Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b

In Nature

http://www.nature.com/nature/journal/v464/n7292/abs/nature09013.html

Quote :

Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b

Kevin B. Stevenson1, Joseph Harrington1, Sarah Nymeyer1, Nikku Madhusudhan2, Sara Seager2, William C. Bowman1, Ryan A. Hardy1, Drake Deming3, Emily Rauscher4 & Nate B. Lust1

Planetary Sciences Group, Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
Department of Physics and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
Department of Astronomy, Columbia University, New York, New York 10027, USA
Correspondence to: Kevin B. Stevenson1 Correspondence and requests for materials should be addressed to K.B.S. (Email: kevin218@knights.ucf.edu).


The nearby extrasolar planet GJ 436b—which has been labelled as a ‘hot Neptune’—reveals itself by the dimming of light as it crosses in front of and behind its parent star as seen from Earth. Respectively known as the primary transit and secondary eclipse, the former constrains the planet’s radius and mass1, 2, and the latter constrains the planet’s temperature3, 4 and, with measurements at multiple wavelengths, its atmospheric composition. Previous work5 using transmission spectroscopy failed to detect the 1.4-μm water vapour band, leaving the planet’s atmospheric composition poorly constrained. Here we report the detection of planetary thermal emission from the dayside of GJ 436b at multiple infrared wavelengths during the secondary eclipse. The best-fit compositional models contain a high CO abundance and a substantial methane (CH4) deficiency relative to thermochemical equilibrium models6 for the predicted hydrogen-dominated atmosphere7, 8. Moreover, we report the presence of some H2O and traces of CO2. Because CH4 is expected to be the dominant carbon-bearing species, disequilibrium processes such as vertical mixing9 and polymerization of methane10 into substances such as ethylene may be required to explain the hot Neptune’s small CH4-to-CO ratio, which is at least 10^5 times smaller than predicted6.

Interesting result here. It's a pity metallicity determinations of red dwarfs are so tricky. I know there have been some interesting simulations of planet formations where the relative abundances of various elements differ substantially from those in our solar system, leading to rather different condensation sequences and thus differences in the bulk compositions of the resultant planets.

Hi Borislav,

this is a Spitzer (before "warm-mission") result.
The observations were done in May 2009.

see this

http://www.sciencedaily.com/releases/2010/04/100421131339.htm

exoplanet wrote:

see this

http://www.sciencedaily.com/releases/2010/04/100421131339.htm

Thanks.

Addition
http://www.jpl.nasa.gov/news/news.cfm?release=2010-137

Quote :

The space telescope has captured the planet's light in six infrared wavelengths, showing evidence for carbon monoxide but not methane.

This spectrum of IRS or summary photometry IRAC and MIPS? Is there somewhere in the picture with the spectrum?

Would it be possible to get HST photometry of this? Seeing what it did for WASP-12 b, I think we could learn a lot about this planet.

Also, it would be interesting to examine the spectrum of Kepler-4 b, a much more highly irradiated planet.

Borislav wrote:

This spectrum of IRS or summary photometry IRAC and MIPS? Is there somewhere in the picture with the spectrum?

If I understand right, the spectrum was observed by taking light curves of the secondary eclipse in several wavelengths. If you detect a secondary transit at some wavelength, then the planet must be emitting that wavelength. At the wavelength where methane emission was expected, no (or a very muted) secondary eclipse was observed, suggesting the planet lacks (readily) observable methane.

Sirius_Alpha wrote:

Would it be possible to get HST photometry of this? Seeing what it did for WASP-12 b, I think we could learn a lot about this planet.

Hubble has found water shortage in this planet.
http://arxiv.org/abs/0810.5731

In the archives of the Hubble is still such a program. Ie probably want to find a hydrogen corona.
http://archive.stsci.edu/proposal_search.php?id=11817&mission=hst

I was thinking of more recent HST photometry, with the new and improved super-amazing HST.

Sirius_Alpha wrote:

I was thinking of more recent HST photometry, with the new and improved super-amazing HST.

http://archive.stsci.edu/proposal_search.php?id=11817&mission=hst
Look at the bottom of this link. It says that observations of this program were held on Jan. 5, 2010, ie after the last expedition Hubble.

In addition, looking at the picture with 6 secondary eclipse remembered that say that the archival Spitzer photometry at 8 microns have found something else was not planned.

The dayside atmosphere of the hot-Neptune GJ 436b
http://arxiv.org/abs/1004.5121

Abstract wrote:

We present a detailed analysis of the day-side atmosphere of the hot Neptune GJ~436b, based on recent Spitzer observations. We report statistical constraints on the thermal and chemical properties of the planet atmosphere, study correlations between the various molecular species, and discuss scenarios of equilibrium and non-equilibrium chemistry in GJ~436b. We model the planet atmosphere with a 1-D line-by-line radiative transfer code with parametrized molecular abundances and temperature structure. We explore the model parameter space with 10^6 models, using a Markov chain Monte Carlo scheme. Our results encompass previous findings, indicating a paucity of methane, an over-abundance of CO and CO2, and a slight under-abundance of H2O, as compared to equilibrium chemistry with solar metallicity. The concentrations of the species are highly correlated. Our best-fit constraints require a methane (CH4) mixing ratio between 1.0e-7 - 1.0e-6, with CO > 1.0E-3, CO2 between 1.0e-6 - 1.0e-4, and H2O < 1.0E-4; higher CH4 would require much higher CO and CO2. Using calculations of equilibrium and non-equilibrium chemistry, we find that the observed high CO abundance can be explained with a combination of high metallicity (30 x solar) and eddy mixing (with Kzz ~ 10^6-10^7), whereas the low CH4 abundance can potentially be explained by photochemistry. Our constraints rule out a day-side thermal inversion in GJ~436b. We emphasize that the constraints reported in this work depend crucially on the observations in the two Spitzer channels at 3.6 micron and 4.5 micron. Future observations with warm Spitzer and JWST will be extremely important to improve upon the present constraints on the abundances of carbon species in the dayside atmosphere of GJ~436b.

Methane in the atmosphere of the transiting hot Neptune GJ436b?
http://arxiv.org/abs/1007.0324

Abstract wrote:

We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5 and microns obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. After correcting for systematic effects of the instrument, we fitted the light curves - including limb darkening effects - using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, HST and ground-based V, I, H and K_s observations available in the literature, the range 0.5-10 microns can be covered.
The temperature distribution of the planet was estimated by using a three-dimensional, pseudo-spectral general circulation model with idealised thermal forcing. Transmission spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such planetary atmosphere, namely water vapour, methane, ammonia, carbon monoxide and dioxide, and hydrogen sulphide. In particular, a new, ab-initio calculated, linelist for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant species after molecular hydrogen, possibly with minor contributions from ammonia, water and other molecules. No clear evidence of carbon monoxide and dioxide is found from transmission photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesised to interpret secondary transit photometric data. In particular we show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.

Well that's a bit of a radical revision: methane going from being substantially deficient to the second most abundant species after methane, and carbon monoxide going from the dominant carbon-containing species to "no clear evidence"...

Now on ArXiv.

Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
http://arxiv.org/abs/1010.4591