NASA announced in 2014 the discovery by the Kepler telescope of an exoplanet of size comparable to the Earth, orbiting in the habitable zone of its star (a red dwarf) and therefore likely to harbor water in the liquid state. A new study from the Georgia Institute of Technology (USA) tells us a little more about this exoplanet 500 lightyears away.
Astronomers announced the discovery 4 years ago of a planet of a size close to that of the Earth – barely 10% larger – located in the zone of habitability of its star. Called Kepler-186f, the latter is part of a system with several planets (five detected so far) revolving around the Kepler-186 star, located some 500 light-years away in the constellation Cygnus. This planet also evolves in a region around its star, where it receives the right amount of radiation, so that if water exists there, it could be in liquid form on its surface. But we did not know much more since this announcement.
Researchers at the Georgia Institute of Technology recently conducted simulations to analyze and identify the dynamics of the exoplanet spin axis. This determines how much a planet tilts on its axis and how this angle of inclination changes with time. Axial inclination contributes to the seasons and the climate because it affects the way sunlight hits the surface of the planet.
The researchers suggest that Kepler-186f’s axial inclination is very stable, just like that of the Earth, making it likely that it has regular seasons and a stable climate. How important is the axial slope for the climate? The great variability of the axial inclination could be one of the main reasons why Mars was transformed from an aquatic landscape billions of years ago into an arid desert that we know today. “Mars is in the habitable zone of our solar system, but its axial tilt has been very unstable – ranging from zero to 60 degrees,” says Gongjie Li, who led the study. “This instability has probably contributed to the degradation of the Martian atmosphere and the evaporation of surface water.”
For comparison, the Earth’s axial tilt – more stable – oscillates more moderately between 22.1 and 24.5 degrees, going from one extreme to the other every 10,000 years or so.
The orientation angle of the orbit of a planet around its host star can be oscillated by gravitational interaction with other planets in the same system. Mars and the Earth, for example, strongly interact with each other. As a result, this can cause significant variations in their axial inclination. But fortunately for us, the Moon keeps these variations in check. It increases the rate of precession of the axis of rotation of our planet and differentiates it from the rate of orbital oscillation. On the other hand, Mars does not have a satellite large enough to stabilize its axial inclination.
Regarding Kepler-186f, it seems that it has a very weak connection with its sister planets. We do not know if it has one or more moons, but even without a satellite, its axial inclination remains particularly stable. A good point in common with the Earth, therefore. Recall that Kepler-186f’s radius is more than 10% less than the Earth’s, but its mass, composition and density are still a mystery. According to NASA, the brightness of its star at noon looks as bright as the Sun just before it sets on Earth.