Hellifiblog

How cool is this?

http://arstechnica.com/news.ars/post/20081113-two-reports-detail-imaging-of-extrasolar-planets.html

Three planets directly observed orbiting distant star

By John Timmer | Published: November 13, 2008 – 01:00PM CT

Over the past decade, researchers have made incredible strides in their quest to identify stars that are orbited by planets—you can track their accelerating progress right here at Ars. But most of these planets have been identified indirectly, either through their gravitational effects or when their orbit takes them between Earth and their host star. The few extrasolar objects that we have seen orbiting stars tend to be big and hot, awkwardly straddling the border between super-Jupiters and brown dwarfs. But today’s issue of Science Express (where the journal Science puts its early, online-only releases) will contain two papers that describe direct observations of extrasolar planets, including three orbiting a single star.
Related Stories

* Earth-like planets orbiting other stars on the horizon
* Seeing a planet through the dust
* Astrobiologists ponder the laws of life

The trick seems to have been knowing where to look. Both of the new systems are centered on young A-type stars, which tend to be fairly bright and are more massive than our sun. This extra weight extends their gravitational influence, allowing planets to form further from the host star. The youth of these stars, estimated at less than 300 million years in both cases, is also critical. Any nearby planets should be equally youthful, and thus still warmed by their gravitational collapse. That warmth should show up as an infrared glow, provided the emissions from the nearby star are blotted out.

In both cases, it took several years of observations to confirm that the planets are gravitationally linked with and are orbiting their host stars. In the first case, that host star is Fomalhaut, which lies about 25 light years from earth and is estimated to be 100-300 million years old. Fomalhaut has a substantial ring of dust and, back in 2004, the Hubble started a series of annual images of the dust while using a coronagraph to block out the star itself. By May of this year, researchers had enough data to know that Fomalhaut b was linked to the star, and they followed up with observations using the Gemini Observatory.

The dust actually turned out to be useful, as calculations set an upper limit on the size of a body that could orbit within it without causing major disruptions: less than three times the mass of Jupiter. Fomalhaut b is orbiting about 115 astronomical units (AU—one AU equals the distance from Earth to the sun) away from its host star, and is likely to have an atmospheric temperature of about 400 K. A brightening of certain wavelengths suggests that it has its own ring of dust, orbiting at a distance similar to that occupied by the moons of Jupiter.

The Fomalhaut dust disk (left) and a time-lapse image of the planet orbiting within it (right)
P. Kalas, Berkeley

As impressive as that is, Fomalhaut’s companion pales in comparison with the three-planet system orbiting the star with the catchy name HR 8799. This star is even younger—the authors estimate its age at between 30 and 160 million years. They started imaging it specifically to spot planets back in 2007, using the Gemini and Keck Observatories. The group employed a technique called angular differential imaging, which is designed to pull the signal from planets out of the background noise. Basically, the telescope is pointed at the star so that the field of view rotates around the star slowly—any imperfections in the mirror get averaged out.

HR 8799 b and c were spotted in 2007; d became apparent in 2008. With that information in hand, the researchers went back and spotted b and c in data going back to 2004. The net result is that there’s a high statistical certainty that HR 8799 b is orbiting at 68AU, and HR 8799 c is at 38AU. The certainty on HR 8799 d’s orbit is much lower (six sigma), but it appears to be orbiting at 24AU. The best fit for the masses, given their optical properties, are 7, 10, and 10 times that of Jupiter—still heavy, but below the brown dwarf cutoff.

The authors argue that the system looks a lot like our own from some perspectives. HR 8799 is roughly five times brighter than the sun; compensating for this luminosity difference makes the adjusted distances of the planets roughly equivalent to those of Saturn, Uranus, and Neptune. There’s even a disk of dust orbiting further out, much like our own Kuiper Belt. Finally, they note that these distances leave plenty of space for smaller planets to orbit closer to the star.

NASA hosted a press conference today to discuss the Fomalhaut results that made a few things more clear. The first is that the possibility of a dust disk around Fomalhaut b is exciting because it may be the raw material for moon formation—one researcher suggested that this may be what Jupiter looked like at the equivalent stage of its formation. The other thing is that better hardware for detecting exoplanets is coming on line within the next decade, which should be able to spot smaller planets closer in to the star. Clearly, HR 8799 and Fomalhaut would top the list of where to point them as things now stand, since we know planet formation has occurred there.

Reflecting on the results, I think it’s important to note that these planets would not have been detected if it weren’t for visual observations. They’re so far out that their orbits take over a century to complete, so the standard methods of indirect planet discovery are simply not going to hint at their presence. As a result, they (and the inevitable successor discoveries) profoundly expand what we know about extrasolar planetary systems, and they’re likely to have a significant impact on our models of the evolution of these systems.