A New Duo of Exoplanet Questions

Yesterday’s successful launch of a SpaceX Falcon, and the subsequent safe return of the Dragon spacecraft after a three hour ride, puts an exclamation point on Dana Andrews’ paper on space and commercial viability, which was discussed here yesterday. We’re a long way from a sustainable space infrastructure — many reports note the fact that what SpaceX did yesterday roughly parallels where humans were in space about fifty years ago, with the early Soviet and American flights — but we are seeing the most promising signs yet of a viable launch business emerging from the commercial sector, with all that implies about eventual use of space resources and future colonization.

Exoplanetary Puzzles

Now we wait for news from NanoSail-D, whose sail deployment should, if my sources are right, be today, but the @NanoSailD Twitter feed has grown quiet. We’ll think good thoughts and, while waiting, move on to the exoplanet hunt, the latest news from which is the discovery that the planet WASP-12b, found last year by the Wide-Angle Search for Planets project, has an atmosphere dominated by molecules containing carbon. That’s a sharply different scenario from our Earth, which contains much smaller amounts of carbon and a great deal more oxygen.

The find makes for interesting speculation about other planets that might exist in the same system. Although WASP-12b is a ‘hot Jupiter,’ orbiting its star in a mere 26 hours, it’s possible that smaller, rocky worlds could form around the star, which is about 1200 light years away. Just what such worlds might be like is a question that intrigues Joe Harrington (University of Central Florida), who analyzed the Spitzer data for the study:

> “A rocky planet in such a planetary system could have an interior abundant in diamonds and a surface littered with graphite and diamonds. The theorists will have fun with this one. Could life thrive in such an environment, with little oxygen or water? That might not be so far-fetched given last week’s announcement by NASA of bacteria that can survive by using arsenic in place of phosphorus, previously thought to be essential.”

A brief pause to note that the NASA work on arsenic is taking flak from many quarters (see the comments on Centauri Dreams’ recent story on the announcement), leaving significant doubts about its credibility. The broader principle — that life may surprise us, though perhaps not in this case — is obviously still in play.

It took an analysis of WASP-12b’s infrared spectrum through Spitzer Space Telescope data to flag the significance of carbon, revealing that the world is the first planet found where the oxygen/carbon ratio is reversed over what we see in our own Solar System. Our Sun’s carbon-to-oxygen ratio is about one to two (half as much carbon as oxygen), and at least among the inner planets, none have been found to have more carbon than oxygen (the ratio for the outer planets has not yet been established). WASP-12b is the first planet to have a carbon-to-oxygen ratio greater than one measured, a significant change from Earth normal.

> “When the relative amount of carbon gets that high, it’s as though you flip a switch, and everything changes,” said Marc Kuchner, an astronomer at NASA Goddard Space Flight Center, Greenbelt, Md., who helped develop the theory of carbon-rich rocky planets but is not associated with the study. “If something like this had happened on Earth, your expensive engagement ring would be made of glass, which would be rare, and the mountains would all be made of diamonds.”

A Riddle Much Closer to Home

Not that WASP-12b wasn’t unusual to begin with. It’s close enough to its star that gravity has elongated it, pulling mass off the planetary atmosphere into a thin disk that orbits with the planet. Temperatures in the range of 2600 Kelvin (2325 degrees Celsius) make it one of the hottest exoplanets yet discovered. But the next step in the WASP-12b story may take place in a colder place. For to understand what’s happening there, we need to know whether planets like Jupiter have a carbon/oxygen ratio we can measure. It’s tricky to make that reading because much of Jupiter’s oxygen is trapped in water and thus not available for spectroscopic observation because it has condensed out of the atmosphere due to the cold at Jupiter’s orbital distance.

All eyes thus turn to Juno, a Jupiter mission that will launch in 2011 and reach the giant planet five years later, where it will map water and oxygen to determine whether Jupiter is as carbon-rich as WASP-12b. Getting the answer for Jupiter will tell us just how unusual WASP-12b really is, as this background story in Nature makes clear. The paper is Madhusudhan et al., “A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b,” published online by Nature 8 December 2010 (abstract).

A Challenge to Formation Theories

Be aware as well of the interesting gas giant found around HR 8799. HR 8799e is one of four gas giants in this system, orbiting its star at 14.5 AU and in the process distinguishing itself from its more distant brethren (at 24, 38 and 68 AU respectively). Gravitational instability can be invoked to explain the formation of the three outer gas giants, but HR 8799e is in too warm a position at 14.5 AU to make it likely, leaving us with core accretion as the formation model.

But core accretion doesn’t fit the outer worlds, the process taking so long to complete that solid planetary cores would not have had sufficient gas from the dissipating protoplanetary disk to form a gas giant. Different formation models in the same system? It seems unlikely, because the masses of these planets are similar and they’re showing clear signs of orbital resonance, leading Christian Marois (Herzberg Institute of Astrophysics) to suggest the planets got where they are today through orbital migration. HR 8799, some 129 light years from Earth, is going to preoccupy planetary formation theorists for some time to come. The paper is Marois et al., “Images of a fourth planet orbiting HR 8799,” published online in Nature 8 December 2010 (abstract).

URL: http://www.centauri-dreams.org/?p=15659&utm_source=rss&utm_medium=rss&utm_campaign=a-new-duo-of-exoplanet-questions

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