Earth is just a tiny planet in the middle of vacuum of the solar system. But it is also a unique world: among the thousands of exoplanets we have discovered, we don't know so far any that look like it and that can harbor life. What makes Earth so unique? To begin with, it is a rocky planet, located at a distance from its star that allows it to hold liquid water on the surface (that's why it is said to be in the habitability zone of the Sun). In addition, its star has been quiet for many hundreds of millions of years. The Earth also has a protective atmosphere and a partially molten interior that provides energy, allows the circulation of materials and generates a protective magnetic field against radiation.
Although it is perfectly possible that there is a different life in places very different from Earth, it is interesting to consider whether what we know can be abundant in the galaxy. Now, a study that has just been published in the Astrophysical Journal, and prepared by scientists from the Georgia Institute of Technology (USA), has studied the possibility that binary stars, formed by two «suns», can house a land whose axis of rotation is stable enough to allow the evolution of alien life. His conclusion is that 87% of the land with two suns, such as the famous planet Tatooine, from the Star Wars saga, could have a rotation axis similar to that of our planet. This indicates that many of these worlds could have suitable climates to shelter living beings. But how many?
"Multiple systems are common," Gongjie Li, co-author of the study, said in a statement to refer to the stars that live in pairs or trios. «About 50% of them are binary stars. So our study can be applied to a large number of solar systems ».
Double stars, fit for life
The weather and the seasons of the Earth depend largely on the inclination of its axis of rotation and how it changes over time. For that reason, the researchers wanted to find out how double stars influence the inclination of the planetary axis of rotation, and if these would allow them to have stable climates or suitable seasonal changes.
"The most important thing in our research is that we evaluate how the inclination of a planet like Earth would vary under very optimistic conditions," Billy Quarles, principal investigator of the study, told ABC. That is, the scientists tried to investigate the effect of the presence of two stars, but did not take into account other factors. In fact, their simulations operated exact replicas of the solar system, in which, simply, another star was introduced: "We have assumed that everything is identical to the inner solar system, with the exception of the presence of another sun," said Quarles.
Hopeless in Alfa Centauri
The results obtained by the researchers suggest that many of the galaxy's binary stars allow the existence of rocky planets with a stable orbit, suitable for potential living things. For this, however, it is necessary that the separation between the stars exceeds a certain threshold. Otherwise, the axis of rotation of the lands seems to "go crazy".
Something like that is what happens in Alfa Centauri, the nearest solar system, located about 4.4 light years. It is formed by a ternary system in which two large stars, Alfa centauri A and Alfa centauri B, orbit the same center of mass, while another smaller star, Next Centauri, revolves around the other two. In addition, the star A has a very elongated orbit and the other B more flat, and both are separated by a distance comparable to that between the Sun and Neptune.
In the simulations, the researchers placed a land in the orbit of stars A and B to use as a binary system model. There they observed how its axis would lean in each case, even adding a moon. Thus they concluded that the "dance" of a land located in Alfa Centauri would have a very pronounced pitch in its axis of rotation. This nod, also known as precession, It is a movement reminiscent of the sway that experiences a spinning top.
But the most important thing is that this pitch has important repercussions for the habitability of a planet: it influences the seasons and a too large swing makes it difficult for a world to conserve its atmosphere and have a more temperate climate.
Why Mars lost its atmosphere
The best proof of this is in the solar system. The axis of rotation of Mars oscillates between an inclination of 10 and 60 degrees every two million years. In the first point, the atmosphere condenses at the poles, in the second, an ice belt could appear on the equator.
The history of the Earth is different. Its orbit experiences a subtle movement of precession, in which its axis passes from an inclination of 22.1 to 24.5 degrees every 41,000 years, while the shape of the orbit lengthens and shortens every 100,000 years. Partly thanks to that, our planet has gone through cycles of ice ages of about 100,000 years.
This stability depends in part on the mass of the Moon: «If we did not have the Moon, the inclination of the Earth would vary about 60 degrees», Explained Billy Quarles. "The Earth would look similar to Mars, where the precession of its axes seems to have helped to erase its atmosphere."
Paradoxically, a land that was in Alfa Centauri and lived accompanied by a large moon would have an even more unstable rotation, unlike in the solar system.
The result would be, in some cases, that a land of Alfa Centauri would have an ice age every million years, but these would be longer and much worse, according to Quarles.
After concluding that in theory many of the binary stars could house planets with the proper rotation for living beings, it would be necessary to know how many lands there are in binary systems, how many will really have an adequate orbit and how many will be the other important conditions for housing .
For now, these scientists will try to enrich their research taking into account the atmospheric composition of the planets and the amount of light they can reflect.
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