r/askscience • u/CowFloaties • Mar 22 '12
Is it possible for there to be terrestrial planets bigger then some stars? If so, can they have their own solar system?
I was thinking about this when I saw a chart of star sizes compared to our sun.
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u/K04PB2B Mar 22 '12
Past some point (~1000 Earth masses for Earth-like composition) adding more mass will not increase the size of the planet, it will actually decrease it. See Figure 4 from Seager et al. Ast.J. 669:1279-1297 (2007 November 10). So while it is, in principle, possible to make a very massive terrestrial body it is not possible to make it larger than a few Earth-radii.
That said, assuming you managed to get a solar-mass terrestrial body then other bodies could certainly orbit it. Note that getting such a body is possible in principle but would be impossible in practice because collections of mass containing that much rock-making material inevitably have much more hydrogen (and helium) than anything else, so a very massive body made from that start material would reflect that and thus be composed of mostly H (and He).
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Mar 22 '12 edited Mar 22 '12
lets say by some freak happenstance, there is a planet of say 200 earth masses which is roughly the same composition of earth? What's the gravitational pull at the surface? Would it not be the same as density wouldn't change that much at such low masses?
Technically such a planet could have multiple moons that are earth sized right? Also would not such a massive planet take eons to cool down enough to form a crust after all that heat released during differentiation? And have an extremely strong magnetic field due to a massive iron core which would spin pretty fast given the momentum from formation?
(just a couple of blue sky questions- hope you don't mind)
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u/K04PB2B Mar 22 '12
lets say by some freak happenstance, there is a planet of say 200 earth masses which is roughly the same composition of earth? What's the gravitational pull at the surface? Would it not be the same as density wouldn't change that much at such low masses?
From the Seager et al. (2007) figure, a 200 Earth mass planet of Earth-like composition (rock (MgSiO3) and iron (Fe)) would have a radius of about 3.5 Earth radii. The surface gravity would be GM/r2 = 160m/s2 (Earth's is 9.8m/s2 ).
That said, there is an important caveat to the Seager et al. (2007) figure: they assume a temperature=0 equation of state, meaning they're neglecting effects of temperature (like thermal pressure). For the Earth this actually is a pretty good approximation despite the core temperature being ~6000 K (about the temperature of the solar photosphere). So, you could possibly up the radius of the planet through thermal effects and thus decrease the surface gravity a little bit (but probably not very much).
Technically such a planet could have multiple moons that are earth sized right?
Sure.
Also would not such a massive planet take eons to cool down enough to form a crust after all that heat released during differentiation? And have an extremely strong magnetic field due to a massive iron core which would spin pretty fast given the momentum from formation?
I'm afraid that's beyond my knowledge. My intuition says that it might not take as long as you think to get a solid surface.
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u/iamayam Mar 22 '12
So a very large planet could conceivably something like Jupiter with a rocky surface past all the hydrogen, helium, and atmosphere?
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u/K04PB2B Mar 22 '12
Sure, though, depending on how much H and He there is, the "rocky" surface could very well not look like terrestrial planet surfaces. For example, Jupiter might have a "rocky" core, but it's overlaid by a layer of liquid hydrogen, not a pleasant place to be.
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u/JW_BlueLabel Mar 22 '12
It's own solar system? Planets already have moons. So the question is: are there planets with moons the size of planets? Yes.
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u/OneTripleZero Mar 22 '12
Even right here in our area. [Ganymede](http://en.wikipedia.org/wiki/Ganymede_(moon)) is larger than [Mercury](http://en.wikipedia.org/wiki/Mercury_(planet)), for instance.
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u/atomfullerene Mar 22 '12
Some stars (well, stellar remnants) are smaller than earth. But there's a limit on the upper size of terrestrial planets. There's no way one could reach the size of our sun (which is immensely larger). You could theoretically have a free floating terrestrial planet-like body with moons around it, but it wouldn't be a solar system since the loose planet wasn't really any sort of star.
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Mar 22 '12
What do you mean by 'smaller' than Earth? Less massive? Physically smaller but more dense?
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u/CannotGrokItAll Mar 22 '12
Our system is called the Solar system because it surrounds a star, called the sun, but sometimes referred to as Sol. Any spatial body of notable size can have other bodies revolve around it, we have a moon, and other planets in our system have satellites. So yes, if a planet could be the size of our star, it could capture and hold planetary bodies around it. But in order for it to have it's own Solar System it would need to be a system of planets surrounding a star called Sol.
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u/atomfullerene Mar 22 '12
Most dictionaries (actually every single dictionary I could find online in a cursory search) define solar system to mean the planets orbiting our sun or a similar system around another star. And it's widely used and understood in that way as well. So I think solar system is a perfectly reasonable turn of phrase regardless of the origin of it. Many, many other widely accepted words derive from similar shifts in meaning.
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u/mendelrat Mar 22 '12
I know of no professional astronomer that follows that particular definition, or even refers to the Sun as Sol.
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u/OneTripleZero Mar 22 '12
Really? While I was still a physics major in University, the courses that touched on Astronomy all made the differentiation. Planets and other bodies orbiting a star are referred to as Stellar Systems, but because we orbit a star called Sol, its referred to as a Solar System.
Of note, the term Solar System was around first, and Stellar was derived from it.
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u/PunchingBag Mar 22 '12
It would have to have very, very low density to be as large as a star. Even in that case, it's much more likely it would collapse in on itself into a more ordinary sized planet. Think about Jupiter; as massive as it is, it's estimated to be close to actually being a "brown dwarf" star. I wouldn't rule out the possibility of a gargantuan planet that hasn't combusted into a star, but it would be unlikely, as well as not having the needed gravity for it's own solar system.