An artist's impression of the Jupiter-size extrasolar planet, HD 189733b, being eclipsed by its parent star.
Astronomers using the Hubble Space Telescope have measured carbon dioxide and carbon monoxide in the planet's atmosphere. The planet is a ‘hot Jupiter’, so close to its parent star that it completes an orbit in only 2.2 days.
This type of observation is best carried out when the planet's orbit takes it behind the star (as seen from Earth), allowing for an opportunity to subtract the light of the star alone (when the planet is hidden behind it) from that of the star and planet together before an eclipse. This allows astronomers to isolate the infrared emission of the planet and carry out spectroscopic observations that chemically analyse the day-side atmosphere.
The planet is too hot for life as we know it. But under the right conditions, on a more Earth-like world, carbon dioxide can indicate the presence of extraterrestrial life. This observation demonstrates that chemical biotracers can be detected by space telescope observations.