Interesting Planet
Shining dimly about 490 light years away, Kepler 186 is a red dwarf star with 5 known planets. Four of them are too close and hot to be habitable. The fifth, Kepler 186f, is one of the most Earthlike worlds yet discovered. It is the first known exoplanet close to ours in size, and lies within a habitable zone.
Kepler 186f is estimated to have a diameter 1.11 times that of Earth, and a mass about 44% greater. It orbits its star at a distance of about .36 to .40 AU, and has a year of 129.9 Earth days. There is a 50% chance the planet is tidally locked. If it is, a 2:3 tidal lock is more likely than a 1:1 lock. A day on Kepler 186f may last nearly three months.
Because of the weak energy output of its dwarf star, Kepler 186f receives only about 32% as much energy as Earth gets from the sun. The exoplanet is practically at the outer edge of the habitable zone. Even Mars gets more energy--43% on average. Kepler 186f however, has a key advantage over Mars.
A planet with 1.44 Earth mass probably has twice our gravity. Given such strong gravity, and the weakness of stellar thermal agitation, the planet almost certainly has a large volatile inventory. Unlike Mars, which has no greenhouse effect, Kepler 186f may have a dense, largely CO2 atmosphere, augmented by water vapor (an even better greenhouse gas). It is estimated that, with an atmosphere of .5 to 5 bar of CO2, Kepler 186f would have an average(?) temperature of 273 Kelvin, or 0 Celsius.
One depiction of Kepler 186f as Mars-like is almost certainly inaccurate in view of its strong gravity, causing retention of volatiles.
Another may be more accurate, although Kepler 186f, even under the best of conditions, is likely to have extensive areas of ice, especially at high latitudes.
No doubt, an atmosphere rich in methane would further enhance greenhouse warming. Unfortunately the high UV output of red dwarfs causes photoevaporation of methane. Kepler 186f may have been bereft of CH4 since its early history, when UV radiation was most intense.
The planet's slow rotation may compensate for weak stellar energy. The length of the daytime--perhaps six weeks--may enable warmth to accumulate on the day side, especially in the fortnight of afternoon. Strong, cold winds from the night side may make conditions too unsettled, however, and interfere with warming.
Another factor is internal planetary heat. A world more massive than Earth would have a vast amount of thermal energy. Strong gravity inhibits the rise of magma. Plate tectonics, if present, would prevent massive volcanism. The Red planet has the biggest volcanoes in the solar system. In contrast, Keplerian volcanoes are probably modest. Yet enormous amounts of energy probably accumulate at or near the surface. Kepler 186f may have extensive areas of thermal vents. These may be most common at the bottom of its seas, which naturally are the lowest places on the planet. Keplerian ecosystems may exist, deriving the vast bulk of their energy from the interior. Given the effect of buoyancy in countering high gravity, and likelihood of ocean floor vents, the bulk of the Keplerian biomass may be oceanic. Able to retain vast amounts of water, 186f may have an even higher ocean-land ratio than Earth. If vents exist in great numbers throughout the ocean basins, they may modify the climate of the whole planet. Conceivably Kepler 186f harbors a rich biota, albeit mostly small organisms adapted to crawl or creep rather than fly.
Kepler 186f is estimated to have a diameter 1.11 times that of Earth, and a mass about 44% greater. It orbits its star at a distance of about .36 to .40 AU, and has a year of 129.9 Earth days. There is a 50% chance the planet is tidally locked. If it is, a 2:3 tidal lock is more likely than a 1:1 lock. A day on Kepler 186f may last nearly three months.
Because of the weak energy output of its dwarf star, Kepler 186f receives only about 32% as much energy as Earth gets from the sun. The exoplanet is practically at the outer edge of the habitable zone. Even Mars gets more energy--43% on average. Kepler 186f however, has a key advantage over Mars.
A planet with 1.44 Earth mass probably has twice our gravity. Given such strong gravity, and the weakness of stellar thermal agitation, the planet almost certainly has a large volatile inventory. Unlike Mars, which has no greenhouse effect, Kepler 186f may have a dense, largely CO2 atmosphere, augmented by water vapor (an even better greenhouse gas). It is estimated that, with an atmosphere of .5 to 5 bar of CO2, Kepler 186f would have an average(?) temperature of 273 Kelvin, or 0 Celsius.
One depiction of Kepler 186f as Mars-like is almost certainly inaccurate in view of its strong gravity, causing retention of volatiles.
Another may be more accurate, although Kepler 186f, even under the best of conditions, is likely to have extensive areas of ice, especially at high latitudes.
No doubt, an atmosphere rich in methane would further enhance greenhouse warming. Unfortunately the high UV output of red dwarfs causes photoevaporation of methane. Kepler 186f may have been bereft of CH4 since its early history, when UV radiation was most intense.
The planet's slow rotation may compensate for weak stellar energy. The length of the daytime--perhaps six weeks--may enable warmth to accumulate on the day side, especially in the fortnight of afternoon. Strong, cold winds from the night side may make conditions too unsettled, however, and interfere with warming.
Another factor is internal planetary heat. A world more massive than Earth would have a vast amount of thermal energy. Strong gravity inhibits the rise of magma. Plate tectonics, if present, would prevent massive volcanism. The Red planet has the biggest volcanoes in the solar system. In contrast, Keplerian volcanoes are probably modest. Yet enormous amounts of energy probably accumulate at or near the surface. Kepler 186f may have extensive areas of thermal vents. These may be most common at the bottom of its seas, which naturally are the lowest places on the planet. Keplerian ecosystems may exist, deriving the vast bulk of their energy from the interior. Given the effect of buoyancy in countering high gravity, and likelihood of ocean floor vents, the bulk of the Keplerian biomass may be oceanic. Able to retain vast amounts of water, 186f may have an even higher ocean-land ratio than Earth. If vents exist in great numbers throughout the ocean basins, they may modify the climate of the whole planet. Conceivably Kepler 186f harbors a rich biota, albeit mostly small organisms adapted to crawl or creep rather than fly.
posted by starman at 4:19 AM
10 comments
