Plausibility check for a planet

Question

I have a question about the plausibility of this planet.

• 2.696246 Earth Masses
• 1.5 Earth Radii
• 1.2g
• 4.4044746136 g/cm3 (0.799 Earth's)

I know that the density falls within a range for a terrestrial planet, between 3 and roughly 9 g/cm3. This suggests a silicate crust and a iron core, but the calculators I am using basically say you need a core with negative mass to get this.

I am guessing what could help is a thicker crust, thinner mantle, and small iron core would be able to explain it?

So my questions are, is this configuration possible? If it is not possible with Earth's exact crust and mantle, would it be possible for the planet to have a different mantle or core with a less dense composition, which still winds up with the proper parametres? (This is meant as one question, just worded weird. I was just clarifing I am open to things not exactly matching Earth.)

Answer 1

It's possible to have a planet with those parameters; it just won't be completely like Earth.

First, the values for surface gravity and mean density are consistent with the mass and radius you've chosen, so quantitatively, you're fine. Where things get interesting is the composition of the planet itself.

Terrestrial planets start their lives with envelopes of hydrogen and helium -- like giant planets. However, their gravity isn't that strong, and various atmospheric processes involving energy from the parent star means most -- including Earth, Mars, and the other terrestrial planets in the Solar System -- lose these envelopes. The dividing line ends up being somewhere around $2M_{\oplus}$ (see e.g. Lammer et al. 2014).

Above $2M_{\oplus}$, you get planets in the murky range between super-Earths and mini-Neptunes. Both have thicker atmospheres than Earth, with mini-Neptunes resembling low-mass gas giants. The boundary between these two groups is somewhere around $\sim1.75R_{\oplus}$ and $\sim2R_{\oplus}$ (Lopez & Fortney 2013) -- larger than your planet. So you're looking at some sort of super-Earth, a terrestrial planet with a thicker atmosphere than you might have expected. (This probably makes sense -- after all, the surface gravity is higher than Earth's making it easier to retain hydrogen and helium!)

As for composition -- numerical models of planets of your mass and radius indicate it's like 5% an iron core with the remainder a mix of water ice and silicates (Seager et al. 2007 -- check out Figure 4!). It's unlikely to be more than 50% silicates; planets about that often have larger iron cores. So perhaps 5%/50%/45% is a reasonable iron/silicates/water ratio, something comparable to Ganymede. My guess is that the calculator was giving you weird core density because you can't attain that mass/radius/density/surface gravity with just a combination of iron and silicates; you need water to lower the mean density.

In short, you've got a partially watery or icy world with an atmosphere thicker than Earth's. It's definitely plausible!

Answer 2

You're going to need a weirder composition. The reason Earth is the densest planet in our solar system is because its own gravity compresses the silicates in the lower layers. A larger planet would compress them even more.

The previous posted suggested water. Note that it will be ices VII and X. This does get you the low density you want, but your volcanos will be much more explosive with "magma" boiling if it gets below 100MPa. Also, the planet may need to be pretty young to have such an unsorted mantle (a more sorted planet would have a several hundred mile deep ocean above its rock layer).

You could also have something less exciting like Lithium Chloride make up a large fraction of the planet. Then it won't matter if it's mostly the upper layers.