Supermassive black holes locked in a stable orbit around each other
13 comments
·October 13, 2025pavel_lishin
> The existence of two black holes in OJ287 was first suggested in 1982. Aimo Sillanpää, then a graduate student at the University of Turku, observed that the brightness of the quasar changed regularly over a 12-year cycle.
Damn, that's about the time it takes Jupiter to orbit the sun. That feels wildly close together for objects that mass 18 billion & 150 million times that of our own sun.
These black holes (according to a calculator I found online) have radii of 53 billion km and 400 million km, so I'm guessing they must be orbiting significantly further away, and significantly faster than Jupiter (which is ~800 million km away from the sun) - which makes sense, given the monstrous 18b figure. I wonder how far apart they are, but I don't really know how to easily calculate that right now.
hnuser123456
Feature Primary Black Hole Secondary Black Hole
-------------------- ------------------------- ----------------------------
Mass 18 billion Solar Masses 150 million Solar Masses
Schwarzschild Radius ~356 AU ~3.0 AU
Orbital Speed ~4,200 km/s (1.4% c) ~502,000 km/s (16.7% c)
Feature Value
-------------------- ---------------------------------
Orbital Period 12 years
Average Separation ~13,800 AU (~0.22 light-years)
IAmBroom
Orbiting at c/6 - WOW!
kmm
In Newtonian gravity, the relation between the orbital period T and the semimajor axis a of the orbital ellipse is a^3 / T^2 = GM / 4π^2, where M is the reduced mass of the system (in this case, with 99% of the mass being in one of the two black holes, it's simply the mass of the heavier one).
Plugging 12 years and 18e9 solar masses gives about 2e12 kilometers, or roughly a fifth of a lightyear. This also means the smaller black hole is zipping around the bigger one at around 6% of the speed of light, which is low enough that the Newtonian approximation is probably reasonable accurate (at least to give a rough idea of how large the distances must be).
ardel95
Kepler’s laws should still provide a pretty good estimate, at least until black holes get much closer. I did a quick back of the envelope calculation, and looks like they’ll be roughly 14k astronomical units, or 0.22 light years apart.
hinkley
How much time dilation do you get at those masses though?
I’m having more trouble visualizing how accretion disks would work for a binary black hole. Because the light is coming from the disks, not the black holes. So those are what are actually pulsing/girating.
ardel95
Unless I screwed up the math, they would be quarter of a light year apart. Plenty of space for each black hole to form its own accretion disk.
pavel_lishin
Yeah, good point on that, too. I bet someone's written a simulator that I could run locally, but I've got a busy day ahead of me :(
I thought that in this case, the light that they detected was coming from the jets coming from the poles, not the disk itself directly.
hinkley
Since black holes are black holes, the jets are generated by the disk.
hinkley
Why “just released” if the paper the image came from is dated 2022?
DaveZale
maybe this:
One more flare happened since then, in 2022, but because of instrumental limitations, it was caught only at a prestage (M. J. Valtonen et al. 2023; M. J. Valtonen 2024). At the same time, more flares were discovered in historical photographic plate studies so that only eight of the expected 26 flares remain unconfirmed (R. Hudec et al. 2013). All the unconfirmed ones are due to lack of known photographs at the expected epochs.
InspGadget4343
*Muse starts playing somewhere in the cosmos
This system, OJ287, is perhaps the most important system we have for understanding what happens to supermassive black holes after a galaxy merger. This is the so-called "Last Parsec Problem."
When two galaxies merge, their supermassive black holes fairly rapidly sink to the center of mass of the newly combined galaxy via dynamical friction and enter into a slow orbit around each other. Over time, the SMBHs kick out interloping stars, which removes energy from the orbit and causes the two SMBHs to come closer together. If the SMBHs were able to get within ~0.1 parsecs of each other, gravitational wave radiation could take over and cause the orbit to shrink fairly rapidly and lead to the merger of the two SMBHs.
However, the theoretical models we have generally predict that at about 1 parsec, the SMBHs have kicked out all the stars in their neighborhood, so the process stalls out. In practice we don't observe many SMBH binary systems (OJ287 being the main exception), so there must be some mechanism that causes these systems to shrink from 1 pc to 0.1 pc. But we don't know what it is. The hope is that detailed studies of the orbit of OJ287 can provide some clues as to what that missing mechanism is.