I was up early this morning, awakened by a deluge of freezing rain that made me lose power, threatened to flood my basement, and dragged tree limbs crashing to the ground in my yard. It's the kind of raw weather that makes me wistful for a little of the warmth on HD 189733b.That place, it turns out, is one of the most interesting exoplanets known. Discovered in 2005 using Doppler spectroscopy, HD 189733b is a"hot Jupiter" that circles just 3 million miles (0.03 astronomical unit) from its host star. This means it's hopelessly too close to its sun to be spotted separately with current technology. However, by good fortune, it is a transiting exoplanet that alternately passes in front of and behind its sun every 2.2 days.
Recently astronomers have cleverly exploited this geometry to learn something about the Jupiter-size world's composition. First they record the combined spectrum of the star and planet together — and then, once HD 189733b has ducked from view, that of the star alone. Subtract one from the other, and — voilà — a crude spectrum of the planet emerges.
This week two teams of astronomers report, based on infrared spectra taken with both the Hubble and Spitzer space observatories, that there must be water on HD 189733b. This great news for everyone struggling to understand the 300+ exoplanets now known, because it brings us one step closer to finding a habitable world. I wouldn't call this particular planet "habitable" — its surface temperature near 2,500°F (1,400°C).
Dynamicists believe that this and other "hot Jupiters" didn't form so close to their host stars. Rather, they assembled farther out in their protostellar disks and then got dragged inward through interactions with the remaining dust and gas around them. No doubt countless of just-formed planets have been lost this way. HD 189733b likely survived either because its disk dissipated just in time or because it didn't extend all the way in.
In any case, it's not hard to imagine water being part of this planet's makeup. What's more puzzling is the signature of carbon dioxide there too. So I asked exoplanet expert Sara Seager (MIT) what she makes of this result. "The apparent detection of CO2 is very surprising," she told me, because it wouldn't be present if all of HD 189733b's gases were in chemical balance. "We think it comes from photochemistry, caused by the star's ultraviolet rays breaking up molecules that then reform as different molecules."
For the record, the two just-released papers involve Spitzer observations by Carl Grillmair and others (appearing in December 11th's Nature) and Hubble spectra collected by Mark Swain and others (accepted for publication in Astrophysical Journal Letters.
But don't try to wade through these; instead, download this highly readable overview by researchers Giovanna Tinetti and Jean-Philippe Beaulieu.
Meanwhile, here on my little corner of planet Earth, the local utility company tells me I might have my power back sometime this weekend. By then my house will likely be frozen over — not something that could never happen on HD 189733b!