Stupid question about density

[ThunderStorm (Alan)]

I am not sure if my understanding: I read the magazine "Discover" September 2015 issue about Super Earths, page 42.
From line 9 on left column -
"Once they knew a planet's size and mass, high school physics would provide its density. (From your old notes: Density equals volume divided by mass)..."

Does it say:
Density = (V/M)?

[Atmos (Colin)]

Density is usually in kg/m^3 or something to that effect. Meteorites are either done in that way or g/cm^3.

[ThunderStorm (Alan)]

It should be mass divided by volume, right?

[bojan]

Originally Posted by ThunderStorm

It should be mass divided by volume, right?

Right

[Huey (Michael)]

Once you've got the diameter you've got the radius. You can then use the formula

4/3 pi r^3
to calculate the volume. Having volume and mass gives the density.

Huey

[rally]

So long as the majority of the planet is made up of the same "stuff" thats OK
But of course some planets have rocky cores and liquid or gaseous outer layers

[rally]

Just wondering- if we are talking about exo planets where its not (yet) possible to detect any moons - is a planet's mass deemed to be inclusive or exclusive of its moon/s ?

Afterall the orbit it makes around its sun is going to be a function of the combined mass of the planet and its moon/s.

[Atmos (Colin)]

Originally Posted by rally

Just wondering- if we are talking about exo planets where its not (yet) possible to detect any moons - is a planet's mass deemed to be inclusive or exclusive of its moon/s ?

Afterall the orbit it makes around its sun is going to be a function of the combined mass of the planet and its moon/s.

At this stage we have enough trouble detecting the planets, let alone detecting something smaller. Ultimately it really depends on how the planet has been detected in the first place.
If it is by a transit method (planet moving in front of its star) then we won't be able to detect any moons.
If by direct observation, it is doubtful that we'll have the resolution to be able to detect a moon around the planet, let alone differentiate a dimmer moon from the already incredibly faint planet

[Robert9 (Robert)]

Originally Posted by rally

So long as the majority of the planet is made up of the same "stuff" thats OK
But of course some planets have rocky cores and liquid or gaseous outer layers

Not necessary. The planet's orbit is defined by its total mass not its density. It does not matter what the combination of solid/liquid components are, we are only calculating an the average figure for density; and yes, Density=Mass/Volume, hence we talk of grams per cubic centimetre or kilograms per cubic metre. I would think that the orbits of detected exo-planets would relate to the combined mass of the planet and moon(s) (if any).
Robert