Selected Questions-Answers From ALL Experts Astronomy Forum (Dark Matter Planets?)

Question: Scientists know that there must be more mass than what is visible in galaxies as they manage to hold themselves together despite not being enough ordinary matter, so they call it dark matter as it doesn't emit anything but is observable through its gravity. Then if this dark matter can generate gravity similar to ordinary matter then can it not interact with itself and make dark matter clumps that are gravitationally detectable. Like a dark matter planet? If this was true i imagine a dark matter planet would be detectable as it would skew the orbits of ordinary matter planets? - Puzzled

Answer:

The conception of dark matter planets or solar systems for that matter is dependent on how well we can distinguish exotic forms from ordinary forms. In the latter case one might reference Fritz Zwicky's measurements of galaxy clusters which  highlighted a ‘missing mass’. He found that the mass needed to bind a cluster of galaxies together gravitationally was at least ten times the apparent mass visible.  Around the same time there were observations of stellar motions in the galactic plane by Dutch astronomer Jan Oort. He found there had to be at least three times the mass visibly presenting in order for stars not to escape the galaxy and fly off into space.

What exactly constituted this 'missing mass'.? The short answer is we don't know but there a number of candidates. By the late 1970s (with Cygnus X-1), astronomers realized there were other forms of dark matter. Among the most discussed candidates were black holes, marking the end stage of evolution for very massive stars. In the black hole, the gravity is so strong that no light escapes and the mass typically is much greater than that of the Sun.

Currently, we are aware of a super black hole at the center of our galaxy with 9.7 billion times the mass of the Sun.  Can such an entity form planets? Hardly! It would more likely 'devour' them.

Dark matter generally occurs in either baryonic or non-baryonic forms, depending on whether the matter reacts with radiation or not. If it doesn’t, it’s non-baryonic. Baryons include protons and neutrons, while non-baryons include electrons and neutrinos.

Non-baryonic dark matter further breaks down into cold dark matter and hot dark matter. The terms hot and cold are not so much indicative of current temperatures, as the phase of the early universe at which the particular dark matter ‘decoupled’ from the hot radiation background. An early decoupling implies a higher ambient background radiation temperature of the primeval cosmos. A later decoupling correlates to a cooler temperature. Perhaps the most widely studied candidate of hot dark matter is the neutrino.

By contrast, cold dark matter candidates tend to have larger mass and amongst the most likely suspects are: gravitinos, magnetic monopoles, and primordial black holes. However, there are a couple of exceptions to this, which include: WIMPs and Axions.

How any of these entities could leave specific gravity signatures that distinguish any one from their dark counterparts is a matter of continued inquiry.  Until such signatures are identified and clarified there is little chance of even remotely confirming on the existence of  "dark matter planets". Especially if it is proposed that such a planet could orbit an ordinary matter (e.g. hot plasma) star.

It is certainly plausible that a dark matter object (like a small black hole) could "skew the orbit" of an ordinary matter planet - or star. But that a dark matter planet could exist within the confines of an ordinary matter solar system is another matter. And if such a solar system was entirely of dark matter, how would one detect it at all?

Your conjecture of dark matter clumps  spinning off from a much larger dark matter mass (e.g. proto- star)  evokes a takeoff on the nebular hypothesis of origin for our own solar system.  The problem is that while such an entity (dark matter proto-planet) might be possible, there is no evidence whatsoever it exists. For example, all of the hundreds of exoplanets found thus far have all been ordinary matter objects - no 'dark matter'.

This is not to say such planets will never be found, but rather the investment of time, technology an resources may be much more than current cost-benefit analyses allow.