These are very exciting times for extrasolar planet hunters. Dozens of new exoplanets have been discovered (some a little larger than Earth, others several times the size of Jupiter) and astronomers are refining their already sophisticated techniques to analyze the composition of the atmospheres of these distant worlds.

Take exoplanet HD 209458b for example. It might not have the most romantic of names (the designation “HD 209458″ is the catalogue number of the Sun-like star it’s orbiting), but this exoplanet is famous in its own right. In 1999, it was the first ever transiting exoplanet to be discovered. A transiting exoplanet is a world that passes in front of its parent star when viewed from Earth, blocking a little of the starlight for astronomers to detect and measure.

By using the parent star’s light to their advantage, NASA scientists have been able to decipher what chemicals HD 209458b’s atmosphere contains even though it is 150 light years from Earth. This exoplanet is a very large gas giant which has a very tight orbit around the star (it is therefore dubbed a “Hot Jupiter”), so life isn’t expected to be a possibility, but HD 209458b contains the basic organic chemicals as used, and produced, by life as we know it.

“It’s the second planet outside our solar system in which water, methane and carbon dioxide have been found, which are potentially important for biological processes in habitable planets,” said Mark Swain of NASA’s Jet Propulsion Laboratory. “Detecting organic compounds in two exoplanets now raises the possibility that it will become commonplace to find planets with molecules that may be tied to life.”

Spectroscopic observations by the Hubble Space Telescope and the Spitzer Space Telescope have been analysed and Swain has been able to deduce the wavelengths of light that has been absorbed by HD 209458b’s atmosphere. These “absorption lines” provide the fingerprints of certain chemicals and it would appear that water, methane and carbon dioxide are all present.

When compared with the first exoplanetary observations of organic chemicals (exoplanet HD 189733b), the quantities of water and carbon dioxide appear to be the same, however, HD 209458b contains more methane. Perhaps this is indicative of some planetary formation process that might be particular to this world. “It could mean there was something special about the formation of this planet,” Swain speculated.

Just because organic compounds are being found on distant worlds does not mean life may or may not be present, it simply means that there are a lot more life-giving chemicals around than we previously thought.

“…the detection of organic compounds will not necessarily mean there’s life on a planet, because there are other ways to generate such molecules,” Swain said. “If we detect organic chemicals on a rocky, Earth-like planet, we will want to understand enough about the planet to rule out non-life processes that could have led to those chemicals being there.”

Image: Artist’s impression of HD 209458b orbiting its star (NASA)