March 25, 2019

• We’ve found 4000 exoplanets but almost zero are right for life

  

  Kepler-452b is the only known exoplanet with the right stuff for life – and even that’s a maybeNASA Ames/JPL-Caltech/T. Pyle

  By Leah Crane

  We have found more than 4000 planets orbiting distant stars, but it turns out that probably none of them have the right conditions for life to evolve, making Earth even more special than we thought.

  We normally consider a planet capable of hosting life if its surface is the right temperature for liquid water. This depends on how close it orbits its star – too far and any water will freeze, but too close and it will boil away. The area around each star where these water-friendly orbits lie is called the habitable zone.

  But just being technically habitable does not mean that a world has the right conditions for life to arise. One leading explanation for life’s emergence on Earth is that ultraviolet light from the sun played a role. The idea is that UV light hitting simple molecules gave them enough energy to react with one another, forming the more complicated compounds required to make a living organism.

  In 2018, researchers introduced a concept called the abiogenesis zone: the area around a star where a planet could get enough UV light to start this prebiotic chemistry, but not so much that the planet would be sterilised.

  Read more: Life on nearest exoplanet may have been wiped out by superflare

  Because one is based on temperature and the other on the strength of particular wavelengths of light, the habitable zone and the abiogenesis zone around any star do not always overlap.

  Marcos Jusino-Maldonado and Abel Méndez at the University of Puerto Rico at Arecibo applied the requirements for the abiogenesis zone to a list of known exoplanets in the habitable zone, of which we have found 49. Only eight worlds matched the criteria.

  “Even these eight are bad because they have a large radius, which means they might not be rocky,” says Jusino-Maldonado, who presented the work this week at the Lunar and Planetary Science Conference in Texas.

  Bigger planets tend to accrete gas as they grow, becoming gas giants like Neptune or Jupiter instead of rocky worlds like Earth or Mars. Previous work has suggested that planets larger than 1.7 times Earth’s radius are likely to be gassy.

  “If the planet is too big, it’d be hard to think how life as we know it could evolve,” says Ramses Ramirez at the Tokyo Institute of Technology. Of the eight planets inside both zones, only one has a radius less than 1.7 times Earth’s: a planet called Kepler-452b, which orbits a sun-like star 1400 light years away. Its radius is 1.63 Earth radii, so it is right on the edge.

  This means that out of more than 4000 exoplanets, we may have only found one where life could evolve – or perhaps even none, if Kepler-452b turns out to be a gas planet. The only two planets we know for sure to be rocky and in both the habitable and abiogenesis zones are Earth and Mars. As far as we can tell, Mars doesn’t have any life.

  This shows just how difficult it is likely to be for life to arise. “If our goal is to find life, we need to be finding a lot more exoplanets than we can see with the technology we have now,” says Ramirez.

  Plus, there is no guarantee that a planet where life could arise will actually have anything living on it. “It’s getting harder to find origins of life,” says Jusino-Maldonado. “It seems very unlikely.”

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