Axial tilts, eccentric orbits, could make life impossible on exoplanets in habitable zones

Planets could enter "snowball states" causing extreme ice ages that make life impossible.
By Laurel Kornfeld | May 16, 2018
Earth-like exoplanets located in their stars habitable zones, defined as the distance from their parent stars at which temperatures allow liquid water to exist on their surfaces, could be uninhabitable if they have extreme axial tilts and eccentric orbits.

Using computer modeling, a team of scientists led by Russell Deitrick, then at the University of Washington (UW) and now at the University of Bern, studied the effects of planets' axial tilts and orbital eccentricities on the potential for life on rocky worlds orbiting Sun-like G dwarf stars.

Axial tilt or obliquity refers to the tilt of a planet's rotation axis in relation to the plane of its orbit and is responsible for planets' seasons.

Orbital eccentricity refers to the shape of a planet's orbit around its star and the degree to which that orbit is circular or elliptical. When a planet has an elliptical orbit, its distance from its parent star changes during its orbit, bringing it both close to, then away from, the star.

Specifically, the researchers sought to understand the effects of an axial tilt greater than Earth's, which is 23.5 degrees, on the habitability of planets at the same distance from their stars as Earth is from the Sun.

While earlier studies indicated a greater axial tilt would make these planets warmer, this latest research shows the opposite.

"We found that planets in the habitable zone could abruptly enter 'snowball' states if the eccentricity or the semi-major axis variations--changes in the distance between a planet and star over an orbit--were large or if the planet's obliquity increased beyond 35 degrees," Dietrick said.

Such a "snowball" state would cause oceans on these planets to freeze and make surface life impossible.

Dietrick's team used more sophisticated computer models than previous studies. These models focused on the growth and retreat of planetary ice sheets.

"While past investigations found that high obliquity and obliquity variations tended to warm planets, using this new approach, the team finds that large obliquity variations are more likely to freeze the planetary surface. Only a fraction of the time can obliquity cycles increase habitable planet temperatures," stated Rory Barnes of UW.

Because ice ages on exoplanets can be more severe than those on Earth, a planet's location in its star's habitable zone is insufficient for determining whether it can host life. Orbital dynamics also play an important role in determining habitability; this is an important factor for scientists to take into account when choosing potentially habitable worlds to observe with powerful telescopes.

A paper on the study has been accepted for publication in the Astronomical Journal.

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