I am not talking about other yellow stars like the sun. Just red ones.
I heard the planet can be tide locked. The dark side is so cold it condenses gases. The daylight side is so hot it boils water.
Is it always true that planets orbiting red dwarfs are tide locked?
There's enough supposition in this question to allow for any answer without any of them being right, or anyone knowing whether any of them are right.
Trả lờiXóaThe evidence from the roughly 260 extrasolar planets so far discovered suggests that red dwarfs may be slightly less likely than other stars to have planets. Of course, given the current state of technology, this really only applies to Jupiter-sized or larger.
Trả lờiXóaFor life as we know it, temperatures have to be in a range where water is liquid at least on part of the planet part of the time. But the zone where this is true will be much smaller and closer to a red dwarf than to the Sun. However, with the number of red dwarfs around, I'm sure some have habitable planets.
By red dwarf, I assume that you mean a spectral class M star, a main sequence star having a mass between 0.08 and 0.45 solar masses. The chief worry is, as you say, about a tidal lock between the rotation and the orbit of a planet that could have liquid water on its surface if it were not tidally locked.
Trả lờiXóaIn order for a planet to be habitable, it must have evolved an oxygen atmosphere, which means it must have previously evolved a form of life that converts CO2 to free oxygen. On Earth, that's done by plants. The people who think about this evolution a lot have guessed that it takes about 3 billion years for a planet to become habitable.
That's 3 billion years in which the star's tidal force is slowing down the planet's rotation to lockage with the orbital period.
The incompatibility between the necessary minimum age of the planet and the necessity of not being in rotational tide lock begins at a star mass of about 0.8. And the lower is the star's mass below that value, the greater is the probability of a tide lock for a planet in the ecosphere (liquid water zone).
The red dwarf stars all have very high probabilities of a rotation-orbit tide lock for a planet in their liquid water zones.
There's a way out, however. If the habitable planet is tide locked to a giant planet, which in turn is tide locked to the star, then the giant planet will generate little magnetic field effects because of its relatively slow rotation, while the little habitable planet might have a reasonably short day/night cycle equal to the synodic period of its sidereal periods around the giant planet and the star.
There might be another way out. A tidal lock between a planet's rotation and its orbit doesn't have to be 1:1. It might be 3:2, like Mercury. The planet has a tidal bulge that gets torqued to tightness at every periastron, but successive periastrons torque the opposite bulge from the one that got torqued the last time through.
But there are other problems. Red dwarf stars flare, just like our sun does. But when a red dwarf flares, the flare raises the total luminosity of the star by a much higher percentage than is the case with our sun. So a habitable planet in the liquid water zone had better have a strong magnetic field of its own (to turn away the star's charged particles) and a way to protect any life on it from significant fluctuations in the stellar flux. Like maybe the residents could be living a long ways underground (in the Underdark), or something.
The "good" range of masses for stars that might have habitable planets is
0.8 < M/Ms < 1.42
Or, in terms of absolute magnitude,
5.72 > Mv > 3.22
Or, in terms of spectral class,
K2V ... F1V
The reason for the upper bound in star mass is that stars above 1.42 solar masses turn off the main sequence before the three billion years, needed by a planet to become habitable, can go by. Coincidentally, that's also about where stars switch about in where convective and radiative energy transport prevail (core vs. envelope).
There's no reason planets like the ones you describe couldn't have some form of life. Most scientists believe that liquid water is required for life to start, and your hypothetical planet(s) could easily have that. Nor are such planets around red dwarfs always tidally locked to the star. That would depend on the mass of the planet and whether or not there are other planets in the system.
Trả lờiXóafirstly it is the red giant not red dwarf [they are the white dwarf]. And the life could not be present on the planets around it because it is very hot and is not much stable.
Trả lờiXóa