From: David Roberson
Ø … does it not seem strange that astronomers can accurately predict the orbits of the planets without assuming any dark matter within the solar system? Dave, they would need to know the net mass and the center of mass in order to predict orbits. But the sequence of events, historically, is that astronomers first refined their knowledge of orbits based on observation over the centuries, going back to prehistory - and then in modern times, worked backwards to determine net mass. As a practical matter, they can assume the mass is composed of 100% normal matter, but it really doesn’t change things if part of the mass is dark – it is still mass. There is no need to differentiate, in order to predict orbits, unless dark matter does not interact gravitationally in the same way as normal matter. Small eccentricities do turn up – which were/are not predictable – even today. The mass of the Kuiper belt is presently low and so far removed that it doesn’t affect any orbit other than Pluto, with slight influence on Neptune. Kuiper total mass is estimated at 1/25th the mass of the Earth. That can tell us something important. This is because the models of Solar System formation predict an initial collective mass for the Kuiper belt of 30 Earth masses (according to Wiki and more elsewhere). Thus, there is missing mass today of about 99% of the former level. If the Kuiper belt had always existed at its current low density, the large objects like Pluto simply could not have formed. Some of that missing mass could be dark matter which was collected and removed by one or more objects. Fast forward to the recent discovery of Planet X, the ninth planet (if we accept the demotion of Pluto). As it turns out, most of the missing Kuiper mass could be found in this one object. Some could also be found in Neptune. Note that the interior of Neptune is over 5000 degrees C, despite it getting almost zero solar irradiation - and its net mass is anomalously large by some formation theories. This could indicate that Neptune too has captured dark matter from the Kuiper belt (assuming that captured dark matter undergoes nuclear reactions to provide the internal heat).
