A new technique to explore the atmospheres of planets outside our solar system

'Tau Bootis b' was one of the first exoplanets discovered in the 90's. Fifteen years later, remains one of the closest known and given that it already has confirmed the existence of 750 planets outside our solar system, which number about 2,000 applicants to join the list.

'Tau Bootis b' is a big 'hot Jupiter' orbiting close to its host star. Although its host star is easily visible, so far this planet could only be detected by its gravitational effects on the star.

An international team of astronomers has devised an ingenious new technique to study the atmosphere of an exoplanet in detail, even without the need to pass in front of its host star. The details of this study are published in the journal Nature. For the first time, have studied the planet's atmosphere, and measured its mass and orbit very precisely, thus solving an obstacle they have had for 15 years.

They have done using the 'Very Large Telescope' (VLT) of the European Southern Observatory (ESO), located at the Paranal Observatory (Chile), to capture directly the faint glow of the planet 'Tau Bootis b'. We combined observations of high quality infrared (at wavelengths of about 2.3 microns) with a new trick to extract the weak signal from the planet much more powerful light emitted by the star.

Like many exoplanets, 'Tau Bootis b' disc does not transit its star (as in the recent transit of Venus). So far these transits were essential to allow study of the atmospheres of 'hot Jupiters': when a planet passes in front of its star properties of the atmosphere imprinted in the starlight. Since there is no starlight shining through the atmosphere of Tau Bootis b towards us, the planet's atmosphere has not been studied before.

Calculation of the mass

In a press release from ESO, the principal investigator of this 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 with the highest level of detail achieved so far. Only 0.01% of the light we see comes from the planet, and the rest comes from the star, so it was not easy. "

Most planets around other stars were discovered by its gravitational effects on the host stars, which limits the information that can be obtained from its mass: only possible to obtain a lower limit for the mass of a planet. So you see the planet's light directly has allowed astronomers to measure the angle of the planet's orbit and, hence, extract its mass accurately.

Composition of the atmosphere

In addition to detecting the brightness of the atmosphere and measuring the mass of 'Tau Boötis b', the computer has studied the atmosphere and measured the amount of existing carbon monoxide and the temperature at different heights by means of a comparison made between observations and theoretical models. One of the most striking results of this study was that the new observations indicate an atmosphere having a temperature that decreases with increasing height. This result is exactly the opposite of inversion, an increase in temperature at higher altitudes, other exoplanets found Jupiters.

Meanwhile, Ignas Snellen, co-author and researcher at the Leiden Observatory, considered from now, astronomers can study the atmospheres of exoplanets that transit their stars and measuring their masses accurately, it which was impossible before, "It's a big step forward," he says.