We left Earth about a decade ago, and I'm starting to regret bringing TWO physicists along. They've recently gotten into a disagreement about the kinds of planets we should expect to survey when we get to our sector. Specifically, one adamantly believes that the majority of the planets we're going to survey will be tidally locked, while the other fervently disagrees. Can anyone back home shed some light on the situation-

Are there more tidally locked planets in the galaxy than non-tidally locked ones?

As far as Worldbuilding goes, I'm trying to justify a galaxy very similar to ours with nearly every planet tidally locked to its star- it makes the discovery of (our non-tidally locked) Earth mind-blowing for the aliens who discover us.

  • $\begingroup$ might be best to have the aliens come from hot worlds close into the sun. Then they might be more interested in looking at planets like Mercury and Venus $\endgroup$
    – Slarty
    Nov 8, 2017 at 9:53

3 Answers 3


No one knows yet, because our main planet-finding techniques (transits and radial velocity changes) are both very heavily biassed in favour of planets that are close to their primaries and thus have short years. There's no way we could yet have discovered an exoplanet that was just like Jupiter, because we don't have anything like a long enough baseline to have data from the two or three Jovian years (24 or 36 Earth years) that are necessary to pick up the repeating pattern. So we don't know how many planets orbit close to their primary (and are thus likely to be tidally locked) compared to the number orbiting further out, because we can't yet detect the ones orbiting further out.

  • $\begingroup$ This answer addresses the main point directly and explains the problem with abductively deriving general observations from the current exoplanet dataset. Note also we can only generally detect exoplanets whose orbital planes are aligned in a specific way w.r.t. to us. $\endgroup$
    – mikołak
    Nov 8, 2017 at 9:35
  • $\begingroup$ Follow-up question: given enough time, would our current methods detect these further out planets? Or are our methods too inaccurate? $\endgroup$
    – Pyritie
    Nov 8, 2017 at 11:30
  • $\begingroup$ @Pyritie: Some of them, yes. All of them, no. And no matter how much our methods improve, detecting planets that are far from any star will always be difficult, because they're dimly lit and moving slowly, and too far from anything we can see more easily for their gravity to have much visible effect. For some perspective, note that quite a few astronomers currently suspect that our own solar system may include a large distant planet that we haven't directly observed yet. $\endgroup$ Nov 8, 2017 at 14:16

On our home system only Mercury shows a 3:2 resonance between the day and the year. All other planets have no locking at all.

Based on mechanical consideration, locking can happen only close to the central star. Then I would say that it's more likely you will find non-tidally locked planets.

Mind however that they are quarreling on

the majority of the planets we're going to survey will be tidally locked, while the other fervently disagrees

Put that on our solar system (1 locked, 6 non locked) with you deciding to survey only Mercury (1 locked surveyed, 6 non locked non surveyed), will make the first one right. So you can decide who is right ;)

  • 3
    $\begingroup$ Our solar system is a bad example, because it's atypical. Only 10% of stars are as big as the Sun or bigger, while more than three quarters are M-class dwarves. To know anything about "most planets", you have to know about the planets of M-class stars, not G-class like the Sun. $\endgroup$
    – Mike Scott
    Nov 8, 2017 at 11:12
  • 1
    $\begingroup$ @MikeScott, smaller star means lower tidal forces. This simply pushes closest to the star the region where one can have tidal locking, or not? $\endgroup$
    – L.Dutch
    Nov 8, 2017 at 11:14
  • 2
    $\begingroup$ Not if planets' orbital radii are reduced to match, which we don't know. Tidal force varies with inverse cube of distance, so a star with 1/4 the mass will exert more tidal force on a planet at 1/2 the distance, not less. $\endgroup$
    – Mike Scott
    Nov 8, 2017 at 11:24

As Mike Scott notes, we don't know yet. Our current exoplanet survey techniques are so strongly biased in favor of planets that are close to their sun (and therefore likely to be tidally locked) that they don't yet allow reliably estimating the fraction of all planets that orbit further out (and therefore probably aren't locked).

That said, you hypothetical aliens probably don't care about the total ratio of tidally locked to non-locked planets, anyway. What they care about is the fraction of life-bearing planets that are tidally locked to their star. That's, of course, even harder to estimate using current data (since we haven't even detected any life-bearing planets other than Earth yet), but we do have the following suggestive observations:

  1. By far the majority of all stars are small red dwarfs, much smaller than our Sun.

  2. Our current exoplanet surveys do seem to indicate that such small stars are capable of having Earth-like planets within their habitable zone.

  3. The smaller a star is, the closer to it its habitable zone lies, and thus the more likely a planet orbiting in that zone is to be tide-locked.

Thus, given how common red dwarfs are, there's statistically likely to be a rather large population of planets around them that would be both tidally locked and, by their size and orbital radius, capable of supporting liquid water on their surface. If we assume that those planets aren't significantly less likely to bear life than otherwise similar non-locked planets around larger stars, then there's a good chance that the majority of all planets bearing water-based life are indeed tidally locked.

Of course, that's a big assumption, but it's one we can't currently either prove or disprove. Furthermore, your hypothetical scenario implicitly provides are least one extra (hypothetical) data point in its favor: the existence of your alien civilization from a tidally locked planet.

All that said, there's a couple of things that may reduce the element of surprise for your alien survey team spotting the Earth:

First, if they're surveying for habitable planets around Sun-like stars in the first place, they're presumably smart enough to do the math and figure out that such planets are not likely to be tide-locked to their star. If they're so convinced that this makes life impossible, why bother even surveying such stars?

Second, given the observed existence of "hot Jupiters" (and their apparent prevalence, which of course may also be partly due to survey bias, since they're the easiest kind of planets to detect), it's reasonably likely that a significant fraction of all theoretically habitable bodies around small stars may actually be moons of large gas planets.

While such moons will almost certainly be tide-locked, they'll be locked to their parent planet, not to the star, and will therefore have a more or less Earth-like day-night cycle. If life on planets around small stars is possible at all, it's likely that such moons can also support it equally well (or better), and thus your alien survey team ought to be aware of the possibility of life on such worlds. From there, it's not a huge leap to suppose that freely rotating planets around large stars could also be viable places for life.

Ps. See also: Habitability of red dwarf systems and Habitability of natural satellites on Wikipedia


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