Oddball Planet Puzzles Astronomers

This conceptual image shows the XO-1b planet, which is very similar to the newly discovered planet XO-3b. Credit: Rice University

A team of amateur and professional astronomers has discovered a mammoth orb more than 13 times the mass of Jupiter that whips around its parent star in fewer than four days and is considered an "oddball" planet among its exoplanet relatives.

The new exoplanet, dubbed XO-3b, was described here at a meeting of the American Astronomical Society. The discovery came out of the XO Project, a collaboration between amateur and professional astronomers.

"Of the 200-plus exoplanets found so far, XO-3b is an oddity in several respects," said XO Project director Peter McCullough, an astronomer at the Space Telescope Science Institute in Baltimore.

For one, it's the largest and most massive planet found in such a tight orbit around its parent star. Secondly, with such a close proximity between the planet and star, astronomers would expect a circular orbit as the parent star's gravity tugs and shapes the orbit. But XO-3b treks along an elliptical orbit.

And during every orbit, XO-3b passes in front of its star, making it a "transiting planet," of which just a few dozen have been identified.

Planet or brown dwarf?

The finding also stirs up an already frothy debate concerning the boundary between brown dwarfs and massive planets.

"We are intrigued that its mass is on the boundary between planets and 'brown dwarfs,'" said Christopher Johns-Krull, an astronomer at Rice University. "There's still a lively debate among astronomers about how to classify brown dwarfs."

Brown dwarfs are too massive to be considered planets, yet they don't meet the "Sumo-weight" requirements for hydrogen fusion (about 80 Jupiters) so they fall short of being stars. The new object's mass is right on the boundary where brown-dwarf status begins, a mass requirement for the burning of deuterium, or heavy hydrogen.

"The controversy lies at the lower end of the scale," Johns-Krull said. "Some people believe anything capable of fusing deuterium, which in theory happens around 13 Jupiter masses, is a brown dwarf. Others say it's not the mass that matters, but whether the body forms on its own or as part of a planetary system."

Brown dwarf desert

Even if scientists determine XO-3b to be a brown dwarf, the object would still have "celebrity" status. Due to their mass, brown dwarfs should be easy for astronomers to spot. That's because when hunting for exoplanets, astronomers usually look for them indirectly via the wobbles in stars caused by the gravitational tug of the orbiting planet or planets. The heftier the orbiting planet, the more it should shake its parent star. However, sky scanners haven't spotted a treasure trove of brown dwarfs and this scarcity has become known as the "brown dwarf desert."

"There are many astrophysical systems out there that mimic transiting planets," McCullough said. "The only way to sort out the real planets from the rest is to observe the stars more carefully," and come up with accurate mass and other measurements.

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