How the Universe Works (2010) s06e08 Episode Script

Strange Lives of Dwarf Planets

All across our solar system, scientists are discovering thrilling new worlds, dwarf planets.
They may be small, but they're full of riddles, oceans of subterranean water, ice volcanoes, and vanishing mountains.
The whole idea that dwarf planets are small and insignificant and boring has just been shattered in the last few years.
Dwarf planets defy many of the rules we thought governed our solar system.
Dwarf planets are very interesting bodies scientifically, but beyond that, they tell us something about the origin of our own world.
Believe it or not, they may harbor life.
Dwarf planets are rattling the cages of scientists and shaking up our understanding of how the universe works.
They may have fed the early planets and even seeded them with the precursors of life.
Dwarf planets just may be the most important objects in the solar system.
captions paid for by discovery communications Our solar system has eight confirmed major planets, but we're discovering many other small worlds called dwarf planets.
We used to think they were just dull lumps of rock, but the more we study them, the more shocking and intriguing they become.
Naively, I would expect these objects to not be terribly dynamic.
They're probably just, you know, airless, rocky, icy worlds, and they're just sitting there, and what we're finding out is that that is not true at all.
There is all kinds of stuff going on.
They're full worlds with really interesting geology and interesting histories that can tell us a lot about the solar system.
Scientists believe there may be hundreds of dwarf planets in our solar system.
So far, we've only identified six.
Five of them Pluto, with its moon, Charon; red-colored Sedna; bright, distant Eris; makemake, and bean-shaped Haumea All live billions of miles from the Sun, out beyond Neptune in the Kuiper belt.
They're just the tip of the iceberg.
There are probably many, many more dwarf worlds that are out there waiting to be discovered.
The sixth dwarf planet, Ceres, lives in the inner solar system.
It orbits around in the asteroid belt.
The asteroid belt is a region of the solar system between Mars and Jupiter, and this is where most of the asteroids are.
This is rubble left over from the formation of the solar system.
In early years of the solar system, small rocks collided with one another, stuck together, and built the rocky inner planets.
Dwarf planets grew in the same way.
Ceres was actually starting to get pretty big.
It was on its way to becoming a planet before it stopped growing, and that makes it stand head and shoulders above everything else there.
So why is Ceres called a dwarf planet and not a planet? To be a planet, it must tick off three cosmic boxes.
First, it needs to be a sphere.
Second, it needs to orbit the Sun and not another body.
Third, it needs to clear its orbital area of orbital debris.
Ceres ticks just two of the boxes.
It is a sphere, but a small one Only 600 miles across.
That's the size of Texas.
It orbits the Sun, but it hasn't cleared its path of debris.
It's surrounded by asteroids, so it misses out on being a planet.
Even though we call these objects dwarf planets, small and dwarf does not equal insignificant.
But being small does have its problems.
When the molten core of a young dwarf planet cools, so does the heat engine that drives geologic activity.
Ceres, we thought, would basically be a big, dead rock.
It's a small body.
It should have cooled off long ago.
Nothing very interesting is going on, and when we actually got out to Ceres, nothing could have been further from the truth.
March 2015, NASA's dawn probe arrives at Ceres.
As the dawn spacecraft pulled up to Ceres, we saw the craters and the surface that we expected to see, and then all of a sudden, something totally mysterious rotated into view.
One of the craters had two bright spots, almost like two eyes staring right back at us.
It was such a puzzle to the science community because what are these doing here? Are they ice? It looks very fresh.
What on earth could it be? Scientists find over 100 of these mysterious white spots.
The largest is in a 50-mile-wide crater called Occator.
They are, unexpectedly, made up of a substance we find on earth Sodium carbonate, a kind of salt.
We believe the salts on Ceres as actually very young.
We think they're as young as 4 million years old, and that's basically like yesterday in terms of geology.
And that is super weird, right? That's happening not on sort of on a geologic era.
It's happening now, today.
What could cause patches of salt on a world long-presumed dead? Planetary geologist Jani Radebaugh believes a clue might be found at mono lake in California.
All right, I'm here looking at this beautiful lake off in the distance and standing on massive white deposits.
These white deposits used to be a part of this lake, at one point.
The lake had dissolved a lot of the materials in it, and then as it receded, it left behind the materials, as it evaporated away, and these things are, you know, salts.
They're kind of granular in texture, and just to make sure, we taste it and yeah, sure enough, it's salty.
The salt at lake mono crystallizes as the water evaporates, the only way it can form.
The researchers believe the same process is taking place on Ceres.
This means there must be liquid water beneath the surface but how, out in the deep freeze of the asteroid belt? These bright spots are located in the centers of craters.
They're located around cracks in the surface, and that is telling us that this material is coming from under the surface and welling up onto it.
Absolutely nobody expected there to be liquid water beneath the surface of Ceres.
We cannot explain what is keeping that water warm.
On some moons, gravitational tugging keeps the interiors warm, but Ceres is not really near anything else that's very large.
So the amazing thing is that we may not even understand how rocky planets work.
There may be another source of energy, another mechanism for heating the interior that we haven't even discovered yet.
To find out how Ceres has liquid water, we have to rewind the clock Debris left over from the formation of the Sun slams together to form the dwarf planets.
As they take shape, the heavier, rocky material sinks to the center and forms a hot, molten core.
Slushy water-ice floats to the top.
For a while, it stays liquid, but once the core cools, it freezes and forms the solid mantel and crust.
That surface should still be solid, so the salt patches remain a perplexing mystery.
We still haven't answered the question, "how could there actually still be liquid water on Ceres?" That's still a hard question to answer.
One way this could happen is if it's not actually pure water, if you've mixed it with something else.
Some scientists have proposed that a salty ocean lies beneath the surface.
The high concentration of salt lowers the freezing point of the water, keeping it liquid.
When asteroid impacts fracture the crust, this salty water oozes up from below.
The liquid swiftly evaporates, but the salt remains, leaving a brilliant white spot on the surface.
In fact, I'm willing to bet there could be water coming up now, bringing salts up to the surface, evaporating away into space, and that means liquid water is very close to the surface of Ceres right now.
Ceres has an even more startling card up its sleeve.
Recent research suggests that it's an immigrant.
It didn't form anywhere near the asteroid belt.
Ceres may have been born alongside hundreds of other dwarf planets, many billions of miles away from the Sun.
So how did it get here? Most of the dwarf planets we've discovered lie far out in the solar system beyond the orbit of Neptune, but Ceres orbits between Mars and Jupiter in the asteroid belt.
But its location isn't the only hint Ceres might be an interloper.
Normally, celestial objects are made of the same materials as the other bodies in their neighborhoods, but that's not the case with Ceres.
The asteroid belt is mostly made up of dry, rocky bodies composed of the same heavy elements that form the rocky inner planets.
Ceres is very different.
Ceres is essentially an icy world, right? It's made out ices instead of rocks, and so that's kind of weird, considering where it is.
The ice on Ceres also contains chemical compounds that in the early years of the solar system, didn't exist in the asteroid belt.
The more we learned about Ceres, the more mysterious it became.
One of the things is that Ceres has quite a lot of ammonia on it, and we don't find ammonia anywhere near the inner part of the solar system.
But we do find it in the outer solar system.
We've detected ammonia on Pluto, its moon Charon, and out in the frozen Kuiper belt, where we find the other dwarf planets.
We think that the origin of that ammonia would've had to be in a very cold part of the solar system, colder than where we find Ceres today.
But how an icy dwarf planet with ammonia came to inhabit a place where ammonia can't form That's a huge puzzle.
This suggests to us that Ceres perhaps formed in the outer solar system and then migrated inwards to its present location in the asteroid belt.
We used to think that planetary orbits were completely immutable, that they simply ran like clockwork and they didn't move around.
Now we know that that's not the case.
In the early stages of planet formation, planets move around through the gaseous disc that encircles the young sun, much like rafts that are pushed around by ocean currents.
Ceres' ammonia suggests that dwarf planets rafted around on the cosmic ocean along with the young planets.
Ceres is sort of a smoking gun that solar systems are much more dynamic, much more dramatic than we know.
There's mounting evidence that Ceres formed farther out in the solar system and something brought this little world in.
What could possibly have done that? The answer is the planet Jupiter.
After it first formed, the giant planet migrated in towards the Sun.
Its massive gravity disrupted the orbits of other bodies in the solar system, including that of Ceres.
The solar system formed out of a disc of gas and dust, and as Jupiter formed, it would've been plowing through this material.
And if it plows through that material, it's experiencing drag.
As it was losing energy, it would start to move in toward the Sun relatively slowly.
Ceres formed in the outer edges of the solar system.
It was dislodged from the Kuiper belt and yanked inwards by the migration of Jupiter, and when Jupiter stopped migrating, so did Ceres.
It settled into a new, stable orbit in the asteroid belt.
Once you realize that something that strange and dramatic can happen, that a dwarf planet can form far out in the solar system and be brought in, it makes you wonder how many times that happened before.
Could there have been other generations of dwarf planets that got thrown in towards the Sun or maybe were thrown out of the solar system entirely? Scientists believe that squadrons of rocks and icy dwarf planets may have hurdled into the solar system.
Hundreds set out.
Only one survived.
If there was a population of small dwarf planets in the outer solar system that migrated inwards, Ceres might be the sole survivor, the only one left.
So if Ceres settled into its new home in the asteroid belt, where are the rest of the icy worlds and the water on them? The idea that Ceres may have moved in from the outer solar system is interesting, but why should it be important to you? And incredibly, the answer might be inside your own body right now.
For the longest time, we've wondered where did the majority of earth's water come from? When you think about where the earth is, how close it is to the Sun, there shouldn't have been any water here.
Understanding the evolution of Ceres, from where it formed to where we find it today, could also lead us to understand how the earth can end up with more water than we would otherwise expect.
When the earth formed, it was too hot for water to exist on the surface.
Perhaps the squadrons of dwarf planets broke up on their journey, showering earth with water-rich lumps of rock, enough to fill earth's oceans.
Amazingly, when we study the chemistry of water, the best match is that the water in your body right now came from asteroids themselves, asteroids and dwarf planets that rained down and hit the earth over billions of years.
Dwarf planets may have brought something else.
February 2017, scientists announce the discovery of organic materials on the surface of Ceres.
On earth, life uses water and organic chemistry, carbon-based molecules.
The intriguing thing about the dwarf planets is that they have both of those.
Pluto and Ceres have organic molecules.
There's liquid water below the surface.
There's a source of energy that warms the interior.
It is not at all impossible that somewhere under these cold, icy surfaces, there could be life.
Could other dwarf planets host life, too? Sedna and makemake both have red-colored patches.
The color comes from something called tholins, organic molecules that could be a precursor to life.
Based on all the interesting chemistry they're doing, these dwarf planets could be like the test tubes of the solar system.
If there are small bodies strewn all about the solar system that had liquid water or ice, it could possibly serve as an Incubator for life, just holding on to it, ready to crash into another body and seed it.
The habitable zone now extends to the entire solar system.
It really expands It greatly expands The stage for the play of life in the entire galaxy.
Maybe we have dwarf planets to thank for our very existence.
All of a sudden, the smallest bodies in our solar system have become some of the most interesting things we've ever seen.
The idea of migrating dwarf planets opens up some intriguing scenarios, including one really out-there possibility.
Some of that water and organic material may not be from our solar system.
The more we learn about dwarf planets, the more they surprise us.
But there's one dwarf planet whose very existence is a mystery.
It's called Sedna, and no one is quite sure what it's doing in our solar system.
Sedna may have my vote for the single most peculiar object in the entire solar system.
Here, we have a world which is about 1,000 miles wide, but it is way far out in the solar system, way past Neptune.
Sedna is the most distant object we've identified in our solar system.
Standing on the surface of Sedna, looking back at the solar system, the Sun would look like a really bright star, but not much more than a really bright star.
Just like Pluto, Sedna has a strange, elliptical orbit.
The difference is, Sedna travels from 7 billion to 93 billion miles from the Sun, and unlike Pluto, its orbit can't be explained by its close proximity to Neptune.
The weird thing about Sedna is its orbit.
How could it have gotten that elliptical when it's that far away from any of the major planets? If you have an object that's close enough to Neptune, Neptune's gravity can affect its orbit and swing it into an elliptical orbit.
The problem is, Sedna never gets that close to Neptune.
It doesn't get anywhere near close enough to be in that kind of orbit, and that means that something else is going on out there.
Sedna cannot be explained using objects that we know.
Everything else, we can understand why its where it is based on, you know, the eight planets and many, many other small bodies.
Sedna cannot be explained by that, and, you know, that's the sign of a good mystery.
Something else must have happened.
Looking at models for how you can change the orbits of objects, there's almost no way Sedna could've formed in our solar system, and then had its orbit change so that it's that elliptical and goes that far out from the Sun.
And that means maybe Maybe it didn't form here.
It may be an alien world.
How could our solar system have snagged an alien world? Long ago, it turns out our sun may have rubbed cosmic shoulders with other stars.
It was born in a stellar nursery Close to many other embryonic stars.
So if the Sun was born in a very dense neighborhood, whereby a lot of other stars were forming at the same time in the same region, it is absolutely possible that material could be exchanged between these stars as they're forming planets.
Sedna may have formed just like any other object around another star In a nice, circular orbit out past the main planets of that alien solar system, but if that star got close enough to the Sun, our gravity may have been able to lift Sedna out and steal it.
It's possible that other dwarf planets were abducted from other systems and that these alien worlds carried alien water and even alien organic materials to the inner planets.
It's so tempting to think that we understand something as basic as our own solar system, our own home.
When you discover something like Sedna, you realize there could be a lot out there that we haven't seen.
There are objects that are small like Sedna that are just so far away that they're beyond our limit to detect them, so there could be hundreds, thousands, tens of thousands of objects out there, new parts of our solar system that are still waiting to be discovered.
Our understanding of the solar system is changing radically.
Newly explored dwarf planets stun us, and even the most famous one, Pluto, reveals new secrets making us ask, could these worlds be as active And alive as our own? Dwarf planets all over the solar system are revealing hidden lives.
In the asteroid belt, salt on the surface of Ceres suggests liquid water beneath the surface.
Farther out, the new horizons mission found subsurface oceans on Pluto.
We used to think that water could only exist in this Goldilocks zone, where it's not so hot that the water boils off and it's not so cold that it freezes, but that's not the case anymore.
We look around, and we find water in the most unexpected places in our own solar system.
This is kind of a revelation of modern planetary science that so many of these worlds in the outer solar system may have subsurface oceans of liquid water.
It kind of boggles the mind to see how far we've come in our understanding of the interior structures of these worlds.
Finding liquid water so far from the Sun left scientists stunned and the surprises keep on coming as we study these distant worlds up close.
Makemake, 2/3 the size of Pluto.
Its surface is covered in ethane and methane ice.
The methane is frozen into It reacts with sunlight, forming organic molecules call tholins.
These color the planet red-brown.
Farther out lies Eris.
It's 9 billion miles from the Sun, and its surface temperature About 400 degrees Fahrenheit below zero.
Eris is an absolutely tantalizing object.
We really don't know much about it at all.
It should be very similar to Pluto, but one of the things we notice is that Pluto's surface is kind of patchy.
There are areas that are very bright, but also areas that are quite dark.
Eris, on the other hand, seems to be almost entirely bright.
Eris is one of the shiniest objects in the solar system, reflecting 96% of the light that hits it.
Scientists wondered why.
A clue comes from its near neighbor, Pluto.
When new horizons flew past, it spotted something strange.
One of the funny little details is that as we flew over Pluto, we realized that there were things that looked a lot like sand dunes down there.
Now, that may not sound incredibly exotic.
You know, what's very interesting about a sand dune? Sand dunes may sound dull, but they can reveal a lot about the mechanics of a planet.
Unlike the dunes we know and love on the earth that are made of sand, the dunes on Pluto are made entirely of particles of ice.
There's only one thing that can build dunes wind.
Dunes are like a visual representation of the wind that's moving across the valley and carrying the sands with it and depositing it into these big, beautiful dune forms.
Our planet is large enough to hold on to an atmosphere.
Air, warmed by the Sun, rises.
Fresh air rushes in underneath, generating winds.
Pluto is so small and so far from the Sun, it shouldn't have an atmosphere or wind or dunes.
The problem Pluto has, like other small bodies in the solar system, is that it's really hard for it to hold on to an atmosphere.
It's just too small.
The very thin, light atmospheric gases basically just escape.
Neither Haumea nor makemake have detectable atmospheres, but when new horizons looked back at Pluto as the dwarf planet passed in front of the Sun, scientists spotted a thin haze of gas.
Turns out, Pluto has an atmosphere.
But this atmosphere is temporary because Pluto's orbit is elliptical.
Some of the time, it's far from the Sun.
Other times, it's much closer and warmer, creating a kind of winter and summer.
Pluto's atmosphere depends on the season.
In the summer, it's warm enough to have an atmosphere, and in the winter, that atmosphere freezes out.
Over the course of just a single orbit around the Sun, the surfaces of these dwarf planets may change significantly, condensing and coding out atmosphere when they're far from the Sun, having that atmosphere revolatilize and redistribute the surface when they're closer to the Sun.
Pluto is currently in its summer phase.
The extra heat during the long super summer evaporates some of the nitrogen ice on the surface, creating a thin, wispy atmosphere.
It turns out that even though the atmosphere of Pluto is very thin, there is wind.
It's really light, but there's just enough wind to be able to carry particles with it once they start moving.
The seasonal cycle could help explain Eris' brightness.
Eris is three times further away from the Sun than Pluto is, but when you put a nitrogen atmosphere three times further away, that nitrogen freezes solid to the surface.
Eris could be an indicator of what Pluto looks like when it enters its winter.
The gases will freeze, and it'll become even more reflective.
In winter, Pluto's dunes will be locked in place Frozen on the surface, unlike the icy features of another dwarf planet and the case of the vanishing volcanoes.
From afar, the dwarf planet Ceres looks uniform and dull but up close, one huge feature comes into view.
One of the strangest objects that we saw when we began to map the surface of Ceres was something called Ahuna Mons.
Now, this was a strange, jutting hill, very, very sharp sides, and it didn't match any of the other terrain on Ceres.
Ahuna Mons is a very peculiar feature on Ceres.
This is a mountain that is standing three miles high, and there's nothing else like it on the entire surface of Ceres.
Ahuna Mons dominates the landscape of Ceres.
With its steep sides and enormous height, it looks a lot like volcanoes on earth, but earth is still geologically active.
Ceres is so small, its molten core should be frozen solid.
Planetary scientist Nina Lanza heads to one of the most volcanically active places on earth Iceland.
She has a drone's-eye view of mount Helgafell, a volcano similar in shape to Ahuna Mons.
So this volcano is what's called a rhyolitic dome, and so it's a type of lava that kind of gets squeezed out through fissures, and then forms this kind of blobby dome feature that gets pushed up by the magma coming up from beneath.
On earth, red-hot magma bubbles slowly out of cracks in the surface, building a steep-sided volcano.
But dwarf planets like Ceres are too small to have a hot core of molten rock to power volcanism.
There isn't molten rock on these smaller worlds that have a lot of ice on them.
Instead, what's molten is water under the surface, and if the water can work its way up through cracks and erupt out in the surface, you get a volcano.
But it's a cold-water volcano.
We call these cryovolcanoes.
Liquid water squeezes up through fissures in the surface.
It quickly freezes, building the mountain.
This volcano, you can see that it's a pretty young feature, and it's not very eroded.
We expect on earth that wind and water will slowly erode this mountain away.
With no wind or weather to erode Ceres' cryovolcanoes, once created, they should remain on the surface for billions of years.
Ahuna Mons is very strange because it's the only tall mountain on Ceres.
Why should that be? You don't typically get just one of something.
You should have dozens of them, and, in fact, Ceres may have had quite a few cryovolcanoes in the past, but they're all gone today.
Just to put that into perspective, imagine if this is the only mountain on earth.
Why would there only be one mountain? What would that mean? This leads us to ask the question, you know, are the volcanoes on Ceres disappearing? The idea that volcanoes are vanishing It just sounds totally science fiction, and really, not realistic at all.
Of course, volcanoes can't just vanish, but actually, in the right context, in certain scenarios, they actually can.
The key to this magic trick Gravity.
It can flatten solid matter.
How quickly depends on the structural composition of the material.
If you want to build a sand castle on the beach, you can't use dry sand.
It doesn't stick together, so you want to mix a little bit of water in there so that when you make the structure, it holds together, but if you mix in too much water, it just dribbles away.
It viscously relaxes.
It slumps.
Even Iceland's rock volcanoes are slowly slumping under their own weight.
Strange as it is to imagine this, it turns out the mountain behind me is actually slowly relaxing back down.
It's just happening very slowly, not on a time scale that we can directly observe.
It may be that there were many cryovolcanoes on the surface of Ceres.
They no longer show any trace of their existence So if we waited around a little bit longer until all of Ahuna Mons had slowly relaxed back into the planet, we'd see no trace of it, either.
Maybe Ahuna Mons hasn't always stood alone.
Maybe it's just the last of its kind.
Ceres continues to confound our expectations, and there are still many mysteries with the other dwarf planets to be solved, such as how they got their moons and why Pluto lies on its side.
Just like their larger cousins, dwarf planets often have orbiting satellites.
We now realize that all of the largest dwarf planets have moons around them, have a moon.
Most of them have one.
Haumea has two.
Pluto has five.
Four billion years ago, the young solar system was chaotic, filled with small bodies orbiting the Sun.
One hit the infant earth, forming the moon.
Smash-ups like these happened throughout the solar system.
The dwarf planet Haumea formed from an explosive collision between two larger objects Which may account for its unusual bean-like shape.
All the dwarf planets suffered huge impacts.
Haumea had this big one that left it spinning.
Eris has a tiny moon, presumably from a giant impact.
Makemake has one.
All these biggest objects have these tiny fragments of moons showing us their history of just getting battered and pieces being knocked off everywhere.
Most dwarf planets' moons are tiny, not much bigger than asteroids, but one moon is very different Pluto's moon, Charon.
Pluto's moon is, if anything, weirder than Pluto itself.
It's Frankenstein's moon.
It looks like somebody tore a moon apart and then just kind of slapdashed it back together.
One hemisphere is smooth.
One is very rugged.
It's got a canyon that's like a notched carved out of the side.
It is really bizarre.
An impact may have formed Charon and left it tied to Pluto in an oddly codependent relationship.
In some ways, you can think of the Pluto-Charon system as almost a binary planet.
There is no other planet in the solar system where the moon is so large in proportion to it and so close.
Like other binary objects, Pluto and Charon orbit around a central gravitational point.
Locked in this gravitational dance, Pluto and Charon always show each other the same face.
One of the really interesting things about Pluto and Charon is that they're what we call tidally locked.
When Pluto and Charon formed, they were probably both rotating on their own axes, but the two worlds actually slowed down their rotation and locked together, with one side constantly facing the other as they orbit around.
But Pluto's rotation is tipped over like a top spinning on its side, so Charon's orbit around Pluto is also tipped over.
Almost every planet in the solar system has an orbital axis that points in roughly the same direction.
Pluto's is tilted down about 120 degrees.
Scientists have long wondered what caused this disparity.
Did Charon pull Pluto over? Or is the tilt a result of the impact that formed Charon? A clue was revealed when new horizons sent back images of Pluto's heart.
One of the more endearing features of Pluto as the new horizon's probe approached it was a gigantic heart-shaped region on the side of Pluto facing the spacecraft.
Sputnik Planitia, a bright, white heart against Pluto's dark, pockmarked surface.
When we got closeups of this, it was completely fascinating.
I gasped out loud.
This is how shocking this was, and I remember saying, "oh, my gosh.
There are no craters there!" It is smooth, like it's a frozen-over lake.
This is indicative of something liquid, something flowing under the surface of Pluto, and what we're seeing is the top, frozen layer of it.
There are even convection cells where the ice appears to be warming and spreading out.
That suggests that underneath, there's a source of energy, and amazingly, there may even be a huge basin of liquid water under that ice.
Sputnik Planitia may hide a giant, subterranean ocean of liquid water.
It's also a gigantic scar on Pluto's surface.
Most likely, given the shape and size, Sputnik Planitia was formed in a giant impact.
Something smacked into Pluto.
Could the combination of subsurface water and an impact account for Pluto's unusual tilt? One theory suggests that an object smashed into the top of Pluto.
The impact shattered the surface, and water oozed up to fill the crater.
The liquid water knocked Pluto off balance, and the gravitational dance with Charon spun this heavy heart out to the opposite side.
One idea is that Sputnik Planitia formed where it is because ices can accumulate in the floor of a giant impact base.
But it's not yet certain whether that's actually the case or not.
Dwarf planets, once thought to be dead lumps, have come alive with mysteries.
They've challenged all our assumptions, and yet, we've barely scratched the surface of these perplexing worlds.
There are many more dwarf planets to discover, and who knows what surprises they may have in store? We don't know the final count of dwarf planets because we're still finding them, but I think that there are probably somewhere between 100 and 200 dwarf planets out past Pluto.
There are probably many, many more as you go even further out in the solar system.
Dwarf planets are, perhaps, the most interesting objects we've found in the solar system.
They're diverse.
They're geologically active.
They contain liquid water.
Just because they're small, that doesn't mean they're insignificant or they should be ignored.
They are where it's at.
For me, that is just the best, the most exciting.
We have all of these new worlds to study that we didn't even dream existed just a few years ago.
That's science.