After a few years I seem to have come back to the topic of the possibility of tidal locking on a planet with life. I love speculative biology and designing these sorts of scenarios but tidal locking I've never been able to find a solid answer to it's ability to support life.

In a post a few years ago, someone suggested that a low speed collision could cause tidal locking with the right speed and angle of collision and wanted to follow up on this in more detail. So on to the question at hand, this is a scenario I've come up with to determine if this is possible; early in the life of a planet orbiting a K-class star in the goldilocks zone, a rogue planet collides into the primary planet at low speed, approaching from a counter-orbital angle. The collision isn't head on but the rogue planet mostly scrapes the surface, then pulled into the other planet due to its gravity. The primary planet is young, but largely at the end of it's molten stage (or soon after depending on the fidelity of the situation). The collision has enough force to slow the planet into a tidally locked orbit around the K-class star.

Would the planet in this scenario be able to support life after this event and stay stable? The core, I would assume, would still be active for a long time, as well as the magnetic field. Although I know more about biology than astronomical bodies so many of these ideas are new to me.

Edit: the time between the planets collision from the formation of the planet would be about 200 million years, tidal locking would be completed after about 2.5-3 billion years(?), with life forming around 1.1-1.6 billion years after the collision.

My apologies for any weird wording of things as well, if anything needs clarification or to be reworded I will gladly do so!

Edit: To clarify, I am asking if a planet tidally locked by a collision, can still support life and stay stable for the foreseeable future.

Edit 2: removed redundancies.

Edit 3: removed the part about the planet not being fully absorbed.

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    $\begingroup$ Any object of a few hundred Km in diameter will become spheroid, unless your planet is really tiny then there's is no bulge counter example to sphere is Iapetus which has a most a ridge around the equator. Tidal-locking takes time, geological time. It's difficult at present to understand what the specific question is, couched in uncertainties and tentative ideas. Can you clarify a fair bit please? $\endgroup$ May 29, 2021 at 4:33
  • $\begingroup$ @ARogueAnt. sorry, the specific question if could a planet tidally locked through a collision still support life and be stable. $\endgroup$ May 29, 2021 at 4:36
  • $\begingroup$ That's two questions, "could a planet be tidally locked by a collision, and can X planet support life" ...Then there's the stability.... $\endgroup$ May 29, 2021 at 4:37
  • $\begingroup$ @ARogueAnt. Im not asking if the tidal locking is possible, this question is asking if the resulting planet can still support life assuming the collision creates the tidal locking. As I am aware tidally locked planets in general tend to have cores long dead by the time locking happens, or often have orbits very close to the parent star. $\endgroup$ May 29, 2021 at 4:41
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    $\begingroup$ How gentle is gentle? For example, in the case of Earth an object coming from space cannot possibly impact with a speed of less than 7 km/sec--and that's if it comes from low orbit: objects coming from far away will come with at least 11 km/sec. 25,000 km/h (16,000 mph) is not what I would call "gentle". $\endgroup$
    – AlexP
    May 29, 2021 at 13:06

3 Answers 3


The way I see it a slow collision can only happen if there is something which can dissipate momentum.

Let's put some numbers on the paper... Any body approaching Earth from very far will at least impact it with a velocity of 11 km/s, which is also the escape velocity. This is greater than the velocity a satellite needs to have in order to orbit Earth.

Since gravity is a conservative force, you need something to shed off that momentum. The most plausible thing is to have the accretion disc of the forming planet to act as dissipating medium, and cross your finger that things go exactly right to have an orbit and not a merging.

Anyway, if the orbiting happens, it is so early in the evolution of the planet that there will be no life yet. What happens after that depends on the specific of the planet chemistry and physics. Think of us: we are here after a very early big impact which created our moon.

  • $\begingroup$ That's one very dense accretion disk... Wondering what's keeping it aloft. $\endgroup$
    – AlexP
    May 29, 2021 at 13:09
  • $\begingroup$ "to have an orbit and not a merging": a merger seems to be their actual scenario (with one somehow remaining as a "area of high elevation"). $\endgroup$ May 29, 2021 at 13:33

Planetary collisions cannot occur at "low speed" The minimum possible being orbital speed of roughly just under 7.8 km/s. Such a collision involving Earth sized astronomical bodies would release so much energy that both bodies would loose their structure. Debris might form a ring or even eventually a Moon, but at "relatively" low speed most material would remain to form a central spherical body under gravitational forces.

There is no "counter-orbital angle" by which one planet can approach another at low speed. An off centre hit is possible and might well throw up vast amounts of debris. Regardless of the planets state of evolution, it would be converted back to its molten stage by the impact.

It is not possible to have any significant area of high elevation at one end of the planet. mountain scale surface irregularities (and possibly a little more for short periods) are possible, but it is not possible to suspend hundreds of miles of planet or planet parts above another planets surface. Gravity would collapse the arrangement into a sphere very rapidly.

Any planetary collision would also sterilise the planet.


  • $\begingroup$ Would a planet still be sterilized if the collision happened during it’s molten state? $\endgroup$ May 29, 2021 at 17:53
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    $\begingroup$ I can't imagine that anything living would be able to survive in super heated molten lava, so I suggest any planet that was in a molten state at any time (with or without collisions) would be sterile. $\endgroup$
    – Slarty
    May 29, 2021 at 21:12
  • $\begingroup$ oh, sorry the way I read it I thought you meant it would permanently sterilize the planet. $\endgroup$ May 29, 2021 at 21:27
  • $\begingroup$ Should be good for life to evolve in a few hundred million years after it cooled down $\endgroup$
    – Slarty
    May 29, 2021 at 22:27
  • $\begingroup$ @SentiCarter: The impact is going to reduce the planet to a molten state, regardless of its previous condition. We also know that a planet & moon can become mutually tidally locked. See Pluto & Charon. $\endgroup$
    – jamesqf
    May 30, 2021 at 3:59

Earth did fine.

Earth got hit by something big early in its planetary career.


Got the Moon knocked out of it, yes she did. And here we are, living the life that life lives. I am anyway. I have a snack coming up! I conclude a big impact does not preclude life coming to exist later on.

Other issues: could impact produce tidal locking? Tidal locking can happen with or without impacts. I imagine impact could facilitate.

Other other issues: can a tidally locked world come to support life? That has been dealt with elsewhere here and is fun to think about but I do not think there is anything about a half and half world that precludes life.

I was hoping this idea was looking for a mechanism to allow a gentle and life-sparing collision between planets but no. I think I have one that will meet L.Dutchly approval but I will nurture it a while more until the right opportunity comes.


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