Could aliens consider Earth a habitable world?

Aliens equipped with technology similar to ours would have a difficult time detecting our world by astrophysical methods. Discovering the Earth next to the Sun is impossible by means of direct images, it would be on the edge of the impossible with radial velocity techniques and it could be feasible if the eclipse method is used with space technology, although only one in two hundred extraterrestrial civilizations would be lucky. to be well placed for it. If they are supposed to have detected us, would they classify Earth as a habitable world?

We ask ourselves this question assuming that we are evaluated by an extraterrestrial scientific community with a level of knowledge and resources similar to ours.

habitability and liquid water

Perhaps the first thing to do is clarify what is meant by a “habitable world.” The criteria used today is limited to qualifying in this way the planets that appear to have a reasonable probability of having liquid water on their surface. It should be noted that even such a seemingly simple concept is faced with very considerable uncertainties.

To begin with, the most important thing to assess whether or not there may be liquid water on the surface is to estimate the pressure and temperature conditions. But both physical magnitudes depend on the composition and density of the atmosphere, and it happens that in the vast majority of cases we do not have any accurate data in this regard. Therefore, we have to assume that the aliens that detect us will also have no information about the structure and conditions of the Earth’s atmosphere.

Given this scenario, several reasonable assumptions are made, but they carry margins of error that should not be neglected. If the Earth did not have an atmosphere that gives us a certain greenhouse effect, then its “natural” equilibrium temperature with solar radiation would place it below freezing point and we would find ourselves with an uninhabitable world. But neither should we go overboard by introducing an excessive greenhouse effect into the calculations or we could find ourselves with a case like that of Venus, a world very similar to Earth but with surface temperatures so high that they could melt lead.

“My house…”

The example of Venus or Mars show us how diverse planetary atmospheres can be and how much they can influence pressure and temperature conditions on the world’s surface. We are living beings that inhabit the Earth and this corner of the universe seems to us the most welcoming paradise, we feel at home. That is why the models are adjusted in such a way that, when doing the math, it comes out that the Earth is habitable.

But the home of the aliens that discover us may be quite different from ours. They will undoubtedly have adjusted their models so that the optimal result applies to their planet, and we must not rule out that their calculations yield a verdict of “not habitable” for Earth.

Color-coded Earth Similarity Index (ESI) for a set of real extrasolar planets. The horizontal dashed lines encompass planets with radii between one-half and two-and-a-half times that of Earth. The vertical lines mark the stellar radiation thresholds considered habitable.
PHL@UPR, Arecibo

Given the criticism that the very schematic and simple concept of identifying habitability with the possible existence of liquid water has received, there are research groups that have proposed more complex indices that take into account other additional parameters, such as the mass or density of soil. planet or some features of its star. This results in the “Earth Similarity Index” (ESI, from Earth similarity index), a product of fuzzy logic that may seem more nuanced but cannot escape, even from the name, its homemade and anthropocentric character. Of course, Earth has ESI = 1 00, while for Mars you get ESI = 0 64 and 0 44 for Venus.

Tee garden system

Whichever criteria is used, among the lists of the most promising potentially habitable worlds, the planet Teegarden b always appears in a prominent position, one of two that orbit next to the star Teegarden, only twelve light-years from the Sun. This planetary system was discovered from Spain, from the Calar Alto observatory, with the Cármenes spectrograph.

With an ESI equal to 0 93, Teegarden b looks promising as a possible home for extraterrestrial life. But it’s not even known if there’s elemental life there, let alone possible intelligent life. Undoubtedly, this system will be the subject of very detailed studies in the coming years. Still, despite the uncertainties, let’s imagine for a moment that there was an alien civilization there: can they know that Earth exists and that it is a habitable place?

It happens that the Teegarden star, in its movement through space close to the Sun, is about to enter the region of the firmament from which it is possible to observe the Earth transiting before the Sun. From the decade of the 40s of this century, this little star will be in that 0 5% of the sky from which the Earth causes eclipses. If there is someone in that system, and if they have technology comparable to ours, they could be about to discover us.

Should we worry? Should we prepare, perhaps, for an invasion? No! Let us remember that we have started from the hypothesis that their technology is similar to ours, which does not allow interstellar travel, not even to cross twelve miserable light years, not even resorting to the wildest of fantasies.

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