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Proxima Centauri, closest system (4,3 light-year). Three stars (G2, K1, M5). Component A is similar to the Sun (a G2 star).
On August 24, 2016, the discovery of an Earth-size exoplanet orbiting in the habitable zone of Proxima Centauri was announced.
The fastest outward-bound spacecraft yet sent, Voyager 1,
has covered 1/600th of a light-year in 30 years and is currently moving at 1/18,000th the speed of light.
At this rate, a journey to Proxima Centauri would take 80,000 years.
The troposphere is the lowest portion of Earth's atmosphere. The average depths of the troposphere are 20 km (12 mi) in the tropics,
17 km (11 mi) in the mid latitudes, and 7 km (4.3 mi) in the polar regions in winter. At middle latitudes,
tropospheric temperatures decrease from an average of 15 °C at sea level to about −55 °C at the tropopause.