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And welcome back to Coast to Coast George Nori with you. Josh Wynn is a professor of astrophysics at Princeton University. His research group explores the properties of planets around other stars and tries to understand how planets form and evolve and make progress on the age old question of whether there are planets capable of supporting life out there. His latest book is called The Little Book of Exoplanets. Josh, it's a great book, by the way, Welcome to the program.
Thank you very much, and I've been looking forward to this conversation.
These exo planets. How many have been discovered so far?
Well, NASA maintains the closest thing we have to an official list of exoplanets, and I checked just before the program and the official number is five thousand, five hundred to two.
That's amazing, it is.
Yes, it's one of the most rapidly advancing fields of astrophysics.
Considering thirty years ago. Do you know any of them?
Right? That's right.
Yes, it's a new field. It's very much technology driven and it was only in the mid nineteen nineties, when technology advanced to the point that we could start discovering planets around distant stars, well outside the Solar System.
What percent would you guess that a star, wherever it may be, would support planets.
Yeah, that's a good question. So there are too many stars for us to search them all, right, there's maybe one hundred billion stars in the Milky Way. So what we do is we search the nearest brightest stars, those are the ones where it's easiest to find planets, and we do it kind of a census, and based on what we've seen so far, it seems likely that at least for Sun like stars, most or all of them
have some kind of planets. To be a little more specific, if you pick a random sun like star, there's at least a thirty percent chance that it has one of the types of planets that we can detect today. And we can't detect all kinds of planets. Some planets are much harder to find than others. Planets that are very small are difficult for us to find. Planets that are in very wide orbits are difficult for us to find.
So there's like a one in three chance that it will have a detectable kind of planet, and that leaves plenty of room for the others that probably have the types of planets we're only just beginning to be able to detect.
And Josh, of those planets that might be out there, what percent do you think might be earth like, Well.
It kind of depends on what you mean by earth like. The one thing you need to know about exoplanet science is that we can only learn a very limited amount of information about each planet. So if we have a planet in the Solar System like Mars, I can show you beautiful images of the surface of Mars and it'd see mountains and valleys, and you can look for water at the ice caps and so forth. For exoplanets, they're so hard to detect that we have none of that
kind of detailed information. Most of the time. All we know is the size of the planet, the overall size, the mass, and we know how far away it is from the star that it's orbiting, We know how fast it's moving in its orbit, and we might be able to measure the shape of its orbit. That's about it. So when you say earth like planet, many of your listeners are probably thinking about a planet that has continents
and oceans, maybe breathable air, that sort of thing. We have no way at the moment of knowing whether those things are true for any exoplanet. But if we just ask, okay, well, what about planets that are the same size as the Earth and that orbit a star like the Sun and are about the right distance from the star to have the same surface temperature as the Earth. That's kind of a working definition of Earth like based on things that we can measure today, And the answer is that they're
probably not. We actually struggle to detect such planets. That's kind of right on the limit of our technology right now to detect planets as small as the Earth. But it seems to be that maybe ten of Sun like stars have such a planet.
Josh, how does a planet even form?
That's a great question, and that is one of the big questions that motivates this field. Planets and stars. They didn't just pop into existence with the Big Bang. They had to form the universe. The early universe was just a kind of formless sea of atoms, and somehow, in a there some process must have created the Sun and the planets and other stars and their planets. And the current theory is that This happens mainly due to gravity.
When we look around the galaxy, we see lots of clouds of material, mostly hydrogen gas, but all kinds of other stuff is mixed in there too, And every now and then, one of those gigantic clouds of gas starts contracting under its own gravity. Gravity, as you know, is an attractive force. Everything attracts everything else with the force of gravity.
And it's universal, right, and.
It's universal as far as we can see, And so that will cause the cloud to start shrinking under its own gravity. And what happens is that if it contracts strong enough and gets to be small enough and dense enough, then the material can ignite and become a star. It can start nuclear fusion reactions that give stars their power sources. But it doesn't just all collapse into a point like a star. Instead, what happens is the material ends up
swirling around this new born star like a vortex. And that spinning pattern of material, that disk of material that surrounds a young star that is supposed to be where the planet's formed. The planets kind of congeal out of this material that swirls around a newborn star for several million years, so that is the essence of the modern
theory of planet formation. You form a star by contracting a giant cloud of gas under its own gravity, and then the material that doesn't quite make it down all the way to the star swirls around for a long time, brillions of years in fact, and that gives enough time for small objects within that spinning disk to conglomerate and join together and get larger and larger and become planets.
And in our Solar system, Josh, the one planet that didn't get its act together and form is probably what they call the asteroid belt, now right.
Yeah, that's right. So there are actually two places in the Solar System where we see a lot of small debris, asteroids and other objects of similar size. One of them is between Mars and Jupiter. That's the asteroid belt that you just referred to. Probably what happened there is we're looking at the material that would have come together under
its own gravity to become a planet. But Jupiter is right next door, and Jupiter is a very massive planet, and that means that its gravity stirs up the material in the asteroid belt and probably prevented it from forming a full fledged planet. The other region that's like that is out beyond Neptune. There's another zone where we find lots of small, rocky and icy bodies. That's called the Kuiper Belt. That's right. Probably what happened there is that
that material might have become a planet eventually too. But everything happens so slowly out there. The further you are from the Sun, the weaker the Sun's gravity, and the slower all of the motions are out there, and so they're just they couldn't get it together quickly enough to form a planet.
Josh, you mentioned the Big Bang, and I got to tell you, I have interviewed so many scientists on this program over the years. I still don't understand the Big Bang. How something would start from nothing? Have you come up with your own thoughts about that?
Yeah, I don't know what came, why something started from nothing. I think that's one of the large kind of imponderable questions that we have today. But I do know the evidence for the Big Bang, the fact that we have pretty good reason to think that the universe is expanding.
That is the when we look at distant galaxies, we see that the distances between galaxies are growing with time, and that means that if we trace back the clock about thirteen point seven billion years I think is the current estimate for the age of the universe, that the universe was an extremely dense state, that all of the matter that we see and spread all over in distant galaxies was actually quite close to each other. So it was a very hot, dense sea, as I call it
before of atoms. There's actually a lot of very solid evidence for that, and the evidence can take us all the way back to the first few seconds after the Big Bang. We understand we think the conditions at that moment in the early universe's history, and the reason is that when we look around the universe, we see it's mostly hydrogen, but there's also some helium. Hydrogen and helium are the simplest kind of atoms that there are, and so there's three quarters of the universe is hydrogen and
the other quarter is mostly helium. And the only way to make sense of that is to suppose that these elements formed in that early, hot, dense phase of the universe. And if you calculate, well, what would be the elements that formed from a hot, expanding fireball with the conditions that we think existed in yearly Universe. You get the right answer, You get that, well, there should be about three quarters hydrogen and most of the rest is helium,
with a little bit of lithium and so forth. So the evidence can take us back to the first few seconds. But then if you ask me, well, what about beforehand, that's where we have to go off of the path of the evidence and start speculation.
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