MOVING RIGHT ALONG: The first visible-light image of what appears to be a planet orbiting the star Fomalhaut, 25 light-years away. In this composite, the object's position in 2004 [below] and in 2006 [above] follows its projected counterclockwise orbit [green lines]. Image: Courtesy of Paul Kalas, University of California, Berkeley
Two groups of researchers searching for extrasolar planets—planets orbiting stars other than our own sun—laid claim today to an astronomy milestone: photographing extrasolar planets directly, rather than inferring their presence through effects on their parent stars.
A team led by astronomer Paul Kalas of the University of California, Berkeley, detected a planetary candidate orbiting Fomalhaut, a star 25 light-years away in the constellation Pisces Australis (the Southern Fish), using visible-light observations from the Hubble Space Telescope. Another group, led by astronomer Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia, used infrared to image a family of three planets orbiting HR 8799, a star nearly 130 light-years distant. (Marois was also a member of Kalas's group.) Both teams report their findings online today in the journal Science.
Of the more than 300 other known exoplanets, all have been detected indirectly by their effects on their parent stars—either a wobble in induced by the object's orbit or a decrease in detected light from the star as the planet passes in front of it. Other photographed objects have been too massive to be conclusively labeled planets, falling instead into the brown dwarf category (objects about eight to 80 Jupiters in size that lack sufficient mass to ignite hydrogen fusion in their cores, thereby never becoming true stars); have been found to themselves orbit brown dwarfs rather than stars; or have not been shown to be gravitationally bound to a star.
"Finally, we now have separate images where you can see, actually see, the planet," says astronomer Mark Marley of the NASA Ames Research Center at Moffett Field, Calif., who did not participate in the research but wrote an article for Science summarizing and analyzing the teams' results. (Marley commented for SciAm.com as a scientist in the field, not as a representative of NASA.) "I've been using the analogy," he says, that "it's like you're in an apartment building and you can hear the people in the next apartment through the walls, so you know they're in there, but now you have opened the door and you can see the people."
Kalas and two of his co-authors, astronomy professor James Graham of U.C. Berkeley and astrophysicist Mark Clampin of the NASA Goddard Space Flight Center in Greenbelt, Md., had ventured in 2005 that Fomalhaut should harbor planet-size objects in its orbit, based on the disk of dust ringing the star. "At that time we hypothesized that there should be a planet shaping the ring," Kalas says. By looking at Hubble images from 2004 and 2006, Kalas and his colleagues were able to track a speck, dubbed Fomalhaut b, inside that ring that seemed to be orbiting the star.
"When you look at Fomalhaut b, its location is consistent with where we expected to find a planet, interior to the dust ring, and it shows orbital motion—and that's also encouraging," he says. "If this speck of dust had moved in a different direction between 2004 and 2006, we wouldn't believe that it was associated with Fomalhaut."
Fomalhaut b is significant for its small size, estimated to be between the mass of Neptune and three times the mass of Jupiter, which would place it squarely in the realm of planets. An object larger than about 13 Jupiter-masses is considered a small brown dwarf rather than a large planet. "The upper bound to Fomalhaut b is unprecedented," Kalas says. "Our upper bound definitively excludes that Fomalhaut b is a brown dwarf or a star." The planet, he says, "can't possibly be more than three Jupiter-masses," because a more massive object would clear its gravitational sphere of debris, meaning that the dust belt would have to be farther away than it is.