The Moon orbits around the Earth. The Earth orbits around the Sun. And out in the distant Universe, astronomers have found a system that takes this logical progression to its most extreme. There’s a system where a supermassive black hole with millions of times the mass of the Sun orbits another black hole with billions of times the mass of the Sun. 

How astronomers discovered this incredible interaction took careful observations, imagination, and the hard work of the Spitzer Space Telescope, taken during its final years of operation.

A regular stellar-mass black hole already boggles the imagination. An object with several times the mass of the Sun compacted down into a volume as small as a city. But astronomers also know of the supermassive black holes that lurk in the hearts of most galaxies. 

Our own Milky Way has a black hole with 4.1 million times the mass of the Sun. Again, just let the scale wash over you. 4.1 million solar masses compacted into a region that’s smaller than the orbit of Mercury.

But our galaxy’s black hole is actually pretty small in the grand scheme of things. In the distant galaxy OJ 287, astronomers have found a supermassive black hole that weighs in at 18 billion times the mass of the Sun. 

And what’s happening at the heart of this galaxy is absolutely fascinating. Orbiting around this monster black hole is another supermassive black hole with merely 150 million times the mass of the Sun.

Here’s how astronomers figured this out.

Galaxy OJ 287 is located 3.5 billion light-years away in the constellation of Cancer. Astronomers have known for a while that it contains one of the most massive black holes ever seen in the Universe, weighing in at 18.35 billion times the mass of the Sun.

OJ 287 is an object known as a blazar. Astronomers have been aware of a series of objects which are very distant and surprisingly bright - known as “active galaxies”. You might be familiar with the term “quasar” which is the bright center of a galaxy which has a supermassive black hole that’s feeding on material.

Blazars are essentially the same thing, but it all depends on the angle at which we see them. If we’re seeing the galaxy edge-on, it’s known as a “radio galaxy”. If we’re seeing it at an angle and can see more of the central accretion disk, it’s a “quasar”. And if we’re looking right down the barrel into the galaxy, face-on, that’s a “blazar”.

OJ 287’s supermassive black hole is surrounded by a huge disk of gas and dust; the accretion disk of material that’s spiraling around the black hole, waiting for its turn to disappear beyond the event horizon and join the singularity.

Astronomers studying OJ 287 noticed a flash of light of radiation coming from the galactic center on a semi-regular basis. Every 12 years or so, there would be a double-flash of brightness, where the system would brighten up by a factor of 4 over the course of 48 hours. 

Sometimes the flashes would happen within a year of each other and sometimes they’d happen 10 years apart. Astronomers struggled to figure out what was causing the flares and tried to predict when the next one would happen.

Finally, in 2018, a group of scientists led by Lankeswar Dey, a graduate student at the Tata Institute of Fundamental Research in Mumbai, India figured out the orbital parameters,  published a study that predicted exactly when the next flare would happen within a couple of weeks of accuracy. 

According to Dey and his team, the more massive 18-billion solar mass black hole is being orbited by a 150-million solar mass black hole on an irregular orbit. Each time that the smaller black hole plunges through the accretion disk of the bigger one, it generates a flash of light brighter than the entire Milky Way. Like a trillion stars shining together at once, fading away in just a couple of days.

But there was a big problem. They predicted that the flare was going to happen in late July 2019. As luck would have it, that part of the Universe was inconveniently located behind the Sun, invisible to any observations from Earth, and would remain so until September 2019, long after the flare was due to fade away.

Fortunately, astronomers had an instrument that would still be able to see that region of the Universe, even though the view was blocked from Earth. And we’ll get to that in a second, but first I’d like to thank:

Patrick Carpenter
Don Novak
Ben Slutsker
Anton Sigal
Dark Water

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As you’ve probably heard, NASA’s long-lived Spitzer Space Telescope was retired in January 2020 after 16-years of observations of the Universe in infrared. It fundamentally changed our view of the Universe, making the first direct observations of atmospheres of exoplanets, seeing through gas and dust to see newly forming stars and planets, and helping astronomers see through the core of the Milky Way to core structures normally blocked by the Zone of Avoidance.

Spitzer was on a path that kept it slowly drifting away from the Earth as it orbited the Sun. By 2019, Spitzer was 254 million kilometers from Earth, allowing it to see the Universe from a completely different vantage point. And galaxy OJ 287 wasn’t obscured by the Sun from its vantage point.

Engineers at NASA directed Spitzer to watch OJ 287 for that telltale flash, and they were surprised to see it on July 31, exactly when the team had predicted it would happen. And then, just a few months later, Spitzer was out of fuel and retired forever, shutting off one of astronomy’s most important views into the cosmos.

I’m sure you’ve played enough Kerbal Space Program to have an intuitive sense that things orbit around other things. So why was it so difficult to predict the movements of one supermassive black hole orbiting another?

In order to make predictions about the movements of black holes orbiting each other, astronomers have to account for gravitational waves, ripples in spacetime caused by the movement of massive objects. The more massive an object is, the faster it’s moving, the more gravitational waves that it emanates. And the OJ 287 system takes this to the extreme.

This has always been a rough calculation, but thanks to the detection of gravitational waves from colliding black holes that were detected by LIGO, physicists could calculate black hole moments with much higher accuracy.

Lay and his team were also able to incorporate modern theories about the shape of a black hole’s event horizon. The “no-hair” theorem proposed in the 1960s by Stephen Hawking and others, said that a black hole’s event horizon should perfectly smooth and symmetrical. Less dense objects, like planets and even neutron stars, can be lumpy and bumpy, with uneven bulges on either side.

Kip Thorne predicted that black holes orbiting each other would interact with each other in different ways depending on whether their surfaces were smooth or bumpy, and you could detect though how the gravitational waves distorted the space around them.

And that’s why, catching the flare within just a couple of hours of their prediction provided an enormous amount of evidence to the “no hair” theorem of black holes. Evidence is mounting that black holes have smooth, featureless surfaces, as predicted by Hawking in the 1960s.

In my opinion, this story really has it all. It deals with objects of incompressible size and mass orbiting around each other, releasing trillions of times the radiation of the Sun every few years. It allowed astronomers to further validate a theory that’s been half a century in the making, taking advantage of the first direct observations of gravitational waves.

It used an aging space telescope, called in for one final critical mission before it could pass on to its inevitable retirement. Thanks for everything Spitzer, we’re going to miss you.

What do you think? Let me know your thoughts in the comments. 

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Thanks to Advanced LIGO, astronomers are detecting gravitational waves every week or so now, and things are only going to get better. We did a whole video on the new gravitational wave observatories in the works, and what the future holds for this science. And you can watch that now.