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The TRAPPIST-1f is in the TRAPPIST-1 system discovered recently with seven planets. Three of them are said to be in the habitable zone. But the data says that the average temperature on the TRAPPIST-1f planet is -54°C. How is it considered habitable then?

Source: https://en.m.wikipedia.org/wiki/TRAPPIST-1f

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    $\begingroup$ Turn on a heater? :) People live at the south pole of Earth. $\endgroup$ – Steve Mar 19 '17 at 14:08
  • $\begingroup$ Well, as far as I know @Steve the Earth's average temperature is 12°C or something like that. $\endgroup$ – KKZiomek Mar 19 '17 at 14:14
  • $\begingroup$ Well, for half the year, the average high temperature of the south pole is around -55°C, according to Wikipedia. Not sure how a whole planet would be viable, but with greenhouses and such, it should be "habitable". "Comfortable" or "convenient" might be another matter... $\endgroup$ – Steve Mar 19 '17 at 14:16
  • $\begingroup$ @Steve And most of the people (say around three-quarters) leave Antarctic research stations during the winter, suggesting things are pretty marginal. $\endgroup$ – David Richerby Mar 19 '17 at 18:05
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None of the TRAPPIST-1 planets are "considered habitable"; we don't know nearly enough about them to say that (any news article which referred to the planets themselves as "habitable" needs firm correction). Three of them are within the habitable zone of the star, which is defined as the region around the star where liquid water could potentially be found on a planet.

A planet with an average temperature of -55C will not be dominated by liquid water, but -- given that TRAPPIST-1f is likely tidally locked, showing the same face to the star all the time -- on one side of the planet the sun is always in the sky, greatly increasing the temperature on that side, particularly in the region where the sun is most directly overhead. Depending on the atmospheric composition, it's likely that water, if present, would be liquid there.

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    $\begingroup$ All this assuming that atmospheric pressure doesn't change the state of potential oceans, and that our lovely hydrogen and oxygen are dominant elements in said atmosphere. $\endgroup$ – Weckar E. Mar 20 '17 at 7:59
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The temperature estimate is based on no atmosphere. For instance, Earth, without our atmosphere, would have an average temperature of -20°C . The equator should be warmer, and any atmosphere will also add heat. Also, the objects are most likely tidally locked, which means that a single side of the planet will always face it's star, making it even warmer in some places.

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  • $\begingroup$ Tidally locked + an atmosphere = extreme weather, no? $\endgroup$ – Daniel Jour Mar 19 '17 at 19:37
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    $\begingroup$ It depends on the location quite a bit. The weather near the terminator would be extreme, but less so along the central point. $\endgroup$ – PearsonArtPhoto Mar 19 '17 at 22:57

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