The Solar System is the name we give to ourlocal cosmic backyard. A better way to think of it is all the stuff held sway by the Sun’sgravity: The Sun itself, planets, moons, asteroids, comets, dust, and very thin gas.
Introduction to the Solar System: Crash Course Astronomy
If you took a step back — well, a few trillionsteps back — and looked at it from the outside, you might define the solar system as: theSun. That’s because the Sun comprises more than 98% of the mass of the entire solar system.The next most massive object, Jupiter, is only 1/10th the diameter and less than 1%the mass of the Sun. But that’s a little unfair. Our solar systemis a pretty amazing place, and you can figure out a lot of what’s going on in it justby looking at it. For thousands of years we had to explorethe solar system stuck on this spinning, revolving ball — the Earth.
The problemwas, for a long time we didn’t know it was a spinning, revolving ball. Well, the ancientGreeks knew it was a ball — they had even measured its size to a fair degree of accuracy— but most thought it was motionless. When a few folks pointed out that this might notbe the case — like the ancient Greek astronomer Aristarchus of Samos — they got ignored.The idea that the sky spins around the Earth seems obvious when you look up, and when greatminds like those of the astronomer Ptolemy and philosopher Aristotle supported that idea,well, people like Aristarchus got left behind. The basic thinking was that the Moon, Sun,and stars were affixed to crystal spheres that spun around the Earth at different rates.While it kinda sorta worked to predict the motions of objects in the sky, in detail itwas really unwieldy, and failed to accurately predict how the planets should move.
Still, Ptolemy’s idea of a geocentric Universestuck around for well over a thousand years. It was the year 1543 when Nicolaus Copernicusfinally published his work proposing a Sun-centered model, much like the one Aristarchus had dreamedup 2000 years previously. Unfortunately, Copernicus’s model was also pretty top-heavy, and had ahard time predicting planetary motions. The last nail in geocentrism’s coffin camea few years later, when astronomer Johannes Kepler made a brilliant mental leap: Basedon observations by his mentor Tycho Brahe, Kepler realized the planets moved around theSun in ellipses, not circles as Copernicus had assumed. This fixed everything, includingthose aggravating planetary motions. It still took a while, but heliocentrism won the day.And the night, too. This paved the way for Newton to apply physicsand his newly-created math of calculus to determine how gravity worked, which in turnled to our modern understanding of how the solar system truly operates. The Sun, being the most massive object inthe solar system by far, has the strongest gravity, and it basically runs the solar system.In fact, the term “solar” comes from the word “sol,” for Sun.
We named the wholeshebang after the Sun, so there you go. The planets are smaller, but still prettyhuge compared to us tiny humans. At the big end we have giant Jupiter, 11 times widerthan the Earth and a thousand times its volume. At the smaller end, we have…well…thereis no actual smaller “end”. We just kinda draw a line and say, “Planets are biggerthan this.” That’s a bit unsatisfactory, I’ll admit, but it does bring up an interestingpoint. I’ve been using the term “planet,” butI haven’t defined it. That’s no accident: I don’t think you can. A lot of people havetried, but definitions have always come up short. You might say something is a planetif it’s big enough to be round, but a lot of moons are round, and so are some asteroids. Maybe a planet has to have moons. Nope; Mercuryand Venus don’t, and many asteroids do. Planets are big, right? Well, yeah. But Jupiter’smoon Ganymede is bigger than Mercury. Should Mercury be stripped of its planetary status? I could go on, but no matter what definitionyou come up with, you find there are lots of exceptions. That’s a pretty strong indicationthat trying to make a rigid definition is a mistake; it’ll get you into more troublethan it’ll help.
“Planet” can’t be defined; it’s aconcept, like continent. We don’t have a definition for continent, and people don’tseem to mind. Australia is a continent, but Greenland isn’t. OK by me. So that’s what I tell people if they askme if Pluto is a planet. I say, “Tell me what a planet is first, and then we can discussPluto.” Pluto is what it is: A fascinating and intriguing world, one of thousands, perhapsmillions more orbiting the Sun out past Neptune. I think that makes it cool enough. All the orbits of the planets lie in a relativelyflat disk. That is, they aren’t buzzing around the Sun in all directions like beesaround a hive; the orbit of Mercury, for example, lies in pretty much the same plane as thatof Jupiter. That’s actually pretty interesting. Wheneveryou see a trend in a bunch of objects, nature is trying to tell you something. In fact,there are other trends that are pretty obvious when you take a step back and look at thewhole solar system. For example, the inner planets — Mercury,Venus, Earth, and Mars — are all relatively small and rocky. The next four — Jupiter,Saturn, Uranus, and Neptune — are much larger, and have tremendously thick atmospheres.
Inbetween Mars and Jupiter is the asteroid belt, comprised of billions of rocks. There arelots more asteroids scattered around the solar system, but most are in the main belt. Then, out beyond the orbit of Neptune is acollection of rocky ice balls, called Kuiper Belt Objects. The biggest are over a thousandmiles across, but most are far smaller. They tend to follow the plane of the planets too.But if you go even farther out, starting tens of billions of kilometers from the Sun, thatdisk of Kuiper Belt Objects merges into a vast spherical cloud of these ice balls calledthe Oort Cloud. They don’t follow the plane of the inner solar system, but orbit everywhich way. So what do all these facts tell us about thesolar system? We think they’re showing us hints of how the solar system formed. 4.6 billion years or so ago, a cloud floatedin space. It was in balance: its gravity trying to collapse it was counteracted by the meagerinternal heat that buoyed it up.
But then something happened: Perhaps the shockwavefrom a nearby exploding star slammed into it, or maybe another cloud lumbered by andrammed it. Either way, the cloud got compressed, upsettingthe balance, and gravity took over. It started to collapse. As it did, angular momentum becameimportant. That’s a lot like regular momentum, when an object in motion tends to stay inmotion. But in this case it’s a momentum of spin, which depends on the object’s sizeand how rapidly it’s rotating. Decrease the size, and the rotation rate goes up. Theusual analogy is an ice skater starting a spin, then drawing their arms in. Their spinis amplified hugely. The same thing happened in the cloud.
Anysmall amount of spin it had got ramped up as it collapsed. It flattened into a disk,much like spinning raw pizza dough in the air will flatten it out. As it collapsed, material fell to the center,getting very dense and hot. Farther out in the disk, where it was cooler, material startedto clump together as little grains of dust and other matter randomly bumped into otherlittle bits. As these clumps grew, their gravity increased, and eventually started drawingmore material in. These little blobs are called planetesimals — wee baby planets. As they grew, so did the center of the disk.The object forming there was a protostar — or, spoiler alert, the protosun. Eventually itscenter got so hot that hydrogen fused into helium, with makes a lot of energy. A lot of energy. A star was born.
The new Sun blasted out fiercelight and heat that, over millions of years, blew away the leftover disk material thathadn’t yet been assimilated into planets. The solar system was born. Closer to the Sun it was warmer. Hydrogenand helium are very light gases, and the warm baby planets there couldn’t hold on to them.Farther out, there was more material in the disk, and the planets were bigger. Since itwas cooler, too, they could hold on to those lighter gases, and their atmospheres grewtremendously, eventually outmassing the solid material in their cores. They became gas giants.
There was also a lot of water out there, farfrom the Sun, in the form of ice. Smaller icy objects formed past Neptune, but spacewas too big and random encounters too rare. They didn’t get very big, maybe a few hundredkilometers across. A lot of them — billions, perhaps trillions of them — got too close to the bigplanets, and were flung hither and yon. Closer in, material between Mars and Jupitercouldn’t get its act together to form a planet either; Jupiter’s gravity kept agitatingit, and impacts between two bodies tended to break them up, not aggregate them together. And there you have it. Our solar system, formedfrom a disk, sculpted by gravity. Echoes of that disk live on today, seen in the flatnessof the solar system. This isn’t guesswork: the math and physicsbear this out.
And not only that, we see it happening, now, today. When we look at gasclouds in space, we see stars forming, we see protoplanetary disks around them, we seethe planets themselves getting their start. We may think of ourselves as the solar system,but we’re really just a solar system. The scenario that happened here so long ago playsitself out daily in the galaxy. We’re one of billions of such systems. And remember: Every atom in your body, andeverything you see around you — every tree, every cloud, every human, every computer,everything on Earth, even the Earth itself — was once part of that dense cloud. We are, quite literally, star stuff. Today you learned that the solar system isone star, many planets, a lot more asteroids, and even more icy comet-like objects.
It formedfrom a collapsing cloud, which flattened into a disk, and that’s why the solar systemis flat. Rocky planets formed closer to the Sun, and larger gas giants farther out. Icy objectsformed beyond Neptune in a disk as well, and a lot of them were flung out to form aspherical shell around the Sun. We see this same thing happening out in the galaxy, too.The motions of the objects in this system caused a lot of confusion to ancient astronomers,but we eventually figured out what’s what. This episode is brought to you by Squarespace.The latest version of their platform, Squarespace Seven, has a completely redesigned interface,integrations with Getty Images and Google Apps, new templates, and a new feature calledCover Pages. Try Squarespace at Squarespace.com, and enter the code Crash Course at checkout for aspecial offer. Squarespace. Start Here. Go Anywhere.
Crash Course Astronomy is produced in associationwith PBS Digital Studios. Seriously, you should go over to their channel because they havea lot more awesome videos there. This episode was written by me, Phil Plait. The scriptwas edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was co-directedby Nicholas Jenkins and Michael Aranda, edited by Nicole Sweeney, and the graphics team isThought Café.
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