Black holes. Giants in space that consume everything around them. Not even light can escape their grasp. Our planet orbits a main sequence star, (the Sun), which is too small to become a black hole when it uses up its fuel. Let's briefly discuss how black holes do form (keep in mind I'm not an astrophysicist, so I will try my best to explain this).
Essentially, black holes form when a massive star collapses in on itself. When a large star dies, it causes a supernova explosion, and black holes form from these supernova remnants. Black holes are essentially points that have VERY large mass condensed in a very small point.
Artist's conception of a black hole
Here's an image about the life cycles of different sized stars and what their fate will be at the end of it:
The life cycle of high and low mass stars
But the real question is here... if our Sun was large enough to become a black hole at the end of its lifetime (and Earth wasn't destroyed by it), could Earth still orbit it (or any planet in general), AND would Earth still be habitable? Could a habitable planet orbit a black hole?
Firstly, let's discuss the possibility of planets even orbiting black holes. The short answer is: yes, planets can orbit black holes. They have a gravitational pull just like any other star. This means that theoretically there could be planets out there that are orbiting black holes, however, if we stop and think about what black holes REALLY are (the remnants of huge stars that ran out of fuel and went supernova), the chances of any planet actually surviving such an explosion would be slim to none. But this is SCIENCE we're talking about, anything is possible! (Well, at least we like to think that, but imagining things like this is fun, so I'm going to keep going).
Earth orbits the Sun in the habitable zone. I have mentioned this multiple times in some of my previous posts, but essentially, the habitable zone is what allows for liquid water to exist on the surface of our planet, thus allowing life to exist and thrive as well. These factors depend on the output of energy from the star and the planet's distance from the star. (Really, we just happened to be lucky). Theoretically, a planet could have a "habitable zone" around a black hole, it just wouldn't be getting its energy from light from a star. In the case of a black hole, the planet could potentially receive light from the accretion disks (the disks of matter and gas that form around large black holes).
Artist's conception of planets orbiting a black hole
Of course, scientists have yet to find evidence of planets orbiting black holes, but if they did, could they still be habitable? Would Earth still be the same? We know that if any object gets too close to a black hole, it will immediately get sucked in and be unable to escape, trapped forever in one of these giant cosmic vacuums. However, if the black hole is spinning fast, it is possible for close, stable orbits to exist. (Another caveat to this is that the black hole would have to be spinning VERY close to the speed of light in order for any planet to be sufficiently close to it in order to receive energy from the Cosmic Microwave Background (CMB). Why is the CMB important? It's the leftover radiation from the Big Bang that is found throughout the universe. According to Astrophysicist Pavel Bakala of the Silesian University in Opava and his colleagues, "The CMB would appear as a bright star on the edge of the black hole's shadow" (sciencemag.org). This is due to the massive gravitational pull from the black hole. It would compress the radiation into optimal wavelengths and funnel it into a narrow beam. This is why the planet would need to orbit very close to the black hole.
Additionally, let's think about how big this black hole would really need to be. Black holes vary in size: some can be a few times the mass of the Sun, whereas others can be millions or billions of times the mass of the Sun (think of black holes at the centers of galaxies for instance). Black holes this size (supermassive) tend to instantly rip planets and stars apart from their gravitational pulls, but if a planet were to orbit around the outside, it would be safe, since it wouldn't get pulled apart until reaching the black hole's event horizon.
The researchers also stated that the space around the black hole would need to be mostly empty, since black holes emit Hawking radiation, which would destroy any nearby planet instantaneously.
Diagram of Hawking Radiation
The thought of the Sun just being a black hole may sound exciting. Theorist Avi Loeb of Harvard University thought of some "fun things to do near a black hole," and I'm sure that sounds really interesting, but here are a few of the things he was able to come up with: "using its accretion disk to burn trash and produce usable energy, or surfing with light sails on the jets coming from a black hole’s poles" (sciencemag.org).
Unfortunately, here comes the disappointing part: it's really not possible for life to thrive on such a planet if it were to exist. Think for a second everything you've learned about black holes, whether you've studied them for a long time or barely even know what they actually are. The conditions surrounding them are so extreme that it just doesn't seem feasible for a habitable planet to be able to thrive if it were orbiting one. As mentioned earlier, the black hole would need to be spinning absurdly fast, and it just doesn't seem like that would even be possible, for anything in space for that matter. Secondly, how would a planet form in such conditions? We said that it wasn't IMPOSSIBLE, but that doesn't mean it is highly likely. "Researchers in Japan hypothesized that planets could theoretically form in the cold disk of gas and dust that exists at some distance around a galactic center, but it just doesn't seem likely that a planet could have a stable orbit near the edge of an active black hole" (sciencemag.org). Due to how powerful black holes are, most planets and stars just wouldn't be able to survive the blasts of UV light as the black hole grows when it consumes the gas and dust around it.
If such planets DO exist, it would be extremely difficult to even detect them. A planet that passes in front of a black hole is going to be very hard to see, due to light not even being able to escape from them. "Bakala says a vast array of radio telescopes, like the one used last year to image a black hole for the first time, might be able to detect such a transit. “Technically it’s not so easy, but in theory it’s possible”" (sciencemag.org).
For reference, here's the image of the black hole:
This image was a HUGE leap for the scientific community, since it was the first REAL image of a black hole. However, the quality is not the best, so even if these telescopes could detect and image planets orbiting black holes, it would probably be very difficult to see them, let alone try and find out if there's life existing on that planet.
Final thoughts: Doing research on this topic made me appreciate our Sun even more. It allowed our planet to be able to support a vast array of life and it didn't obliterate our planet in the process! Obviously, the thought of orbiting a black hole sounds cool and straight out of a sci-fi movie or book, but it's also very terrifying. We've heard about black holes plenty of times and we know what they're capable of. Additionally, even if life were possible on a planet orbiting a black hole, my guess would be that it would be VERY extreme due to the conditions that it would experience. One important thing is that the planet would experience significant time dilation. For someone far away from a black hole, time would appear to move more quickly than for someone near a black hole. This is known as gravitational time dilation. This is why an object that's falling into a black hole appears to move very slowly as it approaches the event horizon. I can't even imagine what time would feel like on such a planet orbiting a black hole. Let's also not forget that black holes LITERALLY bend spacetime around them because of how massive they are and how fast they spin:
Artist's rendition of a black hole bending spacetime around it
As I've said before, I'm really glad we have the star that we do, because without it, who knows if we'd be here? Although, I am looking forward to what comes out of future black hole research, since we're just know beginning to scratch the surface of how they really work and affect our universe.