We've discovered plenty of planets in the habitable zone of other stars, but are any of them actually habitable? Despite the seemingly definitive terminology, "habitable zone" is simply defined as the amount of energy incoming from a planet's star. How that energy interacts with the planet's surface and atmosphere to set a temperature requires us to understand the planet's composition, as well as the details of the light produced by a star.

Fortunately, we've built tools that can tell us how radiation, an atmosphere, and a planet's surface interact. They're called climate models. Unfortunately, most climate models have hardcoded Earth-like conditions, which make them useless for studying other planets. But a little while back, NASA researchers adapted one of the agency's climate models to work with conditions at Proxima Centauri b, a planet orbiting our Sun's nearest neighbor. Now, they've released a set of animations showing how slight changes in our assumptions about the planet can radically change the conditions at its surface.

This isn’t the Earth

Climate models generate their output based on the physics of the atmosphere and its interaction with the energy provided by our Sun. But they do have a lot of assumptions built into them. For example, the amount of radiation sent our way by the Sun only changes within pretty narrow limits, and it comes in a fairly well-defined range of wavelengths. The composition of the atmosphere, with the exception of some greenhouse gases, isn't changing much. Land and ocean areas don't change over timescales that are relevant to the ones the models are examining.

As a result, many of these assumptions have been baked into the code of climate models. But nearly none of them applies to other planets. The surface of an exoplanet could range from nothing but reflective rock to a water world that absorbs copious amounts of incoming light. Its atmosphere could be nearly entirely greenhouse gas, or it could be mostly transparent at key wavelengths. Its distance from the host star could be anywhere within the sphere of that star's gravitational influence. Separately, the star might not be Sun-like; the light it produces could be redder or bluer than what we receive on Earth.

To get a climate model to work with an exoplanet, you'd have to go through the model's code, identify every location where Earth-like assumptions are locked in place, and replace them with a value that you can provide separately. If that sounds like a huge hassle, consider that much of the code in climate models is decades old, in FORTRAN, and written by academics, many without a background in computer science.

Nevertheless, because of the value of the model that would result, NASA researchers have taken the agency's Model E2 Earth climate model and made all the necessary changes, producing the ROCKE-3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics) climate model. And with that ready, they turned their attention to Proxima Centauri b, a planet that's very much unlike Earth.

For starters, Proxima Centauri is a red dwarf, a cooler and (surprise!) redder star than our own Sun, meaning that any planet will receive its radiation in a different range of wavelengths. The planet, Proxima Centauri b (Prox b from here on out), however, gets a similar amount of energy from the star because it orbits much closer, at only about 5 percent of the Earth-Sun distance. That's close enough to ensure that the planet is tidally locked, having one side permanently facing the nearby star. And while we don't know Prox b's mass with any precision, it's very likely to be less than three times that of the Earth.

Checking out the options

While the process of getting the simulations to work was published in 2019, NASA has just gotten around to publishing animations that show the results of those simulations. And the results show how even a slight change of assumptions can lead to dramatic differences.

In the first simulation, the researchers assume a watery world without ocean circulation. The fact that Prox b is tidally locked means that the energy provided by the nearby star always reaches the same location on the planet. And the lack of ocean circulation means that that energy largely stays there. This produces a classic "eyeball earth," with most of the planet an icy white with a darker, near-circular patch of ocean existing on the side closest to the star—the pupil of the eyeball earth. Atmospheric circulation winds up focused over this patch of ocean.

Of course, a static ocean isn't an especially likely situation, so the team enabled that in the model. It made a dramatic difference.

Here, a strong equatorial current forms, transferring warm waters across the entire surface of the planet. This melts the overlying ice, creating a dark band of open water running around the equator. Meanwhile, a counter-flow creates "wings" extending in the opposite direction north and south of the equator. Again, atmospheric circulation is focused on the areas of open water, but this now means that winds are present across the entire planet.

On Earth and formerly on Mars, however, the planetary surface was a mix of land and ocean. There's absolutely no way to guess what land masses on another planet might look like. In fact, given plate tectonics, there's no way of knowing what the Earth's land masses would look like at any point in time without knowing things like its past configuration, active plate boundaries, and so on. So, in the absence of an obvious configuration to try, the researchers simply threw Earth's continents into the simulation.

Here, the researchers put the Pacific Ocean on the star-facing side of the planet. That creates an eyeball-earth-like situation, with most of the Pacific being free of Ice. But now, areas of Asia and North and South America emerge from the ice, as does most of Australia. But, oddly, so does Greenland and much of Africa. And, south of Australia, a large stretch of the Southern Ocean remains ice-free, presumably because it's the only uninterrupted stretch of ocean that circles the entire Earth. Because of all the complexity, atmospheric weather patterns now become nearly global, although some areas covered by ice sheets don't see much in the way of wind patterns.

Of course, there's no reason to favor the Pacific as the location that ends up under the star. So, just to try a different situation, the researchers placed Africa on the star-facing side of the planet.

Now, most of the oceans remain covered in ice, and the majority of Africa and Eurasia emerge from the ice, as do Greenland, South America, and parts of North America. Even more of the Southern Ocean is ice free. But a key difference from the earlier model is that wind patterns start being influenced by the land masses, most notably off the coast of South America.

It's important to note that you can get this much variation without changing the composition of the planet itself. By altering the greenhouse gas levels, it's also possible to radically change how much of the heat generated on the star-facing side gets transferred elsewhere on the planet, potentially causing more of it to emerge from the ice. At even higher levels, they'll set off a feedback with water vapor from the oceans, potentially pulling the entire planet out of its deep freeze.

In some ways, this is kind of depressing. If you can get such a dizzying variety of results just by tweaking a few major planetary features, then it'll be a long time before we're able to resolve enough on Prox b to really understand what conditions might be like there. But it has to be considered in light of the vast number of planets that exist in our galaxy alone. There are enough exoplanets that a lot of the combinations of conditions we could try out in our models could exist.