If humans were to travel from earth to another planet that was an exoplanet 5 times the mass of Jupiter ("Rocky earth like planet"), would they be able to survive?


The planet I was using as a source is HD38858b because the main stipulation for the story is that it is a planet that is within the constellation Orion. After more research there is no confirmation that this planet is "earth like" or rocky, it is merely in the habitable zone distance from its star and its size is calculated to be about 5 times the size of Jupiter and beyond that all that is known is that it has a ring of comets. So I've decided to salvage what realistic details I can and for the purpose of my story, making it at most twice the size of earth and "earth like" for the sake of my story, and to keep the facts of the constellation its in and that it has a ring around it.

Thank you all so much: this was my first use of this site and I love all your ideas, theories, and crazy but realistic work arounds to fit what I vaguely described. I am truly thankful for all the calculations and possibilities brought out

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  • $\begingroup$ Alrighty thank you. I went ahead and trimmed it down to my one main question for now. $\endgroup$ – Dae Knight Oct 12 '18 at 5:03
  • $\begingroup$ Gas giant, or rocky planet? $\endgroup$ – RonJohn Oct 12 '18 at 5:03
  • $\begingroup$ Rocky earth like planet. i was thinking slightly less water. Maybe 50% $\endgroup$ – Dae Knight Oct 12 '18 at 5:04
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    $\begingroup$ Survive as in "not get crushed by the stunningly high -- almost star-like -- gravity"? $\endgroup$ – RonJohn Oct 12 '18 at 5:05

If humans were to travel from earth to another planet that was an exoplanet 5 times the mass of Jupiter, would they be able to survive?


Jupiter's mass is 318x that of Earth, and you want 5x that, which gives about 1600x Earth's mass.

That much mass in an rocky planet requires (using the formula for the volume of a sphere V=4/3πr^3, and presuming that the planet has the density of Earth) a radius 7.25x larger than Earth's.

Plugging the 1600x mass and 7.25x larger radius into Newton's universal law of gravitation (F = Gm1m2/r^2) while taking Earth's gravity as "1", such a planet's surface gravity would be 30.5x grater than Earth's.

You'd be crushed by your own mass.

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    $\begingroup$ You have forgotten the density, that can be much lower. So, you answer is totally senseless. $\endgroup$ – Gangnus Oct 12 '18 at 11:19
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    $\begingroup$ OP did say rocky and Earth like. $\endgroup$ – pojo-guy Oct 12 '18 at 12:07
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    $\begingroup$ @Gangnus he did not forget the density - he did say "presuming that the planet has the density of Earth". Actually, the figure is low: even the pressure at the Earth's core is enough to almost double the density of its nickel-iron solid nucleus. While our super-planet wouldn't be develop pressures such as to reach electron degeneracy, it would be surely far denser on average than Earth. $\endgroup$ – LSerni Oct 12 '18 at 19:41

Not as such

"Rocky Earth-like planet" - that's your problem right there: a planet five times Jupiter's size cannot be a "rocky Earth-like" one, because it is too large for the light gases to escape during its early life.

So, it will become a gas giant - a huge gas giant. Unless you posit some mechanism to strip it of all light gases, but in that case you're left with a rocky ball with a surface gravity more than 30 times Earth's - no way that's colonizable.

A gas giant has promise though. You can imagine a Saturn-like world, with a very dense core and the rest made up of light elements, so that the "surface" is far enough for the nucleus' gravity to fall off to something a human may survive - no more than twice Earth gravity I'd say. You'll still get Aarn Munro's type inhabitants - squat, probably not very long lived due to heart and circulatory problems. But they'll live long enough.

To provide them a reasonably solid surface, you can imagine very large floating "islands" made up of pseudo-algal analogs, evolved to be buoyant and reach the sunlight.

Okay - 5-J rocky planet it is

Can such a planet exist? Yes. Several improbable but not impossible mechanisms could lead to a light-gases-poor, rocky world that is not also the core of an uninhabitable gas giant.

Plenty of oxygen compounds would be there anyway, and conceivably some organism could start from that and get a breathable atmosphere going, if sunlight can reach the surface (in the case of Saturn and Jupiter, for example, it couldn't).

The significant gravitational collapse could be handwaved away given enough time, or hypothesizing a two-stage planetary formation - first a dense, rocky core, then a substantial cooling off, and finally a Heavy Very Late Bombardment with a new increase in core temperature that doesn't affect the outer layers much. These in turn do not collapse as hard; their kinetic energy is vented off by boiling the initial light gases atmosphere (a young star nearby is needed to supply the necessary massive solar wind). In the end we're left with a fiery ember smothered in silicate dust, nitrogen and water ice, completely airless, but slowly outgassing while the surface gets oxidized. Manage to lose the hydrogen somehow.

We would have a 2-to-3 Jupiter-mass nucleus, say 2.5 (i.e. 800 Earths), with heavy metals, mostly nickel-iron and silicates, and an average density around 15 kg/dm3. This gives us a radius (for the nucleus) of 42380 km. The next 2,5 J-masses form a shell with a lower density - say around 4, even if it's mostly light silicates. Gravity at the shell boundary is around 18 G, and pressure is enough to squeeze them to a higher density.

The "mantle" holds other 800 Earth-masses worth of relatively low-density silicates, and has a thickness of around 28880 km. Surface density is around 2.5, but this way the total planetary radius increases to a whopping 72300 km. This gives us a surface gravity of "only" 6.5 G. Especially in the seas, life is more than possible.

Pressure can be as low as we want (just remove atmospheric gases), the gravity well is deep enough that we needn't fear losing atmosphere. Actually, the problem will be losing enough atmosphere so that what remains is more like air and less like dense soup.

Human life might be possible with some geneering; see this answer and its attached tables. Reasonably long-term survivability of up to 4 G for unmodified humans is also demonstrated by life expectancy of unhealthy, morbidly obese people with body mass up to three-four times normal; it's conceivable that healthy humans could fare no worse, and probably much better. Add some gene splicing from giraffes and humpback whales for adaptations to abrupt pressure changes and blood circulation improvement, remember that people managed to thrive with life expectancies as low as 35 years, and the planet is indeed colonizable.

  • The planet's surface would be around 125 times larger than Earth's.

  • A planetary rotation time of 24 hours would lead to a surface speed eleven times Earth's; this would have a marked effect on weather patterns. I haven't calculated the centrifugal effect on gravity, but it would be proportionally higher than Earth's.

  • Escape velocity might be unattainable with ordinary rockets, which might be impractical (again, I haven't done the math).

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    $\begingroup$ OP did asked about 5x Jupiter sized rocky earth like planet. While realistically your answer is correct, can we please provide them with a illustration if they wish for a answer regarding their world? $\endgroup$ – Mr.J Oct 13 '18 at 1:56
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    $\begingroup$ @Mr.J good point. I've tried. $\endgroup$ – LSerni Oct 13 '18 at 13:36
  • $\begingroup$ "Pressure can be as low as we want (just remove atmospheric gases)", Is it safe for me to assume that without the handwaving done to the atmospheric gases, this planet is dense and will crush a human by his own mass? Thank you for your well explained answer. $\endgroup$ – Mr.J Oct 14 '18 at 23:57

Of course, yes. For example, Saturn is 10 times greater than Earth, and the gravity on its surface is smaller than that of the Earth.

g~ m/r2

So, if the planet contains 1500 masses of the Earth, as you wish (about 300x is the mass of Jupiter), it only needs to have its radius about 40x of the Earth or 3x of Jupiter. Simply your large planet is almost empty inside, for example, all its inside is filled by metallic Hydrogen, that is very light, supposedly (nobody had seen it yet), plus a great water ocean on the surface.

And such planet will also have gravity same as that of the Earth.

There will be another serious problem - if the distribution of gases on such planet is traditional, it will have really great pressure on the surface. To compensate this your planet should have some serious collision in its past, that deprived it from the most of its atmosphere. Thin earthlike atmosphere will also help against the third problem - the planet will be warmed from inside. So, you need to place it far from the sun, to have the temperature balance around 20C+- a bit.

Another solution will fit even on the hypothetical rock planet - people can live in some upper layer of the atmosphere.

  • $\begingroup$ As long as you're orbiting, you're weightless. "Evolution will favor traits for 0G over heavy gravity. $\endgroup$ – pojo-guy Oct 12 '18 at 11:24
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    $\begingroup$ @pojo-guy Oh, yes. The author of the question did not said they need to live on the surface. But zero gravity is not good for health, too. $\endgroup$ – Gangnus Oct 12 '18 at 11:28
  • $\begingroup$ OP stated "Rocky earth like planet" in a comment. $\endgroup$ – RonJohn Oct 12 '18 at 13:52
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    $\begingroup$ @Gangnus going "to an exoplanet" means going to it, not going into orbit around it. $\endgroup$ – RonJohn Oct 12 '18 at 14:43
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    $\begingroup$ Ya i need people on its surface colonizing it $\endgroup$ – Dae Knight Oct 12 '18 at 16:40

One way to survive it would be to get only as close as you can handle the gravity, then get closer once evolved to be strong enough. Once on the planet surface (metal hydrogen), one may terraform the planet to replace hydrogen oceans with water, and hydrogen atmosphere with similar balance of earth gases (oxygen, nitrogen, etc).

Once evolved enough to live on the planet surface, one could return to Earth & be like superman without laser emission, and almost ageless. But returning to Jupiter, would have aged a lot (by their time density lines, due to the intense gravity).

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    $\begingroup$ Welcome to Worldbuilding! Can you explain how you could "get only as close as you can handle the gravity"? If you're in orbit, you won't feel any gravity and if you're on the surface, you'll feel all the gravity. Also can you show your calculations where going to a planet 5 times the size of Jupiter is enough to age someone a lot? - it's still a small size/mass on the relativistic scale of things $\endgroup$ – Mithrandir24601 Oct 12 '18 at 8:22
  • $\begingroup$ Why do you mention terraforming the planet? The post does not mention that the planet is unsuitable for life, only asks if they could survive on such a large planet. $\endgroup$ – John Locke Oct 12 '18 at 11:22
  • $\begingroup$ Ya size was my main question. It is to be suitible for life. In fact I was planning on it having its own humans $\endgroup$ – Dae Knight Oct 12 '18 at 16:39

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