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This question is a followup of Is there any theoretical or empirical research about planetary systems with a black hole in their center?;

Since it is possible to detect planets around stars with the change of light they cause, I would think it should also be possible to detect them when they orbit around an active Stellar Mass BH.

From what I (can) understand from the first answer, there is no known case of planets orbiting around Stellar Mass BH. What could be preventing us to observe it?

  • Is there too few active Stellar Mass BH close enough for us to have a chance the system's plan is well aligned with us?

  • Do we only see Stellar Mass BH when they have a stellar companion feeding them, and this discards to possibility for close-by planets?

  • Or is it because the light we can see from them is too fast-changing for a wobble or transition to be observable with today telescopes?

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  • $\begingroup$ is SMBH ususally the abbreviation for Stellar Mass Black Hole, or Super Massive Black Hole ? $\endgroup$ – James K Jul 7 '17 at 15:13
  • $\begingroup$ Oh sorry, I'll edit. I mean Stellar Mass $\endgroup$ – J. Chomel Jul 7 '17 at 15:23
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Stellar Black Holes (SBHs) are really really hard to find. If there's a free-floating one in space with no companion, our chances of finding it are almost nil. However, observations have more or less ruled out the possibility that space is freely floating with huge quantities SBHs. This was one of the proposed sources of Dark Matter, namely MACHOs. We don't believe MACHOs exist at this point because lensing observations towards the galactic core suggest they don't exist in any significant quantity.

The list of known and candidate SBHs contains less than 30 entries. The reason SBHs are hard to find is that, as stated above, they don't seem to be too common, and also that they can only really be observed indirectly through their interactions with a binary companion. Often, one observes the X-ray radiation coming from infalling matter around the black hole, taken from the binary companion, rather than observing the black hole itself.

Is there too few active Stellar Mass BH close enough for us to have a chance the system's plan is well aligned with us?

The answer to this is probably yes.

Do we only see Stellar Mass BH when they have a stellar companion feeding them, and this discards to possibility for close-by planets?

Again yes, we can only "see" the SBHs when they have a companion feeding them. This doesn't count the three cases where we've found SBHs through gravitational wave observations though, but all of those were far outside our galaxy. Whether a planet could exist in such a violent binary system is up for debate. There's only one known case that I know of where a planet is orbiting a similar system and that is PSR B1620-26. This is a white dwarf, pulsar pair with a planet around it. This would be completely analagous to an X-ray binary system with a black hole if only the pulsar were a bit more massive. So certainly it seems possible for a planet to be around such a system, but probably not very likely.

Or is it because the light we can see from them is too fast-changing for a wobble or transition to be observable with today telescopes?

This certainly plays a part too. Even if you find a binary system with an SBH that is active and producing X-rays, you're still going to have a hard time teasing out an exoplanet signal, especially if that exoplanet is small (i.e., smaller than Jupiter). The transit method would be difficult due to the natural variability and hence noise in the X-ray emissions and the radial velocity method may not be sensitive enough to small planets. Such a measurement would be really hard.


As it turns out though, astronomers are thinking about these things. There was recently a paper published almost on this very topic. Check out Irma & Stefano (2017) where they discuss trying to find planets around X-ray binaries that may potentially house a SBH.

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    $\begingroup$ There are probably $\sim 10^8$ stellar mass black holes in the Galaxy. The nearest will be far closer than the most distant stars around which exoplanets have been detected. $\endgroup$ – Rob Jeffries Jul 7 '17 at 22:41

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