Earth Proxima refers the closest confirmed exoplanet to Earth that exhibits similar conditions to our planet, therefore maximizing its chances of being habitable to humans. The distinction is currently given to Proxima b, approximately 4.25 light years from Earth, though its conditions for habitability are largely unknown. The title may change as other Earth-like exoplanets are discovered and their atmospheric compositions are studied and confirmed. If a potentially habitable planet is found around Earth's nearest Sun-like star system, Alpha Centauri, 4.37 light years away, it would be another good candidate for Earth Proxima. The term was coined by the 2016 documentary, "The Search for Earth Proxima."
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Characteristics
To sustain Earth-like conditions—rocky, approximately 0.5 to 1.5 times the size of Earth, an atmosphere and a surface that could potentially hold liquid water—Earth Proxima must orbit in the habitable, or “Goldilocks”, zone of a star. Based on 2013 Kepler data, astronomers estimate that there may be as many as 11 billion Earth-sized planets in the Milky Way orbiting in the habitable zone of Sun-like stars. Of these, the closest known, potentially habitable one is approximately 4.25 light years away.
Previous candidates for Earth Proxima have included Wolf 1061c, 13.8 light years from Earth, and Kapteyn b, 12.8 light years away and twice the age of Earth. However, both exoplanets orbit red dwarfs, which calls into question the likelihood of habitability. In 2015, NASA's Kepler telescope confirmed Kepler 452b, the first Earth-like exoplanet in the habitable zone of the Sun-like star, approximately 1,400 light years away.
Alpha Centauri
The closest possible exoplanet to Earth, if discovered, would orbit the Alpha Centauri star system, which contains two Sun-like stars (Alpha Centauri A and B) and a red dwarf (Proxima Centauri, slightly closer to Earth than its binary siblings at 4.25 light years). Alpha Centauri's close proximity, approximately 4.37 years from Earth, is a rarity; the next nearest Sun-like star (FGK), Epsilon Eridani, is more than two times farther away, at 10.5 light years. Current estimates place the probability of finding Earth Proxima around Alpha Centauri A or B at roughly 85%, although this number remains uncertain. Surveying the habitable zone of Alpha Centauri A and B have been the target of several conceptual missions, including NASA's Space Interferometry Mission (SIM), which was canceled in 2010 due to financial issues.
Most recently, the nonprofit organization 'Mission Centaur' plans to build a lightweight, low-cost telescope to capture images of any Earth-like exoplanets around Alpha Centauri A and B via direct imaging using a coronagraph. Expected to launch by 2020, the telescope will directly image exoplanets and start assessing their habitability. The project is chronicled in the 2016 documentary film, “The Search for Earth Proxima.”
On August 24, 2016, the European Southern Observatory and the Pale Red Dot project announced the discovery of a rocky planet orbiting within the habitable zone of Proxima Centauri. Called Proxima b, the planet is 1.3 times the mass of Earth and has an orbital period of roughly 11.2 Earth days. However Proxima Centauri's classification as a red dwarf casts doubts on the habitability of any exoplanets in its orbit due to low stellar flux, high probability of tidal locking, small circumstellar habitable zones, and high stellar variation.