An article published in the “Astrophysical Journal Letters” offers considerations on the possible advantages in the search for biosignatures such as the presence of oxygen and methane on exoplanets orbiting a K-class star, a bit smaller than the Sun. Giada Arney of NASA’s Goddard Space Flight Center tried to find out what those biosignatures and therefore the signs of the presence of life forms on an exoplanet in that kind of system could look like creating a series of computer simulations to understand where to look for traces of oxygen and methane.
The discovery of exoplanets is now a daily feat and the significant amount of rocky planets known today offers an increasing amount of information that can help understand how to find evidence of the existence of life forms on one of them. The problem concerns not only the planets but also the stars they orbit because their characteristics greatly influence their evolution and therefore their potential habitability.
Given that the only certain example of a planet hosting life forms is the Earth, which orbits a G-class star, a lot of research focused on similar systems but over the years there have been discussions on the subject. Blue giant stars are definitely unsuitable because they blast the surrounding area with heavy radiation and anyway consume their hydrogen at a very high speed so even if they had planets their life would be too short to develop life forms. Many discussions concern stars smaller than the Sun, which consume their hydrogen slowly and are the most common.
The limit case of the tiny star TRAPPIST-1, with the confirmation in February 2017 of 7 rocky planets, indicated that it makes sense to look for exoplanets even around those stars. The problem is to understand which characteristics can favor the development of atmospheres favorable to the birth of life forms similar to those of Earth.
There are scientists who think that stars a little smaller than the Sun, also called orange stars and classified as K-class, are the most suitable for habitable planets. That’s because they can live for tens of billions of years but they don’t have the powerful flares typical of red dwarfs, which are even smaller but despite their reduced mass can be very active, to the point of sterilizing their planets.
Scientists believe that the presence of oxygen and methane at the same time is a biosignature because they tend to interact and therefore to destroy each other so finding them together indicates that there’s a process that produces them, probably of a biological type. The problem is that they must be present in remarkable amounts to be detected from the Earth.
Giada Arney tried to understand which exoplanets could offer the best conditions to find that type of biosignature and the result is that K-class stars have greater possibilities to host them than the ones more similar to the Sun. The models used for this research simulated the chemistry and temperature of a planetary atmosphere and the response of that atmosphere to different types of stars. The ultraviolet light emitted by a K-class star doesn’t generate oxygen so reactive that it destroys methane very quickly so they can be present together in greater abundance. That’s true even for planets orbiting red dwarfs but the problem of stellar flares can make them unsuitable for life forms.
Among the K-class stars mentioned as candidates for follow-up research of biosignatures there are 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026. They’re nearby stars in astronomical terms so the search for rocky planets to examine their possible atmospheres can be simpler. This doesn’t mean ignoring planets that orbit stars of different classes, just giving priority to the ones that offer the best chances of finding biosignatures.