# Two celestial bodies tidal locked - not with the sun

I tried to do my homework and actually thanks to this question and answer, I found myself to this site. But I just found more questions :)

Basically the setting would be close to what Earth - Moon is, but both objects being habitable and with 1G (Earth's gravity).

Preferably one of these would be tidally locked to another but neither to the star, so both would experience day/night cycle.
(I understood that if both are tidally locked to each other like Pluto and Charon, their tidal waves would slow them down and they would spiral into each other?)

1. First, could this work? Another spinning freely while another tidal locked to another?

2. As rotating axis of the tidally locked "moon" would have to be set, can the "planet" have it just about anyway imaginable? For example its pole pointing at the locked "moon"?

3. "Moon's" surface that is pointing at the "planet", would it be just ocean so deep there is no dry land and would another side be barren without any bodies of water?

My initial plan would be to have my world in this tidally locked "moon" so that another side would constantly see the "planet" while another would not. The magnetic axis would point towards the "planet", let's call it the "north pole". I would have the cultural center of this world at the magnetic pole but I am not sure if the "north pole" would be just ocean.

1. Would rotating around barycenter cause seasons as the distance from the star shifts a bit constantly? Or should I throw in an elliptical orbit too?

• Welcome to worldbuilding. Please be aware that, as stated in our help center, we prefer 1 question per post.
– L.Dutch
Dec 1, 2021 at 16:51

1. Not realistic long term under your conditions. Moon is tidally locked to Earth, while Earth is rotating freely (this is like what you want), but Earth is 81 times more massive than Moon. You want both bodies with 1g surface gravity, which necessarily makes their masses much closer. What you ask might be happening relatively early in binary planet system evolution, when one partner is already tidally locked while the other is in the process of it;

2. Yes, until the bigger partner is locked, its axis of rotation can be any. However, the question of how this happened in the first place (is there a realistic protoplanetary collision that would lead to this kind of rotation) is open;

3. No. Actually, tidal waves bulge on both near and far surfaces of a planet. If a planet is tidally locked, then its lithosphere would rearrange in a somewhat oblong shape, making ocean depth more or less uniform in any region of a planet;

4. Yes. If (when) Earth would lock up to the Moon, it will still have seasons, because Earth-Moon rotation plane is different from the Sun-Moon rotation plane, so Earth would still turn different sides to the Sun while going around it.

• Thanks for (3) I never realized this secondary bulge on the other side, let's put a source, oceanservice.noaa.gov/education/tutorial_tides/… Dec 1, 2021 at 20:35
• "Yes, until the bigger partner is locked, its axis of rotation can be any." Not unless the moon planet has negligible mass as a ratio of the main plant. If the mass ratio is significant, the tidal effects of the main planet's crust/mantle will take care of aligning the planet rotation axis orthogonal on the co-orbital plane. For the transient ages in which it doesn't, the planet will be uninhabitable with a high active volcanism. Dec 1, 2021 at 21:51
• @Adrian Colomitchi yes, in the process of tidal locking, planet's spinning axis will be rearranged, and yes, this may cause significant tectonic activity. Dec 1, 2021 at 21:54
• "both bodies with 1g surface gravity, which necessarily makes their masses much closer" - unless you make the moon planet much denser than the main planet, in which case $1/R^2$ relation of the surface gravity on the moon will catch up (to 1g) at a smaller mass of the moon planet. Whether or not increasing the density will keep the moon planet still habitable... ummm... I don't think so, plenty of heavy elements (and not enough of the lighter ones) aren't quite supportive to life as we know it. Dec 1, 2021 at 22:08
• @Adrian Colomitchi if we make Moon out of pure gold, its surface gravity would be close to 1g - but then its mass will have to increase about 6 times. Dec 1, 2021 at 22:40