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19 April 2017

The five best exoplanets in the galaxy to check for alien life

LHS 1140b
LHS 1140b may be right for life



Most of the worlds we have spotted so far are either blazing hellscapes or frozen tundras. But a few, like LHS 1140b, reside in the habitable zones around their stars – where they could theoretically retain liquid water – giving them the potential to host life as we know it.

Some potentially habitable planets are easier to examine than others, however. Some pass in front of their stars – called a transit – allowing us to measure their widths. This also lets us find out if they have an atmosphere, and even what makes up their atmospheres, by measuring the starlight that filters through them. Some are close enough to dream about sending a probe directly, or taking a photograph. Some seem to be too dense to have much of an atmosphere at all, or orbit stars whose volatile natures put any residents at risk of being fried.


With those criteria in mind, here are New Scientist’s five top planets beyond our solar system where we might be able to look for life:

LHS 1140b

This newcomer, pictured above, is already being hailed as the best place to look for signs of life beyond the solar system. It is a super-Earth, about 1.4 times as big as our planet and 6.6 times as massive, which means the planet is probably rocky, rather than gaseous.

Despite a close orbit around its star – it circles once every 25 days – the star’s cool nature means it receives about half as much light as Earth does, so is probably cool enough for liquid water. The star is a dim red dwarf, which are often prone to flares that could damage life on nearby planets. But LHS 1140 is unusually calm, so its planet is less at risk.

At 40 light years away, LHS 1140b is one of the closer potentially habitable exoplanets we have found. Since it is so close and passes between its star and us, it presents an excellent opportunity to look for an atmosphere distorting the star’s light as the planet passes by – and to suss out signs of life in that atmosphere.

Proxima b

Proxima b
Proxima b orbits in 11 days

ESO/M. Kornmesser

Proxima b is the nearest rocky exoplanet we have spotted, at only 4.2 light years from Earth. Its size hasn’t been pinned down, but it is almost definitely just a bit bigger than Earth.

It orbits its star, Proxima Centauri, about once every 11 days, putting it incredibly close. Even though Proxima Centauri is a relatively chilly star, explaining Proxima b’s habitable-zone status, such a tight orbit presents dangers to the exoplanet. It experiences a huge amount of X-ray radiation and stellar wind, and it might even be tidally locked, with the “daytime” side always facing the star – making it hot and bright – and the “nighttime” side always pointing towards the emptiness of space, making it cold and dark.

Another mark against Proxima b is the fact that it doesn’t transit in front of its star, so we will never be able to use its effect on the star’s light to look for an atmosphere. But if it does have an atmosphere, it could protect the planet from many of its star’s dangers and distribute heat more evenly around the surface.

The upside is that, cosmologically speaking, Proxima b is right next door. This could let us look for an atmosphere and signs of life there in other, more direct ways.

GJ 1132b

GJ 1132b
GJ 1132b has water

Dana Berry/MIT

We have already found an atmosphere, complete with water, around GJ 1132b, a planet about one and a half times as big as Earth. It only takes 38 hours to orbit its star, though, so it is extremely close and incredibly hot. This could make GJ 1132b more like Venus than Earth, with a sweltering haze of steam.

It does transit, so further observations will help researchers determine whether its atmosphere and water can make it cool enough to support life.


Three of TRAPPIST’s planets are in the habitable zone


The planets of the TRAPPIST-1 system, 40 light years away, have one stunning advantage: there are a bunch of them. All seven of the star’s tightly packed worlds are larger than Mars and less than 20 per cent bigger than Earth, and all orbit well within the distance at which Mercury circles our sun.

All seven of the planets were discovered when they passed between their star and us, so atmospheres around any or all of them could be detected that way, too. Three lie firmly in the habitable zone, and they are so close together that if one has life, it is possible that it could hitch a ride on a rock and spread to the other two.

TRAPPIST-1 is a small star of a type that is often active and dangerous for planets, though, emitting powerful flares of X-rays and UV radiation. Thick protective atmospheres could be its planets’ only hope for hosting life. Astronomers are already using the Hubble Space Telescope to look for such atmospheres.

Kepler 186f

Kepler 186f
Kepler 186f is chilly


At 500 light years away, it will be difficult to look for signs of an atmosphere or life on the planet Kepler 186f, even with the next generation of space telescopes. But it’s one of the most truly Earth-like exoplanets we have found. It’s only 10 per cent larger than Earth, although its mass isn’t well constrained, meaning we aren’t sure what it’s made of.

Like the other worlds on this list, it orbits a relatively dim star about half the size of our sun, but its year is a bit longer at 130 days. That suggests that its surface is about -85°C, or roughly as cold as Mars – chilly, but not too cold for certain microbes to survive.

And there’s another bonus: if Kepler 186f has an atmosphere and an internal magnetic field like Earth’s, they could interact with its star’s frequent flares to create spectacular aurora displays visible around the globe.

Journal reference: Nature, DOI: 10.1038/nature22055

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