Posted on 09/26/2007 12:01:13 PM PDT by LibWhacker
How large a planet is depends upon its composition and mass. Earth is largely made of silicates, with a diameter of 7,926 miles at the equator. Imagine an Earth mass planet made of iron and you’re looking at a diameter of a scant 3000 miles. Interestingly, the relationship between mass and diameter follows a similar pattern no matter what material makes up the planet. Running the numbers, an Earth mass planet made of pure water will be 9500 miles across.
Sara Seager (Massachusetts Institute of Technology) has been studying these things as part of a project to model the kind of Earth-size planets we’re likely to find around nearby stars. About the mass/diameter pattern, she says this:
“All materials compress in a similar way because of the structure of solids. If you squeeze a rock, nothing much happens until you reach some critical pressure, then it crushes. Planets behave the same way, but they react at different pressures depending on the composition. This is a big step forward in our fundamental understanding of planets.”
It’s a needed step, too, because we often speak of Earth-size planets as if they were likely to resemble the worlds we see in our own Solar System. The team, made up of scientists from MIT, NASA and the Carnegie Institution of Washington, wants to throw out that assumption, going back to the nature of the protoplanetary disks we’re seeing around young stars. Its speculations have produced fourteen different compositions, among them pure water ice, carbon, iron, silicates, carbon monoxide and silicon carbide. Corresponding sizes can be calculated for each planet.
“We have learned that extrasolar giant planets often differ tremendously from the worlds in our solar system, so we let our imaginations run wild and tried to cover all the bases with our models of smaller planets,” says Marc Kuchner (NASA GSFC). “We can make educated guesses about where these different kinds of planets might be found. For example, carbon planets and carbon-monoxide planets might favor evolved stars such as white dwarfs and pulsars, or they might form in carbon-rich disks like the one around the star Beta Pictoris. But ultimately, we need observations to give us the answers.”
Image: Astronomers have calculated the diameters of various types of planets given certain compositions and masses. This image shows the relative sizes of six different kinds of planets with different compositions, and depending on whether they have the same mass as Earth, or five times the mass of Earth. Note that the 5-Earth-mass planets are larger than their 1-Earth-mass counterparts, but they are not five times larger due to the gravitational compression that occurs when a planet’s mass is increased. The planets are shown silhouetted against the Sun, as if they are transiting planets seen from afar. Credit: Marc Kuchner/NASA GSFC.
Comparing a planet’s size and mass with the help of planetary transits is a first step toward determining its composition. The French COROT satellite should be capable of finding planets not much larger than Earth as they pass across the surface of their star as seen from the spacecraft. One tricky call will be a silicate planet vs. a carbon planet — the two model out to roughly the same size for a given mass. Maybe by the time we need to make such distinctions we’ll have the James Webb Space Telescope around for help.
And this comment in the paper’s conclusion on a definition for ’super Earths’ is interesting:
Planets above the H2O [mass-radius] curve must have a significant H/He envelope. We can therefore easily distinguish between exoplanets with significant H/He envelopes and those without, as is the case for GJ 436b. We therefore define a “super Earth” to be a solid planet with no significant gas envelope, regardless of its mass.
The paper is Seager, Kuchner et al., “Mass-Radius Relationships for Solid Exoplanets,” now in press at The Astrophysical Journal, with publication due in late October (abstract).
How big is an earth-mass planet made up of plutonium?
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I don’t know. You’d have to go look up the formula in the original article. Smaller than a pure iron planet, though, I’m pretty sure about that. You can see it in the diagram: Planets made up of heavier elements are smaller than planets made up of lighter elements, for any given mass.
Anyone who is attracted to this thread MUST read
Privileged Planet
It’s an awesome book about our particular place in the universe.
Does anyone know if a planet of pure water would be possible? I’m assuming not, as it would have no magnetic field and if I remember correctly this would lead to the atmosphere (the water) being stripped away by the sun.
Probably about the size of the Moon or smaller..............
The sci-fi writers have spent decades and held innumerable conferences designing planets. Sci-fi planets are better than these scientists’ planets since they are made of more than one thing and are livable.
“How big is an earth-mass planet made up of plutonium?”
Probably about 2000 km, for a millionth of a second, then a kajillion-km ball of hot gas later :)
Rare Earth is another and possibly the first of modern times to calculate the odds that this is the only planet anywhere with higher forms of life such as our exalted selves.
Oh, yeah, and there there is THAT. I forgot about that little detail!
Most likely just an idealization by the authors.
I used to think, with the sheer numbers of stars in our universe, that the occurrence of a system with an earthlike planet with intelligent life MUST be pretty significant.
However, after reading privileged planet, and how the effect of even changing (for example) the orbit of Jupiter slightly would have caused the occurrence of life on Earth to be unlikely. There are numerous other factors as well, like the specific size of the moon, our sun’s position in the galactic disk, and even the timeframe that we’re in (in galactic terms).
“Oh, yeah, and there there is THAT. I forgot about that little detail!”
Some quick googling shows the average density of the Earth as 5.5 g/cc, and plutonium at 19.8 g/cc. Obviously plutonium if it didn’t explode would compress a great deal, so according to my wildass guess of an average density of 30 g/cc you would have a sphere of 1/6 the volume of the earth. My guess of 2000 km diameter wasn’t all that far off.
oops, my previous post should be in miles, not km
OTOH it might be that microbial life is everywhere inside rocky planets. Inside the rock.
Be great for high-diving. Nothing to hit your head on.
It’s possible to have the pure water, but not likely that such a planet would have much diversity of lifeforms.
You need continents and plate tectonics in order to have a “carbon cycle” that supports higher forms of life.
I looked it up and you are correct.
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