The recent discovery of a planet around Proxima Centauri, the closest star to our own sun, created immense excitement. Not only was the new world, called Proxima Centauri b (Proxima b for short), conveniently close to us – only about four light-years away – it was roughly the mass of Earth and just the right distance from its host star. A “habitable planet”!
But don’t fire up the generation ships just yet.
In the study of alien worlds, there is perhaps no designation more hopeful, or more misleading, than “habitable”. While it evokes a vision of a pleasant, temperate world, complete with breathable air and a human-friendly landscape, to an astronomer it means none of those things. While we would certainly classify our own planet as habitable, the term could be applied to any of a wide range of lethal nightmare planets, and Proxima b might be one of them.
We don’t really know what a planet needs to harbour life. Worlds inhospitable to humanity could be teeming with a kind of life we can’t even understand, possibly more alien than the “extremophile” life forms that populate subglacial lakes and hydrothermal vents. All we know is that for the kind of life that exists on Earth, liquid water is a necessity – at least intermittently.
With current technology, we don’t have the capability to conclusively detect liquid water on the surface of any worlds outside our own solar system, so we have to work with the information we have – the temperature of the star and the distance of the planet’s orbit. A planet too close to its star might be so hot that water would immediately boil off. Too far away, and it’s a solid ice world. The habitable zone is the sweet spot, the Goldilocks zone, in which the amount of starlight reaching the planet is just enough to allow water to exist on the surface in liquid form.
But there are some caveats, and they’re big. Distance isn’t everything when it comes to the temperature on a planet’s surface. In our own solar system, both Venus and Mars are often considered to be in the habitable zone. However, Venus has such a suffocatingly thick atmosphere that it’s undergone a runaway greenhouse effect; its surface is a sweltering 460 °C. The present-day atmosphere of Mars is so thin that liquid water can only appear briefly in salty rivulets on crater slopes on the warmest days of the year.
Studying the atmosphere of exoplanets is difficult. So far we’ve only been able to examine a tiny number of atmospheres, and none belong to rocky worlds in the habitable zone. But a problematic atmosphere isn’t the only thing that can render a world uninhabitable. In many cases, we don’t know for certain if a planet has a surface at all. In the case of Proxima b, we can tell that it’s at least 1.3 times as massive as Earth – and no more than three times – but if it’s over two, it’s probably more like Neptune, forgoing any solid surface for a thick gas and liquid envelope over a small, deep, rocky core.
Another wild card for Proxima b is its host star. Proxima Centauri is a red dwarf, much cooler and smaller than our Sun. This means Proxima Centauri’s habitable zone lies very close to it – so close that the gravitational interaction between it and its planet is extreme enough that Proxima b is probably tidally locked. This means that the same side of the planet faces the star at all times, in the same way our moon always shows us its same face.
On such a world, rather than a temperate, circulating atmosphere, the day side might be boiling and the night side frozen. Even worse, Proxima Centauri is a flare star, meaning that it sends out giant flares of stellar material into space with alarming frequency. Even if Proxima b had a perfectly good atmosphere to begin with, it may have been stripped by its star’s unruly outbursts.
With future telescopes, some of them already under construction, we might soon be able to peer directly at Proxima b to analyse its atmosphere. In the meantime, we can only speculate and search for more habitable planets in the hope of learning more about our own origins. Perhaps someday we’ll find unmistakable signs of life on another world.