Earth-like Exoplanet May Have Seasons, Stable Climate

Two exoplanets in the habitable zones of their stars probably have stable climates and regular seasons, scientists have discovered. One of them, astronomers believe, is roughly the size of Earth.

In simulations studied by Georgia Tech and Harvard University researchers, both Kepler-186f and Kepler-62f appear to tilt steadily on their axis like Earth. Both planets, scientists think, are relatively good candidates for life.

The way a planet tilts on its axis affects how much light reaches its surface, which, in turn, affects its climate. A variable axial tilt on Mars, for example, may help explain why the red planet morphed from wet to incredibly dry.

An artist's conception of Kepler-186f. T Pyle/JPL-Caltech/NASA Ames

Although Mars lies within the sun's habitable zone—close enough for liquid water to persist—it is a barren desert. The planet's axial tilt has swung from zero to 60 degrees, study author and Georgia Tech assistant professor Gongjie Li said in a university statement. "That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water."

The complex gravitational gymnastics of other planets in the same system can jiggle a planet's orientation angle as it spins around its star. At the right speed, this can cause the planet's axis to swing to and fro. Satellites such as moons, however, can dampen these swings and settle a planet's axial tilt.

Earth's axial tilt moves from just 22.1 to around 24.5 degrees every 10,000 years or so. Mars faces much larger wobbles without a big enough satellite to tug on its movements.

"It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets," Li said. "We don't know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years."

Kepler-62f lies approximately 1,200 light-years away in the constellation Lyra. The super-Earth is approximately 40 percent larger than our own rocky home. Scientists think it's either terrestrial or covered in water.

Kepler-186f, spotted in 2014, lies just 500 light-years from Earth. This planet orbits its star in a five-planet system in the Cygnus constellation. The first planet discovered with a similar radius to our own, it's just 10 percent larger than Earth. Kepler-186f years are much shorter than our own, as it completes one orbit in 130 days.

"I don't think we understand enough about the origin of life to rule out the possibility of their presence on planets with irregular seasons," said study author Yutong Shan from the Harvard-Smithsonian Center for Astrophysics in the statement.

"Even on Earth, life is remarkably diverse and has shown incredible resilience in extraordinarily hostile environments. But a climatically stable planet might be a more comfortable place to start," Shan said.

The research was published in The Astronomical Journal.