Oxygen ions can be a reliable indicator of life on exoplanets

Exoplanets have recently become the object of attention from astronomers. No wonder - just recently there was no evidence that there are other planets except those circulating around the sun. As it turned out - there is, and in the set. In fact, exoplanets are many billions, if not trillions, only in our Milky Way.

But now scientists face another problem - the search for evidence that life exists in other star systems. The proof is complicated by the fact that everything we know about life, all this information, is all obtained as a result of studying terrestrial organisms. We can only guess about other forms of life besides earthly ones, just as scientists guessed about the existence of exoplanets.

The difficulty is that a sufficiently reliable method of detecting exoplanets has now been developed. This can be done by studying the dynamics of the brightness of distant stars. During the passage of an exoplanet across the disk of such a star, its luminosity characteristics change, and Earth scientists receive almost one hundred percent confirmation of the existence of a planet or several planets at once. As for life, the situation is different here.

In the telescope, even the most powerful, life you will not see. None of what the existing equipment offers, life helps to detect life. And, as mentioned above, we know about it only judging by the terrestrial organisms. Recently, a proposal by a group of scientists about how you can still find life (or rather, signs of its existence) on exoplanets.

First, scientists know where to look for life is not worth it. Namely - on the planets orbiting dwarf stars of M-class. Yes, many of them have planetary systems. And earlier, the search for possible indicators of the existence of life was centered around these planets. But, most likely, there is no life there, since the conditions on exoplanets from M-class stars can hardly be called suitable for life.

Such stars shine and heat too weakly so that life can originate on a more or less distant exoplanet. The habitable zone here is too close to the star. As a result, the planets that could be in this zone fall into a gravitational lock: they always face their star on one side. Yes, just like the moon facing the earth.

In addition, being close to the star, such planets are constantly exposed to X-ray and hard ultraviolet studies. All this is not only not suitable for the emergence and maintenance of life, but also constantly negatively affects the atmosphere of the planets. The atmosphere simply blows into space. Perhaps some life forms are able to exist in such conditions, but still the probability of the absence of all life on the planets around M-class stars is much higher.

Instead of searching for life on such planets, astronomer Paul Dalbah suggests searching on exoplanets orbiting G-class stars. They are much smaller than M, only about 10% of the total number of stars. But the probability of the existence of life near such stars is much higher. In addition, instead of searching for planets that are in the habitable zone, the scientist proposes to pay attention to life indicators, one of which is the presence of oxygen ions O + in the planet's atmosphere, at an altitude of about 300 km from the surface.

As an example, the Earth, Mars and Venus. Only the Earth's atmosphere has positively charged oxygen ions, which rise several hundred kilometers above the surface. According to the astronomer, oxygen ions in the atmosphere of the planet is a direct pointer to the existence of life capable of photosynthesis.

It is worth noting that by the standards of Earth scientists, both Venus and Mars are in the habitable zone. In addition, these are earth-like planets on which, from the point of view of science, life may well exist. However, there is no life there (in any case, it has not yet been found), and there are not too many oxygen ions in the atmosphere of both planets.

If astronomers decide to seriously search for life from other planets, Dalba believes, then the presence of oxygen ions must be taken into account - in this case the probability of life is much higher than in all other cases. True, again it is worth mentioning that this is a sign of the existence of life on Earth. On the other hand, at least some indicator is better than its absence. In the near future, scientists are going to test the method proposed by Dalb. True, there are other factors that lead to the appearance of oxygen ions in the atmosphere - these are reactions involving water and UV radiation.

Nature Astronomy , 2018. DOI: 10.1038 / s41550-017-0375-y