# The creation of the planets in our Solar System **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [August 14, 2025, 9:11pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065 "2025-08-14T21:11:18Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [August 16, 2025, 4:13pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/21 "2025-08-16T16:13:25Z")
> [@dtilque](#): > > Not true at all. We have an example of two objects of similar size sharing an orbit and not where they’re in orbit around each other nor in an L4/L5 configuration. That example is Janus and Epimetheus, two satellites of Saturn. They’re in a co-orbital arrangement that involves them coming close to each other every few years, at which point they “swap” orbits – the inner one moving to be the outer one and vice versa. There’s no reason two planets couldn’t fall into this kind of co-orbital situation. [Janus and Epimetheus](https://www.planetary.org/articles/janus-epimetheus-swap) are in small, comparable mass objects in very close orbit to Saturn, so they can develop resonances like this with relative ease as compared to planets at a distance from Earth’s orbit to the Sun. Long term this likely not a stable configuration but then Saturn is a highly dynamic system to begin with; it has over 270 satellites, a complex system of rings, and the planet itself has a mean density that is less than liquid water. It’s a weird and wild place. > [@dtilque](#): > > There’s a widely accepted hypothesis that the Moon was formed by another planet (roughly 10% the mass of the Earth which they’ve named Theia) colliding with the Earth some 30 to 50 million years after they formed. Well, where was Theia for those 30M years? Could it have been in co-orbit with the Earth? Which orbit was then destabilized by a passing star or rogue planet or something dramatic like that? The impact of the hypothetical Theia into the primordial Earth would have been a violent event for them to not just merge into a single body, instead producing Luna from ejecta at the edge of orbital stability about Earth’s sphere of influence, indicating that Theia must have been in an highly eccentric, overlapping orbit coming from the outer solar system 9or possibly even an extrasolar object although that is considered unlikely). Stranger ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 17, 2025, 8:44am UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/22 "2025-08-17T08:44:45Z")
> [@Stranger\_On\_A\_Train](#): > > The problem is that with two comparable Earth-sized bodies in relatively close orbit (and especially with active, liquid cores and a plastic mantle) the tidal interactions will end up siphoning off gravitational energy via internal friction and they’ll ultimately end up colliding after spiraling down. I don’t see how that could happen. In most cases, the two bodies and their overall rotation will all have angular momentum in the same direction. Energy dissipation from tidal forces will _separate_ the two bodies, because if the angular momentum from the rotation is lost, it has to go into the orbit. And orbits gain angular momentum as they separate. Which is why the Moon is slowly moving away. There are a few other ways for the angular momentum to dissipate, but they aren’t significant on a planetary scale. Gravitational waves are only significant at the neutron star/black hole level, and light pressure effects are only significant for small, irregular bodies. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [August 17, 2025, 1:22pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/23 "2025-08-17T13:22:24Z")
> [@Dr.Strangelove](#): > > I don’t see how that could happen. In most cases, the two bodies and their overall rotation will all have angular momentum in the same direction. Energy dissipation from tidal forces will _separate_ the two bodies, because if the angular momentum from the rotation is lost, it has to go into the orbit. And orbits gain angular momentum as they separate. Which is why the Moon is slowly moving away. It’s my understanding that the Moon is moving away _now,_ but eventually would become tide locked and start spiraling in instead until it reached the Roche limit and breaks up. It’s just that it and Earth will be immolated by the Sun first as it dies, so it doesn’t matter. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 17, 2025, 7:30pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/24 "2025-08-17T19:30:31Z")
That would have to be an _exceptionally_ long-term process, again because there are few places for the angular momentum to go to. Gravitational waves would do it eventually, but the universe would end first, not just the Sun. Same for light pressure effects, dust, etc. Two Earth-sized bodies would become tidally locked fairly quickly but I don’t know of any rapid process after that where they can shed the remaining angular momentum and spiral into each other. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [August 18, 2025, 1:51pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/25 "2025-08-18T13:51:03Z")
> [@Dr.Strangelove](#): > > if the angular momentum from the rotation is lost, Does angular momentum _have_ to be lost, though? As the bodies get closer, they just co-rotate faster around the center of gravity thus maintaining the total angular momentum of the system. As for total energy: the faster the co-rotation, the greater the kinetic energy. But at the same time the bodies are getting closer, so gravitational potential energy decreases. Admittedly this is a somewhat tricky problem and I need to think about it some more… ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 18, 2025, 7:43pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/26 "2025-08-18T19:43:38Z")
> [@xtenkfarpl](#): > > As the bodies get closer, they just co-rotate faster around the center of gravity thus maintaining the total angular momentum of the system. No, the orbital velocity is determined by the masses and the distance between them. For one mass much larger than another (like an artificial satellite), the velocity is just v=\sqrt{\frac{GM}{r}}. And angular momentum is just rmv. Substituting v you get \sqrt{GMm^2r}. Which means that angular momentum scales proportionally to \sqrt{r}, holding everything else constant. It gets a little more complicated with non-negligible masses, elliptical orbits, etc., but the basic principle is the same. And anyway, orbits tend to circularize over time for energy-related reasons. ---
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [August 19, 2025, 1:48pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/27 "2025-08-19T13:48:41Z")
Yes but our moon did not form with the Earth, it is the result of a collision with another body. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [August 19, 2025, 5:39pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/28 "2025-08-19T17:39:02Z")
So what does this lead to in the case of a double planet (a la Rocheworld or Summertide)? Let’s set up a clear gedankenenexperiment. Two exact copies of the Earth in size and composition, co-rotating, stably in the short term, around a barycenter, with a separation of 3 Earth radii between their centers. No other gravitational influences. I would think at that separation they would be tidally locked (though there might be a bit of libration)? At least let’s start with that assumption. How does this system evolve over the long term? PS: of course there are a lot of potential questions about how, or even whether, such a system could come about in the first place. But as I said: this is a gedankenenexperiment… 🙂 ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 9:07pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/29 "2025-08-19T21:07:03Z")
I’m about 90% certain they’d immediately merge into one planet. The tidal forces would be _immense_–a point on the near side of one body would experience 4x the force as one on the far side. So they’d immediately elongate, and that would be a self-reinforcing effect, until the two bodies touch, at which point all hell would break loose, so to speak. Their splatting together would send debris all over and I’m not sure how it would evolve, but most likely a bunch of stuff would fly off and the rest of it would behave chaotically until it settled down into a sphere. I don’t know about Rocheworld but it looks like the position has been delicately arranged so that the planets touch gently. I’m not sure if this is stable, even if it’s actually in equilibrium (think of a pencil balancing on its point). My inclination is that it’s not stable; that some small perturbation would grow until again the whole thing breaks down and it mostly turns into a sphere. **Maybe** it’s possible that the small but non-zero strength of the crust could hold those perturbations in check; normally we ignore that (it’s part of the definition of the planet), but maybe in a case like this it could be an influence. Overall, though, I don’t know. This isn’t a case where I can apply basic Newtonian physics and come up with an answer. If the other planet was 60x the radius away, as the Moon is, I think they’d just become tidally locked pretty quickly and then be stable. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 19, 2025, 9:08pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/30 "2025-08-19T21:08:02Z")
That close together, they might tear each other apart, and at the very least would have extremely distorted shapes. If they did tear each other apart, then you’d have a chaotic spinning debris field, that would eject objects (and hence angular momentum) until the angular momentum of the remains were low enough to settle into a single rapidly-spinning world. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [August 19, 2025, 9:14pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/31 "2025-08-19T21:14:52Z")
Even if farther away (the current Earth/Moon distance, which note is 32 Earth diameters), the issue is that larger mass of the companion will destabilize the axial tilts much quicker than the Moon will, causing all sorts of climate havoc when they do. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 9:22pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/32 "2025-08-19T21:22:57Z")
Well, there are lots of problems. Even if you make the planets out of scrith, the regions between the two planets get almost no sunlight. Having a third ice-cap on the side of your planet is going to make things weird. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [August 19, 2025, 9:23pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/33 "2025-08-19T21:23:02Z")
Contact binary stars are a relatively commonplace type of star; these form curious elongated shapes where the stellar material more-or-less fills the Roche Lobe. To demonstrate that planets _can’t_ form into similar binary configurations, you’d need to show that there is something intrinsically different between close-orbiting planets and close-orbiting stars. [![](https://www.orionsarm.com/im_store/med_s%20antilae.png) ](https://www.orionsarm.com/im_store/med_s%20antilae.png) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 9:36pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/34 "2025-08-19T21:36:31Z")
One significant difference is that there is a vastly higher range of densities in the stars compared to a planet. On Earth it’s about 4x. For a star, it’s thousands of times. So a contact binary is much more like two planets orbiting outside of their Roche limit, but where you’ve added enough atmosphere so that they merge. Interesting but not quite the same thing. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [August 19, 2025, 9:41pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/35 "2025-08-19T21:41:17Z")
> [@Dr.Strangelove](#): > > Well, there are lots of problems. Even if you make the planets out of scrith, the regions between the two planets get almost no sunlight. Having a third ice-cap on the side of your planet is going to make things weird. Huh? They aren’t going to stop in their orbits if tidally locked (orbits matching their orbits around the sun, to be precise). ? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 9:48pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/36 "2025-08-19T21:48:58Z")
The planets will shadow each other to a large extent. There will be a several-hour eclipse that happens every day. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [August 19, 2025, 9:49pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/37 "2025-08-19T21:49:28Z")
Only if their orbital planes coincide with the ecliptic, which for the moon and Earth they don’t. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 9:51pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/38 "2025-08-19T21:51:21Z")
The proposal was two equal-sized planets only 3 radii apart (i.e., only one radius between the surfaces). The tiny angle from the ecliptic won’t make much difference. The Moon doesn’t eclipse on every pass because it’s small and far away. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 19, 2025, 10:15pm UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/39 "2025-08-19T22:15:06Z")
For someone at the darkest spot, the other planet will take up 60 degrees of the sky (compared to 0.5 for the Moon). So ~4 hours of eclipse every day, assuming a 24 hour rotation. And those happen to be the prime daylight hours as well, basically 10 am to 2 pm. That’s a dramatic reduction in light. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 20, 2025, 12:53am UTC](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065/40 "2025-08-20T00:53:02Z")
Even if you imagine the co-rotating plane is 90 degrees to the ecliptic so each planet sees the other as being in a polar orbit about itself you still get eclipes & climatic issues. That plane is aligned w the fixed stars. So as they both progress together around their solar orbit, sometimes one is sunward from the other and vice versa. The real Earth may be a mess, but that place would be very unhealthy to the biosphere. If they even held together at all over a timescale of mere centuries, much less millions of years. [Previous page](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065.md?page=1) [Next page](https://boards.straightdope.com/t/the-creation-of-the-planets-in-our-solar-system/1022065.md?page=3)