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.