# Varying Gravity

Suppose a very large planet orbited a star or other spacial body in an oval shaped path, similar to Earth's but closer to induce a varying gravity during orbit.

The planet has a large gravity well and the star or other body has a stronger one when nearest to it on the planet's orbital path.

1. Is this possible?
2. Assuming it is possible and life has started and it has adapted to the shifts in gravity, would the organisms be pulled towards the star or other body when it is closest the the star or other body but still remain on the planet, creating a stretched effect?
• Unfortunately, I don't think it is possible. Using Newton's law of gravity: g = {\frac{GM}{r^2}} the distance from the star is not taken into consideration. In order for the star to be able to ''pull'' the planet, the planet needs to be pretty close. Probably closer than Mercury. But even at that distance, I'm not sure the star would have an impact of the surface gravity of the planet. Oct 14, 2014 at 19:43
• this question is also about gravity : worldbuilding.stackexchange.com/questions/243/… Oct 14, 2014 at 19:44
• @Vincent: Newton’s law does not take this into consideration, because that’s not what it is about. It does not mean that there is no influence. Oct 14, 2014 at 20:12
• I don't have time to write up an answer, but I strongly believe that one term you are looking for is orbital eccentricity. The closer to zero it is, the more circular the orbit is. (An orbital eccentricity of exactly 0 means the orbit is perfectly circular. See the linked Wikipedia article for more.) For example, Earth's orbit has eccentricity approximately 0.017 according to Wikipedia, whereas Mercury's orbit has eccentricity 0.206 and Pluto close to 0.25 (also Wikipedia). It doesn't normally impact gravity the way you propose, though.
– user
Oct 14, 2014 at 21:28

As an expansion to Mark's good answer: the increased tidal forces (which cause the stretching you want) aren't directly from increased gravity -- a planet in orbit is already in "freefall" and so doesn't directly feel this force. It's "weightless", the same way you would be in a freefalling elevator car.

Tidal forces are caused by the difference in gravity between the front and back of the planet. It is the planet's center of mass that is in orbit. Anything closer to the central mass than the center is slightly deeper in the gravitational well, and feels an additional tug toward the center. Anything on the far side of the planet (not facing the central mass) will be in a slightly weaker gravitational field, and will thus feel a slightly stronger centrifugal force (from the planet's orbital revolution) pushing away from the central mass. This difference of forces creates a stretching effect which, on Earth, causes the tides.

The strength of the stretching effect depends on how quickly the gravity drops off with distance. The closer you get to a central body, the more rapidly gravity's strength increases, so the strength of the tidal forces would increase. The stretching force would always be along an axis towards/away from the central body, so if that body was on the horizon, you'd have a horizontal stretching. As Mark suggests, I'd strongly recommend making the central body be a gas giant (and making the planet actually be its moon) so that you can get close enough to it to feel substantial forces without getting burned.

Such a planet would be "tidally locked" to the central gas giant, although in an eccentric ("oval-shaped") orbit, this would actually end up resulting in a wobbling motion (even the moon has a slight wobbling, called libration). This is because, although the rotational and orbital periods are identical, the orbital angular velocity will vary depending on distance, while the rotational rate will essentially be constant. The wikipedia article I linked to has a neat animation which shows this libration effect, and the moon's orbit is nearly circular. It would be more pronounced for a more eccentric orbit. From your planet's perspective, this would cause probably the large central body to appear to trace a sort of figure-8 pattern in the sky over the course of the orbital period (which would be essentially a month-long day if you're orbiting a gas giant).

As a side note: You wouldn't have to have an eccentric orbit for this stretching effect to exist. You could just orbit close to the central body in a nearly-circular orbit, and you'd feel the effect year round. In an eccentric orbit, though, you'd only feel the strongest effects for a fraction of the year.

But you have another problem (as Neil Slater points out in a comment). The strong tidal stretching forces, coupled with the libration from the eccentric orbit, is going to cause a lot of bending and flexing in the planet itself, which will cause a lot of friction and heat, a hot molten mantle, and a cracked crust. This will almost certainly lead to lots of volcanoes and earthquakes (and in fact, is exactly the reason why Jupiter's moon Io is covered in over 400 active volcanoes).

• As a small aside, a moon close to a gas planet doesn't necessarily be fully tidally locked but could, given an eccentric orbit have a phased tidal lock, like mercury around the sun. Oct 15, 2014 at 6:00

Yes, it's possible.

The limiting case for approach distance is the Roche limit: the distance at which tidal forces will pull a body apart. For two bodies of similar density, this distance is about 2.4 times the radius of the larger body (in the fluid-body case); at this distance, you would see a gravity reduction on the near and far sides almost all the way to zero.

Now, you can't do this with the Sun (the Roche limit is far inside the orbit of Mercury, so the planet's life would get cooked), but a habitable moon around a gas giant could experience varying gravity.

• This has been explored in this novel (which I only know from another question here and have not read). Oct 14, 2014 at 20:14
• A white dwarf or other dense relic star might have a good combination of heat vs distance. Although the planet's rotation would also be tidally locked, and to prevent it flexing (and gaining a lot of internal heat), the orbit would need to have settled to very close to a perfect circle. Oct 14, 2014 at 20:19
• Actually Rocheworld would have constant gravity on the surface, the planets shape changes to give you that. Oct 14, 2014 at 21:04
• Another roche locked planet story: First Cycle - though its not about the physics at all.
– user487
Oct 14, 2014 at 22:43