New trick for probing exoplanet atmospheres

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 For the first time a clever new technique now lets astronomers study  the atmosphere of an exoplanet in detail – even if it does not pass in front of its parent star. An international team with SRON-researcher Remco de Kok has used ESO’s Very Large Telescope to catch the faint glow from the planet Tau Boötis b and measure its mass precisely – solving a 15-year old problem. The team also finds that the planet’s atmosphere seems to be cooler higher up, the opposite of what was expected. The results will be published in the 28 June 2012 issue of the journal Nature.

Artist impression Tau Bootis b

The planet Tau Boötis b was one of the first exoplanets to be discovered back in 1996, and it is still one of the brightest and closest planetary systems known to date. Although the star is easily visible with the naked eye, the planet itself certainly is not and up to now it could only be spotted by its gravitational effects on its parent star. Unlike some exoplanets, this planet does not transit the disc of its star (like the recent transit of Venus), and hence its atmosphere could not be studied up to now. Tau Boötis b is a large hot Jupiter planet orbiting very close to its parent star.

But now, after 15 years of attempting to study the emitted light from hot Jupiter exoplanets, astronomers have finally succeeded and reliably probed the structure of the atmosphere of Tau Boötis b. The team used the CRIRES instrument on the Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile. They combined high quality infrared observations (at wavelengths around 2.3 microns) (1) with a clever new trick to tease out the weak signal of the planet from the much stronger one from the parent star. Co-author Remco de Kok: ‘This method uses the velocity of the planet in orbit around its parent star to distinguish its radiation from that of the star and also from features coming from the Earth’s atmosphere. We have tested this technique before on a transiting planet,  measuring its orbital velocity during its crossing of the stellar disc.’

Lead author of the study Matteo Brogi (Leiden Observatory, the Netherlands) further explains: ‘Thanks to the high quality observations provided by the VLT and CRIRES we were able to study the spectrum of the system in much more detail than has been possible before. Only about 0.01% of the light we see comes from the planet, and the rest from the star, so this was not easy.’

15 year old problem
The majority of planets around other stars were discovered by their gravitation effect on their parent star. These techniques can only give a lower limit on the mass. The new technique pioneered here allows astronomers to measure the angle of the star’s orbit and hence work out its mass precisely. By tracing the changes in the planet’s motion as it orbits its star, the team has determined reliably for the first time that Tau Boötis b orbits its host star at an angle of 44 degrees and has a mass six times that of the planet Jupiter in our own Solar System.

The new VLT observations solve a 15-year old problem. And the new technique also means that we can now study the atmospheres of exoplanets that don’t transits their stars, which was impossible before, says Ignas Snellen (Leiden Observatory, the Netherlands), co-author of the paper.

Planet atmosphere
The team has also probed the atmosphere of Tau Boötis b and measured the amount of carbon monoxide present, as well as the temperature at different altitudes by means of a comparison between the observations with theoretical models. The most surprising result was that the new observations indicated an atmosphere with a temperature that falls higher up. This result is the exact opposite of the temperature inversion — an increase in temperature with height —  found for other hot Jupiter exoplanets (2, 3). De Kok: ‘This might indicate that the strong shortwave radiation from the star causes a breakdown of the chemical components in the atmosphere that otherwise would have caused a temperature inversion.’

The VLT observations show that high resolution spectroscopy from ground-based telescopes is a valuable tool for a detailed analysis of non-transiting exoplanets’ atmospheres. The detection of different molecules in future will allow astronomers to learn more about the planet’s atmospheric conditions. By measuring them along the planet’s orbit, astronomers may even be able track atmospheric changes between the planet’s morning and evening.

This study shows the enormous potential of current and future ground-based telescopes, such as the E-ELT. Maybe one day we may even find evidence for biological activity on Earth-like planets in this way, concludes Ignas Snellen.

(1) At infrared wavelengths, the parent star emits less light than in  the optical regime, so this is a  wavelength regime favorable for separating out the dim planet’s signal.
(2) Thermal inversions are thought to be characterised by molecular  features in emission rather than in absorption, as interpreted from photometric observations of hot Jupiters with the Spitzer Space Telescope. The exoplanet HD209458b is the best-studied example of thermal inversions in the exoplanet atmospheres.
(3) This observation supports models in which strong ultraviolet emission associated to chromospheric activity — similar to the one exhibited by the host star of Tau Boötis b — is responsible for the inhibition of the thermal inversion.

Paper and research team
This research was presented in a paper The signature of orbital motion from the dayside of the planet  Boötis b to appear in the journal Nature on 28 June 2012. The team is composed of Matteo Brogi (Leiden Observatory, the Netherlands), Ignas A. G. Snellen (Leiden Observatory), Remco J. de  Kok (SRON, Utrecht, the Netherlands), Simon Albrecht (Massachusetts Institute of Technology, Cambridge, USA), Jayne Birkby (Leiden Observatory) and Ernst J. W. de Mooij (Leiden Observatory; University  of Toronto, Canada).