Space is honestly a lot messier than those glossy textbooks made it seem back in grade school. We like neat little boxes: you’ve got stars, and you’ve got planets. One glows, the other doesn't. Simple, right? But then you run into brown dwarfs, those "failed stars" that are too big to be planets but too small to ignite like the Sun.
And if you think those are confusing, wait until you look at the neighbors. It turns out there’s a whole spectrum of weirdos out there. If you’re asking yourself if there are other objects similar to brown dwarfs, the answer is a resounding "kinda, and it’s complicated." We’re talking about things like sub-brown dwarfs, rogue planets, and low-mass red dwarfs that basically blur every line we’ve ever drawn in the sand.
The Identity Crisis of the Sub-Brown Dwarf
Imagine an object that looks exactly like a brown dwarf. It’s a ball of gas. It doesn't orbit a star. It formed from a collapsing cloud of dust just like a star would. But there’s one catch: it’s too light to even fuse deuterium (a heavy version of hydrogen).
These are what astronomers call sub-brown dwarfs, or sometimes planetary-mass brown dwarfs. Honestly, the name is a bit of a mouthful. The main difference here is mass. While a standard brown dwarf sits between 13 and 80 times the mass of Jupiter, these "sub" versions are lighter than 13 Jupiters.
But here’s the kicker: they look almost identical to rogue planets. If you found one floating in the dark, you’d have a hard time telling if it was a "failed star" that formed on its own or a planet that got kicked out of a solar system by its siblings.
- Formation matters: Sub-brown dwarfs form "top-down" from collapsing gas clouds.
- Planets form "bottom-up": They grow in disks around stars.
- The Problem: Once they're adults, they both just look like cold, lonely gas giants drifting through the void.
Rogue Planets: The Runaways
You’ve probably heard of rogue planets. These are the "black sheep" of the universe. They started out in a nice, cozy solar system, but gravitational chaos—maybe a passing star or a grumpy big brother planet like Jupiter—tossed them into interstellar space.
Technically, these are very similar to brown dwarfs in appearance. They have weather patterns. They have clouds made of silicate (basically hot sand) or iron. But because they didn't form like a star, some scientists refuse to put them in the same category as brown dwarfs.
Recently, the James Webb Space Telescope (JWST) has been finding a ton of these in places like the Flame Nebula. Some of them are only 0.5 times the mass of Jupiter. When they’re that small, the "failed star" label starts to feel a bit silly, even if they did form from a gas cloud.
Red Dwarfs: The "Successes" Next Door
On the other side of the scale, we have red dwarfs (specifically M-dwarfs). These are the smallest members of the "real star" club. They’re just barely heavy enough—about 80 times Jupiter's mass—to crush hydrogen atoms together and create constant light.
If you were standing on a hypothetical planet orbiting a red dwarf, the sun would look like a big, dim, reddish-orange ball.
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The line between a heavy brown dwarf and a light red dwarf is incredibly thin. Researchers like Nolan Grieves have been studying objects like TOI-148 and TOI-746—objects that sit right on this border. These things are roughly 77 to 98 times the mass of Jupiter. They’re basically the "border patrol" of the stellar world.
One weird thing? Brown dwarfs actually shrink as they get older because they cool down and get "crushed" by their own gravity (a state called electron degeneracy). Real stars don't do that as much because the fusion in their core pushes back.
Why Does This Even Matter?
You might think this is just scientists arguing over semantics. "Who cares if it's a sub-brown dwarf or a rogue planet?"
Actually, it tells us a lot about how the universe builds things. If we find way more "sub-brown dwarfs" than we expected, it means the process of star formation is way more efficient at making small things than we thought.
If most of these objects turn out to be ejected rogue planets, it tells us that solar systems are violent, chaotic places where "evictions" are common.
Spotting the Differences: A Quick Breakdown
Basically, if you want to tell these things apart, you have to look at three things:
- Mass: Is it under 13 Jupiters (Planet/Sub-Brown Dwarf), between 13-80 (Brown Dwarf), or over 80 (Red Dwarf)?
- Fusion: Does it burn Lithium? Deuterium? Or "Regular" Hydrogen?
- Address: Is it orbiting a star, or is it a "free-floater"?
How to Follow the Science Yourself
The field is moving fast. If you're interested in keeping up with these cosmic misfits, here are a few things you can actually do:
- Check out Backyard Worlds: This is a "citizen science" project where regular people help NASA find brown dwarfs and rogue planets by looking at infrared data. You don't need a PhD. You just need an eye for things that move.
- Follow JWST Updates: The James Webb Space Telescope is currently the king of finding these objects because they are mostly visible in infrared (heat) rather than visible light.
- Look for "Planemo" News: "Planemo" is a catch-all term for any planetary-mass object. It’s a great keyword to use if you want to find the latest research on things that aren't quite stars and aren't quite planets.
The universe isn't a collection of neat categories. It's a messy, beautiful spectrum. Whether you call them failed stars, overachieving planets, or just "brown dwarf-ish," these objects are the key to understanding how suns and worlds are born.
Next time you look at a star chart, remember that the dark spaces between the dots are actually filled with millions of these "failed" stars, drifting silently through the cold. They might be dim, but they have a hell of a story to tell.