Rocky exoplanet found in habitable zone of star next door

Extraterrestrial life may be as close as our nearest neighbouring star. Proxima Centauri, the star closest to the sun, seems to possess a rocky planet in its orbit – one that sits squarely in the star’s temperate zone.

The planet, dubbed Proxima b, is 1.3 times the mass of Earth and orbits its star at a distance such that liquid water – and perhaps life – could exist on its surface.

The discovery was announced in Nature. 

Proxima Centauri has been well studied since being discovered more than a century ago. Despite being only 4.25 light-years from Earth, though, it’s too faint to see with the naked eye. This is because it’s a red dwarf – a small, dim, low-mass star.

Observations from 2013 provided hints that the tiny star hosted a planet. The problem was detecting it. 

Exoplanets are generally too small to see with a telescope  – especially small, rocky planets that orbit close to their bright star – so astronomers measure a star’s Doppler “wobble”. 

An orbiting planet exerts a little gravitational tug on its star, which expands or contracts starlight wavelength – not by much, but it can be enough for telescopes on Earth to detect.

In the past few years, the High Accuracy Radial velocity Planet Searcher, or HARPS, at the European Southern Observatory’s La Silla site in Chile found Doppler clues that Proxima Centauri might host a planet.

But there wasn’t quite enough evidence to say for sure – until this year.

Guillem Anglada-Escudé from the Queen Mary University of London in the UK and colleagues trained the HARPS instrument onto Proxima Centauri almost every night between 19 January and 31 March.

At the same time, they observed the star with other telescopes in Chile to make sure any regular wobbles weren’t the result of stellar outbursts such as flares, which red dwarfs are prone to do.

Anglada-Escudé checked the measurements daily: “The first 10 were promising, the first 20 were consistent with expectations, and at 30 days the result was pretty much definitive, so we started drafting the paper!”

The timing and magnitude of Proxima Centauri’s wobbles showed an exoplanet – Proxima b – whizzed around once every 11.2 days. Anglada-Escudé and colleagues pegged the exoplanet’s mass at 1.3 Earth masses, and it orbited its star at a distance of only 0.05 astronomical units (one astronomical unit is the distance between the Earth and sun).

This very close relationship puts it “squarely in the centre of the classical habitable zone for Proxima”, the researchers write, because of the star’s relatively cool temperature.

So what’s Proxima b like? It’s impossible to tell just yet, but the researchers write that how it came to settle in its orbit has a lot to do with the answer.

It may have formed further away from the star then migrated inwards, or planet embryos or pebbles in that orbit may have coalesced to produce the exoplanet there.

Pebble formation, they write, would result in a much drier world than migration or embryo fusion.

Hopefully, the planet’s orbit takes us between Proxima Centauri and Earth, so we can see how the starlight filters through its atmosphere and deduce its composition.

The next generation of telescopes, such as the Extremely Large Telescope, will also provide more clues.

In a News and Views article, Artie Hatzes from Germany’s Thuringian State Observatory writes that such red dwarfs are the most abundant stars in the Milky Way galaxy: “If only a small fraction of [red] dwarfs have temperate-zone planets, our galaxy could be teeming with life.”