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New method reveals contents of an alien world's atmosphere

Astronomers analyze the ever-so-faint light streaming directly from an extrasolar planet to determine what its atmosphere is made of.
Astronomers using ESO's Very Large Telescope have now caught and studied the faint light from the planet Tau Bootis b for the first time.
Astronomers using ESO's Very Large Telescope have now caught and studied the faint light from the planet Tau Bootis b for the first time.ESO / L. Calcada
/ Source: Discovery Channel

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.

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

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

Now scientists have learned not only how massive Tau Boo is — roughly six times the size of Jupiter — but, more impressively, have gotten 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 studying 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 Earthlike 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. They're also looking for other exoplanets.

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

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The instrument, located on the European Southern Observatory's Very Large Telescope in Chile 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 the light from the star, and from Earth's atmosphere, exhibits unchanging wavelengths. Astronomers then can filter out the planet light and look for telltale fingerprints of the chemicals that the light has passed through in the planet's atmosphere.

The research appears in this week's issue of the journal Nature.