[LINK] "First Reconnaissance Of An Exoplanetary System"

[rfmcdonald]

HR 8799, a star 49% more massive than our sun and nearly five times as bright, roughly 129 light years away, stands out for a few reasons. For starters, it's only 30 million years old. More interestingly, as I noted in December 2010, it is known to host four gas giant planets, young massive worlds that are still so hot that their radiated heat lets them be directly detected by telescopic observations. Scientific American's Caleb Scharf notes at his blog that the four gas giants of HR 8799 are now the first planets with atmospheres possessing spectra that have been directly imaged. We know what their atmospheres are made of.

There are signatures of compounds like methane and ammonia, but also of things that might be acetylene and hydrogen cyanide – it’s a real mix. To quote Oppenheimer et al. – their analyses suggest that the planets are like this:

• b: contains ammonia and/or acetylene as well as CO2 but little methane.

• c: contains ammonia, perhaps some acetylene but neither CO2 nor substantial methane.

• d: contains acetylene, methane and CO2 but ammonia is not definitively detected.

• e: contains methane and acetylene but no ammonia or CO2.

You might be glazing over with this, so what does it mean? First, it means that these objects look more like planets than they do brown dwarfs. They’re also clearly, and remarkably, distinct from each other – despite (presumably) all being big, hot, gas giants. The only one that looks vaguely familiar is ‘e’ – whose spectrum is a bit like that of the night-side of Saturn.

Exactly how and why these worlds are so varied is a juicy puzzle. The researchers suggest that it might in part be a result of ultra-violet light flooding the system in bursts from the youthful star HR 8799. A thousand times brighter than the equivalent from our Sun, this radiation can drive all sorts of chemical and physical changes in planetary atmospheres.

The process is described by a paper by astronomer Ben Oppenheimer and colleagues.