The holy grail of exoplanet hunting is to find a planet like Earth orbiting a distant star in its habitable zone (the region around a star where life can exist).
Since 1992 the pace of exoplanet discovery has accelerated, with 538 confirmed planets found as of March 2011, and the space telescope Kepler, launched in 2009, has already made some exciting planetary discoveries, including a 6 planet solar system. And now it may have found the first known Earth-like exoplanets.
Astrobiology is about the search for alien life, we haven’t actually found any yet, but the search has only just begun. Scientists think the best place to find life would be on a planet similar to our own, made mostly of rock and with surface liquid water.
It was announced in February 2011 that Kepler had discovered 1,235 candidate exoplanets in its latest data collection, including 68 Earth-like candidates, with 5 potentially orbiting in their star’s habitable zone. These are candidate exoplanets though, this means that Kepler has found some evidence that these planets may exist, but astronomers will need to investigate further in order to confirm whether they actually are planets, or are signals caused by some other phenomenon.
If Kepler, and other telescopes, do begin to find Earth-like planets, then astrobiologists will be better able to understand how common these planets are, maybe almost every suitable star will have one, or maybe they’ll be incredibly rare? This is important, as it will effect estimations of how likely alien life is to exist in our Galaxy.
The most famous example of this is the Drake Equation, which estimates the number of intelligent civilizations in the Milky Way. A key term in this equation is the number of habitable planets in our Galaxy, the larger the number, the more likely alien life is to exist (see here for more details on the Drake Equation).
If Earth-like planets are found, then astronomers can begin to analyse them to search for evidence of life. This will primarily be done by examining the light the planets give-off with a technique called spectroscopy. The spectra of the planet’s light will help scientists to understand what kind of molecules are present on the planet’s surface or in its atmosphere, and if certain molecules are found to abundant, like methane and oxygen, then this could mean that life is living on that planet, as both these molecules are reactive and short-lived, and would need to be replenished regularly. On Earth this happens continuously as photosynthetic plants produce oxygen, and certain forms of bacteria produce methane.
I’m sure teams of scientists are analysing the Kepler data right now, fingers crossed they find something exciting!
PS. Want to know more about how astronomers find exoplanets? Go here.