Sigelon is a mountainous world that's volcanically-active due to flybys with a larger neighboring planet that has a more eccentric orbit. These volcanoes release enough soot and smog to periodically blot out the sun, but these events happen about once every few years. However, carbon-based life is able to survive, and thrive even, in these conditions. How large would the neighboring planet need to be, and how close will it need to get, to cause intense volcanic activity without disrupting orbits too much, or stripping Sigelon of its atmosphere?

Let's put Sigelon at a radius of 3.6 Earth radii and a gravity of 4.5 g, so its mass would be 58.32 Earth masses. It is terrestrial, and orbits a star with a mass of .3 solar masses at a distance of .122 AU. Use those numbers as your reference.

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    $\begingroup$ Your proposed figures produce a sun with 1.48% the luminosity of sol, but you've reduced the orbital radius to 1.48% of Earth's orbital radius. This does not, however, compensate, because solar radiation intensity obeys the inverse square law. So by being ~100 times as close, you're receiving ten thousand times the radiation intensity. Since that intensity is cut by 98.62%, you're only receiving ~100 times the energy per square metre that earth does, but that's still more than enough to blast all life from the surface. $\endgroup$
    – jdunlop
    Dec 8, 2020 at 6:23
  • $\begingroup$ It's also worth noting that a rocky planet of ~60 earth masses is nearly half again as massive as the biggest rocky planet ever discovered, itself presumed to be a failed gas giant. Comparatively, the rocky/metallic core of Jupiter is only ~12 earth masses. While the initial blowtorch distance from the sun might have prevented Sigelton from becoming a gas giant, placing it at a habitable distance wouldn't save it from that. $\endgroup$
    – jdunlop
    Dec 8, 2020 at 6:32
  • $\begingroup$ jdunlop, I've edited it to put Sigelon further from its sun, where it will be cooler. $\endgroup$ Dec 8, 2020 at 17:45
  • $\begingroup$ The first half of your question appears to be completely irrelevant. Please edit your question to only include pertinent details. Right now it's not entirely clear what you're asking. $\endgroup$
    – Matthew
    Dec 8, 2020 at 18:32

1 Answer 1


It's not going to work as described.

I don't think there is any arrangement that will cause what you're describing, namely tidal heating caused by a separate planet (not the star or a moon, etc) while also strong enough to produce volcanism on an already massive planet.

I suspect you got inspiration for this idea from Jupiter's moon Io. But Io completes an orbit of Jupiter in less than 2 days, not once every few years. So the first thing you would need to do would be to amplify the force applied by the 'neighbor planet' enough to compensate for the lack in frequency. Let's take "few" to mean 3. Io orbits Jupiter about 620 times in 3 years, where your neighbor planet will pass only once. So your neighbor planet has to impart about 620 times as much heating energy per pass.

Next, Io has a mass of 0.015 Earth masses, while your planet has about 58 Earth masses, making your planet about 390 times bigger. So, only considering mass, your neighbor planet needs to heat 390 times the material.

These numbers would be multiplied by each other, since each pass has to heat all the mass. So each pass of the neighbor planet has to impart about 240,000 times as much energy as Jupiter puts in to Io in a each pass.

So if the neighbor planet were 240,000 times the size of Jupiter, and passing as close as Io orbits Jupiter, that would work. But how big is that? Well, the Sun is just over 1000 Jupiter masses, so this neighbor 'planet' would be 240 solar masses. It's not a planet, it's a HUGE star, far bigger than the one the main planet is supposedly orbiting.

BUT ...

Gravitational force is inversely proportional to the square of the separation distance, so distance is going to make a much bigger difference than mass will. So lets trade all that size for a closer pass. (closer than Jupiter/Io orbit?). Io orbits Jupiter at a distance of about .0028 AU. If this neighbor planet were 'only' the size of Jupiter, it would have to pass within about 0.00036 AU, to have any chance at transferring enough energy. For reference, that's only about 33,000 miles ... less than a 10th the distance between Earth and our Moon. But lets shrink it even more, and say these neighbors are identical twins, at only 58 Earth masses. Now it has to pass within 15,000 miles. They'd likely be brushing atmosphers at that distance. Asking for a pass at those distances, "without disrupting orbits", isn't going to happen.

  • $\begingroup$ Gravitational force is inversely proportional to the square of the separation distance... and tidal forces are inversely proportional to the cube of the distance, because they result from the difference in gravitational force across the an object. $\endgroup$
    – AlexP
    Jan 25, 2022 at 23:37
  • $\begingroup$ @AlexP Doesn't that just make the whole situation even worse than what I've described? Just the square was pushing my math limits ... I really don't want to try and re-do it all with it cubed instead. I suspect you could provide a much more accurate answer than I could. But it was a fun one for me to think through. I'm actually quite pleasantly surprised that that's the only criticism of my logic $\endgroup$
    – Harthag
    Jan 25, 2022 at 23:58
  • $\begingroup$ Yes it does. It was a minor point, not a criticism. Upvoted. $\endgroup$
    – AlexP
    Jan 26, 2022 at 0:05

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