[credit: NASA/JPL-Caltech]

Close proximity to red dwarf stars could scupper life’s chances

The search for life beyond the Earth is one of the driving factors behind modern astronomy. For a long time a debate has raged about the habitability of planets around red dwarf stars. This latest study adds fuel to those flames. Such stars are by far the most common type of star in the universe and so, if they can support living planets, they would significantly boost the amount of habitable real estate in our Milky Way.

[credit: NASA/JPL-Caltech]
[credit: NASA/JPL-Caltech]

In the traditional picture, a planet is ‘habitable’ if it orbits at the right distance from its star. Not too close that water boils away, nor too far that it freezes. For red dwarf stars, which are smaller and cooler that our Sun, planets have to snuggle much closer in to keep warm. From an astronomer’s point of view this is a bonus because planet’s orbiting close to small stars are easier to detect.

Alien planets are normally found by one of two methods. The first – the ‘transit’ method – is when a planet passes in front of its star from our perspective, causing the star’s light to dim slightly. Because red dwarf stars are smaller, planets can block out a larger proportion of their light, making such transits more obvious. The second technique is known as the ‘radial velocity’ method, which is where we can detect changes in the star’s light as it wobbles around under the gravitational pull of its planet. Smaller stars are perturbed more easily by their planets, again making their presence more obvious.

Now, new computer simulations created at the University of Washington at Seattle have shown that such alien planets are likely to have their water boiled away from their earliest days – the result of intense heat emanating from their newly formed parent star at fairly close quarters.

The process could also throw some red herring worlds our way. Red dwarf stars are thought to be intense emitters of X-rays and ultraviolet light. This energy can split up the water in a planet’s atmosphere into hydrogen and oxygen. As hydrogen is the lightest element it can easily evade the planet’s gravitational clutches and escape to space. Heavier oxygen, however, it likely to stay put, leaving a world abundant in oxygen but bereft of water. Future telescopes could look into the atmospheres of such worlds and spot a lot of oxygen and perhaps be falsely excited by the prospect of life.

The team behind these simulations call such worlds ‘mirage Earths’ – those offering merely the illusion of habitability.

A pre-print of the research is available on the arXiv server.