(See diagram) Presupposing that a Rocheworld can even exist...

I imagine that sunlight would work more or less the same in the atmosphere of any part of an Earth like planet that was tidally locked with a duplicate planet in a Rocheworld scenario. But what about in the area between them, where the planets share atmospheres?

An atmosphere traps the sun's rays and that's why we get day light and heat etc. But that's also because the surface of the planet is bouncing it back too, right? But the area of shared atmosphere in Rocheworld does not have solid ground, so the light would go right through. What does that do for light, visibility, warmth, radiation, etc?

For arguments sake lets say that the planets are angled somewhat so both the planets can get sun rays.

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Thank you.

  • 2
    $\begingroup$ Just for clarification, "lets say that the planets are angled somewhat so both the planets can get sun rays" seems to imply that the two worlds will have a fixed face toward the sun. This is wrong. They are tidally locked with each other, not the sun. They will be orbiting around there common center of mass quite quickly. $\endgroup$
    – Lex
    Jan 30, 2018 at 18:40
  • $\begingroup$ @Lex. I understand that they are locked to one another, but I'd also been led to believe that the "middle" area between the planets would not get much sunlight because one of the two planets would be blocking the other. So that's not the case? $\endgroup$
    – Len
    Jan 30, 2018 at 18:49
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    $\begingroup$ @len what I think Lex means is that there are 2 orbits going on in such a system; 1. Both planets round the sun (This keeps them from falling into the sun) 2. Both planets round each other (This keeps them from falling into each other). Effectively you have a planet-moon system where the moon got scaled up to planet size. Just like in a lunar eclipse, these bodies will periodical block the light from each other. Only the geometry means this will happen way more often than a lunar eclipse. $\endgroup$
    – Clumsy cat
    Feb 1, 2018 at 16:24

2 Answers 2


An atmosphere traps the sun's rays and that's why we get day light and heat

Not really.

Atmosphere doesn't trap sun light, it is practically transparent to it. Infrared radiation is absorbed by some gases, like water vapor, CO2 and CH4, also called greenhouse gases.

But gases alone are not sufficient to capture light, you need something else to capture visible photons and emit part of that energy at lower frequency. That's also why temperature is higher on the sea level than it is on top of mount Everest, because you have much more absorbing stuff.

In your case you would get the superposition of the temperature gradients, resulting in a minimum temperature somewhere midway between the two planets.

Visibility would not be much different than elsewhere, at least for the inhabitants scale.

  • $\begingroup$ Would everything be half as much in that area of shared atmosphere? Half as much heat, but also half as much light. etc.? $\endgroup$
    – Len
    Jan 30, 2018 at 18:45
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    $\begingroup$ Short answer: no. Less short answer: climate models are a hell of a non linearity and cross dependencies.... $\endgroup$
    – L.Dutch
    Jan 30, 2018 at 18:48
  • $\begingroup$ Is It is a habitat that it survivable by a human, In terms of the affects of the suns rays, or do you foresee major issues? $\endgroup$
    – Len
    Jan 30, 2018 at 18:52
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    $\begingroup$ Sunlight may kill over years because of skin cancer. Lack of oxygen is much faster, it's a matter of seconds. $\endgroup$
    – L.Dutch
    Jan 30, 2018 at 19:32
  • $\begingroup$ Hmmmm... that's what I thought. I'm handwaving the oxygen part, but is it because oxygen as a gas could not exist in such a scenario or that it would escape quickly? $\endgroup$
    – Len
    Jan 30, 2018 at 20:18

I'm affraid this would not be physicaly viable for two planets to share an atmosphere.

This has been debated at length here.

If you went over this limitation, you probably would have no effect on the light inside the planet, but you would have a VERY LARGE moon: the planets would have to be roughly 1000 km appart ( the earth atmosphere is around 500 km large). if you compare to the moon, it is ~380 000 km away.

The light reflected from this 'moon' would dwarf any light from these atmosphere.

  • 1
    $\begingroup$ Note that there's a difference between two planets sharing an atmosphere and a planet and moon sharing an atmosphere (this is the scenario in the Reddit link). The former scenario is plausible, while the latter is not. $\endgroup$
    – Frostfyre
    Jan 29, 2018 at 20:20
  • $\begingroup$ The question presupposes that Rocheworld is possible and the atmosphere is shared. An answer based on saying it isn't possible does not answer the question asked. $\endgroup$
    – T.J.L.
    Jan 30, 2018 at 5:13
  • $\begingroup$ @Gorille, this question piggybacks on several other questions that have been asked about Rocheworlds/tidally locked planets. The verdict is still out. BUT, as far as this question... it seems like you're saying that the planets would bounce light back and forth to one another(...the light reflected...)? With the shared atmosphere being between them that would mean it would be very well lit. What about heat. etc.? $\endgroup$
    – Len
    Jan 30, 2018 at 18:11
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    $\begingroup$ @Len interesting idea ! I hadn't even thought of the light bouncing between both planets. $\endgroup$
    – Gorille
    Jan 31, 2018 at 18:54

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