Last week, two protoplanets in another star system were directly imaged. That seems like a good excuse to talk about planetary formation.
Birth of a Star
Before we move on to actual planetary formation in a star system, we have to talk about the birth of stars.
It all begins with clouds. Not small water vapor clouds like in our skies, but light-year spanning monsters holding molecules totaling a weight millions of times our sun.
Gravity slowly collapses this cloud into smaller clouds. This collapse creates heat and as the clouds become smaller and denser, heat increases. This process, taking hundreds of thousands of years, leads to a protostar.
Protostars don’t do nuclear fusion yet. There are other gravity-driven processes at play, which do generate heat. Eventually, the combination of this heat and increasing pressure cause the star to ignite: nuclear fusion starts to occur and the star becomes one of the bright pinpoints on the firmament which we know and love.
Planetary accretion disc
One thing to note about the process described above is that the cloud of dust and gas that forms into a star will tend to become a disc. Because of angular momentum, the gas starts to spin as it collapses. And like pizza dough being spun into a pizza shape, the gas cloud will flatten out into a circular shape with a large radius and a thin diameter.
This disc around the star contains gas and dust. Slowly this dust will start to come together to form clumps of matter. Small clumps become big clumps because of gravity.
Over time, most of the dust and gas is blown away, and only clumps of rock and gas remain: planets, asteroid belts, and comets.
And that’s how planetary formation works.
About that planetary formation photograph
When you start to image exo-protoplanets, the large gas giants are the most easy to spot. They are bigger than their siblings, and they sweep an entire band of the accretion disc clear quickly. The smaller planets do the same, but slower and on a small scale.
I started this post with the news of a photograph. Now that we know what we’re looking at, let’s see it.
Those two white blobs are two exoplanets slowly sweeping up the accretion disc.
To give a sense of how extraordinary this image really is, we need to look at the scale. All in all, the picture above shows an area some 10 billion km in diameter. The star system is 370 light years away, meaning 3.500.000 billion km. Those blocky shapes are planets several times larger than Jupiter, and the light traveled 370 years to get here. That means the light left the planet around 1650, a century before the US became independent, and when the Netherlands were still a world power (yeah, hard to imagine).
All in all, an amazing achievement. And who knows, in several hundred million years, on the smaller siblings of these planets, aliens might be drinking coffee and watching pictures of our solar system on their version of the Internet.