In the 20 years since the first exoplanet was detected, astronomers have been homing in on an Earth-like planet orbiting another star. The dream is to find an alien Earth on which life flourishes.

Such a planet will almost certainly have plenty of water, large amounts of gaseous oxygen and even chlorophyll or something very like it. All these things are signatures of life on Earth so it makes sense to look for similar signatures elsewhere, at least for the moment.

So NASA and other space agencies are drawing up plans to launch telescopes capable of spotting these bio-signatures. And that raises an interesting question. If an Earth-like planet orbiting another star does have water, oxygen and chlorophyll, would we be able to spot it?

That’s a question addressed today by Timothy Brandt and David Spiegel at the Institute for Advanced Study at Princeton University, New Jersey. These guys have calculated the kind of signal that these features would present on an Earth twin and worked out how good the next generation of space observatories will have to be to spot them.

Astronomers are well aware of the effect that molecules of water, oxygen and ozone have on light— they all absorb strongly in optical wavelengths ranging from 500 to 1000 nanometres (from green to the near infrared). “These molecules are the most prominent absorbers in this spectral range, and are all critical species for terrestrial life,” say Brandt and Spiegel.

Beyond that, in the region of 1 to 4 micrometres, Earth’s spectrum is dominated by the absorption features of water with some broad overlapping features generated by carbon dioxide and methane.

So if an exoplanet is anything like this, light from a nearby star that is reflected from the planet’s surface will bear the signature of these gases.

To find out whether this signature is detectable, Brandt and Spiegel first created a model of the way light would reflects from the surface of such a planet. On the one hand, they consider an ice and desert world and on the other, an Earth analogue with landmasses, vegetation and oceans. In both cases, they consider the case in which clouds obscure 50 per cent of the surface in the case in which no clouds are present.

They then calculate the effect that these molecules would have on the reflection spectrum.

The results make for interesting reading. The easiest molecule to spot by far is water. To pick this out, space telescope must first be able to cope with the contrast between the planet and its parent star, a difference in the region of 1 in 10^10.

It must then be able to resolve the reflected spectrum with a resolution that allows it to distinguish water’s absorption bands. Brandt and Spiegel say this ought to be possible with a resolving power greater than 20, which is well within the reach of today’s instruments.

Oxygen, on the other hand, is more difficult to see. Brandt and Spiegel calculate that the spectral resolving power of a space observatory would need to be in the region of 150, somewhat higher than is possible today, they say. At the same time, detecting oxygen will require three times a signal to noise ratio as detecting water.

But both these goals seem achievable. “Our results suggest that a well-designed space mission could detect O2 and H2O in a nearby Earth twin,” they say.

Spotting chlorophyll, however, will be much harder. Earth’s reflection spectrum contains a “red edge” produced by chlorophyll’s absorption of red light on the surface. An important question is whether the family of chlorophyll-like molecules can work at wavelengths that are significantly different from this.

Even if they don’t, the signature will be hard to see. Brandt and Spiegel conclude that a future mission “would need to be signiﬁcantly more sensitive (or very lucky) to see alien chlorophyll.”

These results point towards an obvious strategy for studying promising exoplanets. Brandt and Spiegel say that such missions should begin by looking for water and then oxygen on the more favourable planets. But the search for chlorophyll should be reserved for later, since it will be much more time-consuming and therefore expensive, and only then on the most promising targets.

When might we begin looking for such planets? Not any time soon.

NASA has developed plans for is space observatory called the Advanced Technology Large-Aperture Space Telescope or ATLAST with this goal in mind. This is envisioned as a flagship mission on a timescale beyond 2025.

That’s some way off. But when space observatories like these finally begin to gather data, it’s quite possible that they will make some of the most astounding discoveries in the history of science.

Ref: arxiv.org/abs/1404.5337 : Prospects for Detecting Oxygen, Water, and Chlorophyll in an Exo-Earth