A team of Dutch astronomers has shown that with a new type of telescope it might be possible to find signs of extraterrestrial life within the next 25 years. Certain gases exhaled by organisms can, in principle, be observed in the atmospheres of exoplanets – planets orbiting stars other than our sun. This idea, which was developed back in the 1960s, has now been linked to a new observation technique that makes use of relatively cheap flux collectors, large reflecting telescopes that cannot make sharp images but collect enough light for accurate spectroscopy to be performed. This study will soon be published in The Astrophysical Journal.
Astronomers have been speculating for several decades about how the observations of exoplanets could provide evidence of extraterrestrial life. About 20% of our earth's atmosphere consists of oxygen. This gas is only present in our atmosphere because plants produce enormous quantities of it via photosynthesis. Without this biological process, the oxygen in the atmosphere would rapidly disappear due to oxidation. Detecting oxygen in the atmosphere of an earth-like exoplanet could therefore provide a first indication for extraterrestrial life.Up until now it had been thought that such observations could only be realized with space telescopes, as the oxygen in our own atmosphere would have too great a disruptive effect for such observations to be made from the ground. However the plans of European and American scientists for a large space mission with this objective, such as Darwin and the Terrestrial Planet Finder (TPF), have been on hold for several years and it is therefore unlikely that such a telescope will be launched within the next 25 years.
A group of astronomers from Leiden University and SRON Netherlands Institute for Space Research have now shown that using a special technique, observations from the ground can realize the same objective. "The oxygen in an exoplanet's atmosphere can be distinguished from that in our own atmosphere by very accurately measuring the wavelengths of the absorption lines," says Ignas Snellen (Leiden Observatory). "Due to the high speed of such a planet
relative to the earth, the oxygen lines will exhibit a Doppler shift and will therefore not be aligned with those from our own atmosphere. Using this approach the telescope does not need to be located outside of our atmosphere, which yields an enormous cost saving."
The team has shown that oxygen in the atmosphere of a hypothetical twin earth, seen against the light of a red dwarf star – cooler and smaller than our sun – can possibly already be observed with the planned European Extremely Large Telescope (E-ELT). "However we would need to have some luck for that," says fellow researcher Remco de Kok (SRON). "This telescope will only be able to detect extraterrestrial life if it occurs frequently and can therefore be found relatively nearby. Even the E-ELT will probably not be big enough."
The team therefore proposes the development of so-called flux-collectors. Spectroscopic measurements of bright stars and their planets do not require the construction of telescopes such as the E-ELT that can produce very sharp images. It is only important to collect as much light as possible. This can be done with large reflecting telescopes of much lower quality, which can be built against far lower costs. "With several such flux collectors, which together will cover the area of several football fields, we can do a statistical study into extraterrestrial life on the planets orbiting other neighboring stars. We still have a long way to go but this is something that we should be able to achieve within 25 years," says Snellen.