Picture the scene, populations rising, destruction of ecosystems and rampant global warming have made the world barren and uninhabitable. It’s the stuff of classic dystopian fiction but, given the state of the world, not outside the realms of possibility. Once this world is used up what happens next, perhaps we might look to the stars, but which ones?
Many worlds and systems have been considered, but what criteria do scientists use to classify habitable planets and what do they look for?
Well, it’s all in the water. Liquid water is considered to be a major prerequisite for habitability and in order to have this the planet must lie in the so called “Goldilocks Zone” (yes, that Goldilocks) where the planet’s surface is the right temperature to play host to liquid water without it either evaporating or being frozen. The exact size of this zone depends on the size and nature of the star which the planet orbits.
The planet must also be “rocky”, i.e. have a solid surface as opposed to “gas giants” such as Jupiter. Neptune, a planet somewhere between the two extremes, has a radius of 1.6 times that of the earth, so many astronomers consider that a radius with less that this should be “rocky”.
The star the planet orbits should be “spectral”, generally meaning mid–range stars like our own sun, who give off a sufficient amount of ultraviolet energy but have a suitably long lifespan for planets to develop. They should also show little variability in the amount of energy they give out.
Ideal candidates will also have a weight and atmosphere roughly similar to that of the earth. They should also have a small amount of “Axial tilt”, the reason we experience seasons. And a stable orbit helps to prevent large temperature fluctuations which may make the planet uninhabitable.
Tidal Locked planets (where one side of the planet constantly faces the star) are largely considered uninhabitable as one side is in constant daylight, the other in perpetual night, meaning one side is much hotter than the other.
So now we know what we’re looking for what are some potential candidates?
We first looked to our neighbour Mars. The red planet has played host many Sci-Fi populations from HG Wells War of the Worlds to Doctor Who’s villainous Ice Warriors. But what is the likelihood of life surviving on mars, or of possible colonisation?
In reality the low atmospheric pressure (less than 1% of the earths) means that the presence of liquid water is extremely unlikely. However, the two poles appear to be capped with permanant ice which may have large bodies of water trapped underneath them. Photos taken of mars have shown landscape features, such as valleys and canyons, which strongly suggest there was once liquid water.
Many unmanned missions have been sent to mars and the possibility of a human mission is one which has been discussed, however it is unlikely that this will occur until at least the late 2020s, and the recent failure of the Schiaparelli Lander reminds us of the potential dangers inherent in space travel.
Even if we can get people to mars it will be a struggle to colonise, mars spends a large amount of its orbit outside of the suns habitable zone and its thin atmosphere leads to continuous bombardment by solar winds
Perhaps we might look a little farther afield. A mere 4 lightyears from us is our nearest star Proxima Centauri, orbiting which is a little rocky planet called Proxima b. This planet had previously been discounted as Proxima Centauri is a dwarf star, consequently Proxima b is much closer, experience strong solar wind pressure and is very likely tidal locked.
However recently data has caused scientist to re-evaluate their assumptions. Data shows that Proxima b has mass 1.3 times that of the earth and a radius of 0.95 – 1.4 times the earth. Also if, and it’s a big if, there is sufficient atmosphere this may protect the planet and disperse the uneven heat, negating some of the effects of tidal lock.
Given what we know it may just be possible for Proxima b to be habitable, either as a vast ocean or as a dense metal planet surrounded by a silicate mantle (similar to our own mercury) or anywhere in-between.
In any case we still have a lot still to discover about our neighbouring star, In particular measuring stellar abundance of heavy metals will allow us to narrow down the possible compositions of Proxima b, it’s still too early to determine if the planet is habitable, however we could very well learn with new search techniques, such as the launch of the James Webb telescope next year.
Looking further afield we turn to NASA’s Kepler mission, a space observatory sent up in 2008 orbiting the sun, searching for exoplanets in the milky way. As of May 2016 Kepler has been responsible for the discovery of 1284 new exoplanets. A 2015 review highlighted the three most likely habitable exoplanets, Kepler 186f, 442b and 62f.
Kepler 186f is the closest to the earth, 500 lightyears away, it is a likely rocky planet, with a radius 1.2 times that of the earth. It is located just inside the outer limit of the Goldilocks zone so receive less solar energy than the earth but enough for there to be a possibility of water.
Another good candidate may be Kepler 62f. A radius 1.4 times that of the earth puts it at good odds of being rocky and it appears to lie within the habitable zone.
Kepler 442b is probably the best candidate exoplanet, it has a radius 1.3 times that of the earth and lies comfortably within estimated of its stars’ habitable zone. However, it is further out, a whopping 1300 lightyears.
But what does all of this actually mean?
There are a staggeringly large number of exoplanets and we have only scratched the surface of discovery, we already have a number of potentially habitable candidates and many more probably wait to be discovered.
However even for those we have highlighted we do not know nearly enough to say for certain whether they may be habitable, we know very little about thing like atmospheric composition which is crucial for maintenance of life.
And even if these planets did turn out to be habitable the current realities of space travel make it very unlikely that a manned mission will go farther than mars in our lifetimes, the, admittedly intriguing, prospect of starting an earth 2.0 far out in the galaxies is, for now at least, very low.