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On airless bodies, the lack of any significant greenhouse effect allows equilibrium temperatures to approach mean surface temperatures, as on Mars, [5] where the equilibrium temperature is 210 K (−63 °C; −82 °F) and the mean surface temperature of emission is 215 K (−58 °C; −73 °F). [6]
LHS 1140 b orbits close to the outer edge of the habitable zone, a region around a star where temperatures are just right for liquid water to pool on the surface of orbiting planets, given sufficient atmospheric pressure. [7] The equilibrium temperature of LHS 1140 b is rather low, at 230 K (−43 °C; −46 °F), as cold as the polar regions ...
The runaway greenhouse effect is often formulated in terms of how the surface temperature of a planet changes with differing amounts of received starlight. [13] If the planet is assumed to be in radiative equilibrium, then the runaway greenhouse state is calculated as the equilibrium state at which water cannot exist in liquid form. [3]
Simulations in 2017 suggested the planet is approximately 20% water by composition, much higher than that of Earth. With such a massive water envelope, the pressure and temperature will be high enough to keep the water in a gaseous state and any liquid water will only exist as clouds near the top of TRAPPIST-1f's atmosphere.
The temperatures of a planet's surface and atmosphere are governed by a delicate balancing of their energy flows. The idealized greenhouse model is based on the fact that certain gases in the Earth's atmosphere , including carbon dioxide and water vapour , are transparent to the high-frequency solar radiation , but are much more opaque to the ...
TRAPPIST-1h orbits the ultracool dwarf star TRAPPIST-1. It is 0.121 R ☉ and 0.089 M ☉, with a temperature of 2,511 K and an age between 3 and 8 billion years. For comparison, the Sun has a temperature of 5,778 K and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with about 0.0005 times the luminosity of the Sun.
Data in the table above is given for water–steam equilibria at various temperatures over the entire temperature range at which liquid water can exist. Pressure of the equilibrium is given in the second column in kPa. The third column is the heat content of each gram of the liquid phase relative to water at 0 °C.
A water vapor atmosphere would need to have a scale height of >100 km (62 mi) and a temperature >1,800 K (1,530 °C; 2,780 °F) to produce the variations seen in the planet's transit depths and its transmission spectrum, and would be vulnerable to photodissociation where CO 2 would not be. Other sources for the effects seen, such as hazes and ...