Planet Pluto knocked down to size, again

Pity poor Pluto the puny former planet is facing yet another indignity.

Demoted from planethood a year ago into a new category of dwarf planet, it now turns out that it isn't even the biggest one of those.

"This is sort of Pluto's last stand," joked Emily Schaller of the California Institute of Technology, co-author of a report in Thursday's issue of the journal Science.

When the International Astronomical Union redefined planets last year, it created the new subcategory dwarf planets, and Pluto was thought to be the largest in that group.

Planetary astronomy professor Michael Brown and graduate student Schaller found otherwise while studying Dysnomia, the moon of Eris.

Using the Keck Observatory and Hubble Space Telescope, they were able to calculate the movement of Dysnomia and, with that information, calculate the mass of Eris at 27 percent more than Pluto. Although Eris tops Pluto, it is 360 times less massive than Earth.

"Pluto and Eris are essentially twins except that Eris is slightly the pudgier of the two," Brown said.

Eris, by the way, is named for the Greek goddess of, among other things, rivalry.

Eris and Dysnomia are the most distant known objects in the solar system, almost 100 times further away from the sunthan Earth.

The discovery of Eris prompted astronomers to decide on a definition for "planet." If Eris had been included in the definition, it may have increased the number of planets in the solar system to as many as 15.

In the end, the International Astronomical Union decided on a definition that excluded Eris and, by necessity, Pluto, demoting the former ninth planet to dwarf planet status.