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Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.
For a stable orbit the ratio between the moon's orbital period P s around its primary star P p must be < 1 ⁄ 9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.
The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
Humans have achieved some mediated presence throughout the Solar System, but the most extensive presence has been in orbit around Earth. Humans reached outer space mediated in 1944 and have sustained mediated presence since 1958 , [a] as well as having reached space directly for the first time on 12 April 1961 (Yuri Gagarin) and continuously ...
The previous record of people in orbit was set in May 2023, when for a brief period of time there were 17 people aboard both the International Space Station and China’s Tiangong space station.
Yet a third option could be to continually move to stay on the night side, as Mercury's 176-day-long day-night cycle means that the terminator travels very slowly. [ 149 ] Because Mercury is very dense, its surface gravity is 0.38g like Mars, even though it is a smaller planet. [ 148 ]
Surface planetary habitability is thought to require an orbit at the right distance from the host star for liquid surface water to be present, in addition to various geophysical and geodynamical aspects, atmospheric density, radiation type and intensity, and the host star's plasma environment. [2]
It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation [7] means it takes longer for it to pass across the surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth.