Have astronomers finally found planet 9? a new clue in the solar system
What’s this planet 9 all about?
imagine our solar system as a big neighborhood with the sun in the center and planets like earth, mars, and jupiter as the houses nearby. way out in the outskirts, past pluto, there’s a mysterious area called the kuiper belt, filled with icy rocks and dwarf planets. for years, scientists have noticed something odd out there: some of these rocks move in weird ways, like they’re being tugged by something big we can’t see. this has led to a theory about a hidden planet, called planet 9, that might be out there, pulling on these objects with its gravity.
this planet 9 idea started getting serious attention around 2016 when two astronomers, mike brown and konstantin batygin from caltech, noticed that a bunch of these far-out objects in the kuiper belt had strange orbits. they were clumped together in a way that didn’t seem random, and their paths were tilted in a similar direction. it’s like seeing a group of kids on a playground all running in the same weird pattern—something’s probably guiding them. the best explanation? a massive, unseen planet, maybe 7 to 17 times heavier than earth, could be out there, acting like a cosmic shepherd, nudging these objects into place.
but here’s the catch: nobody’s actually seen this planet. it’s so far away—estimated to be 300 times farther from the sun than earth is—that it barely reflects any sunlight. think of it like trying to spot a black cat in a dark room with a tiny flashlight. that’s why astronomers have been scratching their heads, trying to figure out how to find this elusive world.
Why infrared is the key to finding planet 9
since planet 9 is so far away, it’s too dim to spot with regular telescopes that look for visible light, like the kind we see with our eyes. but here’s where things get cool: every object, even a super cold one, gives off a tiny bit of heat, which we can detect as infrared light. it’s like how a night-vision camera can pick up heat from animals in the dark. so, instead of looking for reflected sunlight, astronomers use special telescopes that can see this infrared glow to hunt for planet 9.
two big players in this search are the infrared astronomical satellite (iras) and akari, a japanese infrared satellite. iras was launched way back in 1983 and spent about a year scanning the entire sky, picking up infrared signals from all sorts of objects, like stars, galaxies, and maybe even hidden planets. then, in 2006, japan launched akari, which did a similar sky survey but with even better tech, catching fainter signals. these two missions are like cosmic time capsules, giving us snapshots of the sky 23 years apart.
The two infrared surveys, IRAS on the left, AKARI on the right, with the position of the potential Planet 9 marked on each. Credit: Phan et al (2025)
How astronomers searched for planet 9
a team led by terry long phan, an astronomer from national tsing hua university in taiwan, decided to dig through the data from iras and akari to look for planet 9. they had a clever plan: since planet 9 is so far away, it would move super slowly across the sky. imagine watching a snail crawl across your backyard—it barely seems to move, but over a long time, you’d notice it shifted a bit. that’s what they were looking for—a faint object that moved just a tiny amount between 1983 (when iras took its pictures) and 2006 (when akari did its survey).
the team used special software to comb through millions of infrared sources in the data. they were looking for something that showed up in one spot in the iras data and then appeared slightly shifted in the akari data, about 47.5 arcminutes (that’s a tiny angle in the sky, roughly one and a half times the width of the full moon). they had to be super careful because the sky is full of distractions—stars, galaxies, and random noise can trick you into thinking you’ve found something when you haven’t.
after a ton of work, they narrowed it down to 13 possible pairs of objects that seemed to move the right amount. then, they manually checked each one to make sure it wasn’t a false alarm, like a star or a glitch in the data. in the end, only one candidate stood out: a faint dot that was in one spot in the 1983 iras data and had shifted to a new spot in the 2006 akari data. this movement matched what you’d expect from a planet orbiting the sun at a crazy distance of around 700 astronomical units (au), which is about 65 billion miles away—way farther than pluto!
What do we know about this mystery object?
so, what’s this thing they found? based on the data, it could be a massive planet, maybe 7 to 17 times heavier than earth, making it similar to neptune or uranus, which are big balls of gas and ice. out there in the deep freeze of the outer solar system, it’s probably super cold, like -200 degrees celsius, so it’s not exactly a vacation spot. its brightness in the infrared data suggests it’s big enough to be a planet, not just some random rock.
but here’s the deal: this isn’t a slam-dunk discovery yet. they’ve only seen this object at two points in its orbit—one in 1983 and one in 2006. that’s like catching two quick glimpses of a car driving by and trying to guess its entire route. to really confirm it’s planet 9, astronomers need to see it again and track its path over time to figure out its full orbit around the sun. that orbit could be wild, possibly swinging from 280 au to 1,120 au, taking thousands of years to complete one loop.
What’s next for planet 9?
to nail this down, astronomers are planning to use some serious tech. one tool they’re excited about is the dark energy camera (decam) on the victor m. blanco telescope in chile. it’s super sensitive and can take wide, detailed pictures of the sky, perfect for spotting faint, slow-moving objects like this one. other big players, like the upcoming vera c. rubin observatory and the nancy grace roman space telescope, are also set to scan the sky with incredible precision, so if planet 9 is out there, it’s running out of places to hide.
this isn’t the first time someone’s thought they found planet 9. back in 2021, another astronomer, michael rowan-robinson, found a possible candidate in iras data, but it was closer (about 225 au) and smaller (3 to 5 times earth’s mass), and it hasn’t been confirmed by other surveys like akari. what makes phan’s discovery more promising is that it shows up in both iras and akari, which gives it more credibility.
Why does this matter?
if this object turns out to be planet 9, it would be a game-changer for how we understand our solar system. it could tell us more about how planets form and move over billions of years. plus, it would be the first new planet discovered since neptune in 1846, which is a big deal! it’s like finding a hidden room in a house you’ve lived in forever. even if it’s not planet 9, this discovery shows how old data can still surprise us, and it’s a reminder that there’s so much more to learn about our cosmic backyard.
for now, astronomers are keeping their eyes on the sky, hoping to catch another glimpse of this mysterious object. whether it’s planet 9 or something else entirely, this is one exciting step in unraveling the secrets of our solar system.
rewritten based on a study by terry long phan, national tsing hua university, taiwan
