Planet Formation out of Black Hole Belches


I’m Mike Lemonick, and I’m really interested in exoplanets, planets around other stars. I’m Lee Billings, and I too am very interested in planets around other stars. Yeah, well I wrote a book about exoplanets. Really? That’s nice because I did too actually. Ok fine. Anyway we’re here to talk about a really bizarre development in exoplanet science. Some scientists have discovered that it is likely that black holes create planets and shoot them out across the university at millions of miles per hour. That’s pretty crazy but it’s not the first time we’ve seen some craziness with exoplanets. The whole history of the field is kind of filled with surprises. Yeah so let’s talk about how we used to think planets formed: giant cloud of interstellar gas and dust gets nudged by the gravity of a star or something and it starts to collapse and collapses and collapses and spins faster and faster and the center bursts into nuclear fire as a star. And the rest forms this disc, and the dust and the gas in the disc start to clump together- here I’ll show you – to form rocks and those clump into boulders and the boulders clump into mountains, and the mountains clump into things like the size of the moon, maybe, or Mars. And those smash together and eventually they form into something like the Earth! What about a planet like Jupiter? It’s pretty similar actually. So you know you get – wow this stuff’s tough – you get like a core together maybe about Earth’s size or a little bigger. This is rock also. Yeah this is rock as well. This is rock and solid stuff. You get the core together and then what happens is out in the outer parts of the disk you just get – oh wow this is really hard gas – you get some gassy material that kinda gloms onto it…yeah gravitational field just sucks it all together and you get a bigger planet because there’s more stuff. This is called the Core Accretion Hypothesis- the idea that we get these rocky solid cores and discs and close up to the star, where it’s really hot, you mostly have rocks and metals and things like that that form the planets. Whereas further out, you have ices and gases that linger in the cold. So you end up with a giant gassy planet like Jupiter with a rocky core. That’s right. Or you have a rocky planet like Earth which is actually suitable for life. You know maybe we could have viruses somewhere in the cloud tops of Jupiter. Probably not, but maybe. Let’s not get crazy. Okay. Except we learned in the early nineties that it doesn’t always work that way. That’s right! So the very first planets we found around other stars totally upended this model. We had a pulsar planets that were found. Pulsars are remnants of normal stars. They’re neutron stars essentially that are left over after a big supernova explosion after a star dies. Right. And you know when a star dies it blows up and sweeps away all the stuff around it including planets whether it’s a big one or the little one. But we found these small little hunks of rock around these pulsars. Somehow they recondensed and reformed planets from the debris of the explosion. Right, so pretty unexpected. But then in 1995 we found a more normal planet around a more normal star, right? You’d think. You think, but it wasn’t so normal. It wasnt that normal because it was one of these big guys and it was right in like a few day orbit around its around its star. So how the heck do you get something like Jupiter that’s only a few days away from its star, when as far as we know it only forms really far out? Right. So people have theories about how that might have happened, you know it forms far out and then spirals in. Any way, big surprise but we’ve learned to live with it. About that same time, astronomers started to realize there was maybe an entirely different way to make a planet like Jupiter. So instead of little pieces getting bigger and bigger pieces getting bigger and then finally it sucks in the gas, the theory was that you could also form something like Jupiter directly from that cloud of gas and dust just clomped together all at once. Right, much much faster. And there are different ways that this could affect how the planets look but we really don’t have a good sense, really, of I think which one is the prevailing model and chances are they’re probably truth somewhere inbetween. Right so that actually brings us to this new discovery that planets might be able to be formed by black holes like little spitballs that they shoot across the universe. So how could that possibly happen? Well the idea is that these stars that might orbit these supermassive black holes can occasionally fall in. And when they get sucked in, they get ripped apart. About half of them falls into the black hole. The other half gets shot out into interstellar or even intergalactic space. What they found is that essentially these streamers of star stuff that escape the black hole can collapse directly into planets on a timescale of, you know, days or years. Much, much faster than the tens of millions of years that we think about for the accretion of cores, the standard idea. And so the idea is that these things could be floating throughout the universe, free floating everywhere. Well not just floating around, we’re talking about shooting them across the universe at millions of miles an hour so they could be flying around like BBs or something. That’s right, that’s right and astronomers actually have looked and we know that there is this population of free-floating planets out there in surprising numbers. So a fraction of those could actually come from the hearts of far distant galaxies and burping black holes. They could be anywhere! 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