s status, which had been simmering ever since it’s discovery in the 1930’s. The issue was finally resolved once and for all when the IAU ruled on the first ever formal definition of a planet, which excluded Pluto – read more on that here.
However on 5 November 2010, Eris passed directly in front of a distant star and Astronomers were waiting to watch the event through their telescopes. Such occultations provide valuable information. We can measure an object’s size by timing how long the star’s light is blotted out, and we can detect and analyse any atmosphere by seeing if the starlight cuts off abruptly or fades away. Last Friday’s observation revealed something unexpected: We were wrong about Eris’s size, and it is actually smaller than Pluto. We know through other methods that it is much more massive than Pluto, so they’ve clearly got different structures, but the question many people must be asking is this: “Does this give Pluto a chance to be reclassified as a planet?”
Sadly for them, the answer is no. Since the discovery of Eris, we have found many more Trans-Neptunian Objects (TNO’s), and Pluto’s weird orbit only serves to confirm that it is simply not like the remaining planets. While the current definition is flawed, particularly the first line stating that a planet orbits the Sun (so what do we call the 400+ “planets” we’ve found orbiting other stars then?), none of the problems have any impact on Pluto.