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Comment by scld | Hacker News Reader

Layman here: Does this mean that they'll launch orbiters around Neptune and Uranus and, by measuring the doppler shift in comms back to Earth against their simulations, determine if there's enough dark matter to be impacting the doppler shift?

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No, or not exactly. It appears to me that they are suggesting (or rather, modelling) using the doppler shift to precisely measure the trajectories of the spacecraft on the way to those planets in order to get a measurement of the gravitational field that is acting on them and thus a measurement of any dark matter that may be present.

Hmm, but if we’re suspended in a homogeneous “soup” of Dark Matter, the surely such forces would cancel out?

Do we except Dark Matter to be distributed inhomogeneously around the Solar System? I thought it is only in homogenous at ~galactic scales.

https://xkcd.com/2186/

Earths dark matter is equivalent to one squirrel.

Right, but even that link suggests dark matter is pretty homogeneous around Solar System, right? If it’s simply described as a density number.

Is the suggested experiment just measuring that homogeneous density? Or hoping to find some distribution around the Solae System?

We've only been able to infer dark matter densities over galactic scales so I'm not sure that density applies at planetary system scales. It's like trying to use the density of iron to say where the atoms in a chunk of it are.

Warning, I am also a layman, but I don't think we know how clumped dark matter is. We know it has to be spread out on galactic scales, but that doesn't mean it's spread out at solar system scales. Some people even suggest dark matter could be black holes with intermediate masses. If we can figure out how concentrated or spread out dark matter is at the solar system level, it constrains the models that would fit the new data.

How do they propose differentiating the gravity due to normal matter from dark matter?

Basically, we can see the normal matter.

Inside a planet's core? To a degree of precision that allows us to pin any discrepancy on a poorly understood new form of matter? I can see how we can infer mass from orbital ephemeris or gravity measurements, but understood results from both would be affected by both DM and normal matter together.

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