For example, some giant “planet” the size of our sun which has a surface like the moon, floating through space in darkness.
Tangentially, how are we sure that black holes aren’t these? Are we misinterpreting absence of light as black holes when instead they could just be large dark objects?
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Depends on your definition of an object and what you consider to be supermassive. There are plenty of phenomenon in the universe that are huge and not hot or bright, like giant gas and dust clouds. If by object, you mean a single solid object, then, depending on how you consider a black hole’s temperature, there’s supermassive black holes. Otherwise, there’s an upper limit on mass because anything that gets more than roughly 75-80 times the mass of Jupiter becomes a star.
I’m not sure what you mean by your second point. Black holes *are* supermassive and not hot or bright. A black hole is exactly what happens when you get that much mass concentrated in one place. What would we be confusing them for?
For a pretty direct answer to your title question: because it’s harder to find things in space when they’re not letting off a lot of light, and because most supermassive things are hot and bright, which is a direct result of how massive they are.
Heat/light from stars is generated by nuclear fusion, which (in stars) happens when the force of gravity is so strong that atoms basically get pushed together. With any solid object as large as the sun or anywhere remotely near its size (volume), gravity would be strong enough for fusion to happen and thus it would be a star itself.
Neutron stars might be an example of the giant “planets” you’re describing, except they’re tiny by volume- they can have about the mass of the sun but only a diameter about the size of a city. Gravity has basically gone to the extreme and smashed the protons and electrons that are normally in a bunch of atoms into just neutrons. They’re not undergoing fusion anymore so they’re not making energy, but many of them are still hot and bright just from the leftover energy they have from when they used to be stars.
Black holes are also detectable because they don’t just suck light into them, but they also bend light around them. If you have something massive enough to significantly bend light and it’s also completely black and emits no detectable waves, it’s a black hole
Bright things are hot and easy to see, so we find them easily.
If something were the size of our sun, it would be another star.
Stars are so massive that they compress atoms in their core until they start nuclear fusion. Which releases a ton of heat and light, which we can see and measure. Even though stars are made of gas, something that large made of rock just isn’t feasible without a ton of gas as well. The universe 73% hydrogen. Anywhere there’s a solar mass worth of rock, theirs going to be 3x that much hydrogen gas.
Large dark objects that aren’t black holes do exist, they’re called neutron stars. Stars that have died via supernova but aren’t large enough to form a black hole will collapse down enough so their protons and electrons fuse together to make neutrons. This ends up being a massive black ball that spins around really fast, but light can still escape its pull, so it is not a black hole. Neutron stars are made of mostly neutrons, hence the name, we call this material nuetronium.
Black dwarves are also possible, however the universe isn’t old enough for one to exist. When a star like our sun dies, it becomes a white dwarf, and begins to emit light very slowly as it cools down. After about 100 trillion years, we believe a white dwarf will have cooled down enough to not emit light and become a black dwarf. This would be the most massive object that has a surface made of common matter.
Black holes aren’t really made of anything. A black hole is formed when so much matter is packed so densely (like during a supernova) that it creates a gravitational field strong enough that not even light can escape. Theoretically, at the center of a black hole is a point (0 height, length, and width) with all of the mass of a black hole. This is called a singularity, and it is impossible to observe. The big black orb you see when you look at a black hole isn’t actually the black hole, it’s the event horizon. It appears black because there is no light coming from it. If light were to come from behind, it would be sucked into the black hole. If we wanted to bounce light off of it and have that return to us, it would get sucked in. It’s literally a hole through spacetime. There is nothing there.
>supermassive objects which aren’t hot/bright
I don’t know, we have a giant black hole at the center of the galaxy, I wouldn’t say we never hear about it.
>Tangentially, how are we sure that black holes aren’t these? Are we misinterpreting absence of light as black holes when instead they could just be large dark objects?
We don’t detect black holes by looking for absence of light in them. We know black holes are where they are because of the gravitational well they produce, as well as the lensing effect light (and all electromagnetic radiation) go through when traveling next to the black hole.
Stars are what happens when you gather enough mass in a small (relatively) enough space that the force of gravity in the core is sufficient to initiate fusion.
There are dust/gas clouds out there that are many times the size of our sun, but they are very diffuse, so therefore have no surface.
There are such things as brown dwarfs, which are failed stars that didn’t quite have enough mass to initiate fusion, but still give off some heat from the formation process. The line is about 8 times the size of jupiter. Once you get an object with about that much mass, it will start fusion in its core and become a star.
As for the tangent, the black holes we know about are actually some of the brightest objects in the sky. Sagittarius A is a supermassive black hole that is also extremely bright. Once you cross the event horizon, nothing can escape a black hole, including light. But, as mass is sucked into a black hole, it is under such severe strains that a significant portion of it is converted to energy (gamma rays, x-rays, visible light, radio waves, etc).
Something like a rogue black hole wouldn’t give off light, because there’s no matter crossing its event horizon. That we really wouldn’t have a way to detect it.
Objects ~60x Jupiter’s mass are big enough to start fusing and become stars. (See J0523)
So we have to limit “supermassive” to objects less than 60 Jupiters or they emit light.
Exoplanets can fit that bill but until recently our instruments weren’t sensitive enough or been observing long enough to catch planets with long years which means they were all close to their stars = hot.
Black holes and nebulas are objects that are very massive and very dark. They’re detected by their gravitational effect on other visible objects or how they block or scatter the light of objects behind them.
It isn’t clear what you mean by “massive” since you might be shifting between “high mass” (the traditional definition for physics) and “large size” (the semantical use).
A dense massive (high mass) object will be crushed by its own gravity. The extremes of these objects would be black holes and neutron stars (if not quite as massive). There is no escaping the logic of gravity. Something that “starts” with the size of the sun but consists of dense matter like the earth could not hold itself as a planet because the gravity would very quickly lead to it collapsing.
Our sun has an average density of about 1.4 times that of liquid water on earth. Earth has an average density about 6 times that of liquid water. So an “earth” the size of the sun would be around 64 times more massive (in mass) than the sun. The gravity from such a mass at such density would very likely compress itself into a black hole. For a rocky type planet, the theoretical size limit would be (likely, ELI5) around twice the diameter of earth. For a gas type planet like Jupiter, the mass limit would be around 300 times the mass of the earth. (For reference the sun is around 300,000 times the mass of the earth)
There are “objects” that can be far more massive but much less dense and therefore occupy a large volume. They would resemble gas clouds rather than a planet. Some of these clouds will eventually form new stars and solar systems (it isn’t a static picture over time)
The simple explanation is that large mass causes intense gravity so atoms get squished which causes friction, and friction causes heat.
Simply put, once objects get a certain size, they pretty much only can exist as a star, black hole, etc. A solid mass the size of the sun could never exist because it would collapse under its own gravity. And once an even larger mass is achieved, it collapses under its own gravity to become a black hole. And we find black holes because we see objects like starts orbiting a point of seemingly nothing at breakneck speeds.
How would we see a massive dark object? With our giant space flashlight that can shine light across thousands of light years? Unless a dark object crosses in front of a bright object or disturbs it’s path through the sky (via gravity) we have no way of knowing it’s there.
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