View Full Version : Solid Planet Mass

2004-Jul-30, 03:04 AM
Hi all, I was wondering whether it has been theorised as to the maximum size that a Planet could form and remain a solid, as most of the Giant planets we know of are Gas.

2004-Jul-30, 11:07 AM
Parts of the core of the Earth are liquid. Does that mean we exceed the limit? What if Saturn has a largish solid core? do you call the rest of it atmosphere, and say it's a solid planet? It isn't clear from your question what the limits are for what you're asking about.

2004-Aug-03, 03:52 AM
Sorry, my bad, left a shoddy unfinished post and then left for a few days. For Shame!!

I guess what I was asking about was a limit for Planets with solid surfaces, like earth, venus, mars, mercury. We hear talk of Jupiter sized extrasolar planets, but are they all Gas Giants. Have they determined whether it is possible for a planet to form with a solid surface that is the size of jupiter? I just wonder whether a planet this size which is made up of solid iron or rock (with a liquid core presumably) as we see on our inner planets would have a gravitational field so strong that it be struggling not to collapse in on itself

David S
2004-Aug-03, 04:49 AM
Well, when they say that they discovered a planet twice the "size" of Jupiter, what they mean is twice the mass. Since we aren't able to directly see these planets we really don't know how large they are compared to Jupiter, just that they are twice as massive. It's entirely possible (although doubtful) that a planet discovered with twice the mass of Jupiter is a rocky planet that's actually smaller in size then Jupiter, since it would be more dense.

However, since recent planet discovers seem to be more and more from transits of the planet, they can probably now calculate the actual size of the planet based on how much the star dims when it passes.

As far as I know, there isn't anything that would limit how large a rocky planet would grow. There's no danger of it collapsing in on itself, since you would need about 100 times the mass of jupiter to begin nuclear fusion, and about 1.5 times the mass of the sun (which is roughly 1 million times the mass of the earth) before the atoms actually collapsed into neutrons, creating a neutron star. 2.5 times the mass of the sun, and you'd get a black hole.

The limiting factor in how large rocky planets get would seem to me to be how much rocky material is present in the disk of the star when it forms. I believe that our star has a relatively high abundance of the heavier elements (although it's still far less then 1% of the total mass of the solar system) compared with other stars. So you'd probably need to find a star that for some reason has an even higher concentration of elements like silicon and iron before you'd find rocky planets the size of gas giants.

2004-Aug-03, 05:18 AM
Thanks Astro, just what I wanted. using the term collapse was silly, i didnt mean into a black hole as such, more to the point that any subterranean liquid wold be squeezed to the surface due to the rock being so densly packed due to gravity.

2004-Aug-04, 12:28 AM
New work with an old equation may help scientists calculate the thickness of ice covering the oceans on Jupiter's moon Europa and ultimately provide insight into planet formation.

Planetary bodies, such as the Earth and its moon, exert such gravitational force on one another that tides occur, not just in the oceans, but also in bodies of the planets themselves. The surfaces of planets actually rise and fall slightly (Levity / Gravity Balance Disruptions) as they orbit one another.

The standard for calculating how the gravity of one celestial body affects the shape of a second is an equation published

in 1911 by A.E.H. Love.
Sarah Frey, a doctoral candidate at the University of Arizona in Tucson, decided to see if she could figure out the thickness of ice on Europa by using Love's equation to calculate planetary tides.

"Love looked at two cases, which were very well behaved, very similar to Earth's values," she said.

However, Love didn't have the power of modern computers at his disposal.
Working with Terry Hurford, a graduate student in UA's department of planetary sciences and Richard Greenberg, a professor of planetary sciences at UA, Frey used computers to calculate what Love's equations predicted for various spheres that differed from one another in density, compressibility and rigidity. The spheres serve as proxies for planets.
To their surprise, the researchers found that in specific cases, the computer calculations suggested that the sphere would change shape dramatically.
Frey said these special circumstances, called singularities, might ultimately reveal situations that would prevent the formation of planets.

Greenberg said, "If a rocky planet was a little bit bigger than Earth or Venus, it would be in the danger zone where the formula would predict a substantial distortion in the planet's shape.

We're wondering if in some way this regulated the size of the planets.

More resch. needs to be done, in this area

2004-Aug-26, 10:53 PM
From the Universe Today Story "smallest extrasolar planet found" 25/8/2004

"With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets"

this is the info I was looking for

2004-Aug-27, 01:18 PM
It seems likely that large rocky planets would "collect" appreciable atmospheres as time goes by. The atmosphere on the newly discovered 14-earth-mass planet must be substantial.

2004-Aug-27, 06:17 PM
Here is my picture of this planet; it hasn't got a name yet, and I am not entitled to give it one, but for the purposes of fiction I have named it Semele, after the mythical woman who was burned by getting to close to Zeus...
it has high gravity, and a temperature of 900k; probably something like a cross between Neptune and Venus...

An image of this new planet (http://www.forteantimes.com/forum/attachment.php?postid=433087)

2004-Aug-27, 06:47 PM
Originally posted by GOURDHEAD@Aug 27 2004, 01:18 PM
The atmosphere on the newly discovered 14-earth-mass planet must be substantial.
I'm sure that a 14 Earth-Mass rocky planet at 1 AU from a G2 Star would have a very large atmosphere. Mercury orbits in 88 days, this one orbits in 10 days. It should be hot enough to boil off just about any gas made with hydrogen.

Aside from the Noble gasses, Halogens, O2, N2, and CO2, this planet could have an atmosphere of Sodium, Potassium, Sulfur, Phosphorus, Zinc, Arsenic, Selenium, Rubidium, Cadmium, Cesium, Mercury, Astatine, Lead, and Francium. I wonder if any of those are green-house gasses? It would be a hot place with shiny metallic pole caps.

It would probably also have about 5 times Earth's gravity at the hot rocky surface.

Tim Thompson
2004-Aug-27, 08:36 PM
Originally posted by DarkChapter@Jul 30 2004, 03:04 AM
Hi all, I was wondering whether it has been theorised as to the maximum size that a Planet could form and remain a solid, as most of the Giant planets we know of are Gas.
Actually, the large planets that we know of in our solar system are mostly "solid", and only maybe 5% of their mass qualifies as gas. But the gas atmosphere is the part we see. Generally speaking, it goes like this:

A star is anything more massive than about 0.08 solar masses (about 80 Jupiter masses). Anything that massive will have a core tempeature high enough to initiate p-p fusion. Above about 100 solar masses it is probably not possible to have a stable star, and anything star like above that mass (eta carinae (http://etacar.umn.edu/etainfo/history/) maybe) will not last long before being torn apart by its own radiation.

A brown dwarf is anything from the 80 Jupiter mass lower limit for a star, down to about 13 Jupiter masses. Anything that massive will have a core temperature high enough to initiate deuterium fusion, but not high enough to initiate p-p fusion. So brown dwarfs do generate energy by nuclear fusion processes, like stars, but at a much lower yield.

A planet is anything below about 13 Jupiter masses, down to an undecided and controversial lower limit (is Pluto a planet?). Anything in that mass range has a core temperature too low to initiate any nucler fusion reactions. However, there is always nuclear fission, which does not need anything to get started. So planets, like Earth can still generate internal heat by fission, but at an even lower level, and they still have left over heat from formation to get rid of. Big planets like Jupiter can also generate internal energy through settling; as the heavy stuff sinks through the light stuff, that friction generates heat. The bottom of the "planet" range is undecided, but I like John Stansberry's criterion (http://rincon.as.arizona.edu/~stansber/PlutoPlanet.html) that it has to be big enough to make itself at least roughly spherical. Ganymede is about 200 km bigger than Mercury, although only about half of Mercury's mass. Would it be a planet if it wee on it's own? And why can't Pluto be both the smallest planet and the largest KBO?

As for the new, 14 Earth-mass planet, see "The HARPS survey for southern extra-solar planets II. A 14 Earth-masses exoplanet around mu Arae (http://cul.arxiv.org/abs/astro-ph/0408471)", N.C. Santos, et al., submitted to Astronomy and Astrophysics. That's the same mass as Uranus, and the authors say the mass of the atmosphere for their new planet is probably 5%-10% of the total mass. That's definitely not Earth-like, and a lot more Uranus-like.

John L
2004-Aug-27, 08:57 PM

I think the question was what is the maximum mass of a terrestrial planet (i.e. a big rocky world). Will a planet always accumulate a huge gaseous atmosphere above a certain size? Will the surface remain molten because of the gravitationally generated internal heat? Will it tear itself apart above a certain mass? I think that's what the question is...

2004-Aug-28, 09:28 AM
Strangely, assuming an Earth-like density, this world would only have a gravity of 2.4 gee, and a diameter of 30,000 km;
but if the atmosphere is 5-10 percent of the mass, that makes the density of the planet slightly lower...

I think the most likely model is a 'Hot Neptune', with hydrogen and helium present, but gradually boiling off- after a number of billion years this planet will lose most of the light elements, and will lose any water it has by photolysis- so the atmosphere will end up as CO2 and perhaps nitrogen, acidic clouds and so on...
completely inhospitable.