# Surface gravity of Mu Arae-c?

1. Sep 11, 2009

### Researcher X

I was wondering about the possible surface g of this giant rocky planet, and I found this one origin hypothesis on wikipedia.

"If an eroded gas giant, the sun would have boiled the planet from a larger protoplanet, of 20 Earth masses up to half Jupiter's mass. If the latter, its current radius could be as high as 0.6 Jupiter."

With this radius/volume estimate, and the mass (10.5 Earths), put into a density calculator, we get a puny and utterly impossible 189.74kg/m^3. If it's the remaining solid core of a gas giant, shouldn't it be incredibly dense? And it simply cannot be this diffuse.
Balsa wood can sometimes be heavier than this. This is 0.034385647 x Earth's density. Even Saturn is about six times denser.
Using this mean density with the estimated maximum radius, I get only 0.2g.

Before I mixed up kg/m for g/cc and got the g to 34.385646974g and with the early 14e mass of the planet, I got the ridiculous figure of 310g.

So, I've got it either way too heavy or way too light. Where have I gone wrong? Or is the radius estimate on wiki waaaaaaay off. I wouldn't expect it to be that large after all.

I've seen 3.5g for this planet before, but I don't know how that was come to.

2. Sep 15, 2009

### ideasrule

"If an eroded gas giant, the sun would have boiled the planet from a larger protoplanet, of 20 Earth masses up to half Jupiter's mass. If the latter, its current radius could be as high as 0.6 Jupiter."

Note "if the latter", which means half Jupiter's mass. If we use that as the mass, the density comes out to a reasonable 1240 kg/m^3.

3. Sep 15, 2009

### Researcher X

I see, but that is odd because it was already penned at 14 Earth masses, and then revised to 10.5. When it says "20 Earth masses to 1/2 Jupiter's mass". it means that it eroded from that mass of a gas giant down to the current terrestrial planet mass of 10.5.

1240 kg/m^2 would make more sense though. Even still, not entirely, because that would make it less dense than Earth, and we're not talking about a gas giant, we're talking about a solid planet that may have eroded from a gas giant. It's current density should be a lot greater, which is why I also can't console 0.6 Jupiter diameters (the claim for its current diameter) with 10.5 Earth masses (the current mass estimate). It just doesn't add up, and gives it an impossibly low density for a solid planet of that size. If it was 0.6 Jupiter diameters it would be much more massive than 10 times the Earth. Does this mean that that hypothesis is completely wrong?