The closest star to Earth (after the Sun) is well-known: Alpha Centauri, 4.3 light years away. But what about the closest exoplanet to Earth, outside of the Solar System?
Funnily enough, just until recently, the nearest exoplanet Alpha Centauri Bb was believed to be
a planet with a similar mass to Earth's just 4.3 light years away in the nearby Alpha Centauri system.
Source: Nearest exoplanet to Earth may be a ghost world (Aron, 2013). However, according to the article, recent observations have suggested that it may not actually be there. So the answer to this question is not all that clear.
The Paris Observatory has a list of all exoplanets in a sortable database at exoplanet.eu.
According to the Paris Observatory catalogue, if Alpha Centauri Bb does not actually exist, the next closest is the just-larger-than Jupiter sized Epsilon Eradani B (not a hot-Jupiter), at 10.5 light years away. Which according to Nearest Planet Beyond Solar System Might Be Photogenic (Than, 2006):
About 1.5 more massive than Jupiter, the planet takes 7-years to circle its star, Epsilon Eridani. The giant gas planet was originally detected in 2000, when astronomers noticed a rocking motion in the star which they attributed to the gravitational tug of an unseen planet.
There is speculation that more planets may exist in that system as with dust rings.
Earlier this year (2016), scientists used the radial velocity method to discover a planet orbiting Proxima Centauri: Proxima Centauri b. It was announced in Anglada-Escudé et al. (2016). Here are some of its basic properties, as reported by the authors of the paper and known as of August 2016:
- Mass: At least 1.3 Earth masses
- Semi-major axis: 0.05 AU
- Maximum eccentricity: 0.35
- Orbital period: 11.2 days
- Surface temperature: 234 K (see Table 1)
Now, as the authors note, the planet may not be habitable:
The habitability of planets like Proxima [Centauri] b—in the sense of sustaining an atmosphere and liquid water on its surface—is a matter of intense debate. The most common arguments against habitability are tidal locking, strong stellar magnetic fields, strong flares and high ultraviolet and X-ray fluxes; but none of these have been proved definitive.
However, further observations might help us learn more about the planet and determine if it could be habitable for any type of life.
It should be noted that the controversy about planets around Alpha Centauri continued after this question was initially answered by user8. Alpha Centauri Bb was shown to be nonexistent by Rajpaul et al. (2015). However, in another twist, observations by Demory et al. (2015) seemed to show the existence of Alpha Centauri Bc (at the time, Alpha Centauri Bb was still a possibility, so now "Alpha Centauri Bc" would actually be "Alpha Centauri Bb" if it existed), an Earth-like planet. However, there has been no more evidence to confirm the existence of this planet - and at any rate, Proxima Centauri is closer to the Solar System than Alpha Centauri, so Proxima Centauri b wins no matter what.