Innovative technique enables scientists to learn more about elusive exoplanet Tau Bootis b

June 27, 2012
This artist's impression shows the exoplanet Tau Boötis b. This was one of the first exoplanets to be discovered back in 1996, and it is still one of the closest planetary systems known to date. Astronomers using ESO's Very Large Telescope have now caught and studied the faint light from the planet Tau Boötis b for the first time. By employing a clever observational trick the team find that the planet’s atmosphere seems to be cooler higher up, the opposite of what was expected. Credit: ESO/L. Calçada

For the first time a new technique has allowed astronomers to study the atmosphere of an exoplanet in detail -- even though it does not pass in front of its parent star. An international team has used ESO's Very Large Telescope to directly catch the faint glow from the planet Tau Boötis b, solving a 15-year-old problem. The team also finds that the planet's atmosphere seems to be cooler higher up, differently from the expected.

The planet Tau Bootis b was one of the first exoplanets to be discovered back in 1996, and it is still one of the closest exoplanets known. Although its parent star is easily visible with the naked eye, the planet itself certainly is not, and up to now it could only be detected by its gravitational effects on the star. Tau Bootis b is a large "hot Jupiter" planet orbiting very close to its parent star.

Like most exoplanets, this planet does not transit the disc of its star (like the recent transit of Venus). Up to now such transits were essential to allow the study of hot Jupiter atmospheres: when a planet passes in front of its star it imprints the properties of the atmosphere onto the starlight. As no starlight shines through Tau Bootis b's atmosphere towards us, this means the planet's atmosphere could not be studied before.

The video will load shortly.
This animation shows how a system would look when the inclination is varied. For Tau Bootis an inclination of 44 degrees is measured. Note that the animation is not to scale: the size of the planet is exaggerated. Credit: ESO/L. Calcada

But now, after 15 years of attempting to study the faint glow that is emitted from hot Jupiter exoplanets, astronomers have finally succeeded in reliably probing the structure of the atmosphere of Tau Bootis b and deducing its mass accurately for the first time. The team used the CRIRES instrument on the Very Large (VLT) at ESO's Paranal Observatory in Chile. They combined high quality infrared observations (at wavelengths around 2.3 microns) with a clever new trick to tease out the weak signal of the planet from the much stronger one from the .

Lead author of the study Matteo Brogi (Leiden Observatory, the Netherlands) 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".

The video will load shortly.
In this animation, the planet, its orbit and its star are approximately to scale and the inclination of the orbit is as we see it from the Earth. Credit: ESO/L. Calcada

The majority of around other stars were discovered by their gravitational effects on their parent stars, which limits the information that can be gleaned about their mass: they only allow a lower limit to be calculated for a planet's mass. The new technique pioneered here is much more powerful. 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, the team has 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 the planet Jupiter in our own Solar System.

"The new VLT observations solve the 15-year old problem of the mass of Tau Bootis b. 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", says Ignas Snellen (Leiden Observatory, the Netherlands), co-author of the paper. "This is a big step forward."

As well as detecting the glow of the atmosphere and measuring Tau Bootis b's mass, the team has probed its atmosphere and measured the amount of carbon monoxide present, as well as the temperature at different altitudes by means of a comparison between the observations and theoretical models. A surprising result from this work was that the new observations indicated an 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.

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 making measurements along the planet's orbit, astronomers may even be able to 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.

Explore further: Exoplanet atmospheres detected from Earth for the first time

Related Stories

Exoplanet atmospheres detected from Earth for the first time

January 14, 2009

( -- Transiting exoplanets are routinely detected when they pass in front of their parent star as viewed from the Earth, which only happens by chance. The transit event causes a small drop in the observed starlight, ...

Possible water in the atmosphere of a super-Earth

March 5, 2012

A "super-Earth" is an exoplanet (a planet around another star) whose mass is between about two and ten Earth-masses. Planets larger than this are closer to Uranus and Neptune in size (and perhaps in other physical properties ...

Explained: Transiting exoplanets

January 27, 2011

In the quest to find life elsewhere in the universe, planetary scientists have detected more than 500 planets outside the solar system, or exoplanets, over the past 15 years. About one-fifth of those were discovered by scanning ...

Exploring the atmosphere of exoplanet WASP-14b

November 15, 2011

First discovered in 2008, WASP 14b is an interesting exoplanet. It is roughly seven times as massive as Jupiter, but only 30% larger, making it among the densest known exoplanets. Recently, it was the target of observations ...

Recommended for you

The atmospheres of water worlds

October 23, 2017

There are currently about fifty known exoplanets with diameters that range from Mars-sized to several times the Earth's and that also reside within their stars' habitable zone – the orbital range within which their surface ...

Dawn mission extended at Ceres

October 20, 2017

NASA has authorized a second extension of the Dawn mission at Ceres, the largest object in the asteroid belt between Mars and Jupiter. During this extension, the spacecraft will descend to lower altitudes than ever before ...


Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (2) Jun 27, 2012
What is the "clever new trick"?
1 / 5 (1) Jun 27, 2012
chardo137, the paper says that they could use the large change in differential velocity between planet and its star as it orbits it, as well relative to Earth atmosphere. It enabled to sort out with spectroscopy the light of the planet reflecting and absorbing the stars light, from the stars emission & absorption as well as Earth absorption.

May only work for planets close to the star for now, but who knows how far out they can use it. Also, it was _fast_ - only used a few consecutive days for observation.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.