When looking for exoplanets that could potentially support life, astronomers start with those orbiting within the "habitable zone" – the distance from the parent star that's just right for liquid water to exist on the surface, creating the conditions necessary for life as we know it. A new study has found that two of the most promising planets are now even more promising, with axial tilts that likely give them regular seasons and a stable climate.

Whether or not a planet is in the habitable zone is a good starting point for judging exoplanets of interest, but it's not the end of the matter. As it turns out, there are a whole host of factors that can influence the habitability of a planet, including stellar radiation and other space weather, or the planet's atmosphere, volcanic activity, plate tectonics, water content, and axial tilt.

It's that last point that the new research focused on. In our own solar system, Earth is joined in the habitable zone (aka the Goldilocks zone) by Venus and Mars, but only one of those planets is currently life-friendly. Venus has suffered from a runaway greenhouse effect that's left it with a toxic atmosphere of sulfuric acid, while modern Mars is an arid wasteland after its once-hospitable atmosphere was lost to space, thanks largely to the irregular tilt of its axis.

"Mars is in the habitable zone in our solar system, but its axial tilt has been very unstable – varying from zero to 60 degrees," says Gongjie Li, co-lead researcher on the study. "That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water."

For the new study, researchers from Georgia Tech and the Harvard-Smithsonian Center for Astrophysics used simulations to figure out the spin axis dynamics of two of the most Earth-like exoplanets ever discovered: Kepler-186f and Kepler-62f.

An artist's rendition of Kepler-62f NASA Ames/JPL-Caltech/T. Pyle

The former was discovered in 2014, orbiting within the habitable zone of a red dwarf star about 500 light-years away. It's only 10 percent bigger than Earth, and circles its star once every 130 days. Kepler-62f, on the other hand, is 40 percent larger than Earth, and sits 1,200 light-years away, also in the Goldilocks zone of its star.

The researchers found that both exoplanets likely have stable axial tilts. This is largely because they don't interact with their neighboring planets as strongly as planets do in our Solar System. For instance, Mars, Earth, Venus and Mercury all interact gravitationally, potentially messing with each other's axial tilt. The Moon keeps Earth spinning steadily, but Mars isn't so lucky.

"It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets," says Gongjie Li, co-lead researcher on the study. "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."

This would give the planets a stable axial tilt and, by extension, possibly a stable climate with shifting seasons. As always with this kind of work, that doesn't guarantee that either exoplanet is home to liquid water and/or life, but as the researchers say, "a climatically stable planet might be a more comfortable place to start."

The research was published in The Astronomical Journal.

Source: Georgia Tech