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"Super-Earths" Are All Over The Galaxy


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Well, not necessarily higher gravity.  Depends on composition. 


There's a premade plot of mass and radius for exoplanets, here.  Unfortunately, it's not easy to overlay surface gravity on that plot.  You may be able to do so from the raw data, but now you'd have to do the work there.  The root site is https://exoplanetarchive.ipac.caltech.edu/

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If you know the mass of the planet (which you can get from a spectroscopic binary solution for the system, taking repeated spectra of the star), and the radius of the planet (which for eclipsing systems -- that TESS explicitly is searching for -- you get from the eclipse timing and the planet orbit velocity, also a product of the spectroscopic binary solution) then assuming a spherical planet, you know enough to estimate the planet density.  That is interpreted in terms of composition. 


Earth has a density of about 5.5 (g/cm^3 or metric tons per cubic meter ... those are the same number).  Mars has less iron in it, so its density if 3.9 in those units.  The gas giant Jupiter has a density of 1.33, and Saturn 0.7.  The densities of the Galilean satellites of Jupiter are Io 3.5, Europa 3.0, Ganymede 1.9, Callisto 1.8.  Io is essentially water-free.   The others are a mix of rock and ice, increasing ice you go outward from the central planet.  Theoretical models indicate that a pure iron body the size of Earth would have a density of 8 or 9.  We don't have anything in the Solar System with that kind of density, except for very small iron meteoroids (some of which have fallen on Earth and picked up as iron meteorites).


There is a very content-rich illustration in an article in the 2017 volume of Annual Reviews of Astronomy & Astrophysics (but that's going to be behind the paywall for another five years or so) showing a graph of planet radius versus planet mass and including curves from theoretical models showing lines for several fiducial compositions (ranging from pure iron to mixes of MgO2 and iron, MgO2 and water mixes, pure water, etc.) so you can play pin the tail on the donkey with composition if you like.  (The uncertainties are also shown in the graph so you can see the ranges of possibility.)  Venus, Earth, Uranus, and Neptune are also shown on the graph.  The article also makes a point "There is no consistent limit in the literature for the mass or radius divide between the terms mini-Neptune and super-Earth, which often leads to confusion, especially for planets that need a substantial gaseous envelope to fit their radius but have masses below 10 M⊕: These planets are often called super-Earths based only on the 10 M⊕ limit, ..."; i.e., the jargon is pretty sloppily defined and used in some ways.



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17 hours ago, Lord Liaden said:

So in other words, "Krypton" is indeed possible. :P



Its sun wouldn't go supernova, as a red dwarf doesn't have enough mass. It may still become a red giant.


But a smaller star would mean a longer lifespan, so there would be more time to develop an advanced civilization.

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16 hours ago, Lord Liaden said:

In most versions of the story Krypton shattered due to internal pressures, its star Rao did not go nova.

There was one version that had Brainiac being a Kryptonian super computer, that hollowed put Krypton for resources, and then left.  Particularly chilling. 

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