Scientists have long fantasised about finding life outside of our solar system. So it's no surprise then that Wednesday's historic announcement regarding the discovery of a planet - Proxima b - in orbit within the habitable zone of Proxima Centauri, the closest star to our sun, has thrown the search for new potentially habitable worlds outside of our solar system, into the spotlight.

Astronomers, using data from NASA's Kepler mission, have estimated that in the Milky Way alone, there could be as many as 40 billion Earth-sized, rocky planets orbiting within the habitable zones of stars – a range of orbits where the presence of liquid water on the surface is possible. Around 11 billion of those could be orbiting stars similar to our sun.

These mind-bogglingly large numbers might prompt you to think that we have extensive knowledge about such planets, but to date, only slightly more than 3,500 exoplanets have been confirmed in total – largely because they are notoriously difficult to find. Because their signals are so faint compared to stars, scientists usually have to resort to indirect methods to identify them.

Around 2,300 exoplanets have been discovered by the Kepler space observatory alone, which began operations in 2009, with the remainder discovered by various other telescopes. The first confirmed exoplanet discoveries were announced in January 1992 – two planets orbiting the pulsar star PSR 1257+12.

Upcoming missions to find new exoplanets include NASA's Transiting Exoplanet Survey Satellite (TESS) set to be launched in late 2017, and the European Space Agency's Planetary Transits and Oscillations of stars (PLATO), planned for 2024, while next-generation telescopes like the James Webb Space Telescope may be able to better assess the habitability of exoplanets, through more direct imaging.

Of those that have already been discovered, most are hundreds, if not thousands of light years away making them very difficult to study, while only a tiny handful have been deemed potentially habitable. The Proxima b discovery is particularly exciting because it is the nearest exoplanet ever discovered, offering an unprecedented observation opportunity.

So which ones are the prime candidates to potentially support life? The following is a list of planets, very likely to be rocky and lying in the conservative estimates for the habitable zones of their stars – although there around 25 more not included which lie in the more optimistic habitable zone estimates. At present, data about these planets are limited so scientists know very little about them. It is important to note that just because these planets are in the habitable zones, it does not necessarily mean they are habitable. Further observations will be needed to determine this for sure.

Proxima b
This artist’s impression shows planet Proxima b orbiting the red dwarf Proxima Centauri, the closest star to the Solar System ESO/M Kornmesser

Proxima b

Orbiting the Star: Proxima Centauri, red dwarf star, Centaurus constellation

Estimated mass: 1.3 Earth masses

Year length: 11 Earth days

Distance from Earth: 4.22 light years away

Nasa Exoplanets Kepler 7
Artist's impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right Reuters/ESO/L Calçada

Gliese 667 Cc

Orbiting the Star: Gliese 667 C, red dwarf star, Scorpius constellation

Estimated mass: 3.7 Earth masses

Year length: 28 Earth days

Distance from Earth: 23.62 light years away

Kepler 442b
A size comparison between Kepler-442b and Earth Ph03nix1986


Orbiting the Star: Kepler-442, K-type main sequence star, Lyra Constellation

Estimated mass: 2.34 Earth masses

Year length: 112 Earth days

Distance from Earth: 1291.6 light years away

Kepler 452b
Artist's concept of a rocky Earth-sized exoplanet in the habitable zone of its host star, compatible with Kepler-452b’s known data NASA Ames/JPL-Caltech/T. Pyle


Orbiting the Star: Kepler-452, G-type main sequence star, Cygnus constellation

Estimated mass: 5 Earth masses

Year length: 384 Earth days

Distance from Earth: 1402 light years away

Wolf 1061
The Wolf 1061 system, showing all three orbiting planets University of New South Wales

Wolf 1061c

Orbiting the Star: Wolf 1061, red dwarf star, Ophiuchus constellation

Estimated mass: 4.3 Earth masses

Year length: 18 Earth days

Distance from Earth: 13.8 light years away

Kepler 1229b
An artist's impression of the potentially habitable exoplanet Kepler-1229 b with the Milky Way in the background MarioProtIV


Orbiting the Star: Kepler-1229, red dwarf star, Cygnus constellation

Estimated mass: 2.7 Earth masses

Year length: 87 Earth days

Distance from Earth: 769 light years away

Kepler 62f
Seen in the foreground is Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra NASA Ames/JPL-Caltech


Orbiting the Star: Kepler-62, K-type main sequence star, Lyra Constellation

Estimated mass: 2.8 Earth masses

Year length: 267 Earth days

Distance from Earth: 1200 light years away

Kepler 186f
An artist's impression of Kepler-186f Danielle Futselaar


Orbiting the Star: Kepler-186, red dwarf star, Cygnus constellation

Estimated mass: 1.5 Earth masses

Year length: 130 Earth days

Distance from Earth: 561 light years away