Earth-like Sun-Synchronous Planet?

  • Thread starter Bubchubb2
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  • #1

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Hey there friends,

I'm currently world building for a story and potentially a graphic novel. I'd describe the universe as a fair blend of high-fantasy/spirituality/science. While there's a lot of "magic" within the world, I'd like the "material plane" to be pretty solidly grounded in correct and familiar physics. So, I've some questions to pose:

1.) Within a single-star system (unary?), would it be possible for a planet to form and continue to exist with Earth-like size and conditions, in a sun-synchronous orbit (like our moon's geo-synchronous orbit)?

2.) If one could indeed exist, what might the conditions be like? With one hemisphere always facing the sun: What would ocean currents be like? Weather? Plate tectonics? etc...

3.) Would it be feasible for said planet to have one or two small moons orbiting it?

4.) How might these things change/would these conditions be possible in a binary star system?

Any information that anyone could lend would be appreciated immensely!
Thank you!

Answers and Replies

  • #2
Simon Bridge
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1. the Moon is not in a geosynchronous orbit.
A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period of one sidereal day (approximately 23 hours 56 minutes and 4 seconds)
Do you mean it rotates with one face always towards the star?

It is not impossible. You can get coincedence between orbit and rotation so they get locked in integer relations with each other. See: Mercury. But it is usually achieved by tidal locking.

2. the distribution of heat will not be uniform - one side is a blasted desert and the other side is icey.
A lot depends on other factors - distance, presence of moons etc. Considering this is SF, you'd want a melt zone for glaciers which would give strong circulation close to the terminus. Think ice-age Earth but with less livable surface area to go around and no seasons ... the weather may be less changeable I suppose but you'd always have the same prevailing wind for eg.

3. planets can have any number of moons. There will also be objects orbiting in resonance with it.
A large moon will give you more leeway allowing tidal heating and stabilizing the atmosphere (hand-wavey).

4. a close binary may have stable planetary orbits about both stars, you could have one face of the planet always point at the center of mass of the two.

a distant binary may have long-term orbits about one star.

The presence of other large mass bodies in the system can considerably alter orbits.
You probably want to avoid them if you really want a resonant period. Or make sure they are very far away: scaled Mercury-Jupiter type distances for plausibility. Or go for orbiting both stars.

In general, a small body falling in the gravity of two much more massive bodies is a chaotic system with no analytic solutions... OK maybe stuff could accumulate in a Lagrange point. That may be safest and it is a SF staple. Travellers may be able to tell their position by the number and relative position of the suns they see above the horizon.

OTOH: in the distant binary approach, a planet may be tide locked to a small cool close star and periodically warmed by a hotter distant star. May be easier on the grey-matter.
  • #3
Thank you!

Apologies, you're correct, I misused the term "geosynchronous orbit". You're right: I was actually talking about the spin-orbit resonance. I'd figured the desert/ice hemispheres would be a given, considering the situation, but I never thought of a melting zone at all.

Now, is there any kind of natural event that could transpire that would somehow speed up the planet's rotation to assume a standard day/night cycle? (I feel like such a static world of dualistic hemispheres would probably get pretty dreary to write in for too long, haha)

Assuming it was somehow possible (naturally or otherwise) to speed up the rotation, there would certainly be some pretty intense changes to the planet. The changes that I can gather are:

- day/night cycle
- melting of icy hemisphere (eventually leading to rise in ocean levels and diminishing land mass)
- over time, more diverse flora and fauna on desert hemisphere
- intensified weather patterns
- north/south poles would become icy (assuming little to no tilt in the planet's axis)

If you can think of anything else or elaborate/correct me on anything I've listed, once again it would be much appreciated!
  • #4
If your planet gains rotational angular momentum, it'll lose orbital angular momentum (gets closer to the sun.) The extent of this I'm not sure at all of though.

And again, I'll need this line of thinking verified by others but; The Earth's rotating inner core can shift its speed [Kenneth C. Creager Nature Geoscience 6, 424–426 (2013)] and also changes in the distribution of its mass via, say, tectonic activity could be a couple of inciting incidents that'll start the ball rolling (don't forgive the pun.)
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  • #5
Excellent! Thank you everyone!

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