There's a binary star system with a close pair of stars ( A1 V and G2 V), in which there's a rough analogue of our Mercury that orbits both of them - a planet that's supposed to be closest to the pair and very hot. Rocky type with a radius of 4600 kilometers and internal composition roughly analogous to Venus or Earth.

In the concept, the combined effect of the stars luminance with the varying gravitational shifts from them orbiting around each other results in a planet that can't "settle down" and form a permanent crust, being under constant gravitational stress (Like Io is in real life, but much more pronounced) and constantly breaking up and renewing, with lava fields (or even whole exposed mantle zones) so large they're visible from the orbit:

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So the question is, does such a setup sounds plausible? An additional question - how quickly will it be losing its mass due to eruptions and the solar wind?


3 Answers 3


It depends how close the binary stars come within each other... as you say, your planet is close to its star, like Mercury. To have tidal effects from the other star, it would have to come so close it may even disrupt the orbit of the planet's parent star, and unpredictable things would happen. It may be more likely the planet would be flung out of its system rather than have a steady tidal friction that you want.

In my planetary science studies, I've found that the best way to ensure steady active tectonics is to have a rocky planet, about earth size or larger to ensure a hot core, and with a metallic core to ensure magnetic fields.

Info about binary stars and their planets:

From https://en.wikipedia.org/wiki/Alpha_Centauri Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its luminosity.[16] The pair orbit around a common centre with an orbital period of 79.91 years.[17] Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 AU (astronomical units), or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun. Alpha Centauri C is about 13,000 AU away from Alpha Centauri AB. [wont disrupt A or B’s planets]

From https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems : In non-circumbinary planets, if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed. Studies of Alpha Centauri, the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 au distance at closest approach (23 au mean), and both have stable habitable zones. A study of long-term orbital stability for simulated planets within the system shows that planets within approximately three au of either star may remain stable (i.e. the semi-major axis deviating by less than 5%). The habitable zone for Alpha Centauri A extends, conservatively estimated, from 1.37 to 1.76 au[2] and that of Alpha Centauri B from 0.77 to 1.14 au[2]—well within the stable region in both cases."



Just take the Jupiter setup, and scale it up massively.

The central star is massive, and dominates the gravitational forces in the whole system.

Put the planet around this star. Let's call its orbital period 1
This planets takes the role of Jupiter's moon Io.

Replace Europa and Ganymede with suitably scaled much smaller stars. (or supermassive planets) including tailoring their orbits for a 1:2:4 resonance ratio with the planet.

The resonant orbits keep the relative position of both orbiting stars and the planet locked into synchronization, but repeatedly pumps the planet (and incidentally the stars) with tidal forces.

Over really long astronomical timespans the whole system will slowly contract, as orbital energy is converted to heat in the stars and the planet, but it will be stable over several billions of years. Quite likely over a longer timespan than the comparatively brief life of the central star.

P.s. If someone knows better, please correct this.

I believe that just 2 stars + planet is not enough.
As I understand it, you need at least 3 orbiting objects in resonance, for the resonant orbits to be stable on really long timespans.
Otherwise the orbits slowly become more and more eccentric, and eventually the whole thing unsnarls itself.


I don't think that configuration with two close stars and a planet close to them is going to be stable: the planet will be kicked out by the pair in a rather short time.

I have played quite a bit with orbit simulators, and the only way I have found to have a stable system is to have either close stars and far planet or one star far from the other while the planet is close. Otherwise the expulsion happens within few orbital cycles of the planet.

In that short time the tidal heating will be negligible.

  • $\begingroup$ i mean if you put the planet in one of the stable lagranges of the stars then it should get to a stable equilibrium in orbit. its hard to achieve naturally, but can happen. i would place it in L4 of the stars, and give it a moon or two to help multiply the tidal effects. $\endgroup$
    – zackit
    Commented Mar 11, 2021 at 13:55
  • $\begingroup$ @zackit you'll have a problem keeping something planet-sized at a Lagrange point for very long... even millions of years is going to be a stretch. $\endgroup$ Commented Mar 11, 2021 at 15:47
  • $\begingroup$ @StarfishPrime isn't Lagrange stability just a matter of the mass ratios of primary, secondary and tertiary? I recall the ration of 25:1 being required. I.e. for a stable Lagrangian L4 or L5 point, the Primary must be at least 25 times the mass of the secondary, and the secondary must be at least 25 times the mass of the tertiary. The problem with a Binary star system and a planet in the L4 is not that the planet is too heavy, but that the secondary star is too heavy. $\endgroup$
    – PcMan
    Commented Mar 13, 2021 at 19:08
  • $\begingroup$ @zackit planet in the L4 would not experience gravitational tug from the stars. And giving it a moon is not an option, there is no stable solution for a moon around an L4 or L5 object in any sort of long term. Nor for a binary in the Lagrangian position. those orbits can and will get distorted. $\endgroup$
    – PcMan
    Commented Mar 13, 2021 at 19:12

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