Life at Alpha Centauri? Maybe, NASA says

Alpha Centauri is the closest star system to our sun. Chandra data from May 2, 2017 are shown in the inset, in context of a visible-light image of the Alpha Centauri system taken from the ground. Image via Optical: Zdenek Bardon; X-ray: NASA/CXC/Univ. of Colorado/T. Ayres et al.

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With the discovery of thousands of exoplanets orbiting other stars, the search for life elsewhere has entered an exciting new phase. So far, most of these worlds have been found many light-years away (largely due to the fact that the Kepler Space Telescope, which has discovered the majority of them so far, has focused on a specific patch of sky which contains very distant stars). But what about closer stars? Including, of course, Alpha Centauri, the closest star system to our sun, only just over four light-years away. According to Tom Ayres of the University of Colorado Boulder:

Because it is relatively close, the Alpha Centauri system is seen by many as the best candidate to explore for signs of life. The question is, will we find planets in an environment conducive to life as we know it?

Scientists had thought that there was too much X-ray radiation from the stars in the system for life on any planets to be likely. But now, as announced by NASA on June 6, 2018, there is new evidence from NASA’s Chandra X-ray Observatory, that, perhaps, conditions could be more life-friendly than previously assumed.

Artist’s concept of the exoplanet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the solar system. Image via ESO/M. Kornmesser.

While the other two stars, Alpha Centauri A and B, are both similar to our sun, Proxima Centauri is a red dwarf, which emits much more deadly X-ray radiation. That is bad news for its one known Earth-sized planet, Proxima b. However, observations from Chandra since 2005 show that conditions around the other two stars are about the same or even better than around our own sun. In terms of the radiation, the prospects for life are actually better for habitable zone planets around Alpha Centauri A than our own sun, with lower doses of X-rays than similar planets in our solar system, and only slightly worse around Alpha Centauri B, by a factor of five. As Ayres noted:

This is very good news for Alpha Cen AB in terms of the ability of possible life on any of their planets to survive radiation bouts from the stars. Chandra shows us that life should have a fighting chance on planets around either of these stars.

Artist’s concept of super-Earth exoplanet Kepler-22b, which may be habitable. It is not known yet if any other planets orbit the two larger sun-like stars in the Alpha Centauri system, but if there are, some may have conditions suitable for life. Image via NASA.

It is not known yet if there are any rocky planets orbiting Alpha Centauri A or B, but if so, then there is an increased chance of habitable conditions, although other factors come into play as well, such as temperature, liquid water or lack of it, composition of any atmosphere, etc. One problem with searching for planets there is that both stars are bright and currently closer together because of their orbits, making detection more difficult.

For Proxima b however, the situation is different. It receives an average dose of X-rays about 500 times greater than Earth, and up to 50,000 times stronger during a large solar flare. Not exactly ideal conditions for life.

Comparison of the three stars in the Alpha Centauri system and the sun. Image via PHL @ UPR Arecibo.

With so many exoplanets being discovered now, it is natural of course to wonder about the star system closest to us. Could life exist there? For the one planet known to exist there so far, the results are not encouraging. But if there are others, and most planetary systems appear to have more than one planet, then the odds are a bit more in life’s favor. We won’t know for sure until/if we find additional planets in the Alpha Centauri system. But even if we don’t, many other worlds are being discovered on a regular basis now, and a growing number appear to have conditions at least suitable for habitability, if not life itself.

The new results were presented at the 232rd meeting of the American Astronomical Society meeting in Denver, Colorado, and some results were published in January 2018 in the Research Notes of the American Astronomical Society.

Bottom line: The closest star system to our sun, Alpha Centauri, has at least one Earth-sized planet orbiting one of its three stars. Dangerous x-rays from that red dwarf star make life unlikely there, but the prospects may be much better for the other two sun-like stars, if any as-yet undiscovered planets orbit them.

Source: Alpha Centauri Beyond the Crossroads


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June 17, 2018

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