I know of two methods for finding exoplanets: the transit method and the radial velocity method. These two methods work as follows:

  1. Transit: we observe stars and watch for when a planet obstructs the light from the star.

  2. Radial Velocity: detecting the planet by observing the motion of the star and using Kepler's law.

Are these methods also used for discovering black-holes? What are the pros and cons of these methods, I think that the con of transit is that it's difficult to see the black hole. A pro is that it doesn't matter how many stars there are around the black hole.

With the Radial Velocity method, a pro is that it's easy to detect but requires exactly one companion star (I used this source).

  • 2
    $\begingroup$ Ummm. . . These methods are generally used to find exoplanets, not black holes. I have yet to hear of them being applied to the latter. $\endgroup$
    – HDE 226868
    Mar 3, 2016 at 21:57
  • $\begingroup$ @HDE226868 but theoretically, what would the pros and and cons be of each method applied to black hole finding? $\endgroup$
    – Apollo
    Mar 3, 2016 at 21:58
  • 1
    $\begingroup$ Only the second of these is used to find black holes. There are other methods, but black holes are too small to yield a transit signal. Possibly you are thinking of gravitational microlensing. $\endgroup$
    – ProfRob
    Mar 4, 2016 at 18:18
  • $\begingroup$ Edited to reflect these comments. $\endgroup$
    – James K
    Mar 11, 2016 at 18:40

1 Answer 1


We detect black holes mostly by their effects on nearby matter. The "radial velocity" method is important after x-ray observations suggest a likely black-hole.

Matter in orbit around a black hole can form an accretion disc. This disc will become very hot, and glow brightly in X-ray radiation. If we see X-ray radiation coming from a star we look at the radial velocity of the star, and use this to infer the mass of the orbiting object. Since a neutron star cannot exist at more than 3 solar masses, if the object is more than that it must be a black hole.

Compared to planets, black holes are rare. Only about 20 candidates are known in the Milky way.

  • $\begingroup$ Are they rare for real, or just hard to find? Couldn't the first generation stars in general have been large enough to leave black holes behind, so that there are about as many black holes as there are active stars today? $\endgroup$
    – LocalFluff
    Jul 23, 2016 at 5:22

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