We talk with astronomer Dr. Joshua Blackman about the fate of the Earth at the very end of our solar system, when the Sun will render our planet quite uninhabitable. This Pulsar podcast is brought to you by #MOSatHome. We ask questions submitted by listeners, so if you have a question you'd like us to ask an expert, send it to us at sciencequestions@mos.org.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we ask for answers to the most earth-shattering questions we've ever gotten from our visitors. I'm your host, Eric. And whenever we talk about the solar system in our museum programs, students are always interested in what will happen in the future... the far future. Often we get asked: what will happen to the sun? And what will that mean for the Earth? To explore the fate of the solar system I called Dr. Joshua Blackman, an astronomer at the University of Tasmania, who specializes in searching for planets that orbit other stars. One of his team's most recent discoveries gives us new information about what happens to planets when their stars reached the end of their life cycles. Dr. Blackman. thanks so much for joining me on Pulsar all the way from Australia.

JOSHUA: Thanks for having me, my pleasure.

ERIC: This cool discovery that you made, we need a little bit of background before we can really understand it. It kind of involves the evolution of stars and what happens to them at the end of their life. And that is a question we get all the time. So what happens to the sun when it runs out of its hydrogen fuel?

JOSHUA: Right, so the sun is about four and a half billion years old, and in about five billion years, it's going to start to run out of its fuel. And then it will expand into what's known as a red giant. So it's going to expand to about a hundred times its size. And when this happens, it's going to swallow up Mercury and Venus - completely destroy them, and probably destroy the Earth. If it doesn't destroy the Earth, Earth might be covered in lava lakes, the continents will be all messed up. So we're gonna not be having a very fun time, if we're still around then. And then it will contract back down into the corpse of star, which is known as a white dwarf. And so this white dwarf has run out of its fuel. It's about the size of Earth, but half the mass of the Sun. So you can imagine how dense that's going to be. And it just sits there and cools for millions and billions of years. And so the inner planets will be destroyed, but the outer planets, the big gas giants, we expect them to survive.

ERIC: So that sounds like a really rough time to be a resident of the inner solar system, or any star system. Your star gets a hundred times bigger. And you know, that means we'd be inside of it right now. That's crazy.

JOSHUA: Absolutely, absolutely. So if we want to survive, we're going to have to go much further away from the hot star in the middle.

ERIC: But we've got time - you said a couple billion years.

JOSHUA: Yeah, we've got time.

ERIC: We don't have to figure it out. Our kids don't have to figure it out. But in a billion years, we want to start having a plan. And then five billion years from now, I mean, who knows? Maybe we'll be all over the galaxy. And it'll just be something fun to watch by then.

JOSHUA: Hopefully, that would be amazing. If we were all over the galaxy then.

ERIC: So that's the inner solar system is pretty much done. But what about the outer solar system? You mentioned some of those outer planets. Was the theory for a long time that maybe something could survive that? Or would the gravity of the star change enough that we'd see planets kind of escaping and going off into space? Before this new discovery, was it thought possible or likely or unlikely that anything could survive out there?

JOSHUA: So the general theoretical idea is that these big gas giant planets like Jupiter and Saturn will survive. They're probably gonna move slightly further away from the star because the star gets less massive, so the sun will get less massive and the gravity will become smaller, so they'll move slightly further away. But we don't expect the planets to lose their orbit from the sun or anything like that. And our discovery confirms that this general idea that we have about the future of the solar system is probably correct.

ERIC: Awesome. So can you go into that - exactly what you found?

JOSHUA: Right, so we found a solar system, towards the middle of the Milky Way, about halfway to the middle of the Milky Way, which contains a white dwarf. So the corpse of a star, orbited by a planet, about 40% larger than Jupiter, at about a distance similar to what we expect Jupiter to be in about 5 billion years. So this is the first time we found a system around a white dwarf, which resembles what the expected future of the solar system is going to be.

ERIC: So a planet that survived that whole entire process, that planet was just along for the ride. And now is it in a pretty stable orbit around that white dwarf?

JOSHUA: As far as we can tell. Yes, we think it didn't have too rough of a time during this evolutionary process. It wasn't interacted by other planets coming in and pushing it around or anything like that.

ERIC: So it's incredible that it can survive that process. But it's even more incredible that we could find it because we've talked to other astronomers before who search for exoplanets, and some of the main methods that they use are things like a transit. So that's when a planet blocks out the light from a star. But a white dwarf, as you said, it's way smaller than a normal star. It's only the size of the Earth. So it's tiny compared to a regular sized star. I imagine that it's really hard to get transits of white dwarfs. So you had an entirely different method that you're able to discover this planet going around this white dwarf. So can you talk about that and how it works?

JOSHUA: Yes, so the the advantage of the method we use, microlensing, is that it doesn't rely on the brightness of the host star. So that means we can look at white dwarfs because they're very, very faint, because we don't need to see the light from them. So microlensing is based on a theory of general relativity made by Einstein back in the 1930s, he predicted this phenomena. And basically, if you go into the night sky and look up at the Milky Way, the very dense star field of the Milky Way, there's so many stars up there. And if we just point telescopes at that, and wait for a really long time, at some point, there's going to be a star that moves in front of another star, because everything in the Milky Way is moving. And when this happens, the light from the really far away star gets bent by the gravity of the star closest to us. From Earth, we see that as basically an increase in brightness. And so we put our telescopes at the Milky Way, wait for this to happen, wait for this increase of brightness to happen. And then by tracking that increase and decrease in brightness, we can tell things about this system, like maybe there's a planet there. This is what we did 10 years ago. And then then what we did is that we followed this up using the Keck Telescope, which is a giant ten-meter telescope on on Mauna Kea in Hawaii. Because everything's moving, we could actually see these two stars move apart. What we expected to see was just a star like our Sun. We expected to see these two stars move apart, we see this star. But we didn't actually see this star. And because we didn't see it, that means it must be something very faint, which in this case happens to be a white dwarf.

ERIC: So you couldn't actually pick it up with the telescope, even the giant telescope on the top of Mauna Kea wasn't good enough to pick it up?

JOSHUA: That's right, even this giant ten-meter telescope, you know, it's got 36 hexagonal pieces of glass stuck together, even that couldn't see it.

ERIC: So therefore, it had to be something faint. But you actually had the effect. So you knew there was a star there because the lensing happened because of the gravity. So you can kind of deduce that it was something not that bright, but still had enough mass, therefore a white dwarf?

JOSHUA: Absolutely, yes. Very exciting.

ERIC: So these are kind of happenstance. This isn't you saying, oh, I think we should look at this star tonight and see if there's an exoplanet around it. Like you said, you're just looking at lots and lots of stars and seeing when these events happen.

JOSHUA: That's right. So microlensing happens in about one in a million stars. So we have to observe a lot of stars to see these microlensing events. And most of them don't have planets. You know, most of them are just stars coming in front of other stars. And so what we're looking for is that little blip, the little telltale blip that points out that it's a planet and in this case, that's what we saw.

ERIC: So the microlensing itself is pretty rare. But to have that extra planet in there that you can pick out is kind of a super bonus rare.

JOSHUA: Oh, yeah, we only detect a few of these per year. So compared to other exoplanet methods, it's not as common. But the advantage is we can see planets that are much further out. So way further, deep, deeper into the Milky Way, but also planets which are further from their sun. So this is out at the distance of Mars to Jupiter, whereas other methods like transits can't tend to get out that far.

ERIC: Yeah, so we hear about some of the telescopes that are looking for exoplanets. They're looking in our neighborhood, kind of by design, because that's where we'd like to find exoplanets. But also, that's their limit, we can't really discover them that far away. So this sounds like a great way to kind of get a more of a survey of the entire galaxy. I mean, you said most of the way to the galactic center? That's so far away.

JOSHUA: Very, very far away. Whereas the majority of the other planets found by the other methods are within, you know, 20 parsecs.

ERIC: You can look at those maps of where we found all the exoplanets that we found so far, because it's up to almost five thousand last time I checked, and there's lines going all across the Milky Way, and all the lines stop pretty close to where they start. And then there's a couple that are really far away. So I imagine this is probably one of the farthest ones that we've seen so far.

JOSHUA: It's one of the furthest ones. Most of the microlending ones are the far ones away.

ERIC: Why is this in particular exciting, this exoplanet that we found going around a white dwarf? What does it tell us about something like the fate of our solar system?

JOSHUA: So what this tells us is that the picture we had of the evolution of our solar system is likely to be true. So there have only been a couple of other planets detected around white dwarfs, only four other ones, and none of them really look like our solar system. You know, one of them has a planet that's like thousands of times the distance from Mercury away from its sun and the other one is around a pulsar. And so they're kind of unusual objects, unusual systems, whereas this one is kind of probably what we expect to be common in the universe. And so this discovery shows that that yeah, the picture we had is likely to be correct.

ERIC: So it's great when we make a discovery that says, hey, remember what we thought of and remember what we modeled? Remember what we predicted? Here's something that shows that we were pretty much right.

JOSHUA: Yeah, pretty much. I'm sure that the theorists are very happy.

ERIC: So what's next in this field? Are you just kind of doing lots of these microlensing events? And seeing what interesting things come up? Or are you looking for anything specific? Are you hoping to find more stars around white dwarfs? What's kind of on the horizon?

JOSHUA: So this discovery was made as part of a program to prepare for the Nancy Grace Roman telescope, which is a NASA mission, which is going to launch in the mid-2020s. And with this telescope, we hope to discover dozens more of these planets around white dwarfs, which will be very exciting. And in addition to that, with this particular object, we hope to be able to observe it with the Hubble Space Telescope, or the James Webb Space Telescope. We're hoping to see the actual light from the object, because those can see deep enough that we can actually see the white dwarf.

ERIC: Yeah, that's kind of crazy to think about too, that you can make this discovery and say, here's how massive it is. Here's the kind of star. And you've still you've never gotten any light from it. You have no pictures, it's all just deductions.

JOSHUA: Yeah, microlensing is a very strange beast. Yeah, I think the quote is, you know, we're looking for planets we can't see around stars we can't see, which is kind of what microlensing is. Yeah.

ERIC: That's awesome. All right. Dr. Blackman, thanks so much for talking to us about your discovery and your methods.

JOSHUA: Thanks for having me.

ERIC: When you make your next trip to the museum, you can always visit our Charles Hayden Planetarium for the latest exoplanet discoveries. And while you're home, follow the Museum of Science on social media. For more on the James Webb Space Telescopes deployment, and its first look at star systems beyond our own. Until next time, keep asking questions.

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