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Climate models go out of this world, Ars Technica

Climate models go out of this world, Ars Technica
    

      A Proxima-tions –

             

NASA scientists get a climate model to take on Proxima Centauri b.

      

      

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 in to 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

, NASA’s 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.

Prox b as a water world without ocean circulation. 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 while 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.

A water world with ocean circulation.
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.
Placing the Pacific Ocean on the side of the planet closest to the star.
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