I've heard a lot of people say that having two tidally locked planets close together would have adverse affects on both planets. I have two tidally locked planets at a distance of 16,550 miles apart. Or, about 26635 kilometers apart. Both planets are about earth's size, with a similar composition, but one has about 85% and the other 60% of the surface covered in water. (Including polar ice caps.)

How would this affect the conditions on each planet? I believe I've heard that there would be more tectonic activity, and there wouldn't really be tides, and that the water would be pulled towards the opposite planet. How much truth is there to these statements, and what other effects can I expect?

  • $\begingroup$ If life evolved on such a pair of planets, it should be noted that the inhabitants won't consider the effects to be "adverse", in fact they might wonder about the adverse effects of not having a tidally locked companion... $\endgroup$
    – komodosp
    Feb 22, 2016 at 12:16

3 Answers 3


There won't be any tides. The tidal bulge or bulges will be fixed features. Sea level will be higher in some parts of the world's and lower in others compared to how it would be without the companion planet. This will be of no interest to any inhabitants other than a few physics students.

Tectonic activity might start to decline once the planet's get locked ... Over a timescale of hundreds of millions of years. Tidal drag is not the only motive force. Heat from radioactive decay and (possibly) phase changes in the planet's core also provide energy. You can make these drive tectonics if you want to.

The big thing is that it's hard to see how you could get a locked pair of planets with anything like 24-hour-short days. More likely month-long days. That has major implications for weather, climate, evolution. How to survive two weeks or longer of continuous night over an entire hemisphere?

  • $\begingroup$ Getting it is the easy part. My version of hand waving is silicon based aliens creating habitable star systems. Also, on a previous question, someone came up with that distance, where two tidally locked earths would have an orbital period of 24 hours. (And remain stable) $\endgroup$ Feb 20, 2016 at 14:35
  • $\begingroup$ A month-long orbital period for two Earth-mass planets on ≈ 30 Mm distance? Do you really remember how far is our Moon from Earth? $\endgroup$ May 16, 2016 at 18:11

OK your planets are 26,350 km apart (reduced the number fractionally to make the numbers easier), but they have diameters of 12,700km. So the closest point of one planet is only 20,000km from the centre of the other one, while the furthest point is 32,700km away.

We know that the planets generate 1g at a distance of 6,350km from their centres, because they're the same as Earth. So at 20,000km they generate 0.101g, while at 26,350km it's 0.058g and at 32,700km it's 0.038g. Therefore, you're about 4% lighter at the point directly under the other planet, and about 2% lighter at the point directly opposite it, probably not enough to notice but easily measurable even with primitive technology.

The whole of each planet will be stretched by the differential gravity, and they will bulge towards and away from each other. But their hydrospheres and atmospheres are less rigid and will bulge more, resulting in deep oceans and thick atmosphere at the points under and opposite the other planet, and probably no water and thin atmosphere on the circle in between those two points.

  • 1
    $\begingroup$ Actually the rock of the planet will bulge in the same form as it's oceans. Here on Earth the ocean can flow up and down with two tides per day but the rocks cannot. But over geological time rocks will also flow into the same shape as the ocean once the worlds are tidally locked. $\endgroup$
    – nigel222
    Feb 23, 2016 at 22:15

You are very close to the Roche Limit, if not inside it.

A planet has no structural strength on a large scale. Gravity is the only force holding it together. Consider two rocks, one on the surface closest to the other planet, one on the furthest. Now these rocks are in different orbits. The inner one, if the planet wasn't there, would be in a faster orbit that the outer one. It's not going fast enough, so Mg is bigger than v^2/R.



Robert Forward did a pair of books, Roche World and Return to Roche World that takes place on such a binary planet. As I recall he did a good job with the science. (He was a physicist)


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .