Planet Profile: Proxima Centauri b

The closest star to our Sun is called Proxima Centauri. It was discovered over 100 years ago by the Scottish astronomer Robert Innes and is estimated to be only 4.25 lightyears away. Getting a feel for the kinds of distances we’re talking about in astronomy is difficult, so let’s use a mental yardstick to help.

The Voyager 1 space mission launched in 1977. Today, more than 43 years later, the probe has left the Solar System behind and is speeding through interstellar space. Voyager 1 has travelled over 14 million miles–it is the most distant man-made object, the furthest incursion of humanity into space. In that time, Voyager has travelled less than one light-day. If it continues at its present rate, it would take Voyager about 67,000 years to reach Proxima Centauri. (67,000 years ago, Neanderthals were still around. It would take another 20,000 years before humans started creating the famous cave paintings and Venus figurines of the Palaeolithic). And that’s our closest neighbour in the entire universe.

Although Proxima Centauri was first observed in 1915, it took until 2016 for astronomers to discover the star has a planet, Proxima Centauri b. In 2020, researchers found a probable second planet, known as Proxima Centauri c. There may still be others, their presence too faint to yet detect. (If you’re wondering about the naming scheme, it’s standard for the first planet discovered around a star to be called by the star’s name followed by “b.” Subsequent planets are labelled c, d, e, and so on.)

Not only does our closest neighbour have a planet, Proxima Centauri b is inside what is called the “habitable zone” of its host star. This means it orbits at the right distance to have temperatures that would allow water to be liquid on its surface. It doesn’t mean the planet actually has water, much less life–only one basic physical requirement for Earth-like life. Still, finding a temperate planet right next door is a tantalizing promise that such “Earth-like” planets may not be rare.

What is it like on Proxima Centauri b? What can we know about an object so small and distant? Our only source of information is light from the planet or its host star–brighter by a factor of a billion, but still too faint to be seen by the naked eye. Right now, all we know is Proxima Centauri b’s size, the length of its year, and how far it is from its star. The planet is about 1.3 times the mass of the Earth and very close to the star, completing a full orbit in a little over 11 days. Despite being so close in, Proxima Centauri b can still have water-friendly temperatures because Proxima Centauri is much cooler than the Sun.

That’s not a lot of data so far, but amazing astronomical techniques will soon allow us to distinguish light reflected or emitted by the planet’s atmosphere and even take a photo of it (“direct imaging” of exoplanets, as it’s called, is truly cutting-edge science!). Once we have light from the planet itself, we’ll be able to tell what elements make up its atmosphere. Will it have water vapour, methane, carbon dioxide? What about oxygen?

Although telescope observations will be able to tell us about Proxima Centauri b’s atmospheric chemistry, that won’t be enough to give us a picture of what the planet’s surface is like. For that we need models: computer software that can simulate weather and climate using basic physical equations and some input data or assumptions. These programs, called numerical weather modelling software, are used to generate weather forecasts and long-term climate predictions. Several research teams around the world have updated weather models to predict what the climate might be like on an exoplanet, given some information about its size, the amount of energy it receives from its star, and its atmospheric composition.

I use one of these weather models, the Met Office’s Unified Model, to study what the climate and atmosphere of Proxima Centauri b could be like. The planet might have Earth-like temperatures, but in other respects it is radically different from our own world.

Perhaps the most striking feature of Proxima Centauri b, and many of the exoplanets astronomers will observe in the coming decades, is that it is probably tidally locked. Because its orbit is so close-in, the gravitational pull of the host star is disproportionately stronger on the side facing inwards. This exerts a drag that slows down the planet’s spin over time. Eventually, the planet’s rotation synchronizes with its revolution around the star, so that one side always faces the star and the other always faces away–instead of days and nights, there is a permanent “dayside” and “nightside.” In our own Solar System, the Moon is tidally locked to the Earth.

For a long time, this was thought to make tidally locked planets de facto uninhabitable, since the dayside would be unbearably hot and the nightside permanently frozen. Only a thin transition zone between the two faces would be mild enough to support liquid water. This zone, known as the “terminator” (on Earth as well as on exoplanets), would be in perpetual twilight, without days, nights, seasons, or years.

One of the key findings of weather model studies of exoplanets is that tidally locked exoplanets may not have such harsh climates after all. Multiple models have predicted that these worlds develop high winds in the upper atmosphere. This happens because the star’s light heats up the air on the dayside, causing it to rise and flow towards the cold nightside. The winds redistribute heat and reduce the temperature contrast between the two sides.

If the planet does have water, the presence of clouds in the atmosphere or ice at the poles could reflect light from the dayside, cooling it to more comfortable temperatures. Greenhouse gases in the atmosphere, such as water vapour, carbon dioxide, and methane, would also affect the temperature. These elements are common and are found on Solar System worlds like Venus, Titan, and Mars, so it’s no stretch to assume that many exoplanets will have them in at least some amount.

Because of the intricate interplay between the chemistry of the atmosphere and the surface, the motions of the winds, the magnetic fields of the star and planet, and other things we may not even know about, the environment on a given planet can be very sensitive to small changes in these factors. So what’s our best guess about the weather on Proxima Centauri b? The images below are predictions by the model of Proxima Centauri b I use in my research.

There is a “hot spot” at the equator on the dayside. “Hot” is relative, since the model predicts peak temperatures of around 15 to 30 degrees Celsius. If the atmosphere has high levels of greenhouse gases, this could be higher. If the planet has a liquid ocean, clouds form above the hot spot. The sunlight evaporates water from the ocean. The water vapour rises, condenses into clouds, and eventually rains back down, all within the hot spot. Temperatures drop the further you go towards the terminator. In the twilight zone between the dayside and the nightside, there is a ring of snowfall.

The nightside has temperatures comparable to Antarctica at the surface but is much, much drier than even the driest deserts on Earth. The nightside atmosphere higher up is actually warmer than the surface because warm air flows over from the dayside–a thermal inversion. Two vortices called “cold traps” form on the dark side of the planet. These are the coldest areas of the planet, a frozen doldrums. Circling winds could trap air and compounds like ozone that have been transported from the dayside.

These features are general and should apply to any tidally locked Earth-sized planet with a surface covered by water. But what about planets with a mostly dry surface?  What if there are volcanoes and tectonic activity, which can have a huge effect on the climate? What if there are not only clouds of water vapour, but other types of clouds or particles in the atmosphere that can absorb or reflect different types of light? Exoplanet modelling is just getting started, and we have so little data to use as inputs that all these findings are only very rough guesses. The bright side is that the coming decades of exoplanet research are bound to be full of wonderful, exotic surprises–worlds beyond what we can currently imagine.