# Is there Dark Matter inside the Earth?

Since Dark Matter is assumed to interact gravitationally, it follows that the gravitation of massive regular matter bodies should have an effect on it. Even if it is otherwise weakly interacting with regular matter, one might naively expect concentrations of it enter orbit around massive objects, like neutron stars, regular stars, and even (why not) our own planet.

So, a two-fold question:
1) Could there be Dark Matter in orbit around (or within) the Earth?
2) If that were possible, would there be any way for humans to tell if such is actually the case?

• Some scientists recently announced a possible detection of (one candidate for) dark matter particles being emitted from the Sun. It is expected that potentially billions of dark matter particles pass through every square centimeter of earth per second, but being gravitationally bound to Earth would be difficult, but conceivably not impossible. I know of no confirmed local anomalies suggesting dark matter. – zibadawa timmy Apr 14 '15 at 2:25
• This Physics.SE post would also contain a lot of relevant information. The question is addressed more specifically to if we should keep looking as we have been, but this is predicated upon the apparent lack of detectable amounts of dark matter within our neighborhood of the galaxy. – zibadawa timmy Apr 14 '15 at 2:33
• @Serban Nice PhD topic :-) – andy256 Apr 14 '15 at 2:39

## 1 Answer

Yes, there should be dark matter within the Earth, but at very low densities - a few $10^{-22}$ kg/m$^3$ (Bovy & Tremaine 2012), something like one hundredth of the density of the interplanetary medium. The whole of the Earth would contain few hundred grammes!

Of course, this density averaged over a sphere a lot larger than the Galaxy adds up to a lot of mass, but unlike luminous matter, dark matter should not be particularly concentrated in the plane of the Galaxy (and observations of the dynamics of local stars confirm this).

Neither do we expect dark matter to be especially concentrated within the Earth. Although it does interact gravitationally, the kinetic energy it receives from falling in to the Earth's gravitational potential will be exactly sufficient for it to escape again, unless it suffers some inelastic interaction inside the Earth. Some researchers do think that it may be possible that weak interactions may trap dark matter in the core of the Sun (e.g. Vincent et al. 2015) or even more likely, in neutron stars (Guver et al. 2012). But the Earth is not large or dense enough for this to be a likely scenario.