What if two planets orbited each other and also a star?

This situation is quite similar to the current Earth-Moon system. As another answer here says, the two bodies do orbit each other, around a central point called the barycentre. The barycentre of the Terra-Luna system is within the Earth, so it just appears as if the Earth is wobbling but staying in the same place:

If you increase the mass of the Moon until it has the same mass as a planet, the barycentre moves towards the centre of the system, so both planets move, but the Moon's orbit would be less pronounced and the Earth's more pronounced:

You say you're looking for a way to have your planet's gravity change slightly during the day. This is possible, just by changing the planets' orbits slightly. An elliptical orbit means at the furthest point of the orbit, there would be less force pulling people away from the planets' surfaces. When the planets are close together, there is more force pulling people, so gravity feels less.

This works best with more massive planets, since they can exert more gravitational pull.

posted about 9 years ago

ArtOfCode‭ staff

101 reputation 4 65 10 3

The tidal forces would be the main gravitational effects that would change. Tidal forces cause, among other things, tides, and are the result of another object's gravitational pull. The Moon is influential enough. Put the mass of Earth there and the tidal force will increase by a factor of $M_{\oplus}/M_M\approx81$, the ratio of Earth's mass to the mass of the Moon.

I should note that that's only if you assume that the two planets have the same separation as the Earth and Moon do. This could happen, but it's not necessarily the case, especially if you take into account how the Moon formed. This provides a nice segue into the past of the system.

You might have read about the Giant Impact Hypothesis, and it's featured in several answers on Worldbuilding. In short, it says that a protoplanet collided with Earth during the early days of the Solar System. Some debris coalesced around the larger of the two and eventually formed the Moon. Did this system form in a similar way? Are both planets the daughters of some other object, or did they meet gravitationally? This can impact the entire features of both planets.

I am world building and am trying to find a logical way for my planets' gravity to change slightly during a day.

It doesn't seem as if having a second planet would impact the force on a person at the surface of the other planet. If the planets are separated by a distance $d$ and have radii $R$, then the difference in the force felt by one on the near side and the force felt by one on the far side due to the other planet is $$\Delta F=\frac{GM^2}{d^2}-\frac{GM^2}{(d+2R)^2}\approx\frac{GM^2}{d^2}\frac{4R}{d}$$ For Earth, $\Delta F/F=.068$, meaning that the force on a person on the planet from the other planet changes only minutely over the course of a day - and even that is tiny in comparison to the force on that person from the planet they're standing on.

posted about 9 years ago

HDE 226868‭

586 reputation 61 463 77 49