Celestia Forums

In Celestia Chris has set up Mass and Radius for the extrasolar planets...Is it possible to compute the radius of a planet by its mass...

i.e. If a planet is 1.93 Jupiters in mass, how would I compute the Radius? Multiply this by 71398 which is the radius of Jupiter in kilometers...Or is pi used in there somewhere...Basically, what is the formula used to compute radius using mass...

To find out the mass of something from its radius you need to know its density.

Rassilon, only part of the equation which determines the mass of an object (M=dV) can be arrived at trough a measuremnet of radius. This part is the V, or volume. Volume=4/3 *pi*radius cubed. The only problem is that, as mentioned before, nothing is known about the density, the d in the mass equation.

For instance, a sphere made of lead will be heavier than one made aluminum even though the spheres are the same size. This is due to the fact that lead has a higher density i.e more mass or weight per unit of volume, than does aluminum.

I hope this helped

Anonymous wrote:Rassilon, only part of the equation which determines the mass of an object (M=dV) can be arrived at trough a measuremnet of radius. This part is the V, or volume. Volume=4/3 *pi*radius cubed. The only problem is that, as mentioned before, nothing is known about the density, the d in the mass equation.

For instance, a sphere made of lead will be heavier than one made aluminum even though the spheres are the same size. This is due to the fact that lead has a higher density i.e more mass or weight per unit of volume, than does aluminum.

I hope this helped

Well then my guess at density could be 1.93 times the density of Jupiter? Since this is a hypothetical planet...and a gas giant...could I come to this conclusion? I guess my best bet is to just simply assign a radius according to this data here...As I am rendering the planets in Ups Andromedae using the data on this site...
http://www.jtwinc.com/Extrasolar/star.asp?StarID=3

And in turn come up with my best guess...well since its all conjecture anyway

Since this is just guess work, I suppose any size you choose will do just fine:) However, considering all of the extrasolar gas giants to be similar to Jupiter in every way is no doubt a mistake. Just take a look at the differences in the gas giants within our own solar system. For Example, Saturn would float on water, whilst the other three (Jupiter, Uranus, and Neptune) would sink. While the equatorial radius of Neptune and Uranus is quite similar, with Neptune being slightly smaller, Neptune is never the less 2.7 earth masses more massive than is Uranus.

I look forward to taking a look at your texture for ups. And. b. If you are able to duplicate the red glow and the blue color from scattering, it will indeed prove to be a welcome enhancement to this great program.

It would still be nice to know how this giant planet (and others) ended up so close to it's parent star in this star system. Just another little piece of information that warrants consideration when we use our own system as a basis for making assumptions about others.

Anyhow, I look forward to adding your new textures to my Celestia when they are completed. I am sure they will look great on a planet of any size:):)

Anonymous wrote:It would still be nice to know how this giant planet (and others) ended up so close to it's parent star in this star system. Just another little piece of information that warrants consideration when we use our own system as a basis for making assumptions about others.

Right... our solar system might turn out not to be typical at all. The things we take for granted could be, not inevitable consequences of the way solar systems form, but simply prerequisites for our kind of life to evolve in one.

I suppose in hindsight we should have expected to see these close-in giant planets, because, after all, binary stars are known to occur with all manner of different separations, from almost touching to a large fraction of a light year. If small stars, why not giant planets?

I remember thinking, when these planets started to be discovered, that they were the sorts of places Hal Clement might like to write stories about. And, in fact, he wrote one not long after.

Matt McIrvin wrote:

Anonymous wrote:It would still be nice to know how this giant planet (and others) ended up so close to it's parent star in this star system. Just another little piece of information that warrants consideration when we use our own system as a basis for making assumptions about others.

Right... our solar system might turn out not to be typical at all. The things we take for granted could be, not inevitable consequences of the way solar systems form, but simply prerequisites for our kind of life to evolve in one.

I suppose in hindsight we should have expected to see these close-in giant planets, because, after all, binary stars are known to occur with all manner of different separations, from almost touching to a large fraction of a light year. If small stars, why not giant planets?

I remember thinking, when these planets started to be discovered, that they were the sorts of places Hal Clement might like to write stories about. And, in fact, he wrote one not long after.

Thats what makes speculation so enjoyable...I like using the imagination to come up with hypothetical solar systems...But generally like using models of existing ones in Science Fiction...

btw, Matt, do you know where I could get a more detailed description of the extrasolar worlds in Dune?

I have this link so far >> http://www.erinyes.org/gurps/dune/

Rassilon wrote:btw, Matt, do you know where I could get a more detailed description of the extrasolar worlds in Dune?

Sorry, I don't know much about the Dune books-- I haven't even read them.

Matt McIrvin wrote:

I suppose in hindsight we should have expected to see these close-in giant planets, because, after all, binary stars are known to occur with all manner of different separations, from almost touching to a large fraction of a light year. If small stars, why not giant planets?

I suppose it is possible that presumed planets like ups. And. b are merely oscillations in the parent star, rather than actual planets. However, that thought would probably be about as accurate as Sir Fred Hoyle's ideas regarding the expansion of our Universe

Matt McIrvin wrote:

Rassilon wrote:btw, Matt, do you know where I could get a more detailed description of the extrasolar worlds in Dune?

Sorry, I don't know much about the Dune books-- I haven't even read them.

Worth a shot...

If anyone else has any info on Extrasolar systems in Sci-Fi and would like to see them come to life...let me know...

The main variables that determine the radius of a planet are its mass, composition, and for gas giants, temperature. In general, the lighter the atoms making up the planet, the larger the planet will be. There's also a point at which adding more mass to a planet will cause its radius to decrease due to gravitational compression. Jupiter has close to the maximum radius possible for a planet with it's temperature and composition. The only extrasolar planet whose radius we've measure is the one observed transiting HD 209458. It's mass is only 60% of Jupiter's, but because it orbits so close to its Sun, high temperature has inflated it to 1.5 Jupiter radii.

Once I get back home, I'll create a JavaScript thingie to compute the radius of a planet given its mass and composition.

--Chris

chris wrote:The main variables that determine the radius of a planet are its mass, composition, and for gas giants, temperature. In general, the lighter the atoms making up the planet, the larger the planet will be. There's also a point at which adding more mass to a planet will cause its radius to decrease due to gravitational compression. Jupiter has close to the maximum radius possible for a planet with it's temperature and composition. The only extrasolar planet whose radius we've measure is the one observed transiting HD 209458. It's mass is only 60% of Jupiter's, but because it orbits so close to its Sun, high temperature has inflated it to 1.5 Jupiter radii.

Once I get back home, I'll create a JavaScript thingie to compute the radius of a planet given it's mass and composition.

--Chris

And we know precisely what these extrasolars are composed of?