Proxima b, The Closest Exoplanet To Earth, Could Be Habitable

Artist's impression of Proxima b. ESO/M. Kornmesser

The closest known exoplanet to Earth, Proxima b, could have the required conditions for liquid water and even life. That’s according to a new study that used a novel method to study conditions on this world.

Announced in August 2016, Proxima b is about 4.2 light-years from Earth in orbit around our closest star, Proxima Centauri. We believe it is a rocky world and, being so close to us, presents a fascinating opportunity for further examination.

This latest study, published in Astronomy and Astrophysics, was led by the University of Exeter. They applied a model from the Met Office in the UK, normally used to investigate Earth’s climate, to simulate conditions on Proxima b.

“With the right atmosphere, this planet is potentially habitable,” Dr Nathan Mayne, one of the co-authors on the paper, told IFLScience.

The model is called the Met Office Unified Model. Plugging values into a supercomputer at the University of Exeter over several months, they were able to work out the possibilities of Proxima b having liquid water.

At the moment, we don’t know a huge amount about the planet. We know its mass is at least 1.27 times that of Earth, with a radius of at least 1.1 times our planet's. It’s thought to orbit its star in about 11 Earth days, at a distance of just 5 percent Earth’s orbit around the Sun, but within its star's habitable zone.

Its star poses some intriguing problems, being an M-type red dwarf. There has been much speculation of late as to how habitable planets around these stars will be, as while they are dimmer and cooler than those like our Sun, they are prone to extreme flaring events.

Nonetheless, this latest study argues that there is a good chance Proxima b could be habitable. The biggest factor will be how eccentric its orbit around the star is. If it’s between 0.1 and 0.35 – with 0 being a circle and 1 being a parabola with no end – all signs point to the planet being able to host liquid water and having moderate temperatures suitable for life.

A previous artist's concept of the surface of Proxima b. ESO/M. Kornmesser

“We’ve expanded the range of potential configurations it could be habitable in,” Dr Mayne said. He noted this was also more of a thought experiment than concrete evidence of habitability, but it provides a good template for future observations.

However, as so little is known about Proxima b, the team had to make some assumptions in the model. For example, they assumed the entire planet was covered in water, although it likely might have land.

They also relied on it having one of two atmospheres. One was an Earth-like atmosphere, while another was more simple, being comprised of nitrogen and traces of carbon dioxide. We won’t know what the atmosphere of the planet is like for certain until we study the planet in more detail with upcoming observatories like the European Extremely Large Telescope (E-ELT) in 2024.

And they used two possible configurations for the motion of the planet too, one being tidally locked to the star with the same face always pointing towards it. The other was a 3:2 resonance, meaning it would rotate three times for every two orbits, similar to Mercury in our Solar System. The latter was found to have more regions of the planet being in a habitable temperature range.

And, excitingly, the team say this method can also be used to study other terrestrial exoplanets. It has been used before for gaseous worlds like hot Jupiters, gas giants in close orbits around their stars, but never before for rocky worlds. This means we can assess the habitability of other worlds if we know a bit of basic information about them.

Much more work will be needed to understand Proxima b. But as we get a better grasp on its atmosphere, this model might just help us work out what the conditions there are really like. And maybe at some point in the future via a project like Breakthrough Starshot, which wants to use a laser-powered sail to send a probe there in 20 years, we might find out for certain.

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