Could we detect life... on Earth?

2 September 2014 by Joanna Barstow

Earth's Eastern hemisphere (image credit: NASA)

What might Earth look like to an alien living 10 light years away? We're assuming he or she has access to a decent space telescope, of course. A new paper led by Professor Pat Irwin examines what information about Earth's atmosphere might be recoverable from its transit spectrum, observed from 10 light years away using a 10 m space telescope - a bit like a larger, futuristic version of the Exoplanet Characterisation Observatory, a mission concept proposed to ESA. If this telescope could detect absorption features of biosignature gases like oxygen and ozone in the Earth's atmosphere, the presence of life could be inferred from those measurements (see the Atmospheres page on our group's website for details of the technique).

The problem with observing the Earth in transit is that the Earth is small and cold, whereas the Sun is big and hot. That means the signals produced when the Earth transits or is eclipsed by the Sun are really, really small - of order 1:10,000. To find out about the Earth's atmosphere from these observations, we are looking at tiny fluctuations in the transit depth as a function of wavelength, and these are about 100x smaller again - so our signal of interest is only 1:1000,000. To be sensitive to these tiny signals above the noise, Irwin et al. find that we would need to observe 77 transits, or 77,000 eclipses! Given that we would only get one transit and one eclipse per year, that would be a little too long to wait for our result. For Jupiter the case is much more favourable - with only a single transit, we could see absorption features due to gases in its atmosphere, and those would give us a pretty good idea of what Jupiter is like. Sadly, that does mean that the intelligent aliens on the as-yet-undiscovered planet 10 light years away are probably getting very excited about detecting methane on Jupiter, rather than making the career-defining discovery of life on its small, rocky neighbour.

So, does that mean that characterising habitable, Earth-like planets is an unreachable goal? Not at all. We just have to pick our targets carefully. If Earth was in orbit around a much smaller, cooler star than the Sun, we would have a much better chance of getting a measureable signal. Although we would still probably need several transits, these would be much quicker to obtain because the Earth would need to be in a much shorter orbit to maintain warm enough conditions for life.

An alternative observational technique that might be useful is coronography. This is a way of cancelling out the light from the star using adaptive telescope optics, star shields or masks, allowing the signal from the planet to be measured directly even though it is much weaker than the stellar signal. This method is still in its infancy, but it is being developed fast and ground based instruments like GPI on the Gemini South telescope in Chile have recently produced impressive results.

Whatever the method used, Earth-like planet characterisation is likely to be possible in the not-too-distant future, as larger telescopes are built to observe from space or from the ground. Who knows - maybe we'll find the aliens before they find us...

This post originally appeared on the Stars and Planets at Oxford group web page.

Categories: research | exoplanets