Future mission to find extraterrestrial life finds its first biosignatures – on Earth

The techniques that a network of satellites could one day use to find life beyond the solar system have been verified by applying them to the only place we know life exists: Earth. Of course, there is a rather spectacular difference between the distance over which the observations had to be made and the distance over which observations were planned for the future, but it is still a barrier that needs to be bridged.

The astronomers behind the Large Interferometer for Exoplanets mission indicated the scale of their ambition when they chose the acronym LIFE. By combining the powers of five satellites, we hope that LIFE will do what even the JWST cannot: find evidence of biology that occurs on rocky exoplanets (planets orbiting nearby stars).

Like the JWST, the proposed satellites will be positioned at Lagrange Point 2. By using interferometry to combine the light collected by each, for some purposes they will act as a single telescope more powerful than anything we can launch.

This combination won’t be able to do everything a larger telescope can do – but that doesn’t matter if you only have one job. “Our goal is to detect chemical compounds in the light spectrum that indicate life on exoplanets,” the initiative’s leader, Professor Sascha Quanz of ETH Zurich, said in a statement.

To test the idea’s feasibility before spending billions on it, Quanz and three other researchers made observations of Earth through the Atmospheric Infrared Sounder aboard NASA’s existing Aqua satellite. The team examined Earth’s spectrum in the mid-infrared range, where LIFE will operate.

If aliens in another galaxy were to look at Earth with a LIFE-like instrument, they would actually see a pale blue dot with nowhere near the resolution to distinguish oceans from continents, let alone anything smaller. Instead, they would see a spectrum that would be the average for the Earth as a whole. Furthermore, they would have to spend a lot of time looking for enough photons for something useful, so that the spectrum would also be averaged over time, potentially smoothing out seasonal changes.

We also can’t choose the angles from which we see rocky planets, so the team imagined what Earth would look like from a system located above the North Pole (perhaps orbiting Polaris). They then added one over Antarctica and two equatorial views.

By taking a subsample of Aqua Earth’s data that is similar in size to the amount of radiation a telescope would collect at large distances, the team validated LIFE’s approach. Specifically, they concluded that LIFE could detect carbon dioxide, ozone and methane in Earth’s atmosphere at distances of at least 33 light-years in all three orientations.

We know that lifeless planets can have carbon dioxide in their atmospheres, otherwise we would know something great about Mars and Venus. Water is a prerequisite for life, but not a guarantee of it. Methane may have sources other than biological ones, but nevertheless its presence on Earth is greatly aided by living organisms, and there would be no ozone here either if plants or algae were not constantly replenishing the oxygen from the air. In combination, the four gases are a powerful indication that the Earth is inhabited by something, even if you couldn’t tell that it has evolved beyond a single cell.

The spectrum of the gas giant WASP-96 b has peaks where water is expected, but finding something similar for a smaller rocky planet is much more difficult.

The spectrum of the gas giant WASP-96 b has peaks in the places expected for water, but finding something similar for a smaller rocky planet will require something even more powerful.

Image credits: NASA, ESA, CSA and STScI

“Even though atmospheric seasonality is not easy to detect, our research shows that next-generation space missions can assess whether nearby exoplanets with temperate climates are habitable or even inhabited,” Quanz said.

LIFE, as they say, has found a way.

One fly in the ointment is that LIFE may have to stare at the same point for as many as 100 days to collect useful data on these gases. That might be feasible if we already had a really big hint about a specific planet, but if it was just one of many to look at, it would be hard to justify. Fortunately, however, for many priority objectives the time required would be much shorter.

The team is also looking for even bigger giveaways, such as nitrous oxide or methyl bromide, but an accompanying paper suggests that the range at which these can be found may be limited to as little as 16 light-years.

The research has been published open access in The Astronomical Journal.

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