I think the vast majority of space-enthusiasts has heard at some point that in order for a planet orbiting a red dwarf star to be habitable, it needs to be close enough to its parent star to sopport the existence of liquid water. This of course can cause the planet to be tidally locked with one face of the planet akways facing the star and the other facing the night. But what about larger stars like K and G type stars or supergiants like B or O stars? Could they have tidally locked planets orbiting around them?

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    $\begingroup$ I think the main issue is that the habitable zone tend to be far enough out on the brighter-than-M stars that the star isn't going to last long enough (or there literally hasn't been enough time) to normally tide-lock Earthlike worlds at that distance. $\endgroup$
    – notovny
    Aug 7, 2022 at 13:48

1 Answer 1


Many spectral class K main sequence stars would also have part or all of their habitable zones overlapping with the tidal locking distance.

I quote from a recent answer of mine:

How would I make my Quintuple Star System Stable?

So if you accept that tidally locked planets can still be habitable, then you can have tidally locked planets even orbiting the red dwarf, if the other problems with red dwarfs aren't too bad. But if you don't want to risk depicting habitable tidally locked planets you have a problem, since tidal locking will also affect most K class stars. For example, in Habitable Planets for Man, 1964, pages 68 to 72, Stephen H. Dole calculated that the inner part of star's habitable zone would become close enough for tidal locking if a star had a mass of 0.88, the mass of the Sun,and the entire habitable zone would be close enough for tidal locking at a mass of 0.72 times the mass of the Sun.


According to the Wikpedia table, a K0V star would have a mass of 0.88 Sun, while a star with 0.72 the mass of the Sun would be between a K5V and a K4V. So if you don't want to use habitable tidally locked planets, your class K stars have to be between K4V and K0V.


I note that when Dole wrote it was believed that Mercury was tidally locked to the Sun so that one side always faced the Sun. It is now known that is not quite the case and Mercury is locked into spinning three times in two orbits around the Sun.

So possibly not as many K class main sequence stars will have tidally locked planets in their habitable zones as Dole thought, but some should. I also note that the habitable zones of stars might be narrower or broader than Dole believed, thus changing the mass limits of stars which would have tidally locked planets in their habitable zones.


A small change in the mass of a main sequence star will cause a larger change in its luminosity.

Thus a less massive star will be less luminous to a greater degree than it is less massive. Its habitable zone will experience stronger gravitational and tidal forces from the star, thus making its planets tidally lock faster and sooner.

A more massive star will be more luminous to a greater degree than it is more massive. Its habitable zone will experience weaker gravitational and tidal forces, from the star, thus making its planets tidally lock slower and later.

More massive and luminous stars use up their hydrogen fuel faster and spend less time in the main sequence stage before becoming red giants.

So stars more massive than lower mass K main sequence stars should never last long enough in the main sequence to tidally lock planets that orbit in their habitable zones.

However, the rotation rates of planets are largely random, determined by the massive impacts they suffer while their solar systems are forming. So possibly a planet of a more massive star experienced random massive impacts which slowed its rotation to just a tiny little bit faster than being tidally locked, and so the weaker tidal forces that planet experienced were enough over billions ofyears to slow its rotation enough to make it tidally locked.

And I think there may be other factors, such as the rotation rates and wind speeds of dense atmospheres, which can affect the rotation rates of planets.


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