A poorly kept secret is now official: the Hubble Space Telescope has discovered carbon dioxide (CO2) in the atmosphere of a planet outside our solar system. That's a first in the study of extrasolar planets, or exoplanets, which have been quite the hot topic this year.

The exoplanet HD 189733 b, roughly the mass of Jupiter, orbits a star 63 light-years away in extremely close company. Although the planet can't be seen directly, scientists used Hubble data to analyze its atmospheric composition and turn up CO2 as well as carbon monoxide (CO). They did this by comparing the light spectrum from the star with that from the star and planet combined, as the planet passes in front of its star.

Although HD 189733 b is way too steamy for life as we know it—roughly 1,950 degrees Fahrenheit (1,065 degrees Celsius) by one estimate—the finding, leaked to media outlets two weeks ago, shows that techniques exist to find markers of life on other planets. (The paper has been submitted to Astrophysical Journal Letters.)

"The carbon dioxide is the main reason for the excitement because under the right circumstances, it could have a connection to biological activity as it does on Earth," lead author Mark Swain, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a statement. "The very fact that we're able to detect it and estimate its abundance is significant for the long-term effort of characterizing planets both to find out what they are made of and if they could be a possible host for life."

Other chemical signatures familiar to Earthlings have already been turned up by astronomical observations, including on HD 189733 b. Already known to harbor water vapor in its atmosphere, the planet this year also revealed itself to host methane.

CREDIT: ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI