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Usually missions like Kepler are finding exoplanets orbiting (the barycenter of systems related to...) stars at distances that are not common if compared with the Solar System. This finding (as I understand) motivate models in which migration of planets is involved as hypothesis, due to torques (e.g., Lindblad and Co-rotation) and angular momentum exchange.

In this context I ask myself the following hypothetical question: if any exoplanet detection mission (or other related works) find low orbital period planets (e.g., warm Jupiters) orbiting a stellar black hole, would this be (although indirect) "key" evidence of migration processes? (as a star becoming a BH should drag any planet that is close enough, when it collapses)

As new telescopes are being developed, how "far" are we from detecting scenarios such as planets orbiting black holes? It could sound really hard (maybe impossible) to detect a signal of transit or RV without a star emitting light towards us that could reveal motion or a significant drop in the relative flux. Also, mass ratios might complicate this even more but, are there any other possibilities? (High energy doppler? Microlensing? direct imaging?, a system involving also a binary star?, ...)

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    $\begingroup$ Detecting black holes is hard enough. If they aren't close enough to a star to drag material into an accretion disc and so give off X-rays, they are basically black and invisible. $\endgroup$
    – James K
    Dec 24, 2021 at 19:12
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    $\begingroup$ Any situation you describe sounds like some finely tuned, and hence very unlikely, scenario. This is perhaps helpful astronomy.stackexchange.com/questions/12662/… $\endgroup$
    – Daddy Kropotkin
    Dec 24, 2021 at 20:48

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Clearly if there is no light emerging from the immediate vicinity of the black hole, then exoplanet discovery methods involving transits won't work.

Measuring the Doppler effect in the primary object will also not work, but I suppose if the exoplanet were hot enough to be self-luminous then it is conceivable that you could directly measure its Doppler shift and infer the presence of the more massive black hole.

The system will also be emitting gravitational waves, but with very low power compared with two black holes orbiting each other with a similar period, because the power depends on the product of the two masses, not just their summed mass.

Whilst this may sound discouraging, the fact is that many black holes are found in binary systems where the black hole accretes matter. The hot, compressed accreting material emits X-rays from quite a small volume close to the black hole. It may be possible to search for transits in X-rays and a first candidate exoplanet discovery was announced recently.

This exoplanet is not orbiting close to the black hole. The star that became the black hole would likely have passed through a supergiant phase that would have engulfed anything orbiting it within an astronomical unit. On the other hand there are some close-in exoplanets found around pulsars that have similar progenitors. These are most likely formed from debris around the pulsar (after the supernova that produced it) or were captured in some way.

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One more option is gravtional microlensing. If the black hole passess in front of background star, we can see the background star get brighter as the black hole lenses the background star. If there is a planet in the black hole system as well then the planet adds an extra smaller peak to the brightning event.

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    $\begingroup$ How much of an extra effect would the planet add approximately? $\endgroup$
    – Daddy Kropotkin
    Dec 27, 2021 at 10:19
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    $\begingroup$ Randomly finding one of the OGLE planets arxiv.org/abs/astro-ph/0404309. Then the magnification is order the same as the star, but the time frame for the planet is much smaller (sharp peak) rather than a longer more gradual bump for the star. $\endgroup$
    – Rob
    Dec 27, 2021 at 11:36

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