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.
Marley notes that Kalas and his colleagues saw the exoplanet at two different wavelengths from Hubble, but attempts to see it from the ground failed. As a result, they lacked data points describing the object and, so, still had to make some inferences about it.
"If you look at the other exoplanet discoveries, or shall I say the other candidate exoplanets found by ground-based observations, the contrast [between them and their stars] is a factor of 10,000 or 100,000," Kalas says. "The contrast between Fomalhaut b and the central star is 100 million. And it's impossible to get this from the ground" in visible wavelengths. "Our selection is fairly rigorous and conservative. There may be specks in there that are also planets, but we've not been able to confirm them as such. Fomalhaut b is the most rigorously tested."
Marois and his team used ground-based infrared detection to seek out exoplanets around nearby, young, massive stars—those whose planets would have wide orbits and emit significant amounts of radiation as they cool from their relatively recent births millions of years ago. After narrowing some 80 candidate stars to 20 "really, really interesting" ones with infrared excess (indicating the presence of orbiting dust), the researchers settled on a particularly appealing star.
"For HR 8799, the infrared excess was consistent with dust orbiting at around 80 astronomical units," or roughly 80 times the distance from the Earth to the sun (7.4 billion miles, or 11.9 billion kilometers), Marois says. "And that's usually a sign of planet formation, the residue of the planet formation. So that's usually a very good sign."
By comparing data captured by various telescopes in 2004, 2007 and this year, Marois's group selected three objects, all estimated to be below the brown dwarf threshold, orbiting HR 8799. "These are the first images of a multiplanet system, but these are as well the first detections of planets at separations that are similar to the outer planets of our own solar system," Marois says. The innermost exoplanet around HR 8799 has an orbit that would place it between Uranus and Neptune in our solar system. (Fomalhaut b, by contrast, orbits at nearly twice the distance of the farthest-flung planet around HR 8799, albeit around a larger star.)
"Kepler himself would recognize these as planets," Marley says, referring to the German astronomer Johannes Kepler, who formalized his laws of planetary motion in the 17th century. "You can see the planets in close to circular orbits, you can see them moving, and this system also has a dust disk, as people now believe our own solar system out beyond the orbit of Neptune has."
Marley acknowledges that the exact mass of the objects could be called into question, even pushing two of the masses into the realm of brown dwarfs, but does not view that uncertainty as a deal-breaker given the magnitude of the discovery.
Further characterization of the objects from both groups will enhance astronomers' understanding of star and planet formation. Toward that end, Kalas is awaiting the repair of the Hubble's Space Telescope Imaging Spectrograph, which failed in 2004, to give a more detailed look at Fomalhaut b. Hubble could also help confirm the exoplanet's status next year by photographing it farther along its predicted orbital path. As for Marois, "there are actually two other objects that we're tracking" around another star, he says. "We're not sure if these are background objects or planets, so we're going to be confirming these next summer."
UPDATE: Click here to listen to a podcast about the exoplanet discoveries.