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Two recent studies use satellite images to trace a fingerprint of our planet and understand how it could be seen from a hypothetical extraterrestrial civilization. Useful information to search for real “twins”.

It may seem trivial, but if we really want to find the “twins of the Earth”, we must try to understand how the Earth might appear to be seen by the telescopes of a hypothetical alien civilization. And see if it looks like some other place we see out there. In short, we have to take a “selfie” and, like on Tinder, hope for a “match”. After all, ours is the only planet that we know about which life has developed, comparing its “fingerprints” with those of other worlds could be the key to finding out which ones bear the same biological signature. A couple of recent studies have focused precisely on this.

The Earth seen by aliens

Scientists from the California Institute of technology and the Jet propulsion laboratory analyzed the 10,000 images that the NASA satellite deep space climate observatory (Dscovr) took of the Earth at different wavelengths between 2016 and 2017. Then they reduced all this, to look at it as it would appear to aliens who live several light-years away: a spectrum with ten wavelengths drawn over the course of two years. Comparing the data with the globe, they identified which are the specific ones that correspond to the emerged lands and, for example, to the clouds. The result is a nuanced map that roughly follows the division between oceans and land. Of course, we don’t expect to find a true “twin” with the same precise characteristics. The work of the US team (published in The Astrophysical Journal Letters) may, however, serve as a model for identifying the peculiarities of an exoplanet (a planet outside the solar system) inhabited, precisely, like ours, plenty of water, clouds, and ice at the poles.

The fingerprint

This is what astronomers at McGill University in Canada called it, presenting the study published in Monthly Notices of the Royal Astronomical Society: “Earth’s Fingerprint”. In particular, of its atmosphere. After all, the extrasolar planets of terrestrial size are so small, dark and distant, to be confused, in the vast majority of cases, in the light of their star. Therefore it will be difficult, for a long time to come, to be able to distinguish its features like oceans and emerged lands.

What scientists think they have been doing for some time, however, is to analyze their atmosphere, observing them when they pass in front of their sun. Even a few rays of light, filtered by the atmosphere of those planets, come to us bringing the signature of the gases they have passed through, in the form of spectral lines. Again we need to put ourselves on the side of who we are looking for, to understand what the aliens would see them, this time with information from the Canadian satellite Scisat: “Some researchers have tried to simulate the Earth’s spectrum of transit but this is the first empirical spectrum of transit to the infrared – explains Nicolas Cowan of the McGill – this is what alien astronomers would see if they observed a transit of the Earth (in front of the Sun) “.

It will be what the future exoplanet hunters will have in their hands to see if there is any atmosphere out there that looks like ours. We will look for methane, ozone and of course oxygen. Those gases that are defined biosignature, signatures of life, given that they tend to combine very easily and therefore to ‘disappear’, their presence in the atmosphere for long periods of time indicates that there is something (or someone) that produces them.

Waiting for the James Webb

But again, current telescopes are not enough to see these features. The great protagonist of this hunt for alien habitable worlds will be, in the coming years, the James Webb space telescope of NASA. It is the largest space telescope ever built, just finished assembling, and that should take off in 2021.

According to scientists, the JWST would be able to detect traces of gases such as carbon dioxide, water vapor and, observing them for a sufficient time, even methane and ozone in the atmospheres, for example of the planets around the star Trappist-1, about 40 light-years away. One of the most promising planetary systems with several candidates in the so-called “habitability zone”, at the right distance to support liquid water on their surface.

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