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11 March 2014updated 28 Jun 2021 4:45am

Spotting alien worlds that might hold life just got a little bit easier

A possible new method of narrowing down which worlds in other star systems may have life-friendly conditions has been published in a new study.

By Ian Steadman

While astronomers keep finding new planets in other parts of the galaxy – such as the record 715-planet haul that was announced last week by astronomers looking through data from the Kepler probe – there is a sub-section of astronomers who are trying to figure out ways of telling which of these new planets may be candidates for hosting extraterrestrial life. This isn’t a case of beaming radio messages their way and listening for an answer (though that is, of course, a thing scientists do). This is more about planetary classification.

If you imagine looking at our own Solar System from a few lightyears away, how would you work out that it’s the Earth that’s good for harbouring life? The biggest clue is the it’s within the Sun’s habitable zone – that is, it’s not too far from the Sun that water will freeze on its surface (like Mars), but it’s also not too close that any would boil away into space (like Venus). Yet that’s merely one factor of many that could mark Earth as especially life-friendly. What you’d really know, to be sure that the Earth was the life-carrying planet you were looking for, is to know what kind of atmosphere it has.

That’s the premise of a new study published in Astrobiology by a team from the University of Washington. It involves looking for “dimers” – twin pairs of molecules that tend to form within atmospheres, especially in atmospheres which have been influenceed by the Oxygen production of photosynthesising plants. In this case, it’s all about Oxygen dimers, which take the form of O2-O2 pairs. They write:

The absorption by dimers changes more rapidly with pressure and density than that of monomers and can therefore provide additional information about atmospheric pressures. By comparing the absorption strengths of rotational and vibrational features to the absorption strengths of dimer features, we show that in some cases it may be possible to estimate the pressure at the reflecting surface of a planet.”

Oxygen dimers, it appears, are easier to spot on the infrared spectrum than standalone Oxygen molecules – and, what’s more, the percentage of them in the atmosphere is a really good proxy indicator for other atmospheric conditions. It’s not useful for every type of planet, but it is useful for most planets that we would consider “Earth-like”; that is, with atmospheric pressure between 0.1 and 10 bars, with more than 50 percent of the Oxygen that the Earth has now, and which are rocky and Earth-like in mass. Looking for Oxygen dimers is significantly more useful than other methods, like observing a planet’s atmospheric Rayleigh scattering, the study authors contend. That makes dimers a great indicator of life, even if just basic and algae-like.

This is also good news for the James Webb Space Telescope, due to launch in 2018. Named after a former head of Nasa, the JWST is the first major orbiting observatory of the generation that will follow Hubble, Spritzer and Kepler; its massive, supercooled 6.5m-diameter mirror will investigate the birth of galaxies, stars and planets, searching along the infrared spectrum.

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The JWST won’t be hanging around Earth, though – it’s going to be sent out to the far side of the Moon from Earth, to a point in space known as L2. It’s a Lagrange point, a place in space where the gravitational forces from two or more bodies balance each other and mean that an object in exactly the right spot won’t move, relative to one of the objects. Something in L2 will be orbiting around the Sun, but because it’s orbiting at exactly the right point beyond the Earth the combined pull of both planet and star will give it a greater gravitational tug inwards. That translates to greater centripetal motion; so, the JWST will take the same time to orbit the Sun as the Earth, but it’ll do it at a faster speed to make up the extra distance of orbiting just that little further out. And, because the Earth will always be between the telescope and the Sun, it’ll get some nice shade.