Although stars appear as simple points of light in the sky, astronomers now know that almost all of them are orbited by planets. Even without being able to see them, researchers have discovered whole planetary systems and have measured the sizes, masses, and orbits of the planets — some tantalizingly similar to Earth. But do any of them harbor life? Answering that question seems impossible since the planets are too far and too faint to see. Astronomers may soon find an answer by exploiting the power of spectroscopy: spreading out the various wavelengths of light prismatically to see which ones are brightest and which might have been absorbed by something on their way here. When a planet passes in front of its star, some of the starlight arriving to telescopes will have passed through the planet’s atmosphere and gases will have absorbed certain wavelengths. Space telescopes such as Hubble are using this method to detect gases around exoplanets. For now they’ve only managed this with a
few gas giants, not around anything like Earth. To find more constituents of exoplanet atmospheres, astronomers need to block out the light from the star so that they can see the dim planets and gather their light directly. That can be done with a mask inside the telescope called a coronagraph but far more effective will be a starshade. This is a free-flying spacecraft which can unfurl a parasol tens of meters across. When stationed tens of thousands of kilometers away from a space telescope, it can block the light from a star while leaving planets in view. Astronomers hope such space missions will happen in the next decade. But once we can see exoplanets directly what signs should we look for to tell us that life is present? What are the biosignatures? If we take Earth life as a guide, then looking for the byproducts of biological processes, such as oxygen and methane, seems like a good approach. These gases would normally react and turn into something else, so if they are detected in large quantities it is a sign that something, such as life, is replenishing them. But it’s easy to be fooled. Methane can be produced by volcanoes; oxygen by ultraviolet light splitting water. Similarly, it is possible for life to thrive without producing such gases, as microbes did on the primitive Earth for billions of years. Some scientists argue that we shouldn’t be looking out for a duplicate of Earth-based life but should consider all volatile compounds as possible biosignatures. Any atmosphere that is out of balance, with constant replenishment of reactive gases, could be flagging up an alien biology. Astronomers now realize that detecting atmospheric gases is unlikely to be enough to convincingly detect life. They will need a holistic view of a planet: What sort of light rains down from its star? does it have volcanoes? How fast does it rotate? The light reflected from its surface could show a pattern of absorption like that from our green photosynthetic plants. With the help of starshades and giant space telescopes, one day we’ll get images of such a distant world. They may be only a few pixels across but researchers have simulated them using degraded images of Earth and believe they will be able to detect oceans and continents, rotation rate and seasons. From these they can get a better idea of a planet’s climate and suitability for life. Take a close look. A picture like this may be our first glimpse of a world with life that has evolved separately from our own.