Probing an 'Invisible' Exoplanet's Atmosphere

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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.
ESO/L. Calçada

THE GIST

— A new technique can be used to study an exoplanet's atmosphere.

— With more sensitive telescopes, Earth-like planets could be probed for atmospheric oxygen, a sign of life.

— Planet Tau Boötis b was one of the first planets discovered beyond the solar system.

To study the atmospheres of planets beyond the solar system, astronomers have had two choices: pick one that flies across the face of its parent star relative to Earth's perspective (an event known as a transit), or wait for a new generation of more sensitive space telescopes that can directly capture the planet's faint light.

Now, there's a third option.

Using a cryogenically-cooled infrared detector on a telescope in Chile, astronomers ferreted out beams of light coming directly from Tau Boötis b, a massive planet about 50 light-years from Earth.

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Fifteen years ago, Tau Boötis b was one of the first worlds discovered beyond the solar system.

Scientists not only learned just how massive Tau Boo is — roughly six times the size of Jupiter — but, more impressively, got a readout of carbon monoxide in its atmosphere.

"Atmospheric characterization has been done before, but only for transiting planets. However, the majority of the exoplanets do not transit their stars," lead researcher Ignas Snellen, with Leiden University in The Netherlands, wrote in an email to Discovery News.

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Super-hot Tau Boo flies way too close to its parent star to host life as we know it, but the technique used to probe its atmosphere bodes well for searching other planets better positioned for liquid surface water. Water is believed to be a key ingredient for life.

"Maybe with the next generation of telescopes we could probe much cooler and smaller planets — even Earth-like planets. This could then possibly be used to search for oxygen, pointing to biological activity," Snellen wrote.

Astronomers are now looking for other gases, such as water vapor and methane, in Tau Boo's atmosphere, as well as studying other exoplanets.

"We have a method now to look at atmospheres of planets that don't transit," astronomer Simon Albrecht, with the Massachusetts Institute of Technology, told Discovery News.

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The instrument, located on Europe's Very Large Telescope (VLT) and known as CRIRES (CRyogenic high-resolution InfraRed Echelle Spectrograph), can finely split light into its component colors for analysis.

The wavelengths of light from the planet change because of the motion of the planet around the star, while light from the star, and from Earth's atmosphere, are stationary. Astronomers then can filter out the planet light and look for telltale fingerprints of chemicals the light has passed through in the planet's atmosphere.

The research appears in this week's Nature.

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