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Exoplanets have taught us a lot more about planets than our solar system could ever teach us, from what happens when they’re born, to what happens when their stars die.

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Go to  to start streaming thousands of documentaries  and nonfiction TV shows. [♪ INTRO]. Humans discovered the first planets beyond  our solar system in the ancient days of 1992.

And since then, the number of confirmed  exoplanets has ballooned to over 4,300. Thanks to that flood of new information, we’ve learned a lot more about planets  than our solar system could ever teach us, from what happens when they’re born,  to what happens when their stars die. And along the way, we got a few surprises.

First, before we really  started studying exoplanets, we thought we had a clear picture  of the way solar systems formed:. You start with a massive disk of gas  and dust surrounding a newborn star, called a protoplanetary disk. Then, from that, you get planets  and moons and asteroids and so on.

But then came a 2018 paper. In it, a team of astronomers was looking at several hundred solar systems  with similarly-sized stars. They were comparing the mass of all  the planets in the older systems with the masses of protoplanetary  disks in the newer ones.

And what they found was a bit of a mismatch. There seemed to be more mass in the  planets than in the protoplanetary disks. That suggests that the disks  those planets formed from didn’t have enough material to form them.

And since matter can’t just come from  nowhere, clearly something was wrong here. Exactly what happened is still a bit of a  mystery, but, thankfully we have some ideas. One possibility is that the planets got  a mass boost by siphoning off material from the surrounding interstellar  medium; the gas and dust between stars.

Another idea is that there was  enough mass at the very beginning, like in the first few million years. But then, the planets gobbled it up  so quickly our observations missed it. If that were the case, though, the planets would be a lot more  likely to balloon up to Jupiter-size.

And as far as we can tell, the most common  planets are a lot less massive than that. So, a third option is that we’re just  not looking at the right material to get a good estimate of what’s out there. For example, astronomers can extrapolate  the total mass of a protoplanetary disk based on how much carbon  monoxide gas they can detect.

But if a lot of carbon monoxide is  frozen solid, it won’t be detected, meaning the total mass estimate will be too low. Either way, it seems that planets’ births  are not quite as simple as we thought! And neither it seems, are their early lives.

Back in 1995, astronomers detected  an exoplanet called 51 Pegasi b. It’s a gas giant that orbits really close to  its star; once every four-ish Earth days. Today, we call planets like that “hot  Jupiters”, and we’ve found tons of ‘em.

But back in the ‘90s, the existence  of hot Jupiters rewrote the textbooks. See, we used to think planets just  formed in one spot and stayed there. Except… hot Jupiters wouldn’t be able  to form that close to their stars, based on the traditional understanding of things.

Theyre made of hydrogen and helium,  along with ices like methane and water. And that close to a star, there just aren’t  enough of those materials to make a gas giant. So we have had to revise our ideas.

Today, researchers think hot  Jupiters like 51 Pegasi b do form farther away from their stars where it’s cold,  but then, they undergo planetary migration. That’s where the gravitational pull  from dust in the protoplanetary disk, or even from small almost-planets,  changes a planet’s orbit. In the case of a hot Jupiter, it migrates  inward to a much closer orbit around its star.

Finally, studying exoplanets also taught  us something that seems almost unthinkable:. Planets might be able to  survive the death of their star. See, not all stars have the mass to go supernova and obliterate everything around  them, including stars like our Sun.

Instead, they swell into red giants,  consuming nearby planets in the process. And then, they will shed their outer envelope as the core collapses into a  remnant called a white dwarf. It’s a less violent death, for  sure.

Still, fairly violent. For example it can knock  planets out of their orbits, or bring them so close to the white  dwarf that they are torn apart. So when the time comes for our Sun,.

Earth will, in the very best case,  you know be rendered uninhabitable. But in another system, a distant  enough planet might survive this. And according to a paper published in Nature  in 2020, we may have found one that did.

NASA’s TESS space telescope has been monitoring the light emitted by a selection of white  dwarfs to see if their signals periodically dip, which would hint that a planet is  blocking some of the light as it orbits. And the researchers found that telltale  dip in the signal coming from WD 1856, a six-billion-year-old white  dwarf around 80 light-years away. Whatever is blocking that tiny bit of  the starlight orbits every 34 hours, which is way too close for it to  have been there before the star died.

So instead, the team thinks it started  out over 50 times farther away. Then, as its star transformed, it got  knocked out of its standard orbit, probably thanks to a combination of  the star’s changing gravitational pull and the influence of other large planets nearby. There’s just one caveat here:.

Because of observation limitations, this  body might not actually be a planet. It could be a brown dwarf, something too big to be a planet but  too small to really qualify as a star. Still, it’s hinting that something a little  more Earth-like could survive a star’s death, if it gets very very lucky.

So, overall, the explosion of  exoplanet research has made us rethink much of what we thought we knew  about planets’ life cycles. The universe is far more complex and  interesting than we gave it credit for. And if these discoveries are any indication,  exoplanets still have a whole lot to teach us.

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