TITLE-PERFECT PLUTOIDSThe dwarf planet, Pluto, and its counterpart Eris are the first two members of a new class of objects called plutoids. Both lie beyond Neptune and are large enough that gravity pulls them into spherical forms. Any newfound object with similar traits will also fall into this IAU-established, dwarf-planet category.IAU, NASA/ESA Hubble Space Telescope, H. Weaver (JHU/APL), A. Stern (SwRI), the HST Pluto Companion Search Team and M. Brown.

Pluto now has a family of its own, after astronomers have struggled for years to give it a place among its celestial brethren.

The International Astronomical Union announced June 11 that it has accepted the name plutoid to distinguish all dwarf planets lying beyond Neptune. Last summer, the members of the IAU Committee on Small Body Nomenclature, or CSBN, proposed classifying these objects under the new title, which the IAU Executive Committee approved at its recent May meeting held in Oslo, Norway.

“We wanted a rather clear name that related to Pluto,” says astronomer Brian G. Marsden, secretary of the CSBN. “Plutoid fit the bill. And I am happy enough with it. It means Pluto-like.”

A plutoid is defined as a type of dwarf planet that orbits the Sun at a distance greater than Neptune and has enough mass for its self-gravity to give it a near-spherical shape. This characterization means “we have two known and named plutoids: Pluto and Eris,” says Marsden of the Harvard-Smithsonian Astrophysical Observatory.

But there are more plutoids to find and more known ones to name, he adds. “We just needed the protocol to name the objects, and that is what this news does.”

Two years ago, the IAU voted to kick Pluto out of the planet family and demote it to what they called a dwarf planet. But, Marsden says, Ceres, which is in the asteroid belt between Mars and Jupiter, also fit that bill, so scientists needed to distinguish between rocky dwarf planets like Ceres and icy ones like Pluto.

At the IAU meeting in Prague where Pluto was demoted, CSBN committee members tried to agree on labels to distinguish dwarf planets as either Pluto-like or Ceres-like. “We came up with plutonian, but the name did not pass.”

Although that vote was close, plutonian ended up two or three votes shy of acceptance, the astronomer says.

“I have to admit I didn’t vote for it,” he says. “The reason is because I felt that if they wanted plutonian, then we needed a label for Ceres-like objects. It just does not make sense to call Ceres a plutonian. That is why I voted against the name for Pluto-like dwarves back then.”

Marsden says that IAU president Catherine Cesarsky had been hoping that the committee would come up with a title for the minor planets like Pluto for past two years.

“Cesarsky must be very happy with plutoid. It is all over the IAU’s press release and even in italics,” he says.

Ceres is not a plutoid because it does not lie beyond Neptune. But Marsden noted that the committee recommended to the IAU Executive Committee to name Ceres-like objects ceroids. The IAU did not accept that name because current scientific knowledge leads astronomers to believe that Ceres is the only object of its kind.

“That is news to me,” Marsden says. “But maybe if we do find some bigger stuff similar to Ceres, ceroid could be accepted too.”

At any rate, the astronomer says, now that the IAU has accepted plutoids, the CSBN, which is responsible for naming minor planets and asteroids, can start working with the IAU’s Working Group for Planetary System Nomenclature, or WGPSN, to determine the names of new plutoids.

Previously, the two committees worked together to name the dwarf planet — and now plutoid — Eris and its satellite Dysnomia.

The relationship between the committees is important because it helps ensure that objects in different categories aren’t given the same name as any other small body in the solar system, the Marsden says.

“And keep watching,” he adds, “because now that we’ve gotten the go-ahead from the IAU, the third plutoid will be named soon.”

The astronomer did not say what the name of the next plutoid would be. “Announcing that wouldn’t be fair because it is not official,” he says. “But I’ll tell you, the one we are working on now is 2005FY₉ and the name was proposed by its discoverer, Mike Brown. We, the CSBN, like the name and the WGPSN is considering it right now. So we should know in a few weeks.”

Up next for naming, is plutoid 2003EL₆₁, Marsden adds.

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