A recent study suggests a that a primordial black hole may be orbiting the sun at and that they can be common near other star systems.

What measurements and equipment can perhaps find BH's near other stars?

  • $\begingroup$ If they are of at least planetary mass, you should be able to detect wiggles in the star's position as the hole orbits. It'll have to be close in. Eclipse dimming seems a forlorn hope fir such holes. They are small. $\endgroup$ Commented Oct 27, 2019 at 17:27
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    $\begingroup$ Main problem would be the same as with any kind of dark matter: how could they get rid of potential energy and angular momentum so that they'd end up orbiting the star? Most likely when they encounter a star they'd fall down the gravity well and gain velocity, but then climb up from the well using the said velocity. $\endgroup$
    – tuomas
    Commented Oct 27, 2019 at 20:26
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    $\begingroup$ While I'm sure a recent study says that; is it possible cite or link to it? Thanks! $\endgroup$
    – uhoh
    Commented Oct 27, 2019 at 23:05
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    $\begingroup$ Depending on the system, radial velocity, astrometric reflex motion of the host star or gravitational microlensing could detect them. The tricky part is being able to distinguish between a primordial black hole and a planet. $\endgroup$
    – user24157
    Commented Oct 28, 2019 at 19:46
  • $\begingroup$ Orbital anomalies in the Kuiper Belt; search for a 9th planet etc.. Tennis ball size PBH, 5-15 earth mass. Schultz & Unwin arXiv:1909.11090. Anderson, D.; Hunt, B. (5 December 2019). "Why astrophysicists think there's a black hole in our solar system". Business Insider. Retrieved 7 December 2019. $\endgroup$
    – Moggsy8
    Commented Apr 7, 2022 at 4:05

1 Answer 1


Black holes around stars can be found by searching for the reflex motion of the visible star. This reflex motion will be modulated on the orbital period of the system. Distinguishing a primordial black hole from one formed by stellar evolution, or from a lower mass compact, dim companion, could be very difficult

If the reflex motion is measured using line-of-sight velocities, then it is possible to get a lower limit to the mass of an unseen companion ($M \sin i$, where $i$ is the inknown orbital inclination) by combining the velocity amplitude and the orbital period. If that lower limit were greater than 3 solar masses and similar to, or more massive, than the visible star, then this is very likely to be a black hole, since it is too massive to be a neutron star (maximum possible mass is probably about 2.5 solar masses for a neutron star) and if it were a "normal star" it would be visible in the spectrum.

This technique has been used to confirm most of the non-merging black holes (i.e. not those found by gravitational wave detections) in binary systems that we know about. However, these objects are rare and are usually signposted by mass transfer from the normal star and accretion activity around the black hole. I'm not aware of any black hole candidate that has been found by systematic radial velocity surveys of random stars.

A more promising technique is to look at the reflex motion of the normal star as revealed by changes in its position. As we look at it from the Earth, the position of a star will execute a small ellipse in response to the gravitational pull of an unseen companion. The size of the ellipse is bigger for larger unseen companion masses and for wider binary separations. Measuring this ellipse (and the period of the motion), together with a known distance, should give the mass of the unseen companion, especially if then combined with line of sight velocities.

The Gaia mission has the potential to find many such unseen companions with periods of up to about 10-15 years, but we are waiting for the full first epoch-by-epoch position measurements to be released. At present all most Gaia data tells us is when a star has some sort of wobble or anomaly. Nevertheless an initial data release of orbital solutions for about 170,000 candidate binary systems has started to yield firm evidence for black hole companions on much wider orbits than the black hole binaries previously found (e.g. . A couple of black hole candidates have been found (e.g., Chakrabarti et al. 2022; El Badry et al. 2023a, b) of around 10 solar masses; unseen companions of solar-type stars or red giants. The prospects for finding many such objects are good (e.g., Janssens et al. 2021).

However, your question also talks about finding primordial black holes. The candidates above are all thought to be the products of normal stellar evolutionary processes, not primordial. Primordial black holes in binaries could be quite rare - they cannot be straighforwardly captured, requiring some sort of three-body interaction to take some kinetic energy away from the primordial black hole in order to bind it. Nevertheless, if primodial black holes are a significant part of dark matter then surely many such objects will exist (another question perhaps).

How would you identify an orbiting black hole as primordial? It would have to have a mass quite different to that expected from normal stellar evolution. So, less than 3 solar masses or more than 100 solar masses. In the former case it is hard to see how one would discriminate between this and a neutron star or, if it were below a solar mass, from a cold white dwarf, a brown dwarf or even a planet.


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