Upgrading a moon

Pluto’s satellite meets definitions of plutoid and dwarf planet

Normal 0 false false false MicrosoftInternetExplorer4 LAUREL, MD. — Pluto got the boot from planethood in 2006. But one of the dwarf planet’s moons, Charon, could get an upgrade, thanks to discussions August 14 during the Great Planet Debate Conference in Laurel, Md.

“Charon is not a satellite,” Keith Noll, a planetary scientist at the Space Telescope Science Institute in Baltimore, said during the meeting. “It meets the definitions of plutoid and dwarf planet, and if it weren’t orbiting Pluto, it would be a dwarf planet.”

But when the International Astronomical Union redefined planet and created the label dwarf planet, the group listed Charon as one of Pluto’s satellites — Nix and Hydra being the other two. In 2006, the IAU also wrote, “The idea that Charon might qualify to be called a dwarf planet on its own may be considered later,” as a footnote to their definitions of planet and dwarf planet.

Stirring even more controversy into Pluto’s demotion, the footnote was removed from the IAU’s statement of resolutions two days after it was published, says HarvardUniversity astronomer Owen Gingerich, who chaired the committee that worked on defining a planet.

“Charon is one of those loose ends from the 2006 IAU conference that needs to be tied up,” Noll says. “It was one of those decisions that was stripped off and held until later because the decisions were being made in real time and time was running short.”

Charon has a diameter 1,200 kilometers, Noll pointed out at the conference. Therefore, Charon is large enough to, by its own gravity, be round, he said. And that roundness characteristic is one of the first criteria of the IAU’s definitions of planet and dwarf planet.

But Charon isn’t quite a planet either. One IAU criterion for a planet is that it clears its neighboring region of other want-to-be planets, called planetesimals. Charon has not done this since it hasn’t gotten rid of Pluto, Noll notes. So Charon could be defined as a dwarf planet, but not a planet.

Finally, to be a dwarf planet, Charon cannot be a satellite. Charon is now considered one of Pluto’s satellites, Noll said. But, he countered, the IAU decided that when a satellite orbits its parent body, the center of gravity between the two must lie within the parent body.

That is not the case with Pluto and Charon. The center of gravity of the two lies 1,000 kilometers above Pluto’s surface. It is also important to note that Charon meets these criteria of dwarf planet and lies past Neptune, so it also fits the recent IAU definition of plutoid, Noll said.

Based on those considerations, “Charon should be reconsidered, not just for its own case,” he says, “but because of the fact that we are starting to find more and more of these binary systems in the Kuiper Belt,” the ring of dwarf planets and small solar system bodies that lie beyond Neptune. Fifty binaries similar to Pluto and Charon have been found so far with more to come, he says.

Gingerich, however, is unsure whether Charon would get its chance for a promotion at the next IAU General Assembly, to be held in August of 2009 in Rio de Janeiro. “I don’t think it will happen in a specific vote,” he says. “We have more serious problems to deal with.”

Ashley Yeager is the associate news editor at Science News. She has worked at The Scientist, the Simons Foundation, Duke University and the W.M. Keck Observatory, and was the web producer for Science News from 2013 to 2015. She has a bachelor’s degree in journalism from the University of Tennessee, Knoxville, and a master’s degree in science writing from MIT.

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