In 2014, NASA announced the discovery via the Kepler telescope of an exoplanet similar in size to the Earth, orbiting in the habitable area of its star (a red dwarf) and thus potentially containing liquid water. A new study in the Georgia Institute of Technology (United States) has a little more to teach us about this exoplanet 500 light years away.
4 years ago, astronauts announced the discovery of a planet with a size close to that of the Earth – barely 10% bigger – located in the habitable area of its star. Called Kepler-186f, this planet is within a system of several planets (five which have been detected), turning around the star Kepler-186, located 500 light years away in the Cygnus constellation. This planet evolved in the area around its star, where it got enough light for water to be present in liquid form on its surface. But we haven’t heard much since this announcement.
Researchers in the Georgia Institute of Technology recently carried out simulations to analyse and identify the dynamics of the spin axis of this exoplanet. This determines the extent to which a planet tilts on its axis, and how this tilt angle evolves over time. Axial inclination contributes to the seasons and the climate on a planet, as it affects the way in which sunlight hits the planet’s surface.
Researchers suggest that the axial inclination (or tilt angle) of Kepler-186f is very stable, similar to the Earth, which makes it likely that it has regular seasons and a stable climate. So how important is tilt angle for climate? The great variability in axial tilt on Mars could be one of the reasons why it transformed from an aquatic landscape billions of years ago, into the arid desert we know nowadays. “Mars is in the habitable zone in our solar system, but its axial tilt has been very unstable—varying from zero to 60 degrees”, explains Gongjie Li, director of the study. “That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water.”
By comparison, the axial inclination of the Earth – which is more stable – oscillates more moderately between 22.1 and 24.5 degrees, passing from one extreme to the other around every 10,000 years.
The orientation angle of a planet’s orbit around its host star could change due to gravitational interaction with other planets in the same system. Mars and Earth, for example, interact strongly with each other. As a result, this can cause significant variations in their axial inclinations. But fortunately for us, the moon keeps these variations in check. It increases the precession levels of our planet’s rotation axis, and differentiates it from the rate of orbital oscillation. Mars, for its part, does not have a large enough satellite to stabilise its axial inclination.
In terms of Kepler-186f, it appears that it has a very weak connection with its sister planets. We don’t know whether it has one or several moons, but even without a satellite, its axial inclination stays particularly stable, which is something it has in common with the Earth. Remember that the radius of Kepler-186f is not even 10% larger than that of the Earth, but that its mass, its composition and its density remain a mystery. According to NASA, the luminosity of its star at midday appears as bright as of our sun just before sunset on Earth.