Fresh Debate over First Photo of Extrasolar Planet
In a wild turn of cosmic events, a group of astronomers is trying to reclaim the role of having made the first photograph of a planet around another star.
They may have to wait for history to award the blue ribbon, however, since the hunt for planet-like objects is turning out to be easier than developing a classification system for the variable types of orbs being found.
The saga goes back to last September, when SPACE.com broke the news that a team based at the European Southern Observatory (ESO) had made what they said was likely the first picture of an extrasolar planet. That object, 2M1207b, appeared bound to a young but failed star known as a brown dwarf sitting about 200 light-years from Earth.
But it was also possible that 2M1207b was instead a distant background object. More observations were needed to make sure the object was indeed orbiting the brown dwarf star.
Meanwhile, earlier this month, Ralph Neuhaeuser of the Astrophysical Institute & University Observatory said his team had made the first confirmed picture of a planet around another star called GQ Lupi, some 400 light-years away. In this case, also first reported by SPACE.com, the object was observed to be clearly bound to the star.
Importantly, GQ Lupi is similar to our Sun, rather than being a dim brown dwarf star that emits virtually no visible light.
But again there was a hitch: While observations suggest the planet orbiting GQ Lupi is about twice as massive as Jupiter, there is a slight chance it is 42 times the mass of Jupiter - so heavy that it would be considered a brown dwarf. The outlying models, however, are very unlikely to apply to the system, some astronomers said.
"Based on what we know, that image is an image of an object much like Jupiter at an extremely young age," said Ben Oppenheimer of the American Museum of Natural History, who was not involved in either study.
Both claims were appropriately modest, in the sense that the researchers admitted more observations were needed to confirm their apparent discoveries.
Back in this corner ...
Today, the ESO team announced new observations of 2M1207b that show convincingly, they say, that their target is indeed a planet. If so, it could be remembered as the first picture of an exoplanet, since the team had already released the initial image last fall.
"Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system," said team leader Gael Chauvin, an ESO astronomer.
"The two objects -- the giant planet and the young brown dwarf -- are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding," says Benjamin Zuckerman, another ESO team member from UCLA. "I'm more than 99 percent confident."
The object's mass is based on both observations and assumptions about they system based on its age, which is about 5 million years (our solar system, by comparison, is 4.5 billion years old). Leading theory suggests 2M1207b is between three and seven times the heft of Jupiter -- well within the limits of planethood.
What is a planet?
The whole issue is further clouded by the fact that 2M1207b orbits a brown dwarf rather than a regular star. Brown dwarfs stars do not have enough mass to trigger the thermonuclear fusion that powers a regular star. So a planetary-mass object around them exists in an unusual system that is unlikely to have any chance of harboring life as we know it. It also may have had a different formation history.
"Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," Chauvin said. "Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust."
Alan Boss, a planet formation theorist at the Carnegie Institution of Washington, said both findings represent "great stuff" but he's not ready to draw firm conclusions about who wins.
"This object [2M1207b] should be termed a sub-brown-dwarf (star), in order to convey this suspicion about its formation mechanism," Boss said in an e-mail interview. "A number of sub-brown dwarfs have been observed as single objects in regions of recent star formation, but 2M1207 would seem to be the first one in orbit around a brown dwarf (star). All in all it is an excellent discovery of a new class of object, but it is unclear if this object should be termed a 'planet.'"
Boss is careful to note that this is his opinion, however. There is no agreement among astronomers on the definition of the term "planet." A spirited debate dates back to attempts to define the word five years ago.
"I consider it as a planet, regardless whether it formed differently than Jupiter - and regardless it's orbiting a failed star instead of solar-type star," says Christophe Dumas, a colleague of Chauvin. "Actually, this discovery is even more interesting due to the fact that the brown dwarf and the giant planet are not forming a 'traditional' system as we know it from looking at our own solar system. We did not expect to find a giant planet in orbit around a brown dwarf and it's there."
For now, the International Astronomical Union lists 2M1207b as a "possible planetary-mass companion to a brown dwarf. It catalogues the GQ Lupi discovery as "a possible planetary-mass companion to a young star."
The ultimate challenge
Someday history will sort out a winner. Meanwhile, there are more challenges for planet hunters to tackle.
More than 140 planets have been found around other stars. Most are very massive and have been detected by noting the gravity wobble they induce on their stars. Photographing planets around other stars is difficult because the starlight so greatly overpowers any light coming from a planet.
The tricks used to tease the two present contenders out of their respective glares surprised the whole astronomy community.
Scientists expect to eventually photograph obvious planets around normal stars, and even Earth-sized planets when technology allows. Those images will likely come from space-based observatories that are now only in the planning stages.
"We're collecting here the first piece of a puzzle that will lead - within the next decade - to define a big picture of planetary systems," Dumas said via e-mail. "Some will be like our own, but most of them probably not. Some planets will be orbiting close to giant stars and will not be in a position to sustain life. Others will be orbiting far away from much less massive stars like in the 2M1207 system. Others will be Earth-like and will orbit at the right distance from the star to produce the right conditions, so life as we know it could emerge."
The paper describing the 2M1207 observations has been accepted for publication in the journal Astronomy and Astrophysics.
For the record, there was a third player in all this. A Hubble Space Telescope photograph reported by SPACE.com in May 2004 was also said to be a candidate planet. That object, a much more tentative detection than the two recent cases, was later found to be a background object, showing how difficult this quest has been.
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