Planets in the Goldilocks zone may be snowballs

Earthlike planets circling other stars may be prone to wild climate fluctuations that could make them uninhabitable, even if they lie within the so-called “habitable zone”, scientists say. 

Sometimes referred to as the Goldilocks Zone, the habitable zone is the region in which a planet is neither so close to its sun that it is permanently parched nor so far away it doesn’t receive enough energy for liquid water to exist. 

But another factor affecting habitability, says Rory Barnes, an astrobiologist at the University of Washington, Seattle, is the planet’s obliquity – the angle between the planet’s spin and its orbit. 

On Earth, this is the well-known 23.5 degrees that establishes our seasons — winter in the hemisphere tilted away from the Sun, summer in the hemisphere tilted toward it.

For us, the condition is both fairly mild and fairly stable over time. “The moon keeps our obliquity fairly constant,” Barnes says. But small wobbles known as Milankovitch cycles have since the 1970s been associated with the beginning and ending of ice ages.{%recommended 3900%}

A few years ago, Barnes got to wondering how such cycles might play out on exoplanets. In our solar system, the planets follow fairly circular orbits, lying neatly in a single plane. But other solar systems might not be so tidy. In fact, exoplanet hunters are coming to believe that planets could be flying around at all sorts of angles. 

To determine the effect of this, Barnes and colleagues, including graduate student Russell Deitrick, now at the University of Bern, Switzerland, set out to model the effects of such systems’ gravitational interactions on an otherwise earthlike world’s obliquity, and hence its climate. 

The result was a model that not only included forces and torques between planets, but how ice sheets flow across them and deform their crusts. 

“It took years,” he says.

In a few cases, he says, these effects helped keep planets habitable by periodically warming them up. In these, he says, “[They] were good at removing ice.” 

But most of the time, the reverse occurred. “About 75% of the time you have a better chance of going into a complete snowball state, where the planet freezes over,” he explains.

And it’s not just that “winter is coming”, as in the hit TV series Game of Thrones. These winters are permanent. Once a planet has frozen in this manner, it doesn’t thaw again, even if subsequent gravitational torques tilt it back to an obliquity in which it wouldn’t have frozen over in the first place.

Part of the reason, Barnes says, is that ice sheets flow. So, once they get big enough on one part of the planet, they can flow to warmer regions, chilling them as well. Once formed, they also have high “thermal inertia”, meaning that they are hard to melt.

Also interesting, he says, is that the poles aren’t always the coldest places on a planet. If a planet is tilted too much on its side — the magic number turns out to be more than 55 degrees — the equator winds up getting less solar energy per year than the poles. 

“With high obliquity you are more likely to get an equatorial ice belt,” Barnes says.

With unstable obliquity, the results can be climate cycles that would be unrecognisable on Earth. 

“As your obliquity is flopping around,” he says, “you can have ice sheets that grow and retreat. Sometimes they’re from the poles, sometimes from the equator.”

This could simply produce a yo-yo. But if the polar cap or equatorial ice belt has grown large enough, there could come a time when, as the planet passes through the 55-degree boundary, the ice cap takes over completely. 

“The sunlight is then distributed evenly,” says Barnes. “That means there’s no place that’s particularly warm, so it’s easy for the ice sheets to escape from the pole or the equator and take over the whole planet.”

In the future, he adds, astronomers will be able to use this to pick the exoplanets they most want to study for signs of life. 

“As we find more and more systems with potentially habitable worlds we need to identify which are the ones we want to observe first,” he notes. “If there’s a huge possibility of a snowball state because of the orbital dynamics, we may say here’s one to cross off the list and go on to the next one.”

Chris Colose, a climate scientist at NASA’s Goddard Institute for Space Studies in New York, US, calls the new research an “interesting contribution” to our understanding of exoplanet climates, saying that it makes a “compelling case” that in determining an exoplanet’s habitability it may be important to look at such factors as Milankovitch cycles, rather than just looking at “some ‘average’ set of orbital parameters”.

That said, in order to accomplish all of its goals, he adds, it uses a relatively simple climate model, and some of its findings will almost certainly be model-dependent. 

“So the robustness of these conclusions may or may not stand the test of time,” he says. “Simpler models have some history of being too pessimistic at generating abrupt changes.” 

In particular, he notes, heat stored in oceans may help make some planets more habitable. 

“Even when you do have a planet with ice,” he says, “it seems relatively easy to open up patches of liquid water near the polar regions during the summer. The intense seasonality isn’t too much of an issue because the oceans do a good job of storing heat during the warm part of the year and releasing it during the winter.”

The research will appear later this year in the Astrophysical Journal, and is currently available on the pre-print site, Arxiv.