[behnamoh]

I wonder what life looks like on a planet which is over 8 times more massive than the Earth. Do animals have spines at all on a planet with almost 8g gravity? Does life even get to evolve into complex systems like animals under this much gravity? How about plants? Do they grow up or spread out instead?

If one day we get a visitor from this planet, they'll jump on our planet the same way human astronauts jumped on the Moon.

[pdonis]

The planet's surface gravity is not 8 g. Surface gravity goes like mass over radius squared, and the planet's radius is 2.6 times Earth's, so the surface gravity will be 8 / (2.6)^2, or only about 1.2 times that of Earth.

[thot_experiment]

Juuuuusssttt barely below the limit to be able to launch chemical rockets into space (IIRC it's ~1.3g)

[rayrey]

Lol.

Seeing that response and then your username

[adolph]

That is if density remains constant. If the planet were 8x mass but with same radius, gravity would be 8 g, or 8 × 9.795 m/s^s.

  Earth mass:
  5.97×10^24 kg, 6378.137 km yields 9.795 m/s^2

  8x mass:
  (8×5.97)×10^24 kg, 6378.137 km yields 78.36 m/s^2

All calculations: https://www.wolframalpha.com/input?i=surface+gravity+calcula...

[pdonis]

> That is if density remains constant.

No, the calculation I made did not assume constant density. I just used the direct Newtonian formula for surface gravity and plugged in the known mass and radius of the planet. (You could also use that known mass and radius to calculate the average density. But you don't need to do that to calculate the surface gravity.)

> If the planet were 8x mass but with same radius

But we know it isn't. We know the planet's radius is 2.6 times the Earth's radius. That's stated in the article.

[mr_toad]

> If the planet were 8x mass but with same radius

That’s not possible for normal stable matter. The Earth’s density is about 5g per cubic centimetre. Iron is 7.8g per cubic centimetre. Osmium is the densest stable element at 22.6g per cubic centimetre.

[lovecg]

Note that 8 times more massive doesn’t mean it has 8 times surface g, unless it’s exactly the same radius as the Earth. If the planet is larger you’re further away from the center of gravity.

For example, the Earth is 10 times more massive than Mars, but only has 2.6 times surface g.

[jdblair]

The classic sci-fi "Mission of Gravity" explores what life would be like on a rapidly rotating planet where one experiences 3g at the equator and 700g at the poles.

https://en.wikipedia.org/wiki/Mission_of_Gravity

[everyone]

Thanks for that! I need a new novel to read, just downloaded it!

[alexpotato]

The book Dragon's Egg has a species living on a neutron start with millions of G's

https://en.wikipedia.org/wiki/Dragon%27s_Egg

[colechristensen]

As others have mentioned it wouldn't be 8 g. Life would be smaller. There would be speed differences. A lot of optimums and limits depend on how volume scales against area. Like the biggest terrestrial animals are limited a characteristic dimension (height or length) x being proportional to femur area x^2 being proportional to mass x^3. Mass grows proportional to x^3, femur strength (area) grows proportional to x^2, so you have a limit on how big a thing can be when you run out of available femur strength.

Higher gravity means this upper limit will be smaller. All sorts of similar scaling things will change optimum points for structural and energy reasons.

[dylan604]

The gravity question is one I've pondered myself as a thought exercise. There's been discussions on how far up a plant can draw water as the defining limit to how tall a tree could grow. Some discussions as well on how tall an animal could grow based on how high blood could be pumped up. Which is a direction different from the structural support and sizes that I find interesting.

[adriand]

I can imagine an intelligent species on a high-G planet scratching their “heads” and wondering, as they surveil Earth, how anything could possibly survive on such a low-G planet.

[dylan604]

like humans trying to determine how to survive on moon bases or Martian colonies?

[gizmo686]

Gravity is more relevant for land based life. However, we know from Earth that complex life can evolve in oceans.

[wheels]

Gravity is only about 1.25 g, according to Wikipedia. Density matters. The earth and Saturn have about the same surface gravity.

[OkayPhysicist]

Comparing rocky planets, density doesn't really matter at all. The range of possible densities for rocky planets is tightly constrained. What matters is the fact that surface gravity scales sub-linearly with regards to a planets mass.

  M = 4/3*pi*r^3*d
  r = (4/3*pi*d/M)^(-1/3)
  a = GM/r^2
  a = GM(4/3*pi*d/M)^(2/3)
  a = G(4/3*pi*d)^(2/3) * M^(1/3)

[wheels]

We know that it's not (primarily) rocky though because the radius is 2.6 that of earth, but the mass is only 8 times. So it's about about 46% as dense as earth.

[joshlemer]

It would be really cool to run an experiment like this. Have some population of rats living in a large enclosure that is held in a large centrifuge for decades and see how they evolve.

[kemmishtree]

Just put e. coli in an ultracentrifuge and do a few hundred generations of serial passaging and see what evolves, for starters. You could do this pretty cheaply without waiting decades or killing mammals. Apparently e. coli can proliferate happily at almost half a million g... https://phys.org/news/2011-04-bacteria-extreme-gravity.html#....

[watersb]

I wonder how much of an outlier we may be, shuffling around on dry land, when most of the biosphere of our planet is in the ocean.

Higher gravity certainly means higher pressure gradient, more pressure per vertical meter of ocean. And high pressure affects protein structure.

It's life, but not as we know it.

[0xfaded]

Assuming roughly comparable density to earth, the surface gravity would only be 2g

[littlestymaar]

In addition to what others have said about the fact this planet doesn't have 8g at its surface, at 8g you could still have many lifeforms that exists on earth, but only the small ones. Gravity grows roughly as the cube of your size (because your volume does), but bones resistance only get n² (because it's the surface that counts), so the bone resistance / weight ratio is inversely proportional to your size.

[fmobus]

I think 8g would be pretty hard to escape from, at least with chemical rockets.

[ericbarrett]

Earth's surface gravity is really on the edge of what's feasible for chemical rockets; IIRC the limit is around 1.4g. Though as other commenters have mentioned, it's possible to have a much more massive planet that's also got a larger radius and thus has comparable surface gravity.

Some fun trivia—the planet Kerbin from Kerbal Space Program is the opposite case. It has a radius of 600km, versus Earth's 6378km, but is exactly 1 Earth g on the surface. This implies it's over 10x as dense.

[ethbr1]

To expand, isn't that limit "the most efficient possible chemical rockets, launched from sea level"?

I.e. air-breathing aircraft + chemical rockets would work, as would other exotic solutions

[ericbarrett]

I think an air-launched rocket would only be an incremental help. Chemical rockets on Earth are barely at 1/8th of orbital speed before they're out of most of the atmosphere (~60km altitude). You can't accelerate more without ascending because drag increases exponentially with velocity at a given air density.

Another way of looking at it—on a body with no atmosphere, the most efficient way to attain orbit is to be on the equator, point your spacecraft "east" (prograde to rotation), and elevate the nose just enough to avoid lithobraking on that mountain in the distance. If Earth were such a beast it would take roughly 7000 m/s delta-V to do this. IRL, because you need to get over the atmosphere first, it takes about 9000; the "gravity turn" is a compromise between losing energy to gravity/steering versus losing it to drag. So any exotic system—air launching a Saturn V is definitely exotic!—would help with efficiency, but I don't see that it would radically alter the situation.

[ethbr1]

This seems to work through the numbers and equations, albeit for answering a slightly different question: https://space.stackexchange.com/questions/20054/fuel-needed-...

In summary, as you said, the altitude is less important than the base velocity increase and atmospheric density reduction.

The former, because you're pushing maximum mass at t=0 (i.e. all the future fuel you need to burn), so any added velocity at rocket ignition time would compound throughout the rest of the burn cycle (or, to think of it another way, you've already overcome fully-fueled vehicle inertia with the benefit of atmospheric oxygen combustion).

Similar to how a multistage vehicle operates more efficiently, albeit without the benefit of atmospheric oxygen.

The latter, because you're essentially getting atmospheric density reduction for "free" (in terms of saving your on-vehicle propellant), and your propellant efficiency (in terms of propellant:velocity increase) scales better.

[BurningFrog]

The g force isn't that important for this.

Assuming life develops in an ocean, like we did, organisms in water are essentially weightless, regardless of the g force.

[mulletboy]

Can anyone recommend a good book on exoplanets speculative biology, flora & fauna, etc?

[thelittleone]

Perhaps they're gas like.