Last year, astronomers discovered an Earth-like planet orbiting the star nearest to our own, Proxima Centauri. The planet, called Proxima b, is about the same size as the Earth and orbits inside its star's "habitable zone," the region where liquid water can exist. Many were excited about the prospect of a potentially habitable planet next door to our own, and there are even plans to send a probe to visit it in the near future. But a closer look reveals that the planet might not be habitable after all.

The habitable zone of any star is the region where it's neither too hot nor too cold for liquid water to exist. If you have a planet thats too close to a star, like Venus in our system, it overheats. Too far, and you wind up as an icy world like Mars. Proxima b sits right in the middle of its star's habitable zone, which means it is—or rather, was—a prime candidate to search for life.

But liquid water isn't the only ingredient for life, and the habitable zone might be smaller than we thought. A team of NASA scientists looked at what happens to planets that orbit small red dwarf stars like Proxima Centauri, and found that intense stellar flares and coronal mass ejections (CMEs) might shrink or eliminate the habitable zone.

This solar weather was once thought to be the exclusive province of larger stars like ours, but observations of Proxima Centauri a few months ago revealed that ejections of hot plasma occur on small red dwarfs too. Because Proxima Centauri is much smaller than our sun, Proxima b and the habitable zone are much closer to the star, which makes these solar storms even more dangerous to any possible life on the planet.

Unless Proxima b has a very strong magnetic field, the intense solar flares and CMEs will blast away much of the planet's atmosphere. Many crucial ingredients for life like hydrogen, oxygen, and nitrogen would be lost to space. Without these elements, it doesn't matter if the planet is within the habitable zone, because there wouldn't be any way to make or retain water.

Importantly, the severity of solar storms is dependent on the age of the star. Stars are violent when they're young but tend to calm down as they age. If a planet forms later in the star's life cycle, it's probably safe. But if a planet forms early, it could lose too much of its atmosphere to ever support life.

All of this is bad news for Proxima b. Its star, Proxima Centauri, is still very young, which means the planet is constantly bombarded by severe solar weather. The NASA scientists conclude that the atmosphere on Proxima b is disappearing fast—if it isn't gone already—and that it's highly unlikely that it supports any kind of life.

Source: NASA

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