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Exoplanet LHS 1140b may be most habitable yet found

A “rocky super-Earth” orbiting a cool nearby star may hold conditions suitable for life, writes Andrew Masterson.

An artist’s impression of “rocky super-Earth” LHS 1140b and its red dwarf host star.
M. Weiss/CfA

Until this week, if you asked an astrobiologist to nominate the likeliest targets for life beyond our solar system the most common answers would have been Proxima b or the recently discovered TRAPPIST-1 system.

Now, however, for at least some of the astrobiological gambling community, the odds have changed, perhaps dramatically, following the announcement of a newly discovered exoplanet orbiting a red dwarf star some 39 light years from Earth.

News of the planet, described as a “rocky super-earth” and dubbed LHS 1140b, is revealed in Nature, and is the result of research by a large team led by Jason Dittmann of the Harvard Smithsonian Centre for Astrophysics in Massachusetts, US.

This chart shows the location of LHS 1140 in the constellation of Cetus (The Sea Monster).

The first hint of the planet’s existence was gathered by Harvard’s dedicated red dwarf survey, MEarth. Further observations were then made by the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS), from which, among other things, astronomers were able to calculate LHS 1140b’s size, density and orbital period, as well as take an informed stab at its age.

LHS 1140b is 1.4 times the size of Earth, with a diameter of approximately 18,000 kilometres. Its mass, however, is almost seven times that of our own planet, leading to the assumption that it likely comprises rock encasing a solid iron core.

Its density is one of the main reasons for considering the planet a possible location for extraterrestrial life. Dittman’s team estimates its age at around five billion years, which means it long ago survived a major drawback associated with its host sun – and, thanks to its hefty mass, may have done so rather well.

Many astronomers consider planets orbiting red dwarf stars to be excellent candidates for extraterrestrial life – more so than planets orbiting stars similar to the Sun – in part because red dwarfs are by far the most numerous type around.

In the Milky Way, for instance, 50 of the 60 stars closest to our own solar system are red dwarfs. Some estimates suggest they account for three-quarters of the stars in the galaxy.

Because red dwarfs are dimmer and cooler than Sun-type stars, however, their planetary habitable zone is much closer in. In their early lives, their energy outputs vary wildly, with modelling indicating massive solar storms capable of generating runaway greenhouse effects on nearby planets, burning off atmospheres and boiling away any liquid water.

LHS 1140b’s dense metal core, however, might mean that it was covered by an ocean of magma during its host star’s crazy youthful period. After the star settled into a stable middle age, the magma would have continued to generate vast amounts of steam, potentially creating a stable atmosphere and, through precipitation, bodies of liquid water.

“This is the most exciting exoplanet I’ve seen in the past decade,” says Dittmann. “We could hardly hope for a better target to perform one of the biggest quests in science – searching for evidence of life beyond Earth.”

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Andrew Masterson is news editor of Cosmos.
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