Pluto's years-long identity crisis just got more complex on Wednesday.
The International Astronomical Union has decided on the term "plutoid" as a name for Pluto and other objects that just two years ago were redefined as "dwarf planets."
The surprise decision is unlikely to stem ongoing controversy and confusion, astronomers say.
Sidestepping concerns of many astronomers worldwide, the IAU's decision, at a meeting of its Executive Committee in Oslo, comes almost two years after it stripped Pluto of its planethood and introduced the term "dwarf planets" for Pluto and other small round objects that often travel highly elliptical paths around the sun in the far reaches of the solar system.
"Most of the people in astronomy and planetary science community had no idea this was going to come out," said Hal Weaver of the Johns Hopkins University Applied Physics Laboratory. Weaver called the new definition "sort of outdated, outmoded, archaic."
A meeting in August at the Applied Physics Laboratory is slated to debate the entire topic of defining planets. Meanwhile, other astronomers said the new definition needed more definition or that it might simply not be used.
"This seems like an unattractive term and an unnecessary one to me," said David Morrison, an astronomer at NASA's Ames Research Center who, in 2006, said the IAU's actions on Pluto have created major rifts among astronomers.
The new definition
The name plutoid was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III and by the IAU Working Group for Planetary System Nomenclature (WGPSN), and approved by the IAU Executive Committee at its recent meeting in Oslo, according to a statement released today.
Here's the official new definition:
"Plutoids are celestial bodies in orbit around the sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit."
In short: small round things beyond Neptune that orbit the sun and have lots of rocky neighbors.
"Rather than resistance to 'plutoid,' I think we'll just be hearing groans," said Stephen J. Kortenkamp, senior scientist at the Planetary Science Institute in Tucson.
One IAU leader recognizes it is adding to an ongoing controversy.
The IAU has been responsible for naming planetary bodies and their satellites since the early 1900s. Its decision in 2006 to demote Pluto was highly controversial, with some astronomers saying simply that they would not heed it and questioning the IAU's validity as a governing body.
"The IAU is a democratic organization, thus open to comments and criticism of any kind," IAU General Secretary Karel A. van der Hucht told Space.com by e-mail today. "Given the history of the issue, we will probably never reach a complete consensus."
Van der Hucht said the new designation is not a further demotion for the once-favorite planet of grade-school children: "Pluto is now the prototype of a very interesting category of outer solar system bodies."
IAU Division III President Edward L.G. Bowell of the Lowell Observatory said the ruling stems from unfinished business from the forging of a planet definition in 2006. Bowell said there is no agreed-upon way to define "dwarf planet" yet, so "officers of the IAU thought it would be a good idea to adopt alternative criteria that would at least allow those large bodies to be named as though they were dwarf planets."
It remains to be seen whether astronomers will use the new term.
"My guess is that no one is going to much use this term, though perhaps I'm wrong," said Caltech astronomer Mike Brown, who has led the discovery of several objects in the outer solar system, including Eris. "But I don't think that this will be because it is controversial, just not particularly necessary."
Brown was unaware of the new definition until the IAU announced it.
"Back when the term 'pluton' was nixed they said they would come up with another one," Brown said. "So I guess they finally did."
Reactions were not all negative, however.
"It seems like a reasonable decision to me, and given the excitement generated by New Horizons [a NASA probe headed for Pluto], it's in everyone's interest to favor the largest Kuiper belt objects with their own categorical designation," said Gregory Laughlin, an extrasolar-planet researcher at the University of California, Santa Cruz.
"The only fly in the ointment that I can envision is if a plutoid larger, than, say, Mars is detected," Laughlin pointed out. "In that case, I think we'd see a big flare-up of the what-is-a-planet debate."
More debate coming
The dwarf planet Ceres (which used to be called an asteroid, and before that was called a planet!) is not a plutoid, as it is located in the asteroid belt between Mars and Jupiter, according to the IAU. Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind, the IAU stated. Therefore, a separate category of Ceres-like dwarf planets will not be proposed at this time, the reasoning goes.
Weaver, the Johns Hopkins researcher, has helped organized a meeting, planned for Aug. 14-16 at Johns Hopkins University Applied Physics Laboratory, that aims to bring astronomers of varying viewpoints together to discuss the controversy.
"We're not trying to slam the IAU, it's just that we also don't want to lead people to the idea that there's a handful of people who decide where science should go," Weaver said. The meeting is designed to "address this question in terms of a scientific conference." He said no votes will be taken at the meeting. Rather, it's a time to "sit back and take stock of everything we've learned in the past couple of decades."
The term plutoid joins a host of other odd words — plutinos, centaurs, cubewanos and EKOs — that astronomers have conjured in recent years to define objects in the outer solar system, whose appearance seems to grow more complex every year.
Kortenkamp wonders if "plutoid" isn't just one more confusing term in the cosmic lexicon.
"So Pluto is a Kuiper belt object, a plutino (the unofficial but nearly universally accepted name for objects in the 2:3 resonance with Neptune), a dwarf planet, and now also a plutoid?" he said. "If the IAU is trying to make things more clear, I think it needs to try again. This is just another layer of confusion that will feed the 'Pluto is a planet' camp at the [Johns Hopkins] meeting."
Kortenkamp also thinks the new defiinition leaves Ceres up in the air: "And this '-oid' classification doesn't apply to Ceres?" he asked. "Okay, so does that means we continue calling Ceres an ASTERoid?"
Asked whether Ceres remains a dwarf planet and is not an asteroid, Bowell, the IAU official, said: "I think so!"