Exoplanet Atmosphere Examined With Pioneering Technique

An artist’s impression of the exoplanet Tau Boötis b. (ESO/L. Calçada)

The atmosphere of Tau Boötis b, a large “hot Jupiter” and one of the closest known exoplanets, has been analyzed using a new method that allows its weak planetshine to be directly detected.

Using the European Southern Observatory’s (ESO) Very Large Telescope (VLT), an international team determined its exact orbit and mass, as well as properties of its atmosphere.

Previously, hot Jupiter atmospheres could only be studied when exoplanets cross in front of their parent stars. However, as Tau Boötis b does not transit its sun, the astronomers had to distinguish the planet’s light from bright starlight.

“Thanks to the high quality observations provided by the VLT and CRIRES [CRyogenic InfraRed Echelle Spectrometer] we were able to study the spectrum of the system in much more detail than has been possible before,” said study lead author Matteo Brogi at the Netherlands’ Leiden Observatory in a statement.

“Only about 0.01 percent of the light we see comes from the planet, and the rest from the star, so this was not easy.”

The astronomers found that Tau Boötis b orbits its sun 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 the 15-year-old problem of the mass of Tau Boötis b,” said study co-author Ignas Snellen, also at Leiden Observatory, in the statement.

“And the new technique also means that we can now study the atmospheres of exoplanets that don’t transit their stars, as well as measuring their masses accurately, which was impossible before. This is a big step forward.”

The team also examined the temperature of the exoplanet’s atmosphere, and found it may be cooler at higher altitudes. This finding is in contrast to the atmospheres of other known hot Jupiters, which show a temperature increase with altitude.

“This study shows the enormous potential of current and future ground-based telescopes, such as the E-ELT [European Extremely Large Telescope],” Snellen said.

“Maybe one day we may even find evidence for biological activity on Earth-like planets in this way.”

The findings will be published in Nature on June 28.

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