We’ve discovered an Earth-like planet—will it host life?

Sometimes the corny wisdom that you can search far and wide for what’s right in front of you all along turns out to be true. For astronomers studying extrasolar planets—those around other stars, of which more than 3,500 are now known—the holy grail is an Earth-like world: small, rocky, and about the right distance from its star for liquid water to exist on the surface.

Now they have found one as near as could be. True, it is still a little more than four light years away—that’s around 25 trillion miles—but it is orbiting our Sun’s nearest neighbouring star, Proxima Centauri, in the constellation of Centaurus.

For one thing, the discovery drives home the message now abundantly clear from studies of extrasolar planets: terrestrial-like planets must be pretty commonplace throughout the cosmos. It’s now believed, on the basis of planet surveys to date, that around one in every five Sun-like stars has a planet of similar size and composition to Earth within their so-called habitable zone, where the temperature is right for liquid water to exist.

What’s more exciting is that it is not absurd to imagine sending a spacecraft (unmanned, of course) to Proxima Centauri to take a closer look. Indeed, earlier this year billionaire entrepreneur Yuri Milner announced a $100m project called Starshot to do just that. He believes a probe could potentially make the journey in just 20 years, although it would take any information sent back at the speed of light a further four and a bit years to reach us. The new discovery is sure to lend a huge boost to proposals of that kind.

Proxima Centauri is actually not a very Sun-like star at all: it is a red dwarf, which is smaller and cooler, with just 12 per cent of the Sun’s mass and only 0.15 per cent of its brightness. All the same, it has a habitable zone where planets could support liquid water, although this is correspondingly closer to the star. That’s where the new planet has been spotted. The planet’s mass is estimated to be 1.3 times that of Earth, and it orbits the star once every 11 (terrestrial) days or so.

In some ways the discovery is not so surprising: based on the numbers so far, it was expected that the nearest Earth-like extrasolar planet would be only around 12 light years away. All the same, the news has been hard to keep under wraps, and rumours have abounded in the past few weeks. The paper, published in Nature, now gives full details. The work was carried out by an international team, led by astronomers at Queen Mary University of London, using instruments at the European Southern Observatory in Chile.

For dimmer, dwarf stars like Proxima Centauri, orbiting planets can be detected by the wobble that their gravity induces in the parent star. This wobble can be seen as a Doppler shift in the wavelength of light emitted at highly specific wavelengths from atoms of particular elements in the star’s atmosphere. In effect, this light gets a little bluer and redder as the planet circulates, which also reveals the duration of the orbit. And from the orbital period, it’s possible to work out the planet’s distance from the star.

If there’s water, could life exist there? Some scientists believe that, with liquid water and a blend of Earth-like ingredients at a planet’s surface, life will be almost inevitable. Others say that there’s much more to it: things like seasons (caused by the tilt of the Earth’s rotation axis) and a magnetic field to shield a planet from high-energy particles in a solar wind are essential too.

Proxima Centauri’s planet, called Proxima b, might have a particular need for a magnetic field—which on Earth is produced by the churning of molten iron in the core. This type of star is more “active” than our Sun, being prone to powerful solar flares which could shower a planet this close with intense bursts of particles and radiation. Because of such flares, it’s estimated that the planet will be exposed to 400 times the dose of X-rays that the Earth receives from the Sun. So life would face a very harsh environment.

More clues would come from studying the planet’s atmosphere to figure out what chemical compounds it contains. That’s challenging, but possible. If the planet happens to pass in front of the star from the perspective of the Earth, we might see which wavelengths of starlight get absorbed around the planet’s fringes and deduce from that what’s in the atmosphere. But the chances of such a transit are very low. Alternatively, for a planet this close we might hope to glimpse the light it reflects or radiates directly.

While it’s probably wise not to hold out too much hope for the discovery of Proximan civilizations just yet, there can be little question that we’re living through a revolution in understanding of our place in the universe comparable to the discoveries of other galaxies and of the expanding universe itself. In some respects, our world no longer looks quite so special. But are we?