They're up to 13 times the mass of our biggest planet, Jupiter, and they roam at will — unattached to any star.
For over a year, the 13 mysterious bodies discovered around the Orion Nebula have befuddled scientists, but today two astronomers claimed they can confirm they are, in fact, planets. The evidence? Water vapor.
The puzzling planet-like bodies were discovered inside the Orion nebula Trapezium star cluster, a collection of gas and dust 1,500 light-years away from Earth.
"If they were stars, they'd be too warm to host water vapor," reasons Patrick Roche, an astrophysicist at the University of Cambridge, England, who teamed with Phil Lucas at the University of Hertfordshire in Hatfield, England, to detect the signature markings of water vapor around the bodies. The pair made their observations in infrared light, using the United Kingdom infrared telescope at Mauna Kea Observatory in Hawaii.
Since a star would burn off any nearby water vapor rather than absorb it, Roche and Lucas concluded the large, free-floating bodies must be planets. While some astronomers embrace the observations as new evidence the bodies are planets, others remain unconvinced.
And, if they are planets, they are strange ones.
As astronomers have understood in the past, planets are objects no larger than about three to 10 times the size of Jupiter. And, as in our solar system, planets normally orbit around a star.
These newly discovered bodies, which range between six and 13 times the size of Jupiter, break both those rules.
"It's like anything in life," says Alan Stern, an astrophysicist at the Southwest Research Institute in Boulder, Colo., about the discovery of the perplexing bodies. "It's like studying Roman history and then going there and finding out there's an order of magnitude of more detail to learn."
In the face of newly discovered complexity, Stern and other astrophysicists say the definition of a planet should remain simple.
"The single and elegant criteria for planethood puts an upper limit at the mass," explains Stern. Anything above that upper limit, he explains, would generate enough heat to trigger the fusion of gasses hydrogen or deuterium inside the body and cause the constant burning that makes stars bright. "These bodies," says Stern, "are on the planet side of that equation."
The Orion Nebula is known as a veritable star factory. Its vast cloud of gas and dust is visible as a fuzzy star at the center of the constellation Orion's sword. Stars form as clumps of the gas and dust accumulate and collapse in upon themselves by their own self-gravity. During this collapse, material inside the massive clump fragments into smaller clumps, generating heat and setting off the stars' continual combustion.
That's how stars form, but then a remaining big question is how would the strange, large planets form in the same region?
Orphans or Wanna-Be Stars?
One theory is these free-floating planets are "orphans" that have been jettisoned from distant solar systems. Scientists have found evidence that our own solar system has ejected orphan planets in the past, although most of these planets were much smaller in mass.
Another idea is the planets formed in the same way stars form — by the gathering of massive gas and dust clouds — but never grew large enough to become stars.
Brown dwarfs are planetary bodies that never accumulated enough mass to become stars but still emit a faint, brownish glow. Alan Boss, an astrophysicist at the Carnegie Institution of Washington, D.C., believes these newly-discovered planets are more like brown dwarfs than planets.
Past calculations have indicated that planets are too low in mass to trigger the reactions that form a star. But Boss has done new number crunching and included the physical influence of magnetic fields — a factor not previously considered — and concluded that objects as small as five to ten times the mass of Jupiter can in fact form as stars. He argues an appropriate name for the mysterious bodies around Orion should then be "sub brown dwarfs," not planets.
"Sub brown dwarfs may not be as sexy a name, But what can you do? I think it's more accurate," says Boss, who adds, despite his disagreement with the astronomers' conclusion that the bodies are planets, he finds the discovery of water vapor around the bodies "interesting."
Roche, who spends most of his time making observations rather than analyzing them, says despite the ongoing debate over what to call the bodies, the important thing will be to keep watching them.
"People get worked up about what to call these things," Roche says. "But analyzing them will eventually tell us how stars form and perhaps what planets looked like at the beginning of our own solar system. That's the exciting part."