# Predicting Worlds With Lower Gravity

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1. Jul 27, 2017

### Handerson Tjia

So, recently I tried to write a novel. It is an ongoing project, and very far from completition. I've read so many Sci-Fi stories before, and some do not follow general physics very well. I want to make a world that is believable, but also scientificly correct. It doesn't need to be so precise, but it has to differ a little bit. I wonder if this forum can help giving some enlightenment on the subject, despite it being Sci-Fi stuff.

I may ask for some thought exercise, what would happen in a planet that has 0.92 less gravity than Earth or 9 m/s2?

I've come up with some rough estimates, such as
- Trees being slightly taller
- People from Earth jump slightly higher
- People from this planet suffer fatigue and knee injury on Earth

So, if you have other ideas on what will happen to this planet and Earth people that move to it, let me know! I might ask for rough formula to your theories, for example h = 1/g on jump height.

Please feel free to go crazy on this thread! I might need all details I can get. Thank you.

2. Jul 27, 2017

### scottdave

The atmosphere may be a little thinner, like living in the mountains, perhaps. Have you read any of Larry Niven's books? Neutron Star (I believe that's the name) is a collection of short stories. There are different worlds. There is a group of people called Belters in one of the stories. They live in the asteroid belt, mining the asteroids. Integral Trees is another interesting one, with low gravity.

3. Jul 27, 2017

### newjerseyrunner

Trees wouldn't necessarily be bigger, that has much more to do with the energy output of the sun.

There doesn't strike me as anything that would be obviously different at 92% gravity. I'd bet after only a few minutes on the surface, you wouldn't even feel a difference anymore. The planet would be about the same size as Earth.

4. Jul 28, 2017

### snorkack

Venus has slightly lower gravity, but much denser air.

5. Jul 28, 2017

### scottdave

Nice point. But is that due to the gasses which make up the atmosphere, or the gravity?

6. Jul 28, 2017

### newjerseyrunner

Titan also has a much thicker atmosphere than Earth and has about a sixth of the gravity.

7. Jul 31, 2017

### stefan r

The surface tension of water has a much larger effect. I am fairly certain you subtract gravity. So lower gravity means the capillaries can transport water higher. Some trees break the rules. On the California coast water will condense directly out of the air. The redwoods are ridiculously huge because they bypassed the capillary limits. Some bromeliads take water out of the air and do not bother with soil at all.
One impact of lower gravity may be continental drift. A smaller planet could have a cooler mantle/core. Without moving plates and subduction zones the mountains would have eroded and would not get replaced. Probably a boring world.

0.92g tells us very little. Earth has density 5.5 g/cm2. Uranus has surface gravity 0.89g but it also has 4x earth diameter. A water world could have a lower density than earth but the effect of surface gravity would be trivial in your story. There is active debate about the survivability of atmospheres close to stars. You could get around that by increasing the size of the star and moving your planet's orbit out slightly. Whether or not the planet is inside the habitable zone will have a huge impact on what sort of life would be possible.

A tidal locked binary planet would have variable gravity. The away side has higher gravity. Technically gravity on earth is also variable because of tides. Rotation also makes the effective of gravity lower at earths equator. A rapidly rotating planet could have 0.92 m/s2 at the equator and 1.0m/s2 at the pole.

8. Aug 2, 2017

### snorkack

Neither Mars nor Venus has moving plates. Both have mountains.
Though recently little liquid water. If Mars had still flowing rivers in Martian valley networks, how far would Tharsis have eroded?
Would trees have grown taller on young Mars, when the rivers flowed?

9. Aug 2, 2017

### stefan r

The mountains are volcanic. Poor choice of words on my part. Mars also has ancient meteor impacts.

Devils Tower [Bear Lodge Butte] in Wyoming is dated 41 million years. So the surrounding plains eroded at least 265 m in 42 million years. Glaciers could move a lot of material from the highest spots.