00:00Now, if Earth were the size of a nickel, Neptune would be about as big as a baseball.
00:06That's impressive, but it's nothing compared to the giant ninth planet that might be hiding out there.
00:12Now, our Planet 9's discovery saga might have finally come to an end.
00:16Turns out, the problem wasn't where we were looking, but how.
00:20And researchers might have just made the biggest solar system discovery in decades.
00:25You see, the idea of finding a planet beyond Neptune has been around for years now.
00:30I know, back in school, you probably learned that the ninth planet was Pluto.
00:35But forget about Pluto, it's been demoted to a less fancy category since 2006.
00:41What I'm talking about is the Planet 9.
00:44A hypothetical giant planet that could be our cosmic neighbor.
00:48We've never actually seen it, but scientists are pretty sure it's out there somewhere,
00:53hiding in the outer solar system.
00:56To explain why they think that, we need a sheet of paper.
00:59Hey, here's one.
01:00We place a coin on it, and suddenly, it starts to move.
01:04Unless we're talking about some kind of superpower,
01:07there's no way this coin is moving on its own, right?
01:10So, you figure someone must be on the other side, holding a magnet and moving it around.
01:16You can't see the magnet, but it explains why the coin is moving.
01:20That's kind of what's happening with Planet 9.
01:23We can't see it, we can't prove it, yet.
01:26But its existence could explain the strange movements we see out there in space.
01:31For example, the planets in our solar system orbit on a flat plane
01:35that's tilted about 6 degrees relative to the sun.
01:39But why 6 degrees?
01:41Well, no one really knows for sure.
01:43Some scientists think those orbits might be slightly tilted
01:46because of the pull of a ninth planet.
01:48Its existence could also explain the unusual paths of smaller objects in the distant Kuiper Belt,
01:55a region full of icy debris that stretches far beyond Neptune's orbit.
02:00Back in 2016, researchers from Caltech published a study about Planet 9.
02:06They suggested it could have a mass about 10 times that of Earth and follow a highly elongated path.
02:12It's a scientific way of saying it takes forever to make one trip around the sun.
02:17Because of that, it would be located in the outer solar system.
02:21I mean, far, far away.
02:23It would orbit the sun about 20 to 30 times farther out than our most distant planet, Neptune.
02:30According to scientists, Planet 9 would take up to 20,000 Earth years
02:34to complete just one full orbit around the sun.
02:37Since it's crazy far out there, it's almost impossible to see it.
02:41So, as cool as this hypothesis sounds,
02:44we've never found solid evidence that Planet 9 actually exists.
02:49But we might be pretty close.
02:51In May 2025, a team at National Tsinghua University
02:55might have made one of the most exciting solar system discoveries ever.
03:00Alright, so there's a hypothesis that Planet 9 hasn't been discovered yet
03:04because we haven't been using the right method.
03:07This new study raises an interesting question.
03:11What if Planet 9 actually looks brighter in infrared light than it does in visible light?
03:16Let me explain.
03:17Scientists have been trying to spot Planet 9 by its reflected light.
03:21But here's the problem.
03:23The spotted invisible wavelengths,
03:25sunlight would have to travel all the way out to Planet 9,
03:29bounce off its surface,
03:30and then travel all the way back to Earth.
03:32If a Neptune-sized planet were about 10 times farther away than Neptune,
03:37it would look about 10,000 times fainter.
03:40But a planet's own thermal radiation,
03:42I mean, its heat,
03:43only has to make a one-way trip.
03:46So, in infrared light,
03:47Planet 9 would only be about 100 times fainter.
03:51That's why it makes more sense to look for it
03:53using space-based infrared telescopes.
03:56And that's the idea behind this new study.
03:58The team started digging through archives,
04:01searching for old infrared sky survey data.
04:04And they pulled it from two main sources.
04:06First, IRIS,
04:08a satellite launched in the 80s that scanned the sky for almost a year.
04:12Then, the Japanese satellite Akari,
04:14another infrared observatory that operated between 2006 and 2011.
04:19So, basically, they compared objects that showed up in IRIS database
04:24and noticed which ones had moved by the time Akari took its observations.
04:29And by doing that,
04:30the researchers found something incredible.
04:33An object that might just be our long-lost ninth planet.
04:37Because if something moves,
04:39it could be a planet orbiting the Sun, right?
04:42And yeah,
04:42they did find some celestial objects showing tiny movements.
04:46But before jumping to conclusions,
04:48they had to rule out the parallax effect.
04:51And that's something really important
04:52when we're talking about solar system discoveries.
04:55To explain it,
04:56I need you to do something.
04:58Hold one finger in front of your face.
05:00Close one eye.
05:02And now switch.
05:03Your finger seems to move a tiny bit, right?
05:05That's because you're looking at it from a slightly different angle,
05:09from one eyeball to the other.
05:11The same thing happens when we look out into space from Earth.
05:14Since our planet orbits the Sun,
05:17our view of very distant objects shifts just a little.
05:21That effect is called parallax.
05:23My point is,
05:24because of this effect,
05:25planet 9 would appear to move across the sky
05:28as Earth goes around the Sun.
05:30On any given day,
05:31it might seem to be in one spot.
05:33But six months later,
05:35when Earth is on the opposite side of the Sun,
05:37it would look like it shifted.
05:39Six months after that,
05:41it would seem to move back again.
05:42Scientists had to account for parallax.
05:45Actually,
05:46they had to remove its effects.
05:48So the team looked at images of the sky
05:50taken on the same date every year.
05:53Because on that same date,
05:54Earth is always in the same spot in its orbit.
05:57That means if planet 9 is real,
06:00it would show up in the same place in those images every year.
06:03No fake wiggle from parallax,
06:05just its real position.
06:07This careful search led them to a single object,
06:11a tiny dot in the infrared data.
06:13This strange little spot
06:15had moved slightly along its orbit around the Sun
06:18over the 23 years between Iris and a carry.
06:21Wait, could that be it?
06:23Did astronomers just confirm a hidden planet?
06:26Well, maybe.
06:28But don't get too excited.
06:30The data we have on its motion over that time
06:32is not enough to figure out its full orbit.
06:36So, for now,
06:37we still can't say for sure
06:39if this mysterious object is really planet 9.
06:42It's definitely a strong candidate, though.
06:45Based on how bright the object appears,
06:47the team estimates it could be pretty massive.
06:50And that came as a big surprise.
06:53You see,
06:53previous NASA research ruled out
06:55any Jupiter-sized or Saturn-sized planets
06:58hiding out there.
06:59But a smaller world would have gone undetected.
07:02So, the scientists were looking for something
07:04just a bit bigger than Earth.
07:06But it turns out,
07:07this mysterious planet
07:08might be more massive than Neptune.
07:11Now that they've discovered planet 9,
07:13supposedly, at least,
07:14the plan is to keep tracking it
07:16and collecting new data.
07:18But don't think the road ahead will be easy.
07:21Because since the Acheri satellite
07:22spotted it for the first time,
07:24that object didn't just sit still.
07:26It's been moving slowly through space ever since.
07:29So now,
07:30scientists need to use regular telescopes on Earth
07:33to look for it again.
07:34And here's the tricky part.
07:36They're not exactly sure where it moved to.
07:39So they have to search a pretty big patch of the sky.
07:42And so far,
07:43no matching object has been found.
07:46Whether this really turns out to be
07:47the Planet 9 discovery or not,
07:50only time will tell.
07:51For now,
07:52its existence is still up for debate.
07:54But with powerful new technology on the way,
07:57like NASA's Nancy Grace Roman Space Telescope,
08:00astronomers are more determined than ever
08:02to uncover the truth.
08:04And if Planet 9 really is out there,
08:07hiding in the cold, dark edges of our solar system,
08:10it's running out of places to hide.
08:13That's it for today.
08:14So hey,
08:15if you pacified your curiosity,
08:17then give the video a like
08:18and share it with your friends.
08:19Or if you want more,
08:20just click on these videos
08:21and stay on the bright side!
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