Megan's Blog

In the news this month... the strange atmosphere of GJ 436b

Most known extrasolar planets are massive gas giants orbiting close to their parent stars. If one of these planets happens to pass directly between us and its parent star during its orbit, then sensitive spectroscopy can be used to determine the chemical make-up of its atmosphere. Models of such atmospheres predict which gases should be present and in what relative abundances, based on physical conditions such as the temperature. Recent infra red observations carried out with the Spitzer Space Telescope have provided the first details of the atmospheric composition of a so-called hot Neptune.

The planet, known as GJ 436b, orbits an M-type dwarf star in the constellation of Leo. It is similar to Neptune in size, but orbits its parent star in just 2.6 days. Previous observations of the planet showed that its surface temperature was estimated to be 712 K, higher than predicted due to stellar heating alone, and the new observations (reported in the April 22nd issue of Nature) suggest that its atmosphere may not be in equilibrium. The team, led by Kevin Stevenson at the University of Central Florida, observed the planet's day side as it passed around the far side of the star and examined the infra red spectrum for various chemical signatures. What they found was a high abundance of carbon monoxide and a deficiency of methane compared to predictions from atmospheric models at this temperature for an atmosphere thought to be dominated by hydrogen. In an atmosphere such as this, methane (one carbon atom and four hydrogen atoms) should be the main carbon-bearing molecule, but the observations show the actual abundance is less than that predicted by a factor of seven thousand. The large amount of absorption due to carbon monoxide is also unexpected, the results suggesting that the atmosphere may not be in thermochemical equilibrium.

One alternative explanation considered by the authors is that the atmosphere may not be dominated by hydrogen, but this is unlikely given the dominance of hydrogen in planet forming disks. Another possibility is that vertical mixing within the atmosphere may dredge up carbon monoxide from lower, hotter parts of the atmosphere, although the authors point out that, in order to explain the observed abundances, the amount of mixing would have to be large. These new data will provide useful information for future atmospheric modeling.

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This blog post is a news story from the Jodcast, aired in the May 2010 edition.

Stevenson, K., Harrington, J., Nymeyer, S., Madhusudhan, N., Seager, S., Bowman, W., Hardy, R., Deming, D., Rauscher, E., & Lust, N. (2010). Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b Nature, 464 (7292), 1161-1164 DOI: 10.1038/nature09013