Artist impression of a brown dwarf much like ASASSN-24fw (Credit : NASA/JPL-Caltech)
What would you think if a star that shone reliably for decades suddenly lost almost all its light — and stayed that way for nine months straight? Would you call it a glitch? A cosmic coincidence? Or something far stranger?
Welcome to FreeAstroScience, where we take the universe's biggest puzzles and break them into pieces you can actually hold. I'm Gerd Dani, and together with our team, we've put together this deep look at one of the most extraordinary stellar events astronomers have seen in recent years. Whether you're a space enthusiast, a curious student, or someone who just loves a good mystery — this story is for you.
A star called ASASSN-24fw, sitting 3,200 light-years away in the constellation Monoceros, dimmed to just 3% of its normal brightness at the end of 2024. It didn't bounce back for nearly 200 days. That's not supposed to happen. Not like this. Not for this long.
Scientists now believe they've cracked the case. And the answer? It's one of the most exotic explanations imaginable.
Stick with us to the very end. This story has hidden companions, colossal ring systems, "failed stars," and a bonus discovery nobody expected.
📖 Table of Contents
1. What Happened to ASASSN-24fw?
Picture this. You've been watching a particular light on the horizon for decades. It never flickers, never fades. Then one day — almost overnight — it drops to a faint whisper. Just 3% of its former brightness .
That's what astronomers witnessed when ASASSN-24fw, a star roughly twice the size of our Sun, began to dim dramatically at the end of 2024 . The star sits in the constellation Monoceros (the Unicorn), about 3,200 light-years from Earth .
Now, when a planet crosses in front of a star — what we call a transit — the dip in brightness usually lasts a few days. Maybe a couple of weeks at the extreme end. But the dimming of ASASSN-24fw dragged on for nearly 200 days . That's over nine months of near-total darkness from a star that had behaved perfectly for as long as anyone had been watching.
Astronomers were baffled. Something huge was blocking the light. But what?
How the dimming unfolded
According to Dr. Sarang Shah, the lead researcher at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) in Pune, India, the dimming didn't start with a sudden blackout. It was gradual . The outer edges of whatever was passing in front of the star were thin and translucent. Only when the denser core regions swept across the star's face did the brightness plummet to that shocking 97% loss .
Think of it like a curtain being slowly drawn across a window — first a thin gauze, then thick velvet.
The team published their findings in the journal Monthly Notices of the Royal Astronomical Society , and the answer they propose is beautifully wild.
2. What Exactly Is a Brown Dwarf?
Before we get to the solution, we need to talk about brown dwarfs — because they're at the heart of this mystery.
Brown dwarfs sit in a strange no-man's-land of astrophysics. They're too massive to be planets, but not quite massive enough to sustain nuclear fusion like real stars do . They can't keep fusing hydrogen atoms in their cores, which means they can't shine the way our Sun does. Some people call them "failed stars," though — as Mark Thompson from Universe Today rightly points out — that label feels a bit unfair .
They're fascinating in their own right. They glow faintly from leftover heat of their formation. They can have weather patterns, clouds, even storms. And, as we're now learning, they can carry enormous ring systems.
Why brown dwarfs are so hard to spot
Here's the challenge: brown dwarfs don't produce much visible light. They're dim. They're cool (relatively speaking). They hide in plain sight against the dark background of space. So we often detect them indirectly — by noticing what they do to the things around them.
In this case, the brown dwarf around ASASSN-24fw revealed itself not by its own glow, but by casting a shadow .
3. How Enormous Are These Rings?
Let's talk scale. Because the ring system around this suspected brown dwarf is genuinely jaw-dropping.
The rings extend about 0.17 astronomical units (AU) from the central object . If that number doesn't hit home immediately, let's put it another way: that's roughly half the distance between our Sun and Mercury.
Saturn's rings, which we think of as grand and spectacular, span roughly 282,000 kilometers from edge to edge. The ring system we're talking about here dwarfs Saturn's rings the way an ocean dwarfs a bathtub.
1 AU ≈ 149,597,870.7 km (the average Earth–Sun distance)
The researchers describe the structure as a giant "saucer" passing in front of the star . Imagine Saturn's rings scaled up about 90 times — dense enough in the middle to block nearly all starlight, thin and wispy at the edges.
The math behind the dimming
The dimming tells us something precise. When astronomers measure a 97% drop in flux from a star, they can model the size and opacity of the blocking object. Here's the simplified relationship between the observed flux drop and the effective blocking area:
Fractional Flux Drop During a Transit / Occultation
Where ΔF / F = fractional decrease in brightness (~0.97 here),
Rblocking = effective radius of the obscuring structure,
Rstar = radius of the host star.
A 97% drop means the blocking structure's effective cross-section
covers nearly the entire stellar disk as seen from Earth.
A 97% brightness drop means the obscuring object — rings and all — covered almost the entire visible face of the star from our vantage point. That's an enormous shadow.
4. Brown Dwarf or Super-Jupiter — What's the Difference?
The research team offered two possible identities for the companion object:
- A brown dwarf — with at least 3 times Jupiter's mass
- A super-Jupiter — a gas giant so massive it blurs the line between planet and brown dwarf
So what separates these two categories? The boundary is fuzzy, and astronomers still debate where to draw the line. But here's the general picture:
MJ = Jupiter masses. Boundaries are approximate and debated among astronomers.
The companion around ASASSN-24fw weighs in at more than 3 Jupiter masses . That places it somewhere in the super-Jupiter to low-mass brown dwarf range — right in the grey zone.
The team leans toward the brown dwarf explanation, based on their photometric and spectroscopic modeling . But regardless of the label, the real star of the show (pun intended) is the ring system.
5. What Bonus Surprises Did the Team Uncover?
Good science often works like this: you set out to solve one riddle, and you stumble into several more. That's exactly what happened here.
A circumstellar mystery
The team discovered that ASASSN-24fw itself has a circumstellar environment — material very close to the star, possibly leftover debris from past or ongoing planetary collisions .
This is strange. The star is probably more than 1 billion years old . By that age, you'd expect all the leftover dust and debris from the star's early formation to have cleared away — either swept up by planets, blown away by stellar winds, or gravitationally scattered.
So why is there still debris hanging around? Did something smash into something else relatively recently? Are we seeing the aftermath of a collision between two planetesimals — or even young planets? The team doesn't have a definitive answer yet, but the clue is tantalizing.
A red dwarf neighbour
While studying the dimming event, the researchers also serendipitously discovered a red dwarf star in the neighbourhood of ASASSN-24fw .
As Dr. Jonathan Marshall from Academia Sinica in Taiwan put it: "While studying this dimming, we also serendipitously discovered that ASASSN-24fw also has a red dwarf star in its vicinity."
Sometimes the universe gives you a two-for-one deal.
6. When Will ASASSN-24fw Dim Again?
Here's a detail that makes this discovery both exciting and bittersweet.
The researchers estimate that the companion object's orbit will bring it back in front of the star in about 42 or 43 years . That means the next stellar occultation from this system won't happen until roughly 2067 or 2068.
For some of us, that feels like a very long wait. But for astronomy, 42 years is the blink of an eye. And when that next dimming event does arrive, we'll be ready. Our telescopes will be better. Our data analysis tools will be sharper. We'll know exactly where to look and what to measure.
The team is already planning ahead. They want to use the European Southern Observatory's Very Large Telescope (VLT) in Chile and the James Webb Space Telescope (JWST) to study the system in the meantime — measuring the star's temperature, chemical composition, evolutionary stage, and age .
7. Why Does This Discovery Change How We Study Planetary Formation?
At its core, this story isn't just about one dimming star. It's about a new window into how planetary systems form and evolve.
Dr. Marshall said it best: "Large ring systems are expected around massive objects, but they are very difficult to observe directly to determine their characteristics. This rare event allows us to study such a complex system in remarkable detail."
Think about what happened here. The star's own light acted as a backlight, illuminating the ring structure as it passed in front . It's like holding a piece of stained glass up to a lamp — the light reveals details you'd never see otherwise.
This gives astronomers clues about:
- How rings form around sub-stellar objects
- How dense and structured those rings can be
- How debris behaves around aging stars
- What the boundary between giant planets and brown dwarfs really looks like
Each of these questions connects to the bigger story of how solar systems — including ours — came into being.
What this means for exoplanet science
We've discovered thousands of exoplanets over the past few decades, mostly through transit and radial velocity methods. But detecting ring systems around exoplanets remains extremely difficult. Events like this one are rare — they require a nearly perfect alignment between the star, the ringed object, and our line of sight from Earth .
When we do catch one, it's a goldmine of information. The shape of the dimming curve — how quickly it begins, how deep it gets, how long it lasts, how it recovers — tells us about the ring's geometry, density, opacity, and tilt.
The ASASSN-24fw event is, in that sense, a gift. A rare, exceptionally long, deeply detailed shadow show, played out over nine months on a cosmic screen 3,200 light-years wide.
8. Final Thoughts
We started with a simple question: why did a star almost vanish?
The answer took us on a journey through the lives of brown dwarfs, the physics of ring systems, the debris fields of ancient stars, and the quiet companionship of a red dwarf hiding in the background. What began as a puzzling data point turned into one of the most detailed studies of an exoplanetary ring system ever conducted.
And there's a lesson tucked inside this story — one we believe in deeply here at FreeAstroScience. The universe rewards curiosity. The team led by Dr. Sarang Shah didn't just see a dip in a light curve and move on. They asked why. They modeled, they tested, they looked closer. And they found something extraordinary .
That's the spirit we try to carry here at FreeAstroScience.com — the idea that complex scientific ideas can be explained in simple, human terms. We believe education should never be locked behind jargon or paywalls. We write these articles specifically for you — curious minds who refuse to stop asking questions.
Because, as Francisco Goya reminded us centuries ago, the sleep of reason breeds monsters. Keeping our minds active, awake, and wondering — that's how we fight back against ignorance and indifference.
The star ASASSN-24fw will dim again around 2067. We'll be here. We hope you will, too.
Until then, keep looking up. And come back to FreeAstroScience.com whenever your curiosity needs a home.
Sources
Thompson, M. (2026, February 15). "The Mystery of the Fading Star." Universe Today. https://www.universetoday.com/articles/the-mystery-of-the-fading-star
Royal Astronomical Society (2026, February 12). "Unseen planet or brown dwarf may have hidden 'rare' fading star." RAS Research Highlights. https://ras.ac.uk/news-and-press/research-highlights/unseen-planet-or-brown-dwarf-may-have-hidden-rare-fading-star
Original research paper: Shah, S. et al. (2026). "The nature of ASASSN-24fw's occultation: modelling the event as dimming by optically thick rings around a sub-stellar companion." Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf2251
This article was written and curated by Gerd Dani for FreeAstroScience.com — where we explain the universe in words everyone can understand. © 2026 Free AstroScience – Science and Cultural Group.
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