Stars like the Sun brighten over the course of their history, a trend that has significant consequences for the habitability of Earth and other bodies both in our Solar System and beyond. An icy world on the far edge of the habitable zone may turn into a temperate paradise given enough time.

Or, it could go straight to being a Venus-style hell if a new study turns out to be right. The study's authors turned a full-planet climate model loose on a planet covered in ice. They find that, under a level of incoming light that's sufficient to melt the ice, the planet reaches a greenhouse state that would cause it to lose all of its water to space and possibly head straight into a runaway greenhouse.

The only thing that saved Earth from a runaway greenhouse is, ironically, the presence of greenhouse gases in its atmosphere.

Modeling an icebox

While there has been lots of talk of habitable-zone exoplanets, their location in that zone doesn't tell us much about whether they are actually habitable. Habitability is characterized by having the right temperature for liquid water to exist on the planet's surface. But that temperature isn't just set by the planet's distance to its host star. Instead, it depends on things like how much light its surface reflects into space, details on its clouds, and how much of its atmosphere is composed of greenhouse gases.

We do have tools for trying to understand these things: they're called climate models. Unfortunately, most of those were set up to look at Earth, and they don't allow researchers to just plug in any arbitrary values for things like the Earth's gravity. But with the explosion of exoplanet discoveries, that's starting to change. Here, the team of researchers were able to use a model called CAM 3.0 (you can download the CAM 3.1 source code if you want to have a look). CAM allowed them to try out a variety of conditions, and researchers had used it successfully to model times in the Earth's past when its waters had all frozen over.

For the starting conditions of their model runs, the researchers created a watery world that was frozen solid without enough incoming light to melt it and an atmosphere free of greenhouse gases. These conditions equilibrated to an icy world with a furrow around the equator. This happened because there was enough light there to sublimate the ice into a gas, which then re-condensed closer to the poles.

The team then gradually ratcheted up the light arriving from the planet's star, trying out different star types—smaller, redder stars produce light that's more readily absorbed by ice, which made a difference. But in all cases, the world stayed frozen until there was more incoming light than the Earth receives. That's because the ice itself is so effective at reflecting light back to space that increasing its intensity didn't make a lot of difference.

When the ice finally started to melt, however, things progressed rather quickly. The reflectivity of the planet's surface dropped dramatically, enhancing the warming. This put lots of water vapor in the atmosphere, and that potent greenhouse gas warmed the planet even further. For most cases, this put the planets in what's called a "moist greenhouse." Here, water vapor reaches the upper atmosphere, where the star's higher energy light splits the water into oxygen and hydrogen, which can then escape into space. The planet would lose all of its water, though it might take more than a billion years to do so.

Smaller planets make things worse, as there's less gravity to hold onto its atmosphere. But for certain stars (G- and F-classes), it doesn't matter what size the planet is. The model shows they would all go to a complete runaway greenhouse, reaching surface temperatures of more than 1,000 degrees Celsius and boiling off all of their surface water.

Thinking locally

So why didn't this happen to Earth? After all, it went through several global glaciations, periods of time known as "snowball Earths." The key factor turns out to be carbon dioxide. Its role in warming the planet means that the snowball would start melting with lower levels of incoming sunlight. It also keeps a lot of the Earth's warmth in the lower atmosphere, which cools the altitudes where water vapor would be lost to space.

On the Earth, the loss of ice would allow some warming due to the feedback, but the newly exposed rock would undergo weathering, reacting with the carbon dioxide in the atmosphere and pulling it back out. This has the overall effect of moderating the warming before any form of greenhouse climate results.

Eventually, of course, the Sun will age into a red giant, at which point the Earth will lose all its water. But if you think that might be a great opportunity to look for beachfront property on one of the watery bodies of the outer Solar System, the authors would suggest you think again. Because of their low gravity, they'd quickly lose all their water: "Europa and Enceladus will have no habitable period. They will transit to a moist or runaway greenhouse state when the Sun becomes a red giant in 6–7 billion years, at which time the stellar flux at the location of Europa will reach the snowball-melting threshold."

This obviously has consequences for habitability elsewhere, too. For exoplanets with low greenhouse gas concentrations, entering a snowball state would mean staying there, even at levels of starlight that keep the Earth at moderate temperatures. And in most cases, exiting again would mean there would only be a narrow window of time (in astronomical terms, at least) before they lost all that water.

Nature Geoscience, 2017. DOI: 10.1038/NGEO2994 (About DOIs).