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When they are looking for exoplanets that could potentially support life, astronomers start with those who are orbiting the "habitable zone" – the distance from the parent star that allows the planet to move. liquid water to exist on the surface. we know it. A new study has shown that two of the most promising planets are now even more promising, with axial inclinations that probably give them regular seasons and a stable climate.
The presence or absence of a planet in the habitable zone is a good starting point for judging exoplanets of interest, but this is not the end of the matter. In fact, there are a whole series of factors that can influence the livability of a planet, including stellar radiation and other atmospheric conditions, or the atmosphere of the planet, volcanic activity, plate tectonics, water content and axial inclination.
It is this last point on which the new research is concentrated. In our own solar system, the Earth is joined in the habitable zone (aka the Goldilocks zone) by Venus and Mars, but only one of these planets is currently adapted to life. Venus suffered from an uncontrolled greenhouse effect that left him with a toxic atmosphere of sulfuric acid, while modern Mars is an arid land after its once hospitable atmosphere was lost in space, thanks largely to the irregular inclination of its axis.
"Mars is in the habitable zone of our solar system, but its axial inclination has been very unstable – ranging from zero to 60 degrees," says Gongjie Li, co-principal investigator of the study. "This instability has probably contributed to the degradation of the Martian atmosphere and the evaporation of surface water."
For the new study, researchers at Georgia Tech and the Harvard-Smithsonian Center for Astrophysics used simulations to determine the dynamics of the spin axis. two of the most telluric exoplanets ever discovered: Kepler-186f and Kepler-62f.
The first was discovered in 2014, orbiting the habitable zone of a red dwarf star about 500 light-years away. It is only 10% larger than Earth, and surrounds its star once every 130 days. Kepler-62f, on the other hand, is 40 percent larger than the Earth, and is 1,200 light-years away, also in the Goldilocks zone of its star.
The researchers found that the two exoplanets probably have stable axial inclinations. This is largely because they do not interact with their neighboring planets as strongly as the planets in our solar system. For example, Mars, Earth, Venus and Mercury interact all gravitationally, potentially playing with the axial tilt of the other. The Moon keeps the Earth in constant rotation, but Mars is not so lucky.
"It seems that the two exoplanets are very different from Mars and Earth because they have a weaker connection with their sister planets," says Gongjie Li, principal researcher on the study. "We do not know if they own moons, but our calculations show that even without satellites, the Kepler-186f and 62f spin axes would have remained constant over tens of millions of years."
This would give the planets stable axial tilt and, by extension, possibly a stable climate with varying seasons. As always with this type of work, this does not guarantee that one or the other exoplanet is home to water and / or liquid life, but as the researchers say, "a planet climatically stable could be a more comfortable place to start. "
The research was published in The Astronomical Journal .
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