Mega Mass

[Johnny Pneumatic]

I was watching a show on Discovery Sci. last year. On the show their is a high mass planet(a rocky world the size of jupiter). I read that such a planet is impossible. Is it? If so, why?

 

[LucyR]

Impossible sounds a bit strong. I'd say "unlikely" instead.

I don't have much idea but I can guess:

I think tidal forces would result in such a large, dense, object being unstable - an interaction with another object, even a small one, could well be disastrous.

Perhaps it also has something to do with the accretion process. Available material accretes into moderate-sized (let's say Earth-sized), objects at the expense of small objects. The moderate-sized objects cannot combine because the energies involved are too high. They would destroy each other instead.

It would be interesting to hear what The Bad Astronomer has to say.

 

[wollery]

Good question, by "the size of Jupiter" do you mean the same mass as Jupiter (smaller diameter because denser) or same diameter as Jupiter and therefore more massive?

I think either one would be virtually impossible. LucyRs idea about the accretion problems is good, but even assuming that you could accrete that much matter without problems the question is whether there's enough of the right type of material available. Jupiter has a mass which exceeds that for all of the terrestrial planets, the asteroid, comets and Kuiper belt put together (and probably the Oort cloud too!). To accrete this mass the planet would have to pick up all of the rocky material in the solar disk, which would require it to migrate through the whole disk, accreting extremely efficiently as it went, on a fairly short timescale (maybe only a couple of million years).

Okay, so let's assume that it could do this, you then have to consider core heating, which would be very significant. Earth and Venus both display active Volcanism, and even Mars (1/10 Earths mass) display a volcanic history. A planet much more massive would probably feature volcanic activity that could practically tear itself apart. This would be pumped by the tidal forces LucyR mentioned (cf Jupiters moon Io).

My considered opinion - Even if you could make such a planet, which I doubt, it wouldn't be around for very long.

 

[toddjh]

One big problem is that, as planets start getting larger, their escape velocity increases. Eventually, it'll get to the point where even the lightest gases (hydrogen and helium) will stay put, and you'll have a runaway reaction as the planet sweeps up material from the solar system's initial gas cloud.

I've heard that there's probably not much middle ground between large rocky planets like Earth, and smaller gas giants like Uranus or Neptune. I'm no expert, but I seem to recall that the biggest rocky planet you could get would have about twice the mass of the earth.

Jeremy

 

[wollery]

One big problem is that, as planets start getting larger, their escape velocity increases. Eventually, it'll get to the point where even the lightest gases (hydrogen and helium) will stay put, and you'll have a runaway reaction as the planet sweeps up material from the solar system's initial gas cloud.

I've heard that there's probably not much middle ground between large rocky planets like Earth, and smaller gas giants like Uranus or Neptune. I'm no expert, but I seem to recall that the biggest rocky planet you could get would have about twice the mass of the earth.

Jeremy

Sweeping up gas won't prevent it from continuing to accrete rocky material though!

 

[toddjh]

Sweeping up gas won't prevent it from continuing to accrete rocky material though!

Yeah, but it's a matter of how much material is available. Rock is hard to come by in the solar system, but hydrogen is cheap, so any solid surface will be buried under thousands of kilometers of gas.

Compare the masses of the solid planets vs. the masses of gaseous bodies (don't forget the sun!), and you'll see what I mean.

Jeremy

 

[SGT]

Would not a rocky planet the size of Jupiter have enough mass to start nuclear fusion, thus becoming a star instead of a planet?

 

[wollery]

Yeah, but it's a matter of how much material is available. Rock is hard to come by in the solar system, but hydrogen is cheap, so any solid surface will be buried under thousands of kilometers of gas.

Compare the masses of the solid planets vs. the masses of gaseous bodies (don't forget the sun!), and you'll see what I mean.

Jeremy

Umm, yeah, I covered that in my first post!

Originally posted by SGT
Would not a rocky planet the size of Jupiter have enough mass to start nuclear fusion, thus becoming a star instead of a planet?

No, because it wouldn't contain enough low mass material. Have you got any idea how hard it is to fuse iron? It only happens in particular types of Supernovae.

 

[toddjh]

Umm, yeah, I covered that in my first post!

Then I don't understand. What part of my post do you disagree with?

Jeremy

 

[wollery]

Then I don't understand. What part of my post do you disagree with?

Jeremy

Maybe my brain cells aren't working in tandem today , but your original post seemed to imply that sweeping up gas prevented accretion of rocky material! If that isn't what you meant then I apologise.

 

[toddjh]

Maybe my brain cells aren't working in tandem today , but your original post seemed to imply that sweeping up gas prevented accretion of rocky material! If that isn't what you meant then I apologise.

Oh, no, sorry if I sounded like that. No, I just meant that once the planet reaches the point where it can retain hydrogen and helium, the amount of gas that's swept up will overwhelm the rock, and so they'll tend to be mostly gaseous instead of mostly rocky. I'm sure Jupiter and the rest have solid cores a lot bigger than the earth.

Jeremy

 

[Soapy Sam]

Isn't it a little early in our exploration of the universe to be making assertions about what can and can't happen during planetary formation?

I think I'll wait a bit before deciding on this one.

Ten thousand years sounds reasonable.

 

[LucyR]

Isn't it a little early in our exploration of the universe to be making assertions about what can and can't happen during planetary formation?

I think I'll wait a bit before deciding on this one.

Ten thousand years sounds reasonable.

You’re joking, right? Either way these are fatuous remarks. I don’t think anyone has made assertions. We have merely made reasonable suggestions.

 

[Johnny Pneumatic]

Okay, so let's assume that it could do this, you then have to consider core heating, which would be very significant. Earth and Venus both display active Volcanism, and even Mars (1/10 Earths mass) display a volcanic history. A planet much more massive would probably feature volcanic activity that could practically tear itself apart. This would be pumped by the tidal forces LucyR mentioned (cf Jupiters moon Io).

My considered opinion - Even if you could make such a planet, which I doubt, it wouldn't be around for very long. [/B]

But how could it tear itself apart? its gravity would be huge, hundreds of Gs.

 

[toddjh]

But how could it tear itself apart? its gravity would be huge, hundreds of Gs.

It takes a lot of energy to tear a planet apart. A lot a lot. So it'll be staying in one piece, especially with a mass that big. Still, if the surface is regularly covered by lava flows and giant calderas, it's not going to be a very hospitable place. I wouldn't want to breathe the atmosphere, either.

Jeremy

 

[drkitten]

Would not a rocky planet the size of Jupiter have enough mass to start nuclear fusion, thus becoming a star instead of a planet?

Rock doesn't fuse.

Even in a star like the Sun, there's very little fusion going on that isn't simply the conversion of hydrogen to helium. Since elemental hydrogen isn't a major component of rocks, there's not going to be enough hydrogen around on a rocky planet to fuse. (You're trying to drive the car on an empty tank).

 

[Johnny Pneumatic]

Sort of why I'm asking this is what is the highest G planet that life could be on. Also how weak can a planet's gravity be and keep it's nitrogen/oxygen atmosphere?

 

[wollery]

But how could it tear itself apart? its gravity would be huge, hundreds of Gs.

Umm, yeah, good point (guess my brain really wasn't working that day!), but I seriously wouldn't want to be anywhere near the surface of that planet.

Sort of why I'm asking this is what is the highest G planet that life could be on.

Don't know what limit there is on high gravity and life, kind of depends on what you mean by life!

Also how weak can a planet's gravity be and keep it's nitrogen/oxygen atmosphere?

Well Mars has a tenuous CO2 atmosphere with traces of O2 and N2 at 0.1 Earth masses, so as a pure guess I'd say that a planet would probably require about half an Earth mass to retain a reasonable atmosphere, but I stress that that is no more than a guess.

 

[LucyR]

wollery,bd,toddjh

re tidal instabilities, please consider:

http://www.sciencenews.org/articles/20040117/mathtrek.asp

 

[ceptimus]

A rocky planet the size of Jupiter would sweep up comets and other stuff. It would accumulate a very thick atmosphere, eventually becoming a gas giant much bigger than Jupiter, with a Jupiter sized rocky core. If it swept up enough stuff, nuclear reactions would kick in at the core, and it would become a brown dwarf star.