# How could we detect planets orbiting black holes?

Detecting planets orbiting around stars can be made with several methods, transit photometry seems to be by far the most popular method nowadays.

I wonder how we could detect planets orbiting black holes. Which methods are we left with? Has it been done before?

• Just a thought but can't photometry be used on a different spectrum? Dec 25 '15 at 11:11
• Since all galaxies have black holes at their centers, aren't almost all planets orbiting black holes? Dec 28 '15 at 0:36

Gravitational microlensing is a way of finding planets that does not care how luminous or otherwise the hosts of the exoplanets are.

The way it would work is that you stare at a dense background field of stars; then when a foreground black hole passes in front of a background star, the light from the star is magnified by gravitational lensing. Typically, the lensing event takes a few weeks for the magnification to develop and subside.

If the black hole has a planet, it may be possible to "see" its graviational potential, which will manifest itself as an asymmetry in the lens light curve or possibly even a little extra spike in the light curve lasting a few hours.

This technique is well established and is already used to detect planets around unseen objects. It is more sensitive than the transit and Doppler methods to planets orbiting a fair distance from their parent star.

The difficulty here is not finding planets around a black hole, it is proving that the planets you found were actually orbiting a stellar sized black hole. The microlensing event would only be seen once and the black hole system would likely be invisible. There is a possibility I suppose that it may undergo some sort of accretion activity after having been found by lensing, and this accretion might then be picked up by telescopes trying to identify the lensing source. I guess a good place to start would be events where a stellar-mass lens is inferred, but no star can be seen after sufficient time has elapsed that the lens and source star ought to be resolved.

Theoretically we could use radial velocity using X ray or gamma ray spectra, but it would only work with relatively active black holes. If a black hole doesn't have an accretion disk detecting planets around it is impossible, because all methods require at least partial visibility of an object and black holes are invisible unless you can see their influence in their surroundings (stars orbiting invisible objects, accretion disks etc.). Besides, chances that a planet could exist near a black hole are very slim, detalled in this post (1st answer), because black holes are formed in supernova explosions which aren't very planet friendly.

The only way a planet could exist near a black hole is if it's a captured planet far enough away to not get torn by tidial forces or if one of the stars orbiting a black hole has planets, while we could detect the latter kind of planets using one of the known methods they wouldn't count as planets orbiting a black hole.

So far no planets orbiting a black holes were found due to reasons stated above.

• If the planet can be viewed directly, then its lightcurve should completely disappear when it transits behind the passive black hole (or rather, funny relativistic effects appear, actually, every orbit should be a microlensing event, facilitating detection). Radial velocity should also work. And planets can maybe form in the disks after the supernova. Dec 25 '15 at 16:35
• I don't see almost any detail in that post of why planets around black holes would be scarce if they aren't captured; only that during the creation of a black hole there's a big explosion that blows almost everything apart. (The guy also talks about the center of the Universe...) What about the probability to capture a planet? I guess it would depend on where the black hole is in a galaxy and in which galaxy. At first glance I don't see why such a probability would be slim (i.e. lower than say $10^{-8}$). Dec 25 '15 at 16:41
• The probability of capture may not be that slim, but some percentage of captured planets are going to be ripped apart by tidial forces. However, we're not discussing weather planets can exist orbiting black holes or not, but about possible ways to find them. Gravitational microlensing can only work if there are background stars and it's not the easiest method. And how can radial velocity work if you can't see the object planet is orbiting. Dec 25 '15 at 17:36