Last updated June 29, 2018 at 3:26 pm
With similar mass and a stable tilt, Kepler-186f is as close to a twin to Earth as we’ve found.
Artists impression of Kepler-186f. Credit: NASA Ames/JPL-Caltech/T. Pyle
Scientists from the Georgia Institute of Technology have revealed that Kepler-186f, the first Earth-sized planet found outside our solar system orbiting a star in the habitable zone, may be even more similar to Earth than we previously thought.
Discovered in 2014, Kepler-186f is less than 10 percent larger in radius than Earth, has a year that lasts 130 days, and according to NASA, the brightness of its star at high noon would be as bright as the sun an hour before sunset here on Earth.
It also exists within the habitable zone, meaning it’s the proper distance from its host star for liquid water to pool on the surface, and is strongly thought to be rocky.
The researchers have now also found that the tilt of its axis is very stable, making it likely that it has regular seasons and a stable climate.
While these results don’t prove that conditions on the planet are the same as Earth, or that life exists, it increases the possibility of the planet being an effective twin to our own.
Spinning without wobble
The Georgia Tech study used simulations to analyse and identify the exoplanet’s spin axis dynamics, which determine how much a planet tilts on its axis and how that tilt angle evolves over time.
The researchers suggest that Kepler-186f’s axial tilt is very stable, much like Earth.
Axial tilt contributes to seasons and climate because it affects how sunlight strikes the planet’s surface, with a stable tilt providing the conditions for regular seasons and a stable climate.
The link between axial tilt and climate is thought to be critical. Large variations in axial tilt could be a key reason why Mars transformed from a watery landscape billions of years ago to today’s barren desert.
“Mars is in the habitable zone in our solar system, but its axial tilt has been very unstable — varying from zero to 60 degrees,” said Professor Gongjie Li, who led the study at Georgia Tech.
“That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water.”
Earth on the other hand exhibits a very stable tilt, oscillating between 22.1 and 24.5 degrees, and going from one extreme to the other every 10,000 or so years.
The degree and stability of a planet’s tilt is affected by the gravitational interaction with other planets in the same solar system.
In the case of Earth and Mars, strong gravitational interactions between the two as well as with Mercury and Venus, means that the tilt of the planets should wobble wildly back and forth as they orbit the Sun.
Fortunately, however, the Moon keeps Earth’s variations in check. Mars on the other hand doesn’t have a moon large enough to stabilise its axial tilt.
In the case of Kepler-186f, it is thought to have a weaker interaction with other planets in its system, leading to less wobble.
Not the only exoplanet with stable tilt
The Georgia Tech team thinks that a second exoplanet, Kepler-62f, also shows a similarly stable tilt as Earth and Kepler-186f.
Artists impression of Kepler-62f. Credit: NASA Ames/JPL-Caltech/T. Pyle
Kepler-62f was the most Earth-like exoplanet until scientists noticed 186f in 2014. It’s about 40 percent larger than our planet and is likely a terrestrial or ocean-covered world, orbiting a star about 1,200 light-years away from us.
“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li.
“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.”
The results do not say whether either exoplanet has water, let alone life. But both are relatively good candidates.
“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 Yutong Shan from the Harvard-Smithsonian Center for Astrophysics, who also worked on the study.
“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 research has been published in The Astronomical Journal
