This artist's conception shows a hypothetical twin Earth (with moon) orbiting a Sun-like star.
Does planet Earth have a twin?
27 Mar 2009
Does a twin of Earth exist somewhere in our galaxy? Astronomers are closer to finding out, following the recent launch of NASA's Kepler spacecraft.
Once Kepler finds some Earth-sized planets, the next questions driving research will be: Are those planets habitable? Do they have an Earth-like atmosphere?
Answering those questions will not be easy, and will have to wait for the next generation of space telescopes, such as the James Webb Space Telescope (JWST) scheduled for launch in 2013. With its large mirror and special location in space, it will offer astronomers the first real possibility of finding those answers.
In a new study, Lisa Kaltenegger (Harvard-Smithsonian Centre for Astrophysics) and Wesley Traub (Jet Propulsion Laboratory) examined the ability of JWST to characterise the atmospheres of hypothetical Earth-like planets during a transit, when part of the light of the star gets filtered through the planet's atmosphere.
They found that JWST would be able to detect certain gases called biomarkers, such as ozone and methane, only for the closest Earth-size worlds.
< The Kepler spacecraft will detect planets moving in front of distant stars.
In a transit event, a distant planet crosses in front of its star as seen from Earth. As the planet transits, gases in its atmosphere absorb a tiny fraction of the star's light, leaving fingerprints specific to each gas. By splitting the star's light into a rainbow of colours, or spectrum, astronomers can look for those fingerprints. Kaltenegger and Traub studied whether those fingerprints would be detectable by JWST.
The transit technique is very challenging. If Earth were the size of a basketball, its atmosphere would be as thin as a sheet of paper, so the resulting signal from light seeping through that air would be incredibly tiny. Moreover, the method works only when a planet is directly in front of its star, and each transit lasts for only a few hours at most.
'We'll have to be really lucky to decipher an Earth-like planet's atmosphere during a transit event so that we can tell it is Earth-like,' said Kaltenegger. 'We will need to add up many transits to do so—hundreds of them, even for stars as close as 20 light-years away.'
Kaltenegger and Traub first considered the scenario of an Earth-like world orbiting a Sun-like star. To get a detectable signal from a single transit, the star and planet would have to be extremely close to Earth. The only Sun-like star close enough is Alpha Centauri A.
The James Webb Space Telescope will analyse the light from distant planets to look for signs of life. >
The study also considered planets orbiting red dwarf stars. This kind of star is the most abundant in the Milky Way—far more common than yellow, type G stars like the Sun. They are also cooler and dimmer than the Sun, as well as smaller, which would make finding an Earth-like planet transiting a red dwarf easier.
An Earth-like world would have to orbit close to a red dwarf to be warm enough for liquid water to exist. Being close in, the planet would orbit more quickly and would undergo more transits in a given amount of time. Astronomers could add the data from several transits, thereby improving their chances of detecting the atmosphere.
An Earth-like world orbiting a star like the Sun, however, would undergo a 10-hour transit only once every year. Accumulating 100 hours of transit observations would take 10 years. In contrast, an Earth orbiting a mid-sized red dwarf star would undergo a one-hour transit once every 10 days. Accumulating 100 hours of transit observations would take less than three years.
'Nearby red dwarf stars offer the best possibility of detecting biomarkers in a transiting Earth's atmosphere,' said Kaltenegger.
'Ultimately, direct imaging—studying photons of light from the planet itself—may prove a more powerful method of characterising the atmosphere of Earth-like worlds than the transit technique,' said Traub.
In a best-case scenario, Alpha Centauri A may turn out to have a transiting Earth-like planet that no one has spotted yet. Then, astronomers would need only a handful of transits to decipher that planet's atmosphere and possibly confirm the existence of the first twin Earth.
Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics / David A. Aguilar (CfA).
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