Our universe is populated by many types of stars. Each type of star has a different colour, and their life varies from some million years to billions or trillions of years. So, the best place to search for life would be around the most long-lasting stars, where life would have more time, and therefore more possibilities, to develop itself. These stars, where, due to the long life they have, would be good places for the search for life, are the named red dwarfs.
However, we have to keep in mind different issues at the moment to think of finding life on planets that are orbiting red dwarfs. Given what we know about these stars, the conditions that would exist on the planets in their orbit seem not to be the best to find life on them: red dwarfs have their habitable zone is much closer than Mercury finds itself from the Sun.
The habitable zone of a star is defined as the zone of its orbit where liquid water could exist. Therefore, it is the region around a star in which, life as we know it, could develop. The larger the star, the further away its habitable zone will be, and the smaller, the closer to it. In the case of the red dwarfs, the smallest stars of all, with less mass and less luminosity, their habitable zone is very close to the star. This, in the moment of analysing in detail the situation, presents some problems.
Around red dwarfs, the planets that find themselves in the habitable zone of the star will be in synchronous rotation. That to say, they will take the same time to turn on their axis as to make a revolution around the star. In this situation, a hemisphere of the planet would always be illuminated, while the other one would be permanently submerged in darkness. This is the same phenomenon that we can observe between the Earth and the Moon. We always see the same side of the Moon, as it is in synchronous rotation with the Earth. In the case of a star, this can difficult thing. One hemisphere would have extremely high temperatures and the other one would be frozen. Between the two hemispheres, there would be a zone of permanent twilight.
Through years, different theories have been published about how the atmosphere of a planet in these conditions would behave. According to some of them, the air masses of the two hemispheres could collide violently during the constant twilight strip. This would provoke the conditions to be very harsh where liquid water could exist.
In other cases, it is suggested that, with a dense enough atmosphere, the winds moving from one hemisphere of the planet to the other could equilibrate the temperatures on the planet, and there could exist liquid water seas throughout all its surface. However, we have to keep in mind that a red dwarf star isn’t a star like the Sun.
Despite their small size, red dwarfs are extremely violent stars. They have the capacity to create stellar eruptions much more strong than those of the Sun, and the caused solar winds could be 2 000 times more powerful than those that arrive on Earth. These winds would make disappear the atmosphere of a nearby planet. Planets around red dwarfs would need to have an extremely powerful magnetic field, and a very dense atmosphere, to survive the solar winds and not lose the atmosphere. Without this one, life on the planet couldn’t exist.
Fortunately, during the last years, it has been observed that not all the stars of this type are so violent. It is thought that the older these stars are, the fewer eruptions take place on them. Proxima Centauri (the nearest star to us apart from the Sun) and TRAPPIST-1 (situated at a distance of 39 light-years from us, in the Aquarius constellation) are very active red dwarfs. In contrast, other red dwarfs, such as Ross 128, are calmer, with a stellar activity similar to the one of the Sun.
The giant planets, also named gaseous, could offer us a solution to the different presented problems. A planet doesn’t need to be orbiting around a star. It could orbit, as well, around a much bigger planet, converting it into one of his satellites (or the only one). For the moment, it hasn’t been confirmed that any exoplanet (a planet out of the solar system) owned any moon, but there aren’t reasons to think that they couldn’t have one.
Until recently it was thought that the red dwarfs couldn’t have giant planets (the size of Jupiter for example), orbiting them, but in 2019 a group of investigators discovered one planet of these characteristics around the red dwarf GJ 3512. Maybe a giant planet, located in the habitable zone of one of these stars could have a moon with a size and a mass similar to the one of Earth or even greater (to be able to have a dense enough atmosphere and a powerful magnetic field). This moon the size of the Earth wouldn’t be in synchronous rotation with the star, and maybe the magnetic field of the planet it would be orbiting could help the moon to deviate the solar wind that arrived in it.
Many red dwarfs
The red dwarfs are the most common stars of our universe. They represent 75% of all the stars, and, in addition, they are the ones that have a greater time of life. If we conclude that around the red dwarfs there can’t be life, we are eliminating an enormous quantity of possibilities to find life in our universe. However, if we conclude that they can have habitable worlds, we find ourselves in front of a scene where life could be abundant in the universe.
Regarding this issue, there are diverse theories. Some give us hope to find life around red dwarfs, and others make us discard the possibility of finding it there.
A possible alternative would be to search for life around the orange dwarf stars, stars less massive than the Sun, but with more mass than the red dwarfs. The orange dwarfs have a bigger life expectancy than the one of the Sun, and they are calmer stars than the red dwarfs from the point of view of their stellar activity. Around these stars, there could exist ideals places for life to develop.