Technologies that a network of satellites might one day use to find life outside the solar system have been verified by applying them to the only place we know life exists: Earth. Of course, there is a rather striking difference in the distance over which observations must be made versus those planned for the future, but there is still a barrier to be cleared.
The astronomers behind the Large Exoplanet Interferometer mission indicated the scale of their ambition when they chose the abbreviation LIFE. By combining the powers of five satellites, LIFE will hopefully do what not even the James Webb Space Telescope has been able to do, which is find evidence of biology on rocky exoplanets (planets orbiting nearby stars).
Like the James Webb Space Telescope, the proposed satellites would be positioned at the Lagrange point 2. Using interferometry to combine the light collected by each, they would for some purposes serve as a single telescope more powerful than anything we know how to launch.
This combination won't be able to do all the things that one larger telescope can, but that doesn't matter if you only have one job. “Our goal is to detect chemical compounds in the optical spectrum that indicate the presence of life on exoplanets,” initiative leader Professor Sascha Cowans from ETH Zurich said in a statement.
To test the feasibility of the idea before spending billions, Cowans and three other researchers took observations of Earth with the Atmospheric Infrared Sounder aboard NASA's Aqua satellite. The team explored Earth's spectrum in the mid-infrared range, where LIFE will operate.
If aliens in another star system were looking at Earth with a LIFE-like instrument, they would really see a pale blue dot that's nowhere near as precise as distinguishing oceans from continents, let alone anything smaller. Instead, they will see a spectrum that will be average for Earth as a whole. Furthermore, they would need to spend a long time searching for enough photons for anything useful, so the spectrum would also be averaged over time, which could smear seasonal variations.
We also can't choose the angles from which we see rocky planets, so the team thought about what Earth would look like from a system located above the North Pole (possibly orbiting Polaris). Then they added one view over Antarctica and two tropical views.
By taking a subsample of the Aqua Earth data whose size is equivalent to the amount of radiation the telescope would collect at large distances, the team validated the LIFE approach. Specifically, they concluded that LIFE would be able to detect carbon dioxide, ozone and methane in Earth's atmosphere at distances of at least 33 light-years in all three directions.
We know that dead planets could have carbon dioxide in their atmospheres, or we would know something big about Mars and Venus. Water is a condition for life, but not a guarantee of it. Methane can have sources other than biological ones, but nevertheless, its presence on Earth is greatly enhanced by living organisms, and there would be no ozone here either if plants or algae were not constantly replenishing atmospheric oxygen. Together, the four gases are a strong indicator that the Earth is inhabited by something, even if you can't tell that it has evolved beyond a single cell.
The gas giant planet WASP-96 b's spectrum peaks where water would be expected, but finding something similar for a smaller, rocky planet will require something much more powerful.
Image credit: NASA, ESA, Canadian Space Agency, and STScI
“Even if atmospheric seasonality is not easily observed, our study shows that next-generation space missions can assess whether nearby temperate terrestrial exoplanets are habitable or even inhabited,” Cowans said.
Life, as they say, finds its way.
One catch in the ointment is that LIFE may need to spend up to 100 days staring at the same spot to collect usable data on these gases. This might be workable if we already had a very big hint about a particular planet, but if it was just one planet among many to look at, it would be difficult to justify. Fortunately, the time required for many priority objectives will be much less.
The team is also seeking larger flashes, such as nitrous oxide or methyl bromide, but the accompanying paper suggests that the range at which these can be found may be limited to just 16 light-years.
The study is published in open access in the Astronomical Journal.
