Brown dwarfs: How do they form, and how far away can we see them?

January 22, 2024   Issue #672

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SciAm What SciAm

Stuff I’ve written for Scientific American

On Friday, January 19, 2024, Scientific American published a column of mine about astronomers who found a brown dwarf that may have an aurora, and if so it’s indirect evidence it might have a tectonically active moon! It’s a super cool story, so please go read it.

It was too late to get into that column, but last week I found a couple of more brown dwarf items that’re pretty fun.

First: Brown dwarfs are objects intermediate between planets and stars. They have between 13 and 75 times the mass of Jupiter — people call them “failed stars”, a term I dislike, since they might just be super successful planets.

But that’s part of the issue with them. Are they more like stars or planets? I had an argument with someone years ago on this topic that opened my eyes to the fact that how we define things guides how we see them, and it’s best to keep an open mind or else you might pigeonhole something unfairly (I know I’ve written about this but can’t find it anywhere; maybe I’ll put that in a future newsletter).

But in reality they’re neither; they are their own thing. But how do they form? One idea is they are born directly from clouds of gas and dust, which collapses, fragments, and those individual clumps form brown dwarfs. This is pretty much how stars form. Another idea is that they form from disks of material orbiting stars, much like planets do.

Some new research looked into these ideas, trying to figure out by indirect means which method is more likely. Many brown dwarfs orbit stars. If we see that orbit edge-on, the brown dwarf will transit the star, making a mini-eclipse. While stars are far too small and distant to see this directly, there are indirect ways to tell what part of the star the brown dwarf crossed. This is cool: The star spins, so one half of the star is rotating toward us, and one half rotating away from us. The half headed toward us will have its light blue-shifted, and the half headed away red-shifted. As the brown dwarf transits, it blocks one part of the star, then the other. If it orbits in the same direction as the star, it’ll block the blue-shifted half first, then the red-shifted one, so we can see that drop in light in the spectrum of the star. This is called the Rossiter–McLaughlin effect.

The point is that if you see a brown dwarf in a close orbit around a star, and its orbit is aligned with the star’s equator and moving in the same sense as the star’s rotation, it likely formed like a planet. That’s because it would’ve formed with planets in the system, and migrated slowly toward the star as it interacted with those planets gravitationally. We’ve seen this with other planets, including ones in our own solar system.

On the other hand, if the orbit is highly tilted it more likely formed like a star and was captured somehow by the host star. The approach angle would be random, so you’d expect the orbit to be tilted.

What they found is that for one particular transiting brown dwarf called GPX-1b, the orbit is not tilted. That means it likely formed like a planet! We’re also pretty sure some form like stars, since we see them just out in space by themselves, so it looks they can form both ways.

Cool. And it reminds me of another discussion I had with a scientist. He was saying (more or less) that if a brown dwarf formed like a planet we should call it a planet, and if it formed like a star we can call it a brown dwarf. His argument has merit, because how things form can change their behavior. However, my argument was that if you have two objects that are exactly alike, and one formed like a planet and the other like a star, he’d call one a brown dwarf and the other a planet. He said yes. I said that strikes me as silly, because they’re exactly alike. We need a better system to classify things if two objects like that go into different bins.

It looks like my argument still works. If GPX-1b formed like a planet but is clearly a brown dwarf, well. It’s a brown dwarf. Go me.

The other news story is that astronomers found a bunch of brown dwarfs in JWST data. Now this has happened before, like when JWST was used to look at the Orion Nebula, where we expected to see them. But in this case the observations were to look for very distant galaxies, and that’s important. Those galaxies may look like little red dots in the observations, but so too will brown dwarfs! You have to be careful to distinguish the two.

The observations in this case used a bunch of different filters to look at how bright objects are at different wavelengths — astronomers call this their colors. Galaxies have different colors than brown dwarfs, which is one way to separate them. But in this case they actually saw the motion of the brown dwarfs through space! Over time, the physical motion of a brown dwarf as it orbits the center of the Milky Way changes its position in the sky very, very slightly, but with JWST’s incredible resolution this can be seen.

And they saw it! They found 21 candidate brown dwarfs by looking at colors, and of those 7 showed motion. Here’s one:

Note how the upper left is green and the lower right is red. This object was seen by the Spitzer Space Telescope years ago, and that is shown in green. The newer JWST observation, in red, shows it’s moved ever so slightly over the years between. Cooool.

They find this brown dwarf is about 800 light-years away, but others they found range out to a distance of over 13,000 light-years! That’s a fantastic distance. Brown dwarfs are faint, so seeing any that far away is amazing.

That excites me. I studied brown dwarfs back in the day, and they were hard to detect and observe. JWST is an absolute machine for finding them. Finding ones close by is great, but seeing them at greater distances means we see ones that formed in different parts of the galaxy and have evolved differently over the eons. That’s a great development for science, and over the next few years I expect we’ll learn a lot more about them.

My thanks to lead author Kevin Hainline on Blue Sky for linking to his journal paper on this!

Et alia

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