Chapter Content
Okay, so, like, welcome, welcome to the Solar System, right? It's kinda mind-blowing what astronomers can do these days. Seriously, if you lit a match on the moon, they could see it! They can, like, figure out the size and stuff, even if a planet could have life, just by looking at the teeny tiniest blips from stars way, way, WAY out there. I mean, we're talking so far away, it'd take us, like, two and a half million years in a spaceship to get there. They can even pick up the tiniest bit of radiation, so small that all the energy we've collected from outside our solar system since, like, 1951, well, according to Carl Sagan, it's, like, less than the energy of a single snowflake falling.
Basically, not much gets past 'em, you know? Which makes you wonder, like, how come nobody noticed that Pluto had a moon until, like, 1978? So, this guy, James Christy, a young astronomer at the US Naval Observatory in Flagstaff, Arizona, was, like, checking out some photos of Pluto that summer. And he sees something, a little something, you know, kinda fuzzy, not really clear, but definitely not Pluto. So he, like, chats with a colleague, Robert Harrington, and they figure out he's looking at a moon. And it's not just any moon, it's the biggest moon in the solar system compared to its planet.
That was, like, a bit of a blow to Pluto's status as a planet. I mean, it was never really that solid to begin with. Turns out, that moon was, like, kinda taking up the same space as Pluto, which meant Pluto was way smaller than anyone thought, even smaller than Mercury. Actually, like, seven moons in our solar system, including our own, are bigger than Pluto.
So, you're probably thinking, like, why did it take so long to find a moon in our own solar system? Well, it has to do with where astronomers are pointing their telescopes, what they're looking for, and, like, Pluto itself. The most important thing is where they point their stuff, you know? As this astronomer, Clark Chapman, said, most people think they just scan the skies all night. But that's not true! Almost all the telescopes are looking for tiny stuff way out there, like quasars or black holes, or some galaxy super far away. The only telescopes really scanning the sky are the ones the military uses.
Also, we tend to think the pictures are super clear and sharp, because of, you know, artists and stuff, but that's not how it is in astronomy. In Christy's photos, Pluto's, like, faint and fuzzy, just a little, like, puff of space fluff. And the moon wasn't, like, a perfect ball, like you'd see in National Geographic, with a bright background and clear lines, sitting next to Pluto. It was just a tiny, super fuzzy blob. Actually, it was so fuzzy that it took, like, seven years to see it again and prove it was actually there.
But here's a cool thing: Christy made his discovery in Flagstaff, where Pluto was first discovered, back in 1930. And that discovery was mostly because of Percival Lowell. Lowell was, like, from this super old, super rich family in Boston, you know, the one that's, like, in that rhyme about Boston being the home of the bean and the cod. The Lowells only talk to the Cabots, and the Cabots only talk to God, that one! He funded this observatory named after him, but he's most famous for thinking that Mars was covered in canals built by hard-working Martians to irrigate their, like, dry, but fertile, land.
Lowell also thought there was a ninth planet, way past Neptune, and he called it Planet X. He thought this because he saw some, like, weird stuff in the orbits of Uranus and Neptune. So, he spent the last years of his life trying to find this gas giant. He was sure it was out there.
Sadly, he, like, died suddenly in 1916, probably from working too hard. His family started fighting over his inheritance, so the search stopped for a bit. But then, in 1929, partly to get rid of the Mars canal thing, which was, like, embarrassing at that point, the head of the Lowell Observatory decided to start the search again and hired this young guy from Kansas named Clyde Tombaugh.
Tombaugh wasn't, like, a trained astronomer, but he was, like, really hardworking and smart. After a year of searching, he finally saw this faint dot of light in the sky, and it was Pluto! It was, like, a total miracle. It was even more amazing because it proved Lowell was wrong. He predicted there'd be a planet way past Neptune, but Tombaugh figured out right away that this new planet wasn't a giant gas ball. But, like, nobody cared. Everyone was so excited! It was the first planet discovered by an American! Some people said it was just a tiny ice ball way out there, but nobody wanted to hear it. They named it Pluto, partly because the first two letters are Lowell's initials. Lowell was, like, remembered as this total genius, while Tombaugh was mostly forgotten, except by, like, planetary astronomers, who, like, really respected him.
Now, some astronomers still think there might be a Planet X out there, like, a really huge one, maybe ten times the size of Jupiter, but so far away we can't see it. There's not enough sunlight hitting it to reflect back. They don't think it's a normal planet, like Jupiter or Saturn, because it's so far out. It might be, like, seven point two trillion kilometers away, more like a failed star. Most star systems in the universe are actually binary, you know, two stars together, which makes our sun seem kind of lonely.
And nobody's really sure about Pluto itself, you know? How big it is, what it's made of, if it even has an atmosphere, or what it even IS. Many astronomers think it's not really a planet at all, but just the biggest thing we've found in this, like, junkyard belt out there, called the Kuiper Belt. And, well, in 2006, there was this big conference in Prague where all the astronomers got together and, like, argued about it. And they decided that Pluto wasn't a planet anymore. That's because a planet has to orbit the sun, be round because of its own gravity, and clear its orbit of other stuff. Pluto is round and orbits the sun, but it hasn't cleared its orbit. So, now it's a dwarf planet. And the Kuiper Belt theory was actually first suggested way back in 1930 by this astronomer named F.G. Leonard, and named after this Dutch guy working in America, Gerard Kuiper, who developed the theory. And the Kuiper Belt is where short-period comets come from, the ones that zip by every once in a while. The most famous one is Halley's Comet. The long-period comets, like Hale-Bopp and Hyakutake, come from this place even further out, the Oort Cloud, which we'll talk about later.
So, Pluto, like, doesn't act like the other planets. It's small, it's faint, and its orbit is all weird. Nobody's really sure where it's going to be in a century. All the other planets orbit on, like, the same plane, but Pluto's orbit is tilted, like, seventeen degrees, like a hat sitting sideways. And it's orbit's all wonky, so for a long time during each orbit, it's actually closer to us than Neptune. In fact, for most of the 80s and 90s, Neptune was the farthest planet from the sun. Pluto only went back to being the outermost planet in 1999, and it's going to stay there for, like, 228 years.
So, if Pluto's a planet, it's, like, a really weird one. It's tiny, like, four hundredth of the size of Earth. If you put it over the US, it wouldn't even cover half of the lower 48 states. It's just weird that we have four rocky planets close to the sun, four gas giants further out, and then this tiny little ice ball way out there all alone. And then, like, after Christy found Pluto's moon, astronomers started looking more closely at that part of space. And by early 2002, they found, like, over 600 more objects out past Uranus. One of them, named Varuna, is almost as big as Pluto's moon. Astronomers think there might be, like, billions of these things out there. The problem is, they're mostly really faint. They only reflect, like, four percent of the light that hits them, about the same as a piece of charcoal. Except these charcoal chunks are, like, six billion kilometers away.
How far is that? It's, like, almost impossible to imagine. Space is just so incredibly HUGE. For fun, let's just pretend we're gonna take a trip in a rocket. We won't go too far, just to the edge of our solar system. But first, we gotta understand how big space is and how tiny we are.
Okay, bad news, we won't be home for dinner. Even if we could travel at the speed of light, which is, like, 300,000 kilometers a second, it would still take seven hours to get to Pluto. And we can't travel that fast, of course. We have to go at spaceship speeds, which are really slow. The fastest anything made by humans has ever traveled is the Voyager 1 and 2 spacecraft, which are going, like, 56,000 kilometers an hour.
They launched the Voyagers back in 1977 because Jupiter, Saturn, Uranus, and Neptune were lined up, which only happens, like, every 175 years. That allowed them to use, like, a gravity assist technique, where they got flung from one gas giant to the next, like a cosmic slingshot. Even then, it took them nine years to get to Uranus and twelve years to get past Pluto's orbit. Good news is, if we wait until 2006, which NASA is planning for a New Horizons mission to Pluto, we can use, like, a good Jupiter alignment and some better technology to get there in, like, ten years. Getting back home would probably take a while, though.
Either way, it's a long trip.
The first thing you'll notice is that space is, like, really well-named. It's a lot of nothing. The liveliest place for trillions of kilometers is our solar system, and all the stuff you can see, the sun, the planets, their moons, the asteroid belt with a billion rocks, the comets, and all the other space junk, only fill up less than one trillionth of the space. You'll also realize that all the pictures of the solar system you've ever seen are, like, totally not to scale. In most pictures, the planets are all close together, but that's just so you can fit them all on one page. Neptune isn't just a little bit past Saturn, it's REALLY far past Saturn, like, five times further away than Saturn is from us. It only gets, like, three percent of the sunlight that Saturn gets.
The distances are so huge that there's no way to draw the solar system to scale. Even if you had a million pages in a textbook, you couldn't get close. In a solar system picture to scale, if the Earth was the size of a, like, pea, Saturn would be, like, over 300 meters away, and Pluto would be, like, two and a half kilometers away. That's, like, the size of a bacteria, so you wouldn't even be able to see it. And the nearest star, Proxima Centauri, would be, like, 16,000 kilometers away. Even if you shrunk everything down so Saturn was as small as a period and Pluto was the size of a molecule, Pluto would still be, like, ten meters away.
So, yeah, the solar system is HUGE. By the time we get to Pluto, we're so far away that the sun, our warm, sun-tanning, life-giving sun, would be the size of a pinhead. It wouldn't look much bigger than a bright star. You start to see why even important things, like Pluto's moon, got missed for so long. It's not just Pluto, either. Before the Voyager missions, we thought Neptune only had two moons. Voyager found six more. When I was a kid, we thought there were only 30 moons in the solar system. Now, there are at least 60, and a third of them have been discovered in just the last ten years. We don't even know what's in our own solar system, so we have to remember that when we're looking at the rest of the universe.
And now, as we fly past Pluto, you'll notice we're FLYING PAST PLUTO! If you check the itinerary, you'll see we're going to the edge of the solar system, and we're still nowhere near there. Pluto might be the last thing on the chart, but it's not the end of the solar system. Not even close. To get to the edge, we have to go through the Oort Cloud, which is, like, a gigantic cloud of comets. And, sorry to say, that'll take another 10,000 years. Pluto's not the edge, like the charts make you think, it's only, like, one-fifty-thousandth of the way there.
We're not planning on taking that trip, of course. Going to the moon, which is like 386,000 kilometers, is still a big deal for us. President Bush, the first one, suggested going to Mars once, but that didn't happen. People think it would cost, like, 450 billion dollars, and it would probably kill everyone on board because they can't block the high-energy particles that would shred their DNA.
Based on what we know and what makes sense, nobody's ever going to go to the edge of our solar system. It's just too far. We can't even see the Oort Cloud with the Hubble telescope, so we don't really know what it's like. We just think it's there.
So, all we can say about the Oort Cloud is that it starts beyond Pluto and goes out about two light-years. The basic unit of measurement in the solar system is the astronomical unit, AU, which is the average distance between the sun and the Earth. Pluto is about 40 AU from us, and the center of the Oort Cloud is about 50,000 AU from us. It's really, REALLY far away.
But let's just say we got to the Oort Cloud. First thing you'd notice is that it's really quiet. We're so far away from everything, even our own sun, that it doesn't even look like the brightest star in the sky. It's amazing that that tiny little twinkle in the distance has enough gravity to hold all these comets. It's not a strong gravity, so the comets just move really slowly, only about 354 kilometers an hour. Sometimes, because of tiny changes in gravity, maybe from a star that passes by, one of these lonely comets gets knocked out of its orbit. Sometimes they get flung into space and never seen again. But sometimes they get into a long orbit around the sun. About three or four of these comets, the long-period comets, come through the inner solar system every year. These lost visitors sometimes hit things, like Earth. And that's why we're here now. Because the comet we're going to see is starting its long fall towards the center of the solar system. It's headed towards Manson, Iowa, of all places. It'll take a long time to get there, three or four million years, so we'll just leave it for now and talk about it later.
So, that's our solar system. What's outside of it? Well, maybe nothing, maybe a lot of stuff, depending on how you look at it.
For a short distance, there's nothing. The best vacuum we can make is less empty than interstellar space.
There's a lot of that emptiness until you get to the next "something". Our nearest neighbor is Proxima Centauri, which is part of a three-star system called Alpha Centauri, 4.3 light-years away. That's not much in galaxy terms, but it's still a hundred million times further than going to the moon. It would take at least 25,000 years in a spaceship to get there. And even if you made it, you wouldn't really see anything. Just a cluster of lonely stars hanging in the middle of nowhere. To get to the next significant landmark, Sirius, it's another 4.6 light-years. So, that's how it would be if you wanted to go "star hopping" through the universe. It would take way longer than we've been around to get to the center of our own galaxy.
I'll say it again, space is huge. The average distance between stars is over 30 trillion kilometers. Even at the speed of light, it's a long, challenging distance for anyone who wants to travel. It's fun to think about aliens traveling billions of kilometers to make crop circles in Wiltshire or scare some poor guy in a pickup truck on a lonely road in Arizona, but it's probably not going to happen.
Still, from a statistical point of view, there's a pretty good chance that there's intelligent life out there. Nobody knows how many stars there are in the Milky Way, estimates are between 100 and 400 billion, and the Milky Way is just one of about 140 billion galaxies, and many of them are bigger than ours. In the 60s, a professor at Cornell named Frank Drake, excited by these huge numbers, came up with a famous equation to calculate the possibility of advanced life in the universe, based on a series of shrinking possibilities.
In Drake's equation, you divide the number of stars in a part of the universe by the number of stars that might have planets. Then you divide that by the number of planets that could theoretically have life. Then you divide that by the number of planets where life has actually appeared and developed intelligence, and so on. Every time you divide, the number gets smaller, but even with the most conservative inputs, the number of advanced civilizations in the Milky Way alone is always in the millions.
It's a really cool and exciting thought. We might just be one of millions of advanced civilizations.
Unfortunately, space is huge, so the average distance between any two civilizations is probably at least 200 light-years. That's hard to get your head around, so let me explain it a bit more. First, that means that even if these creatures know we're here and can see us through their telescopes, they're only seeing the light that left Earth 200 years ago. So, they're not seeing you and me. They're seeing the French Revolution, Thomas Jefferson, and people in stockings and wigs. People who don't know about atoms or genes, just people rubbing amber with fur and thinking it's fun. The messages we get from these observers will probably start with "Dear Sir," congratulating us on our fine horses and skill with whale oil. 200 light years is just unfathomable.
So, even if we're not alone, we're still pretty lonely. Carl Sagan figured there might be as many as a hundred million trillion planets in the universe that could have life, which is way more than we can even imagine. But what's also way more than we can imagine is the size of the universe they're scattered in. "If we were randomly inserted into the universe," Sagan wrote, "the chance that you would be on or near a planet would be less than one in a billion trillion trillion trillion." The world is really precious.
So, here's some good news, maybe: Pluto, it's officially a planet, as of February 1999, according to the International Astronomical Union. The universe is big and lonely. We'll take all the neighbors we can get.