Theoretical Planet

I need some help from the smarter folks in the forum (I believe that covers everyone?).

I’m theorizing a planet that is immense. Try the size of a star system, say the size of our star system for simplicity’s sake. I want to theorize some properties of this planet, but I’m not sure how to go about doing (i.e. -which questions have to be asked and answered, and which can be ignored).

I imagine the planet to have an atmosphere conducive to carbon-based life forms, ones that breath oxygen, nitrogen, carbon dioxide or any other gas that would not preclude human life if present in great abundance (ammonia?).

I’m wondering the following (assuming a planet this immense could exist given our current knowledge and theories):

Would the gravitational pull of this planet be sufficient to capture stars and other bodies into orbit?

Could this planet have an axis and therefore rotate upon it?

If stars could orbit it, and assuming that it did not orbit some other more immense body, what would its days and seasons and tides be like?

Thanks in advance for entertaining this admittedly fantastic (not in the overused superlative sense) suggestion.


Nope. Sorry, that’s way too big. Anything much bigger that about Jupiter-size is big enough to light off a fusion reaction at the core, so you’d have a star, not a planet. And the largest star known, AFAIK, is less than 4 AU in diameter (1 AU=approx. 93 million miles, the distance from the earth to the sun). The solar system is about 100 AU in diameter.

If you want to play with really large livable spaces, check out Larry Niven’s Ringworld, or some literature on Dyson Spheres.

Check out Poul Anderson’s A Stone in Heaven for an interesting planet, nowhere near the size you’re talking but near to the limit for breathable-atmosphere worlds. It was the core of a superjovian world with the original H/He atmosphere burnt off by a very close supernova, a world about the mass of Jupiter (and a 7-G surface gravity) that had produced a breathable atmosphere by outgassing and the evolution of earthlike life.

I agree with Ferrous, your planet would collapse under its own gravitational forces. And fusion can only run as long as their are elements lighter than iron to fuse. After that, it would become a black hole.

Ringworld was a ribbon of neutronium as large as the orbit of a planet, but relatively narrow. Neutronium is the only material that (might be) strong enough to build such a ring.

Why didn’t ringworld collapse on itself? What would its mass be and how rigid is neutronium, anyway? Did Niven address those issues?

Another large livable space I read about was an artificial planet made of concentric spheres. It was ‘only’ the size of our moon, but since there were hundreds of layers, the living space was much greater than Earth’s. I think the story was called “Small Fish in a Big Pond” and was published a looong time ago in Asimov’s Scienc Fiction Magazine.

I think the concentric sphere idea is the best if youwant large living space. The construction would be difficult, but not impossible. And you don’t have to worry about creating ridiculous quantities of neutronium or having your planet turn into a black hole.

I think the Ringworld was supposed to be stabilized by its spin.

I did some checking, and apparently the critical size for a star to collapse into a neutron star is between 1.4 and 3 times the mass of the sun. More than about 3 times the mass of the sun, and the gravitational force exceeds the strength of the material, and it collapses into a black hole.

Ringworld was made from ‘Skrith’, not neutronium. Skrith is a magical material which Niven conveniently defined to be sufficiently strong for the ring to not fall apart. And yes, ferrous is right: It was stabilised by it’s spin.

Incidentally, a back of the envelope calculation suggests that your planet would have a mass of about 10^21 solar masses. So I think the answer is, yes, it would capture stars. Oh, incidentally, the gravitational acceleration at the surface is about a gigameter per second squared. Is that a problem? :slight_smile:

(Note: These are both under-estimates, and were working under the false assumption that such a system could exist).

If you want to theorise some sort of magic-tech you can have a hollow force-field type sphere coated with a layer of solid material. But if you’re going to do something like that, just make up the details as you want them to be.

10[sup]21[/sup] solar masses would weigh more than the entire observable Universe.


Considering that this site ( ) says that the whole Galaxy weighs less than 2 X 10[sup]9[/sup], and if we assume that the entire Universe has 100 billion (10[sup]11[/sup]) galaxies, we find that the Universe weighs about 2 X 10[sup]20[/sup] solar masses.

Your idea is hypothetical, so it doesn’t matter anyway. :slight_smile:

OK, here are some interesting tidbits, if we can play pretend.

First of all, it’s interesting to note that a huge star like a red giant (such as Betelgeuse, 4au in radius, roughly the size of Mars’ orbit) is not so much a different type of star as a star in a very different phase of stellar evolution. Our sun, as mentioned, has a fusion reacive core, in which superhot condensed hydrogen atoms fuse to form deuterium, and eventually helium. However, over the course of many millenia, the fuel for this nuclear fusion is gradually expended. The result is that the reactions occur less and less, and, as mass is now also less, the graviational pull of the star is weakened. The surface (“corona”) expands vastly further from the core in the weaker gravitational field. Also, with decreased nuclear reactions in the core, the star simultaneously cools at it expands, thus the change from a yellow sol star to a red giant. This is just one phase of solar evolution, by the way, very simplified. Blue Giants are in fact closer to just being different types of stars.

Now, your planet. ASSUMING we could have a terrestrial planet the size you are talking about, made of non-reactive nickel, iron, and carbon, like our own planet, here is what you might expect:

Everything comes back to gravity. A planet that large would have an immense pull, allowing it to maintain a VERY diverse atmosphere. See, Earth’s atmosphere developed, it is believed, over the course of the Earth’s cooling. Geological processes associated with molten components reacting in Earth’s early molten stage, and her subsequesnt cooling, yielded gaseous biproducts, creating the atmosphere sustained by the Earth’s gravity. The most notable were CO2, hydrogen and helium. Single-celled photosynthetic life forms then furnished the atmosphere with the O2 we enjoy today. Lighter elements, such as H and He, eventually escaped the Earth’s gravity, explaining their relative scarcity today (H also probably combusted often). The moon, in turn, (as well as Mercury and to an extent, Mars) is thought to have long since lost whatever atmosphere it might have had, due to its weak gravity. A planet that size would have an immense field and thus maintain a very diverse and thus unstable, reactive, and overall hostile atmosphere at an unimaginable pressure. It is therefore basically impossible that a planet this large would be able to support any for of life that we on earth are familiar with. Would some other form of life evolve to meet these conditions? Hey, nothing’s impossible.

With this in mind, I won’t (and frankly can’t) go into lunar phases, tides, etc. but hypothesize that stars and probably galaxies could orbit this planet.

All this is conjecture, by the way, based on my Conecptual Astronomy class Sophomore year. IANA astronomer.


Just for the record, I made a slight error on my original calculation:

My estimate was based on assuming that the planet, being of roughly earthlike composition, had the same density as earth. Say the planet is 10 AU in radius, that’s about 10^12 meters. The earth is about 10^4 meters in radius (I’m rounding shamelessly here - Order of magnitude is all that matters), so the scale factor is about 10^8. Mass scales as radius cubed, so that means the mass is a factor of 10^24 greater than that of the earth. Earth masses about 10^-7 solar masses, so that would be a mass of ‘only’ 10^17 solar masses, which is still ludicrously large.

I got the surface gravityright 'though. At constant density, surface gravity is proportional to radius. Gravitational acceleration on the surface of the earth is 10 m s^-2, so gravitational acceleration on the surface of your planet would be 10^9 ms^-2.

Incidentally, why on earth do you need a planet so big? It’s a horribly inefficient use of materials - Besides which, even if your population bred like rabbits I’d be very surprised if they could fill up the available living space within the life-time of the universe. It would have a surface area a factor of 10^16 times greater than that of earth.

Sorry to just throw maths at you, but the idea really is hugely impossible.

Thank you all. Of all forums I have visited or in which I have participated, this one continues to be the most informative.

Well, I’d been told by a couple of physicist friends that this was wholly impossible, but I take convincing and they didn’t provide the hard data and links that you have.

Okay, so let’s assume that I have a ‘solution’ for the problem of this being impossible given hard science, and have my planet of impossible mass and size.

I am curious about the types of climates one could possibly see. I don’t care if only small fractions of the planet are habitable for relatively small periods.

I’ll come clean : this is related to a story (a set of stories, actually), that I’m writing. This planet wouldn’t have a continuous presence in these stories, but it plays a sigificant part in the backstory and in some of the stories’ dramatic arcs.

I guess my goal is to not violate hard science where possible, but I’m not being orthodox about it, like say the Orion’s Arm project. Still, I’m classify what I’m writing as more fantasy than science fiction due to other elements. I hope I’m communicating the concept well enough to allow you to bear with me.

As most of the previous posts indicate, any “atmosphere” (and by extension, climate) is going to be pretty exotic, at best. Even at the surface of your world, you’ve got adequate pressure to keep hydrogen in its metallic state.

But your problems start at a much more fundamental level. You’ve got more than enough mass to create a black hole bigger than any of the mammoth ones thought to exist at the centre of many [most?] galaxies. Unless you have some form of rigid shell (like a Dyson-sphere) around the BH, all of the planet’s material is going to fall in.

I don’t have the background to do the math, but perhaps you’d be better off with a diffuse gaseous planet of a size near what you’re suggesting. Figure out at what “depth” in the atmosphere would allow for habitation (theoretically, there should be an altitude that offers enough density to allow a vessel to “float”). I imagine heat/energy and light would have to come from the planet itself, perhaps from exotic interactions deeper in the atmosphere.

Good luck!

Ferrous, I just recently reread both Ringworld and the sequel The Ringworld Engineers. Niven did do some calculations to set up the Ringworld, and credits several people for corrections and extensions to the idea, some of which he included in the sequel. In fact, the sequel is dedicated to those who contributed.

Here’s a little quote from that dedication: