Ask Ethan: If Dark Matter Is Everywhere, Why H…

Ask Ethan: If Dark Matter Is Everywhere, Why Haven’t We Detected It In Our Solar System?

“All the evidence for dark matter and dark energy seem to be way out there in the cosmos. It seems very suspicious that we don’t see any evidence of it here in our own solar system. No one has ever reported any anomaly in the orbits of the planets. Yet these have all been measured very precisely. If the universe is 95% dark, the effects should be locally measurable.”

You know the deal with dark matter: it makes up 85% of the mass of our Universe, it has gravitational effects but no collisions with normal matter or itself, and it explains a whole slew of cosmological observations. But why, then, if it’s everywhere, including in an enormous, diffuse halo around our Milky Way, doesn’t it affect the motion of our Solar System in an observable way? Surely, when you say that matter is distributed all throughout our galaxy, that will include the Sun’s neighborhood, right? The truth is, it actually does! Dark matter must exist throughout the Solar System, but that doesn’t mean its effects are observable. Contrariwise, you have to do the calculation to know what its density is, and to quantify the effects it would have on the planets. We can actually do this ourselves, and the results we find tell us, under a variety of conditions, exactly what we’d expect. Dark matter should be in our Solar System, and our best observations aren’t yet able to test whether it exists or not!

Combined, everything within the orbit of Neptune only adds the mass of a large asteroid; that’s not nearly enough. Come get the full story on this week’s Ask Ethan!