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"A solar system similar to Earth's; eleven planets. Number four seems to be... class M. Oxygen atmosphere".
One of the opening lines in Star Trek: The Original Series refers to Talos, an Earth-like exoplanet that hosts other forms of life, among which, vegetation. The idea of other worlds in the Milky Way capable of hosting life sound quite familiar in the current millenium: this is partly thanks to iconic lines such as Mr. Spock's one above, partly to the work of hundreds of scientists who have discovered a few exoplanets, planets orbiting stars other than our Sun, that look somewhat similar to the Earth.

As different species of planets dwell in the Galaxy, so do several kinds of stars: our Sun is a class G, an intermediate size beast between the colder K and M stars and the hotter F and A ones. As a matter of fact, there are other two classes of stars: O and B. Generation of astronomers, notoriously lazy creatures, remember these stellar types memorizing the famous "Oh, Be A Fine Girl/Guy, Kiss Me". Different types of stars emit most of their light in different frequencies and this impacts several processes on the planets orbiting around them.

An example of the influence of different stellar radiations on its surrounding planets can be shown for photosynthesis: the process through which plants convert water, corbon dioxide and stellar light into nutrients. In our work, we have studied this effect in terms of the amount of energy that can actually be used for the photosynthesis by hypothetical vegetation on Earth-like exoplanets (so-called Exergy), such as TOI 700 d, compared to the total energy that different kinds of stars radiate in the Photosynthetically Active Range (PAR), a portion of the light of the star (400-800 nm) that is useful for photosynthesis.

We have found that Earth is by large the rocky planet with the largest PAR flux and with the highest known efficiency for the process. In general, it seems that photosynthetic efficiency raises with the temperature of the host star and that the lowest M-stars might have too low a PAR flux to sustain an Earth-like biomass: this could lower the chances of finding Earth-like vegetation on planets around this kind of stars, such as the famous Trappist-1 worlds.
We also specifically looked at other ten known exoplanets assuming an Earth-like, cloudless atmosphere. We found that one of them, Kepler-442 b, receives a PAR flux slightly larger than the one necessary to sustain a large biosphere, one similar to the Earth's. It is also worthy to notice that Kepler-442 b is not tidally locked, i.e. it does not show always the same face to its parent star like close-in planets around smaller stars often do. This makes this planet a promising target for search of biosignatures, atmospheric tracers of the presence of life on the planet.

Even though the findings are promising, we should bare in mind that we haven't got any exhaustive information on small, rocky exoplanets atmospheres yet. Large programs to study exoplanets gas envelopes are already at work and new high-tech solutions to this detection problem are being developed. A deeper understanding of atmospheres on exoplanets orbiting the habitable zones of their stars is needed to smooth our current models and unlock the mysteries of these hopefully-"class M", intriguing worlds.

Author: Luca Cacciapuoti