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:
To sustain life, a planet would not necessarily have to be identical to Earth in terms of temperature and atmospheric composition. Life as we know it is based on carbon. However, it is conceivable that life forms based on other elements could evolve. Silicon and boron seem to be the most likely candidates. Some life forms on Earth have adapted to extreme temperatures. Ones on other planets could do the same.
I've heard of silicon based life but if I remember correctly, it would have certain drawbacks like inefficient use of energy.
Extremophiles indicate life can ADAPT to harsh conditions. But I don't know if life can initially evolve under such conditions. I'd assume Kepler 186f is uninhabitable if conditions there are markedly different from those of Earth e.g. if the whole planet is frozen.
Fascinating! I don't have time now but will certainly comment more when I do, although this one thing I noticed seems erroneous, which is that the surface gravity is proportional to radius times density, so this planet should only have about 10% higher gravity than Earth I believe.
Anyhow also Tim I have begun to write up your concept of terraforming Venus on my spacedreams.org website and will let you know when I'm done so you can check it for accuracy etc....
Only 10% more gravity than Earth despite 44% more mass? (Of course dimensions are just estimates.) In one book, a hypothetical planet with twice Earth's mass has triple its gravity.
As for the spacedreams report, OK standing by. :)
Hi Tim--I'm only just this week getting back into this--I was out for several weeks due to medical problems, sore leg and feet which hopefully are now beginning to heal. Anyhow so thanks for bringing Kepler 186f to my attention! Wish it weren't so far away but since AFAIK we can only measure planets whose orbits happen to line up with ours, I guess there's a high probability that there exist similar worlds much closer. If its temperature averages anywhere from -50 C to +50C, there is still a good chance that somewhere on its surface there are comfortable temperatures. One thing that surprised me was you say that red dwarfs emit so much UV radiation--wouldn't you expect the opposite, that they would emit a great deal of infra-red instead? Also the effect of gravity on magma is two-sided--high gravity should help magma RISE TO THE SURFACE, but slow down its rise above the surface; and I think the super heights of the Martian volcanos are more due to (1) the absence of weathering erosion and (2) the fact that the crust of Mars is immobile so the cones stay above the same hot spots for billions of years--by contrast with Earth, where the hot spot under Hawaii has resulted in 7 or 8 lower volcanic peaks as the Earth's crust moved.
Interesting to speculate about possible flora and fauna but I think that if the atmosphere is very dense, that would help promote the evolution of things that fly as well as creep and crawl. Anyhow it's going to be fascinating to learn more about this planet hopefully as still more advanced telescopes get sent up!
Hi Roger. Sorry to hear about the medical problems, good to see you back! I heard red dwarfs emit a lot of UV during their early stages. Naturally infrared output is greater afterwards. How does high gravity help magma rise to the surface? If it does but slows down its rise above the surface, that may mean Kepler 186f has substantial, long-lived hot spots. Depending on how numerous or extensive they are, these hot spots may help warm the planet or at least local areas. Imagine submarine volcanic activity warming an entire sea. A dense atmosphere may cause gliding adaptations to be selected for.
That was interesting what you wrote about why Martian volcanoes became so big. (At first I didn't sign in with my usual account.)
Hi Tim, good to be back and get your response already! My understanding is that magma rises at depths below the surface due to its buoyancy--liquid rock being lighter than solid rock, so high gravity should help (it would pull down more strongly on the heavier material, allowing the lighter stuff to rise more, relatively.). Interesting how red dwarfs put out more UV in their early stages, do you know anything more about that, or I'll try to look it up. Yes it would be very interesting if large land or sea areas were heated by magma upwelling from below, it seems odd in fact that this doesn't happen on Earth, at least I have never heard of this effect affecting the climate of any place, not even Yellowstone where you might expect it; but during the vast ancient lava flows of the Columbia River area, Siberian "traps" and India's Deccan Plateau, of course the surface became too hot for anything to survive and it seems like it should have affected the climate at least for 10-100 km downwind?
Hi Roger, did you notice yesterday some kind of glitch causing this blog to appear as a featureless black screen? I'm relieved this morning to see that problem cleared up. I read somewhere that, had Earth been smaller with less gravity, volcanic eruptions would've been much more violent. It seemed true in light of the lunar maria and Tharsis volcanoes on Mars, both appearing on smaller worlds. But what you say makes sense; thanks for that explanation.
As you know, there is widespread agreement that Siberian trap volcanism caused the great end-Permian crisis. See e.g. Benton WHEN LIFE NEARLY DIED. Since that book came out, I understand the extent of the area covered by magma was twice as great as previously thought. I don't believe Deccan volcanism played much if any role in the Cretaceous-Tertiary mass extinction. Researchers found no evidence of extinction in the last 400,000 years of the Hell Creek deposition--nor in the last 250,000 years as far as I know. Dinosaurs continued until the impact horizon. Dinosaur diversity does appear diminished in the last one million years or so but that had nothing to do with the final big eruption in India. It might however have caused extinction c 200 Ma or 50 Ma after the end Permian die off.
Anyway the point is, volcanic activity can be detrimental to life (except for enriching the soil)
. It would only be beneficial if magma stayed just below the surface over a wide area, heating it for a long time. Maybe that would be possible on worlds with thick crusts, hard to breach.
Correction, I meant VOLCANISM might have caused extinction c 200 Ma or 50 Ma after the Permian.
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