This week researchers from Queen Mary University of London announced the discovery of Proxima b, a nearby Earth-like exoplanet that may be able to sustain life.
Proxima b is certainly not the first of its kind, however. We take a look at some of the celestial candidates that could be called home by some lifeforms out there somewhere.
What makes a planet habitable?
There are a number of different conditions that determine if a planet could potentially sustain life.
Planets have to be at an ideal distance from their host star for liquid water to exist on the surface, have the correct conditions that ensure radiation levels are not damaging, and the correct biochemical conditions to allow life to form, replicate and become self-sustaining.
The correct distance that allows planets to sustain these factors is called the habitable zone, or sometimes the Goldilocks zone, as the conditions have to be just right for life to flourish.
1 Gliese 667 Cc
Gliese 667Cc is one of several exoplanets orbiting within the habitable zone of red dwarf star Gliese 667 C, around 23 lightyears away. Although the most Earth-like exoplanet in the Gliese solar system, Gliese 667Cc is larger and more massive than Earth, and also is subject to increased tidal heating.
The heating of the surface of Gliese 667Cc, due to orbital and rotational energy of the exoplanet, is around 300 times greater than that on Earth, so chances of it being habitable are not too promising.
2 Kepler 186-f
Residing in the habitable zone of star Kepler 186 around 560 lightyears away, Kepler 186-f has a radius similar to that of the Earth, although it rotates a lot more slowly than we do.
However, researchers are unable to determine its atmospheric composition – an essential factor in determining if life could survive on any planet.
Another likely candidate for supporting life, Kepler-442b orbits star Kepler-442 around 1,120 lightyears away, and, in terms of temperature and size, has previously been described as being one of the most Earth-like exoplanets.
Again, being so far away, researchers are yet to determine if it has an atmosphere.
Sometimes referred to as Coruscant after the fictional and heavily populated planet in Star Wars, Kepler-452b orbits the Sun-like star Kepler-452 around 1,400 lightyears away.
Although it is unconfirmed whether this exoplanet has a rocky surface suitable for sustaining life, it is twice the size of Earth, and receives a lot more energy from its host star than we do from the Sun.
5 Wolf 1061c
Before the announcement of Proxima b, Wolf 1061c was considered the closest exoplanet that could potentially sustain life.
Only 13.8 lightyears away, Wolf 1061c orbits red dwarf star Wolf 1061 and has an equilibrium temperature of around -50 °C, slightly higher than that of Mars.
This exoplanet orbits star Kepler-62 around 1,200 lightyears away, and although it lies towards the edge of its host stars habitable zone, is likely to be an ocean-covered planet.
Researchers at the University of California, Los Angeles, have tested several scenarios that would allow life to exist on the planet, some involving an atmosphere 12 times thicker and with carbon dioxide levels 2,500 times greater than that on Earth.
7 Tau Ceti e and f
These two unconfirmed exoplanets orbit the star Tau Ceti, a nearby star under 12 lightyears away and with characteristics very similar to our Sun.
Although often listed as a target for missions looking to search for extraterrestrial life, little is known about these exoplanets other than their orbit and mass, although calculations place both Tau Ceti e and f in the habitable zone of their host star.
So what about Proxima b?
Proxima b orbits the red dwarf star Proxima Centauri, the closest star to the Earth, apart from the Sun. A year on the planet only lasts 11 Earth days, but the planet does have temperature suitable for liquid water to exist and is similar in mass to Earth too.
Only being four lightyears away makes it the closest candidate for hosting alien life, but it will be a long while yet until we know for certain.
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