An Alien Planet's Tilt Could Kill Life, Even in a 'Habitable Zone'

Despite the real estate adage, location isn't everything, at least not when it comes to making planets fit for life.

That's the finding of a new paper accepted for publication in The Astronomical Journal. The study looked at how a second planetary trait—the tilt of its axis—inflences its climate.

While you may not know it by that terminology, you're intimately influenced by the tilt of a planet's axis: It's just that trait that gives Earth its seasons. In June, for instance, our axial tilt points the northern hemisphere toward the sun, bringing us summer. Earth's axial tilt is relatively mild at 23.5 degrees, although it may not feel that way at the height of whichever season you dislike most.

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But other planets are much more tilted—Uranus is essentially lying on its side. And planets in other solar systems also come at a variety of angles. So the team behind the new paper wondered whether that might change how temperate they were.

They focused on planets in the so-called habitable zone: far enough from their sun that they can hold liquid water on their surface, but not so far away that any water would freeze. In addition to axial tilt, they also considered how the shape of a planet's orbit—more circular or more oval—could impact its climate.

05_16_exoplanet_tilt_climate An artist's conception of what an ice-covered exoplanet might look like. A study looked at how a second planetary trait—the tilt of its axis—influences its climate. NASA's Goddard Space Flight Center Conceptual Image Lab

The new study focuses on planets orbiting what's known as "G type" stars, the class that includes our own sun. It built a model of how ice sheets would grow and shrink for planets in the habitable zones of these stars, given a range of different orbital characteristics.

And the authors found that those characteristics really matter—in fact, that they might make planets that just by distance should be habitable too cold for life to survive.

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