Ignoring tidal heating and such, how can one calculate the habitable orbital ranges for M, L, T, and Y spectral class stars?

The reason I ask is that I'm also curious if any of these zones encroach on their respective Roche limits.

  • $\begingroup$ Side note, do any of these zones encroach on their respective Roche limits? $\endgroup$
    – Enoch
    Dec 1, 2021 at 21:22
  • $\begingroup$ I've edited your question to include some links and to make the question more interesting to answer by ask "How to calculate...?" rather than just "What's the answer?" And I think your question about the Roche limit is so compelling that it should be included in the question itself! $\endgroup$
    – uhoh
    Dec 1, 2021 at 21:57
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    $\begingroup$ Brown dwarfs are not stars, any moe than they are planets or planetary mass objectsi. They are a different catebory of objects intermediate between planets and stars, with somewhat vague borders. But I upvoted the question because the answers would be interesting. $\endgroup$ Dec 2, 2021 at 18:44
  • $\begingroup$ @M.A.Golding I didn't know that! Wikipedia's article Stellar classification includes them so it's easy to think naively that they are kinds of stars. So I guess neutron stars are not stars either? Related to the issue of vague borders: Do astronomers generally agree that the distinction between comets and asteroids is not so clear? $\endgroup$
    – uhoh
    Dec 3, 2021 at 0:01

1 Answer 1


As mentioned in the comments, your question kind of has two parts.

First are M stars. These are proper stars, having hydrogen fusion, and are actually quite terrible for any kind of life. The reason for this is the incredibly intense magnetic fields and fully convective nature of these stars. The dynamo effect (moving charged liquids creating magnetic fields) is largely to blame here, and is especially bad since from surface to core you have full convection (whereas in larger stars, you have confection regions of smaller fractions of the star). These magnetic fields lead to terrible, terrible stellar weather that would likely wipe out any living thing, especially given the fact that a planet would have to be so close to be in the Goldilocks zone.

Next are the L, T and Y ‘stars’. These objects are brown dwarfs, but brown dwarf specialists often still refer to them as stars, and rightfully so to an extent; brown dwarfs still have fusion happening in them, but not hydrogen fusion, mostly deuterium and sometimes lithium fusion. This isn’t enough to keep the brown dwarf at a stable temperature, however, and so brown dwarfs progressively cool down throughout their lifetime. They can get pretty cold in comparison to hydrogen fusing stars, and so the habitable zone will change over time, making the zones much less Goldilocks than we’d like for anything to realistically survive. Below is a visual that shows much better than I could say exactly how it evolves. Credit to Bolmont et al. for the data, and this website for the visualization :


Habitable Zone for Brown dwarfs

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    $\begingroup$ Some L-stars are stars and some M stars are brown dwarfs. Depends on their age. $\endgroup$
    – ProfRob
    Dec 3, 2021 at 21:43
  • $\begingroup$ True, will edit to reflect that. $\endgroup$
    – Justin T
    Dec 4, 2021 at 0:08

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