Two exoplanets in the habitable zones of their stars probably have a stable climate and regular seasons, scientists have found. One of them, astronomers believe, is about the size of the earth.
Both Kepler-186f and Kepler-62f appear to tilt uniformly about their axes in simulations explored by researchers at Georgia Tech and Harvard University. Both planets, scientists think, are relatively good candidates for life.
The way a planet tilts around its axis affects how much light reaches its surface, which in turn affects its climate. For example, a variable axial tilt on Mars could explain why the Red Planet morphed from damp to incredibly dry. 
Although Mars lies within the sun's habitable zone – close enough that liquid water remains – it is barren Desert. The planet's axial tilt ranged from zero to 60 degrees, said author and Georgia Tech Assistant Professor Gongjie Li, said in a university statement. "This instability has 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 shake the orientation angle of a planet as it rotates around its star. At the right speed, this can cause the axis of the planet to swing back and forth. However, satellites and moons can dampen these fluctuations and stabilize the axial tilt of a planet.
The axial inclination of the earth moves from just 22.1 to about 24.5 degrees every 10,000 years. Mars faces much bigger wobbles without a satellite enough to pull on its movements.
"It seems that both exoplanets are very different from Mars and the Earth because they have a weaker connection to their sibling planet," Li said "We do not know if they have moons, but our calculations show that even without satellites, the Kepler-186f and 62f axes would have remained constant for tens of millions of years."
Kepler-62f is about 1,200 light-years away in the constellation Lyra. The Super Earth is about 40 percent larger than our own rocky home. Scientists think that it is either terrestrial or covered with water.
Kepler-186f, discovered in 2014, is only 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 radius similar to ours is only 10 percent larger than Earth. Kepler-186f years are much shorter than our own as it completes an orbit in 130 days.
"I do not believe we understand enough about the origin of life to rule out the possibility of their presence on planets with irregular seasons," added 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 exceptionally hostile environments, but a climatically stable planet could be a more comfortable starting point."
The study was published in The Astronomical Journal .