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I would like to know please if there is any habitable exoplanet close to Earth.

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    $\begingroup$ Habitable, in the context of exoplanets, means "terrestrial and water could exist in liquid form". It does not mean we could actually live there. It doesn't mean that there is actual water, or oxygen, or an atmosphere. There are three "habitable" planets in the solar system, But you couldn't live on Venus $\endgroup$
    – James K
    Commented Mar 4, 2018 at 8:58
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    $\begingroup$ There's no such thing as "close" when you're talking other star systems. Even the closest star system, 4.2 light years away is ridiculously far. $\endgroup$
    – userLTK
    Commented Mar 4, 2018 at 13:30
  • $\begingroup$ Yes, but potentially habitable. The closest potentially habitable exoplanets are Proxima b, Ross 128 b, Gliese 273 b, Gliese 3323 b and Gliese 667 C c. Here you can find a video about them: youtube.com/… $\endgroup$ Commented Mar 6, 2018 at 20:45
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    $\begingroup$ I think this is a perfectly reasonable question, it doesn't deserve down-votes. An exoplanet is, by definition, outside our own solar system, so "close" is easily understandable as meaning something like "nearby", i.e. in the stellar "neighbourhood". I'm sure we all realise that interstellar distances are big. Anyway, I've posted a comprehensive answer which SE readers might find useful as a summary. $\endgroup$ Commented Mar 9, 2018 at 1:57

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Habitability is related to size or mass (i.e. likely to have a rocky composition and maintain surface water), and distance from the host star (the “habitable zone” – not too hot, not too cold – which depends on stellar temperature and stellar flux). The Planetary Habitability Laboratory (University of Puerto Rico at Arecibo) publishes a catalog of potentially habitable exoplanets.

The PHL catalogue is divided into two lists. The first list covers the “conservative” habitable zone, where the combination of stellar temperature and flux is optimal (equivalent to an orbit between Earth and Mars around a Sun-like star). Exoplanets are also given an ESI (Earth Similarity Index) rating, a measure of how similar the planet is to the stellar flux, mass, and radius of Earth (Earth = 1.0).

As at March 2018 there are only 10 confirmed exoplanets within the conservative habitable zone, the closest (in order) being:

After these 6, the next nearest in the list is Kepler-186 f at a distance of over 561 ly.

Note that while Proxima b might be “in the zone”, it’s unlikely to be habitable. At only 7 million km from its host star (compare with Mercury, 46 million km from the Sun at perihelion), it’s likely to be tidally locked, restricting any habitable area to a thin border wedge between the two extreme regions. It also receives 400 times more X-ray radiation than the Earth; and the stellar wind pressure is 2000 times that of Earth, sufficient to blow away any atmosphere. Not an ideal candidate to look for life!

Kapteyn b (12.8 ly) is in PHL’s conservative list and at 12.8 ly would be the 4th closest habitable planet, but PHL lists it as “unconfirmed”, as recent research has cast doubt on its existence: Robertson, Mahadevan & Roy (2015) concluded that "Kapteyn b is not a planet in the Habitable Zone, but an artifact of stellar activity."

PHL’s second list covers exoplanets that are a bit less likely to be habitable, either because they are less able to maintain surface liquid water due to their composition (based on size/mass), or because they’re in a less hospitable orbit (the “optimistic habitable zone”). There are 33 confirmed exoplanets in this list, the closest being:

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There are some possible planets in the Trappist-1 solar system. Particularly planet 1e looks to be in the habitable zone but we won't know until the Webb telescope is launched next year.

Close to Earth is a relative term, Trappist 1 is about 40 light years away.

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