Brown dwarf stars are bizarre objects, which straddle the gap between fully functional stars, and massive planets like Jupiter. Brown dwarfs aren’t massive enough to start nuclear fusion in their cores, which is the process by which our own Sun reaches such tremendous temperatures. Without a large source of heat in their cores, brown dwarfs can’t do anything to maintain a stable temperature, and cool over time as they radiate their heat away into the void.
In many ways, we look to Jupiter as a model of what a small brown dwarf might look like. But, like many boundaries in astronomy, the border between a large Jupiter-like planet and a small brown dwarf is very, very fuzzy. Do you count a brown dwarf formed with a much larger, brighter star as a dual star system, or as a single one with a very massive planet?
There are a few things we know should change. Brown dwarfs should be hotter than Jupiter, and this is largely because they contain more mass than Jupiter - typically at least ten times more than what Jupiter contains. But how does this mass mean that they’re warmer? I said earlier that brown dwarfs, like Jupiter, are incapable of burning hydrogen to create their warmth. Brown dwarfs rely on another mechanism, which is the crushing force of gravity.
If you compress a fixed amount of gas into a smaller space, through any means, its temperature is driven higher. This works if you’re dealing with a canister of air on the Earth, or with an entire planet. For both Jupiter and the brown dwarfs, gravity is continually crushing the gas down, into a smaller and smaller core, until some force resists it. (The force that does the resisting depends on how much crushing pressure you have to start with. For a brown dwarf, it’s the pressure of electrons resisting getting any closer to each other.) This gravitational compression of the gas which makes up the brown dwarf (or the Jupiter) increases the temperature of the gas of that dwarf.
Unfortunately, this gravitationally triggered temperature increase isn’t particularly good at making anything very warm. Jupiter clocks in at minus 234 F (-145C) at its cloudtops (the core is warmer, as you might expect). The coolest brown dwarf known so far is still in negative temperatures, but the sort of negative you can feasibly get in a nasty cold snap on our habitable Earth - between -54 and 9 degrees Fahrenheit (-48 to -13 Celsius). There’s a suite of brown dwarfs that function at very human temperatures - roughly body temperature, or the temperature of a cup of coffee are often invoked. Even the warmer brown dwarfs tend to get up to the temperature of an oven on broil, so you can pretend to visit a brown dwarf in the comfort of your own home by setting your oven to max and opening the door. (Obviously don’t visit for very long, I don’t want to be responsible for your oven-inflicted sunburns.)
So if we wanted to pull one of these things close to the Earth and see what effect that would have on our planet, we can already see that we’d have to pull them very close to the Earth, which is not advised for a number of reasons. If we simply replaced the Sun with a brown dwarf, we would very rapidly freeze, as the sudden drop in light from our replacement Sun wouldn’t be enough to keep water at a liquid temperature on our planet. On the other hand, once you start throwing massive objects around within the solar system, the likelihood that any individual planet will stay on its current orbit drops massively, and the window within which water will remain liquid is a narrow one. In fact, if you wanted to replace the Sun with a brown dwarf, and keep liquid water, you’d probably have to put it where the Moon is, assuming a star that’s a hundred thousand times less luminous than the Sun. You’d lose lots of what we take for granted from our Sun, like visible light, but the ambient heat may just keep water flowing, with the brown dwarf dominating our skies.
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