New research from the Georgia Institute of Technology and the Harvard-Smithsonian Center for Astrophysics suggests that exoplanets Kepler-62f and Kepler-186f — both of which reside in the habitable zone around their host stars — have very stable axial tilts, much like the Earth, making it likely that each has regular seasons and a stable climate.
The planet orbits its host star every 267 days, is 1.4 times bigger than Earth and is likely a terrestrial or ocean-covered world.
Kepler-62f was the most Earth-like exoplanet until astronomers noticed Kepler-186f in 2014.
Kepler-186f is part of the five-planet system. It is about 1.17 times the radius of Earth, but its mass, composition and density remain a mystery.
It orbits Kepler-186, a M1-type dwarf star located in the constellation Cygnus, about 492 light-years away, once every 130 days.
The brightness of the star at high noon, while standing on Kepler-186f, would appear as bright as the Sun just before sunset here on Earth.
“Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds,” said Dr. Gongjie Li, from the Georgia Institute of Technology and the Harvard-Smithsonian Center for Astrophysics.
Dr. Li and her colleague, Yutong Shan, used simulations to analyze and identify the exoplanets’ spin axis dynamics. Those dynamics determine how much a planet tilts on its axis and how that tilt angle evolves over time. Axial tilt contributes to seasons and climate because it affects how sunlight strikes the planet’s surface.
The researchers suggest that Kepler-186f’s axial tilt is very stable, much like the Earth, making it likely that it has regular seasons and a stable climate. They think the same is true for Kepler-62f.
“How important is axial tilt for climate? Large variability in axial tilt could be a key reason why Mars transformed from a watery landscape billions of years ago to today’s barren desert,” the scientists explained.
“Mars is in the habitable zone in our Solar System, but its axial tilt has been very unstable — varying from zero to 60 degrees,” Dr. Li said.
“That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water.”
As a comparison, Earth’s axial tilt oscillates more mildly — between 22.1 and 24.5 degrees, going from one extreme to the other every 10,000 or so years.
“The orientation angle of a planet’s orbit around its host star can be made to oscillate by gravitational interaction with other planets in the same system,” the astronomers said.
“If the orbit were to oscillate at the same speed as the precession of the planet’s spin axis, the spin axis would also wobble back and forth, sometimes dramatically.”
“Mars and Earth interact strongly with each other, as well as with Mercury and Venus. As a result, by themselves, their spin axes would process with the same rate as the orbital oscillation, which may cause large variations in their axial tilt.”
“Fortunately, the Moon keeps Earth’s variations in check. The Moon increases our planet’s spin axis precession rate and makes it differ from the orbital oscillation rate. Mars, on the other hand, doesn’t have a large enough satellite to stabilize its axial tilt.”
“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” Dr. 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 Kepler-62f would have remained constant over tens of millions of years.”
“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,” Shan said.
“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.”
The study was published in the May 17, 2018 issue of the Astronomical Journal.