A new way to learn about exoplanets

Astronomers have used a new technique to study the atmosphere of a ‘hot Jupiter’ exoplanet.

Tau Bootis b was discovered in 1996, but up to now has been observed only through its gravitational effects on the star. Like most exoplanets, it doesn’t transit the disc of its star, which meant it was impossible to learn about its atmosphere through the atmosphere’s effects on the starlight.

Now, though, astronomers using the CRIRES instrument on the Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile have succeeded in reliably probing the structure of the atmosphere of Tau Bootis b and deducing its mass accurately for the first time.

They combined high quality infrared observations at wavelengths around 2.3 microns with a new trick to separate out the weak signal of the planet from the much stronger one from the parent star.

“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,” says Matteo Brogi of Leiden Observatory in the Netherlands.

“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.”

Seeing the planet’s light directly has allowed the astronomers to measure the angle of the planet’s orbit and hence work out its mass precisely. By tracing the changes in the planet’s motion as it orbits its star, they’ve determined reliably for the first time that Tau Bootis b orbits its host star at an angle of 44 degrees and has a mass six times that of Jupiter.

“The new VLT observations solve the 15-year old problem of the mass of Tau Bootis b,” says Leiden’s Ignas Snellen.

“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’s also managed to probe the planet’s atmosphere and measure both the amount of carbon monoxide present and the temperature at different altitudes, by comparing the observations and theoretical models.

Surprisingly, the new observations indicated an atmosphere with a temperature that falls higher up. This is the exact opposite of the temperature inversion found for other hot Jupiter exoplanets.

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