Space is big. Like, "break your brain" big. When we talk about the biggest planet ever found, we aren't just looking for a slightly larger version of Jupiter. We’re looking for monsters that blur the very definition of what a planet even is. Honestly, if you look at the night sky, you’d never guess that tucked away in the Ophiuchus constellation—about 460 light-years from your backyard—sits a gas giant so massive it makes our entire solar system look like a toy set.
It’s called ROXs 42Bb.
Most people think of Jupiter as the king. It’s huge, sure. You could fit 1,300 Earths inside it. But ROXs 42Bb? It’s basically Jupiter on steroids, weighing in at roughly nine times Jupiter's mass and boasting a radius that’s estimated to be about 2.5 times larger. It’s a behemoth. But here’s where things get weird: scientists aren't even 100% sure we should be calling it a planet.
The Identity Crisis of ROXs 42Bb
When Thayne Currie and his team discovered this object in 2013 using the Keck Observatory, they sparked a massive debate that still rages in the astrophysics community. The problem isn't just the size. It's the birth story.
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Basically, there are two ways to make a big round thing in space.
You have "core accretion," which is the standard way planets like Earth and Jupiter form. Dust hits rocks, rocks hit boulders, and eventually, you have a core strong enough to suck in a massive envelope of gas. Then you have "disk instability." This is a more violent, rapid process where a cloud of gas just collapses under its own weight. This is how stars are born.
ROXs 42Bb is so far away from its host star—about 150 astronomical units (AU)—that it shouldn't really exist. For context, Pluto is only about 40 AU from the Sun. At that distance, the standard "planet-building" rules don't really apply. This leads many astronomers to suspect it might actually be a sub-stellar object or a very low-mass brown dwarf rather than a "true" planet.
Why Size Isn't Everything in the Exoplanet Game
You’ve probably heard of other contenders for the title of the biggest planet ever found. Names like GQ Lupi b or PDS 70b often pop up in clickbait headlines. But the "biggest" tag is slippery.
Mass and volume are different things.
Take HAT-P-67b. It’s roughly twice the width of Jupiter, but it’s a "puffy planet." It has the density of a marshmallow. It’s huge in size because it’s so close to its star that the heat has inflated its atmosphere like a hot air balloon. ROXs 42Bb is different because it’s genuinely massive and wide.
- Jupiter: The baseline.
- HAT-P-67b: The "Puffy" Giant (Low density, high volume).
- ROXs 42Bb: The Heavyweight Champion (High mass, high volume).
- Brown Dwarfs: The "Failed Stars" that often get confused with planets.
Scientists use the "Deuterium Burning Limit" to draw the line. If an object is more than 13 times the mass of Jupiter, it gets heavy enough to start fusing deuterium in its core. At that point, it’s officially a brown dwarf. ROXs 42Bb sits at about 9 Jupiter masses, which keeps it safely (barely) on the planet side of the fence. But let's be real—it's right on the edge.
The Tech That Found This Monster
Finding these things isn't like looking through a telescope at the moon. You can't just "see" them. Most exoplanets are found using the Transit Method—watching for a star to dim as a planet passes in front.
But ROXs 42Bb was found via Direct Imaging.
This is incredibly hard. It’s like trying to see a firefly hovering next to a lighthouse from three miles away. Astronomers have to use "coronagraphs" to block out the blinding light of the parent star to see the faint glow of the planet. Because ROXs 42Bb is so young (only a few million years old), it’s still glowing with the heat of its own formation. That internal heat makes it visible in infrared light.
The Contenders: Are There Bigger Ones Out There?
If we look at the data from the NASA Exoplanet Archive, we see objects that make ROXs 42Bb look like a pebble. But there’s a catch.
HD 100546 b was once thought to be a planet with a radius 6.9 times that of Jupiter. If that were true, it would be the undisputed king. However, more recent studies suggest it might not even be a planet at all, but rather a "protoplanetary disk" or a complex structure of gas and dust that's just becoming something.
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Then you have DENIS-P J082303.1-491201 b. (Catchy name, right?) It has a mass of nearly 29 Jupiters. By the 13-mass rule, that is definitely a brown dwarf. Yet, it’s listed in some databases as a planet because it orbits a sun-like star.
This is why the search for the biggest planet ever found is so messy. The more we look, the more we realize that nature doesn't like our neat little boxes. There is a smooth gradient from "small rock" to "gas giant" to "brown dwarf" to "star."
Why You Should Care About These "Super-Jupiters"
You might wonder why we spend billions of dollars on telescopes to find big balls of gas trillions of miles away. It’s about our own origin story.
By studying ROXs 42Bb, we learn what the early days of our solar system might have looked like. We learn about the limits of gravity. If a planet can grow to be nine times the size of Jupiter, what does that mean for the stability of other planets in that system? Could a "Super-Earth" survive in the neighborhood of such a monster? Probably not.
Every time we find a "biggest" planet, it challenges the math models we’ve used for decades. It forces us to admit that we still don't fully understand how the universe builds worlds.
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Actionable Next Steps for Space Fans
If you're fascinated by these cosmic giants and want to keep up with the ever-changing leaderboard of the biggest planet ever found, you don't have to be a NASA scientist.
First, bookmark the NASA Exoplanet Archive. It’s a live database. You can sort by mass or radius and see the raw data as it’s confirmed. It’s surprisingly fun to see how often the "record" changes.
Second, check out the Eyes on Exoplanets app. It’s a 3D visualization tool from NASA that lets you "fly" to these distant worlds. Seeing the scale of ROXs 42Bb compared to our own Sun is a humbling experience.
Third, keep an eye on the James Webb Space Telescope (JWST). We are currently in a golden age of discovery. The JWST is uniquely designed to look at the infrared signatures of these massive planets. It’s highly likely that within the next year or two, we will find something that makes ROXs 42Bb look small.
The universe is under no obligation to make sense to us, and that’s what makes the hunt so exciting. We are looking for the outliers—the weird, massive, "shouldn't exist" planets—because they are the ones that tell us the most about the rules of reality.
Current Top Specs for ROXs 42Bb:
- Mass: ~9.0 Jupiter masses
- Radius: ~2.43 Jupiter radii
- Distance from Star: 150 AU
- System Age: ~7-10 million years
Stay curious. The record for the biggest planet is likely sitting in a data packet right now, waiting for a human to look at the numbers and realize they've found a new king.