Exoplanet’s rotation speed may hold key to life

An artist’s impression of the surface of Proxima-B.
ESO / M. Kornmesser

How fast the exoplanet Proxima-B rotates on its axis could determine its climate and the possibility of it containing life.

The planet, discovered in August 2016, orbits the star Proxima Centauri, 4.2 light-years from the Sun and is thought to be the closest candidate beyond the Solar System for hosting extraterrestrial life.

A new study published in Astronomy and Astrophysics, using the complex weather and climate modeling tools that comprise the UK Met Office’s Unified Model, indicates Proxima-B’s atmospheric stability is affected by how often it rotates compared to how often it orbits its host star.

The research, led by Ian Boutle, found Proxima-B’s rotational speed – known as “resonance” – significantly affected the area of the planet’s surface that could sustain liquid water, considered to be a critical precursor for life to exist.

Previous studies have picked Proxima-B as a prime life candidate because the Earth-sized planet orbits its star within the “habitable zone”, a distance far enough from Proxima Centauri to prevent water vapour boiling away but close enough to stop it turning to ice.{%recommended 4683%} 

Whether the planet actually has water vapour is, of course, unconfirmed – a probe would have to travel more than 50 million kilometres to get close enough to take a cloud sample. 

The researchers therefore derived their conclusions from two possible atmospheric models – one similar to Earth’s, and another comprising just nitrogen with a small amount of carbon dioxide. They then used the Unified Model to run the numbers on a range of possible orbit-to-rotation variations – known as “eccentricities”.

One possibility considered was a “tidally locked” scenario, in which the rotational period matched the orbital period. The Moon is an excellent example here: its rotation of the Earth lasts exactly as long as its orbit, which is why we always only see one side of it.

Another modelled eccentricity is known as a 3:2 resonance. In this scenario Proxima-B would rotate three times during every two orbits around the star. This is similar to Mercury’s behaviour.

The researchers also factored in the fact that light from Proxima Centauri sits much more towards the infrared end of the spectrum than light from the Sun. 

“These frequencies of light interact much more strongly with water vapour and carbon dioxide in the atmosphere,” explains co-author James Manners, “which affects the climate that emerges in our model.” 

The results of the computer simulations show very stable atmospheres produced by both the tidally locked and 3:2 models, but with the latter resulting in much larger areas of the planet being potentially habitable.

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