Habitable Planets Orbiting Black Holes

Habitable planets orbiting a black hole are the setting for the Christopher Nolan film “Interstellar.” Find out what scientists found when they considered the possibility in real life.

I remember reading Jules Verne’s From the Earth to the Moon in public school. It would have been around the time of the Apollo lunar landings.

Our teacher emphasized how science fiction writers like Jules Verne and H.G. Welles had successfully predicted the future. Their fictional works had foreseen tanks, submarines, nuclear energy and automatic doors among other things.

Many discoveries and advances have been predicted in advance by speculative fiction novels. Jonathan Swift predicted that two moons would be discovered around Mars, Mary Shelley foresaw organ transplants, Hugo Gernsback imagined solar power and Aldous Huxley forecast the widespread use of drugs for mood disorders.


This week, we learned that a science fiction movie may have inspired another discovery. Christopher Nolan’s 2014 film Interstellar tells the familiar story of a team of astronauts who go planet hunting to replace the doomed Earth.

One thing that’s unique about Interstellar though, is that it depicts the astronauts visiting a solar system orbiting a black hole instead of a star. The astronauts explore a range of forbidding landscapes on these bizarre, hostile but habitable planets.

The film planted a seed in the mind of astrophysicist Pavel Bakala of Silesian University in the Czech Republic. He decided to pose a thought experiment to his students to challenge them to apply their understanding of thermodynamics. Could alien life exist on a habitable planet orbiting a black hole?

thought experiment to challenge students

There’s a lot to think about before we can answer this question. One way to think about life on Earth is that it exists in the difference between the energy our planet absorbs from the sun and the surplus heat it diffuses back into frigid outer space.

With a black hole instead of a parent star, the situation around potential habitability would be reversed. Jim Whiting explains the dynamics in his book Mysteries of the Universe: Black Holes. A black hole would have virtually no surface temperature and would play the role of the heat sink.

That means that energy would have to come from the Cosmic Microwave Background (CMB) instead of a sun-like star. As we’ve discussed in previous stories, this is the faint trace of radiation left behind by the big bang.

energy from cosmic microwave background

The CMB is very faint and almost as cold as a black hole. Despite this, the black hole’s gravity would compress the CMB’s wavelength and turn it into a beam of visible light.

If you were standing on the surface of one of the black hole’s habitable planets, the CMB would look like a bright star on the edge of the black hole’s shadow. The thing is, for this to work out, the planet would have to be awfully close to the black hole’s event horizon.

As readers might guess, earth-sized planets close to the event horizon would get sucked into the black hole by its unimaginably powerful gravity. A planet the size of Earth could avoid that though, if the black hole was spinning rapidly.

planet could avoid gravity of spinning black hole

Mind you, it would have to be spinning mind-blowingly fast. By that we mean it would have to be rotating at the speed of light minus just one one-millionth of a percent.

The black hole would also have to be incomprehensibly huge. It would have to have 163 million times the mass of our sun.

A run of the mill supermassive black hole like the one at the centre of our Milky Way galaxy weighs about 4 million solar masses. Their gravity rips habitable planets apart with tidal effects.

gigantic spinning black hole looks feasible

If there was a black hole big enough, the tidal action wouldn’t happen until objects were inside the event horizon. So if a black hole was gigantic and spinning at just under the speed of light, the idea starts looking feasible.

There couldn’t be anything nearby this black hole and its planets. If there were, they would be sucked into the black hole and give off bursts of radiation, destroying any living things on the habitable planets.

They would be very exotic worlds for us. There wouldn’t be daylight and a night sky. The black hole’s pitch black event horizon would fill up half the sky all the time.

black hole’s event horizon would fill half the sky

There would also be unfathomable time dilation from the black hole’s gravity. A year on one of the planets would be the equivalent of thousands of years in the frame of reference of a habitable planet orbiting an ordinary star.

It would be very difficult to detect a planet like this from Earth. Unlike when a planet passes in front of a star, a habitable planet passing in front of a black hole wouldn’t make it any dimmer. For example, the Transiting Exoplanet Survey Satellite (TESS) could not detect such a planet because not even light can escape a black hole.

On the other hand, Professor Bakala points out that an array of modern radio telescopes managed to photograph a black hole for the first time last year. Theoretically, it might be possible to use the same transit method to detect a habitable planet in front of a black hole if we get lucky.

might be able to spot planet transiting black hole

Professor Avi Loeb of Harvard is skeptical about Professor Balaka’s ideas. He reminds his peers that there is no known cause for a black hole to spin that fast.

Professor Loeb also doesn’t see how a planet, even a super-Earth could end up in orbit so close to a black hole. Besides, a black hole that close to a habitable planet would wipe out its atmosphere and any liquid water by giving off radiation blasts from the dust and gas it would absorb.

To be clear, nobody has ever seen a habitable planet orbiting a black hole. This isn’t a real discovery, just a thought experiment.

not a real discovery, just a thought experiment

It is worth thinking about, though. It has led scientists to consider how they might be able to detect habitable planets and other small objects near the black holes at the centres of galaxies. If we apply the above ideas about gravity lensing to objects that are several billion years old like neutron stars, we might think of ways to detect them.

We need a new fact-based story that we can all tell each other about the origin and nature of our universe. Sometimes making up fictional stories using science helps to guide us in the right direction.

Many of the fancy toys we saw on Star Trek in the 60s actually exist today, like tablets, desktop computers and smartphones. Speculative fiction can influence our thinking and lead to making discoveries and tools along the lines of what its authors only imagined.

We always have more to learn if we dare to know.

Learn more:

Science Magazine (AAAS)
Life under a black sun
Habitable Zones around Almost Extremely Spinning Black Holes (Black Sun Revisited)
Mysteries of the Universe: Black Holes
Astrobiology: 3 Questions We Need to Answer
Why Mars? Why Not Life on Venus?
Is Planet X a Black Hole?
Exoplanet Giant Orbiting Tiny White Dwarf
Three Black Holes Colliding in Galaxy’s Core


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