How Do They Build A Planet?

More than once, I’ve mentioned my son’s love of (not to say obsession with) Star Wars. I can’t really complain that he does, either, since I’m the one that introduced him to the movies. He also loves Legos, which is why he really, really loves playing the Lego Star Wars games. And I get roped into playing with him, because he knows I love Star Wars and video games as well.

As we’re playing through the game one day, we get to the animated cut scene where the death Star blows up Alderaan. He’s seen the movie, so he knows it’s coming, but this time around it seems to strike a chord in him.

“Why did they blow up the planet?” he asks.

“Well,” I say, trying to strike a balance between answering the question and having to have an in-depth discussion on the nature of evil with my five-year-old son, “they’re bad guys. And they’re trying to make Princess Leia tell them where her friends are.”

“They’re not making good choices,” he decides upon hearing that, which makes me smile. Then, the level starts and we’re frantically button-mashing. But he’s still thinking. “Can they fix it?”

“Fix what?” I ask.

“The planet.”

“No, you can’t really put a planet back together.”

He thinks about that again. “Well,” he asks, “how do they build a planet?”

A Star Is Born

ALMA image of the young star HL Tau and its protoplanetary disk. This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas.

ALMA image of the young star HL Tau and its protoplanetary disk. This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas.

The current accepted model of solar system formation is the nebular hypothesis. It’s particularly considered a good model these days, thanks to the image above, which is an image of a planet-forming disk captured by the Atacama Large Millimeter/Submillimeter Array(ALMA). In brief, here’s how it works.

Solar systems begin as enormous clouds of gaseous matter – mostly, but not entirely, hydrogen – floating in interstellar space. These clouds are actually referred to as “molecular clouds” (or, sometimes, “stellar nurseries”), and they are huge – anywhere from 15 to 600 light years in diameter with masses of 100 to 10,000,000 solar masses. Which means that, despite their vast size and mass they are extremely diffuse, possessing between a hundrd and a thousand particles per cubic centimeter.

All matter exerts gravity, even matter as small as molecules and atoms. Over time (read “time” as “millions of years”), the particles begin to clump together and form molecular cores (with a density of 10,000 to 1,000,000 particles per cubic centimeter). As a molecular core grows bigger it generates more gravity, pulling more matter to it and making it generate more gravity. It also begins to spin, thanks to the principle of the conservation of angular momentum (the same principle that causes whirlpools and ice skaters to spin faster and faster).

If conditions are right, then eventually (over hundreds of thousands of years) the molecular core will grow dense enough that it becomes incompressible, and further attempts to compress it simply generate more heat. As more of the molecular cloud collapses into the heated molecular core, it eventually forms a protostar (which is just a young star that is still absorbing mass from the molecular cloud).

So Where Does A Solar System Come From?

When the protostar forms, there are three important facts to remember. First, it is spinning. Second, it has gravity. Third, it hasn’t eaten all of the molecular cloud yet. These are important facts, as you’ll see momentarily.

All of the particles in the molecular cloud are moving in a more-or-less random manner. However, the spinning protostar and its comparatively massive gravity lend a little uniformity to these random directions – all of the particles, in addition to their starting random direction, also move at least slightly in the same direction as the spin of the protostar. This leads to a phenomenon called accretion as all of the particles in the molecular cloud do one of three things:

  1. Fall into the protostar.
  2. “Go with the flow”.
  3. Get thrown away from the protostar until it leaves the star’s orbit completely.

Most of the particles will take option one, and be eaten by the protostar (our own sun has more than 99% of the entire of the mass of our solar system). Some of what is left will be flung out beyond the effective borders of the fledgling solar system. The particles that are left continue to engage in inelastic collisions, which cause them to lose momentum as they impact each other. Since they can’t effectively eliminate the rotational speed (they’re being dragged by the spinning of the protostar) they fall towards each other – flattening into an accretion disk in the process.

And The Planets?

So, we’ve got a flat spinning disk around the protostar at this point. The planets begin to form because of molecular cores. See, the core that formed into the protostar isn’t the only core that formed. Molecular clouds have any number of these structures, and the one that formed the protostar is simply the biggest, meanest one in the local area. Smaller cores get treated like any other particle in the cloud – they either get eaten, go into orbit, or get chucked out. The ones that go into orbit begin collecting other particles that are in orbit near them, since they’re the biggest nearby source of gravity (a process that can be observed by how satellites orbiting the Earth will fall towards the Earth more than they fall towards the Sun). They can’t accrete enough mass to ignite into a protostar as well, but they can become noticeably bigger. If they get big enough, they form into planets.

Could we make a planet?

Sure. In theory, at least. All we’d need is enough mass to clump together, and a way to move all of that mass close enough to get it to clump together. As a for example, Mercury (the smallest planet in our solar system) only weighs 60,830,000,000,000,000,000 tons. How hard could it be?

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