This is a show we wanted to do since we started Astronomy Cast but we always thought it was too early. We wanted you to know that we’re positive, happy people with enthusiasm for astronomy and the future. It’s time for some sadness. It’s time for a grim look to see what the future holds for the Universe. This week we stay close to home and consider the end of humanity, the Earth, the Sun, and the entire Solar System. Next week we’ll extend out to the very end of the Universe.
Fraser Cain: This is going to be a sad one.
Dr. Pamela Gay: It is. But isn’t Astronomy all about death and destruction?
Fraser: I guess it is. This is a show that we’ve wanted to do since we started Astronomy Cast. But we always thought it was a little too early.
I think we wanted all the listeners to know that we’re very positive, happy, well-adjusted people with families that we love and love us. We have a great enthusiasm for Astronomy and the future.
Pamela: But now that it is spring and life is abounding everywhere [Laughter] we decided to discuss that. [the end of the Solar System].
Fraser: Exactly, the inevitable long-term destruction of everything that we all love forever. So, time to take a grim look at what the future holds for the Universe. This week we’re going to stay home and consider the end of humanity, the Earth, the Sun and entire Solar System. Then next week we’ll extend out to the very end of the Universe and blow your mind.
If anyone wants a sneak peek, this is actually based on an article I did at Universe Today called â€œThe End of Everythingâ€. It is so fascinating that we wanted to go into it in great depth. So Pamela, let’s start talking about the end from the beginning. [Laughter] Let’s start with the end of humanity.
Pamela: So the end of humanity you can look at as coming from a number of different things. There’s always the â€œwell we could blow each other up tomorrow just because we can, we have the technology to do that.â€
But hopefully the world political systems are a bit more stable than that. Then there’s always the idea of releasing a super disease to wipe out all of humanity next year.
Fraser: Asteroid? We could be hit by an asteroid.
Pamela: We could get hit by an asteroid. There are all sorts of different wonderful ways we could die.
Fraser: So the end of humanity could happen in a blink.
Pamela: In a blink.
Fraser: Who’s to say what is gong to happen. There’s no probability, this is the future. I think we can all imagine a bunch of awful ways that we could instantaneously or within a few hundred years, end humanity as we know it. So, we’re not going to deal with that. Letâ€˜s talk about more of the inevitable stuff.
Pamela: Well, statistically we try to put predictions on how long can a species last. One of the things we do, just because we can, is start from the basic statistical premise: â€œLet’s assume half of all the human beings who will ever exist have already existed.â€ Assume we’re mid-way through our society’s life span on this planet. Then project forward saying that half have already existed, we know how many are to come. We know how fast we’re reproducing (roughly). And that gives us about ten thousand more years on the planet Earth.
Fraser: Okay, so we’ve had a million years so far to get to where we are and then another 10,000 and thenâ€¦..
Pamela: And then we’re gone.
Fraser: That doesn’t seem very feasible to me because population rates aren’t going to necessarily increase forever and if we could find some sustainable level then we could last a lot longer.
Pamela: And it is a purely statistical argument. The truth is we don’t know what’s going to destroy us. We don’t know what’s going to kill us.
So, it could be that we’re here up until the point that the planet can no longer sustain large life at all. That gets us a little bit more time.
Fraser: Right, and I think the other possibility is that we can always evolve or even technologically change ourselves into something else.
Pamela: We can always jump this Solar System and find someplace better to live. We’re just not there yet.
Fraser: Right. When will the Earth stop being a nice place to live and why?
Pamela: Well, our Sun, right now we don’t notice it so much. In fact anyone who tries to say that global warming is caused by the Sun is not so much in line with what scientists are saying. But long term, our Sun is going to heat up. It is going to increase its energy output. It is going to make our planet warm up.
We’re also going to run out of the ability to effectively cycle carbon in and out of our atmosphere. So, with the breakdown of the carbon cycle and with our slowly heating Sun, in a billion years, water on our planet is toast and really we can’t live on the planet.
Fraser: Why is the sun heating up?
Pamela: It’s part of its evolution. As it changes what it is burning, how it is burning in its’ core, it will get a little bit hotter, a little bit hotter, until it runs out of fuel in its’ core. Stars on the main sequence don’t just sit there and say: â€œI’m going to burn hydrogen in my core, the exact same temperature for the next 10 billion years.â€ But rather they go from: â€œI’m going to start burning hydrogen.â€ And they flare up and they do erratic things and eventually settle on to the main sequence.
While sitting on the main sequence, they slowly get a little bit warmer and a little bit warmer. It is just part of the natural evolution of running through the fuel in the core. As the chemical properties in the core change, as you build up more helium as just the mix of elements changes, the Sun is going to run a little bit hot.
Fraser: So where we stand right now is just a snapshot of the Sun’s temperature. It just happens to be that right now the temperature of the Sun is very comfortable. But in like 500 million years, the Sun will have gotten so hot to the point that the whole Earth will just pretty much be a desert.
Fraser: You can imagine the habitable zone around the Sun which we were right smack dab in the middle is pushing outward with the Earth no longer in the middle and starting to kind of be on the inside of that habitable zone.
Pamela: This isn’t to say that Mars is going to become a better place to live because while the temperature situation on Mars is going to be a little bit better, Mars still doesn’t have an atmosphere. So, it’s not that we can go there and be safe either. Mars not only doesn’t have an atmosphere, but doesn’t have a magnetosphere. So, even if we do jump ship and go to Mars, we still have problems with radiation to contend with.
Fraser: You said that the oceans would be toast, what is that process?
Pamela: As you warm up the planet, just increasing the temperature of the planet 50 degrees Celsius you start running into massive evaporation. Once you get too much water vapor in the atmosphere, then you start a runaway greenhouse effect and eventually, our oceans evaporate into the atmosphere and just between normal collisions between particles you will start losing some of the lighter elements away from our atmosphere. It’s sort of like every time you lose a helium balloon, that’s helium that is no longer a part of our planet.
We’re eventually going to run out of helium and other lighter elements are going to go away. So, you evaporate the water and if it breaks into H2 and O in the atmosphere, those H’s are going to go away and you lose the ability to make water.
Fraser: And that’s what’s happened to Venus.
Pamela: That’s what happened to Venus.
Fraser: Right. So that’s the future. Now that is global warming.
Pamela: That’s definitely global warming. That’s the type of global warming we can’t survive.
Fraser: Yes. You look at a desert and you can see that no large creatures can really enjoy themselves. Things have to hide underground. Life doesn’t flourish in the same way it does in a tropical rainforest or in the ocean. So then what does the future hold for life?
Pamela: Eventually it will start moving progressively more and more toward the poles. We will end up with today’s prairie creatures, today’s desert creatures eventually huddling around the North and South Poles. I don’t know what they’re going to be huddling on with the North Pole because there is no land there.
Probably most of them will be combined to whatever can get itself to Antarctica somehow. Eventually even there they will run out of resources to keep themselves alive. Our planet will become a dead desert world.
Fraser: So, is that it? There will be no life on Earth whatsoever?
Pamela: No intelligent life, no large life. Who knows what bugs will exist in the bottoms of coal mines someday. Who knows what will manage to find ways to survive underground. But large life doesn’t exist well seven miles under the surface of the planet.
Fraser: No, but small life does. I’ve heard estimates that there’s more microbial life under the surface of the Earth than there is on top of it.
Pamela: And this is where you make the distinction between large life and microbes. Microbes could be just about anywhere. I’ve heard some microbiologists say there are more microbes on or in your body than human cells on and in your body.
Fraser: Right, you’re really just a colony of microbes. You’re not a person. [Laughter]
Pamela: Something like that. We’re a walking island for bacteria.
Fraser: A robot, an island, yeah.
Pamela: But, where are the elephants going to go? Where are the wolves going to go? Where are the bats going to go? I’m sure there might be mice living underground, but I can’t imagine, and most of the research papers I looked at, just don’t see a future for large multi-celled individuals in the future. Our planet just won’t have the resources to support them.
Fraser: I think it’s funny because people say: â€œOh, 7.5 billion years from now, the sun is going to turn into a red giant.â€ And we’ll get to that in a second. That’s how long we have. But the reality is that we only have about 500 million years. That I think for people feels very different. They’ll both be comprehensively large amounts of time.
When you look back at how long Earth has been here, say for say 4.5 billion years you say that large life really only has another 500 million years. That seems very shocking and very extra sad. You think you have lots of time, but there isn’t lots of time. Maybe only one shot at large life. This is it. Here we are. This is our chance to make large, multi-cellular organisms happen. And then 500 million years from now, that’s that.
Pamela: Part of what makes it so sad is as we look back over our own archaeological record, we can see here’s where the first man was found. Here’s where the first plant was found; here’s where the first multi-cellular object was found. You can only go back so far. But that distance that you can go back does incorporate millions of years. It doesn’t incorporate billions of years for the multi-cellular things.
Microbes first appeared about 3.5 billion years ago. However, you can go back 500 million years and find a fossil record. When you say: â€œAh, life has 7.5 billion years to go!â€ Well, that’s longer than the Earth has been around. That says we haven’t reached the halfway point yet. We’re not middle-aged yet as a planet with life.
When we say 500 million that means that we’re past the halfway point. That means that we should be getting the black balloons at our planet’s birthday party and signing up for planetary Medicare and Medicaid. [Laughter] Once you pass the halfway mark there’s something bittersweet.
Fraser: Now, the Sun though is continuing to get hotter and hotter. So, what does the future hold then for the Sun?
Pamela: Our Sun’s total main sequence life, the amount of time it will be happily burning hydrogen to helium in its core. It’s about 11 billion years. We have about, as you said, about 7.5 billion years to go.
Once the Sun runs out of hydrogen to burn in its core, it will burn a shell of hydrogen around that core. In the process, it will bloat itself out and become what we call a â€œred giant starâ€. It’s not too huge at this point. It’s big, but the Earth is probably okay.
Then it’s going to go through other phases. It will actually ignite helium in its core and become what we call a horizontal branch star. So, it stays the red giant for about 1.3 billion years and then it burns helium after that as a horizontal branch star for about a hundred million years.
Then it becomes what we call an asymptotic giant branch star. This is a star like what the Mira of red variable stars are. It only spends about 20 million years in this stage. But it’s huge. This is where we have to start worrying because a lot of crazy stuff is going to happen.
The Sun, as it gets old, can’t hold onto its atmosphere very well. It’s going to be blowing its atmosphere out. Before it dies, the Sun will actually shrink to almost half of the size that it was when it was born. As it’s blasting this material out, the orbits of the planets are going to get a little bit bigger and bigger because they’re not going to be held on to as tightly by the mass of the Sun which is diminishing. The Sun’s mass actually gets to be less mass.
So even though it is big, gravitationally all we care about is where its center is and where the center of the Earth is. When that mass gets less and less because of this mass loss, this wind that is blowing material out, that amount of mass yanking on the planet Earth gets less and so we drift further and further away.
Fraser: Right, I know in articles about these red giants in this phase, they lose hundreds of times the mass of the Earth every year. That has to have an effect with the total gravity of the star itself.
Pamela: So the question that people have been trying to answer is how does this tug of war between the Earth orbiting the Sun and the Sun losing it’s grip gravitationally as it loses it mass play out as it expands.
Does the Earth move far enough away that we don’t get eaten? Or does the Earth stay just close enough that we get consumed by the Sun as it bloats itself out. There are a lot of different conflicting papers.
The two things that are so hard to understand are: 1) what amount of mass loss is going to take place? And 2) what sorts of things might affect the size of Earth’s orbit? For instance, there could be tidal locking where the Earth, Sun, Moon system gravitationally yanks on to one another and the Earth’s orbit gets slowed down until it is actually on a smaller orbit when you start taking into consideration things like angular momentum. This tidal drag, this slowing down due to tidal affects, could shrink Earth’s orbit just enough comparatively that we get eaten. That’s what some of the latest models are showing. But just a year ago the models were showing no, we’re fine.
I think this is something that’s not going to be settled for a long time because different stars do different things and it’s hard to predict exactly what our little star is going to choose to do.
Fraser: Even if it survives, the amount of damage is just immense. The Sun is gigantic; it’s giving off tremendous amounts of heat. The Earth may be able to survive. But it’s not far away from the Sun in the best case. It’s inside the Sun’s envelope in the worst case.
Pamela: Yeah. We’re going to be that burnt biscuit that someone forgot to take out of the oven.
Fraser: Right, even that microbial life deep under the surface is going to be really wondering if it should have moved a long time ago. [Laughter]
Pamela: And so the future for our planet is well, we either get eaten, which could be kinda cool. You can imagine human beings have moved on somewhere else and we’re looking back at our Sun and the moment the Earth gets eaten, the Sun chooses to do something kind of cool. That might be nice.
Or it could be that we’re left behind as a crispy nugget of former planet. That can also be kind of cool. You can imagine future humans should we choose to survive as a race coming back through the Solar System and pointing out the burnt out rock and pointing out â€œthat used to be the home of all humanity.â€
Sort of like nowadays you can walk through parts of England and Scotland and go â€œthat rock used to be part of the Roman Wall.â€ The Roman Wall is not really there so much in all the areas but the rocks are.
Fraser: Now, is there any way to prevent this?
Pamela: No. It’s a star.
Fraser: Right, I know. I guess you can’t prevent the star itself, but is there a way to prevent the cooking of the planet? [Laughter] I mean like one of these Solar System engineering projects might be possible.
Pamela: There is always the idea that you go out, grab yourself a whole bunch of metallic asteroids and you mill them in space and make a giant sunshield. Then you might be able to prevent the planet from getting over-heated and the oceans from evaporating. But, once the star starts to get into the asymptotic giant branch phase and things like that, it’s going to cool off but it’s blasting so much energy out that you have to wonder if there are enough asteroids out there to build a big enough solar shield to prevent our planet from getting crispied.
Fraser: Didn’t you say that there would the habitable zone would move out so far that places would start turning into liquid water.
Fraser: Like moons of Jupiter and Saturn, right?
Pamela: As the Sun starts giving off significantly more luminosity, admittedly at much cooler temperatures, its total output will increase as it becomes a horizontal branch star. Yeah, we’re going to start doing things like melting the moons of Jupiter which is kinda cool to think about.
Fraser: Except for Jupiter’s terrible radiation. This is all hopeless. [Laughter] This is all pointless, I mean, you know its funny how you search for little glimmers of hope.
Pamela: Yeah, other Solar Systems.
Fraser: All this will be destroyed, the oceans will boil away, and the Sun will have consumed Mercury and Venus, possibly the Earth. But, maybe, just maybe incelladus will take over be kind of slushy. [Laughter]
Pamela: We all go live on Titan? [Laughter] It will melt, it will be no more.
Fraser: All right. And then what will happen to the Sun?
Pamela: Well, so eventually, the Sun is going to become what is called a planetary nebula and a white dwarf. So what happens is the outer atmosphere of the star pops off, floats away, becomes a pretty nebula and the core of the star, the part that fused into helium, the part that perhaps fused into carbon, that inner core is going to be a white dwarf.
It’s no longer going to be generating any of its own energy. It’s just going to be sitting there going â€œI’m hot, wait, I’m not hot anymore, I’m getting cold.â€
Fraser: Right, the star has ceased fusion and now whatever is left just starts to cool down like when you take a pie out of the oven and put it on the counter. Any warmth you are getting from the pie is just it cooling down.
Pamela: And so we’re going to get to sit back and watch from another Star System the white dwarf that was the Sun slowly cooling off and slowly cooling off. Perhaps over several hundred billion years, perhaps over a trillion years, we’re still working out the details of white dwarf cooling. The white dwarf is going to cool off such that it is the same temperature as the background radiation of the Universe.
It’s just a rock; a really dense rock. It is perhaps the largest diamond known if it’s a carbon white dwarf. Our Sun is not going to become a solid carbon white dwarf, but it’s just going to be this dead thing after enough time.
Fraser: Right. They call that a black dwarf, right?
Pamela: Yeah. Now what’s kinda cool though is for about 10,000 years, this hot white dwarf is going to be surrounded by beautiful nebula. Think of the Ring Nebula. Our own Sun could form a future Ring Nebula and for about 10,000 that gas is going to be close and glowing, beautifully lit up.
Over time though, it’s going to float off and eventually it will get consumed and recycled and used to make future generations of stars.
Fraser: Then what will happen to the planets?
Pamela: Well, they will keep orbiting the white dwarf and they will be sad and cold.
Fraser: But would they be able to orbit the white dwarf forever? I mean like a billion or a trillion years? Is there some point where they just have tiny interactions and they eventually just get flung out of the Solar System or smashed into the star?
Pamela: The real question is â€œWhat are the future orbital interactions that our Solar System is going to have with other stars?â€ If our Sun existed in complete isolation where it was just our Solar System and no other stars to interact with then the much smaller Sun could still hold on to the planets and everything would just orbit at a different distance.
But, because we do live in a galaxy with other stars and our galaxy over time is going to get bigger as it collides with other galaxies and consumes their stars, this leads to the potential for all sorts of interactions. You could get perhaps a two-body interaction where two stars interact and their gravitational interactions fling some planets off, suck some planets in, perhaps exchange planets between the two stars.
We don’t know what angles, masses or velocities are going to be involved in these potential possible futures.
Fraser: The time could be so long.
Pamela: And time is so long that anything could happen. It’s just a matter of what chooses to happen first.
Fraser: Right. I mean isn’t it almost inevitable then, given enough timeâ€¦I mean if the Universe has only been around for 13 billion years, 13.7 billion years [Laughter]. But if you look ahead a trillion years to the time that the Sun has cooled down to a black dwarf, the chances of it keeping its planets must be super remote.
Pamela: It’s not super remote.
Fraser: Oh really?
Pamela: Simply because Space is mostly empty, the chances of it keeping all of its planets are somewhat lower. It is harder to hold on to Neptune and Uranus. They’re just a lot further out. It’s easier for them to be grabbed and yanked away. But, the possibility of keeping things that stay closer in is a little bit more helpful.
Fraser: And what we get is another little glimmer of hope because I know that planets have been seen forming around white dwarfs.
Pamela: That’s exactly right. Planets have been found, and what’s really cool is by studying where you find planets around other white dwarfs it helps us understand what the possibilities of our own planet being consumed just might be.
So while the future isn’t hopeful if we stay in our Solar System, at least we can look out and we can see there are other white dwarfs that held onto their worlds. So even if there is no life, there’s still the romantic hope that our planet will exist.
Fraser: Right and I think then you have a much more stable situation if you can survive and you can get to the right distance from your white dwarf then you’ve got hundreds of billion years to sit around a slowly cooling down star, taking in whatever heat it’s got to be able to give off.
Pamela: This is where the realm of science fiction starts to do some of the most fascinating story work. For instance, you can imagine a future where you take an asteroid, hollow it out, mount engines on it and you fly to where it’s safest to be, mount a wall of glass and grow a great greenhouse inside this asteroid that you’ve turned into our new, very small home.
You can almost see yourself being able to manually keep that asteroid of humanity someplace that you’re getting the right amount of thermal output from the Sun. It’s kind of bleak, it’s kind of small and cramped, but it’s possible.
Fraser: Yeah, I mean it’s still kind of bleak. You’re really grasping at straws here [Laughter] with what the future prospects are.
Pamela: I’m trying to leave hope for our listeners. I’m trying not to tell them that humanity is toast in the next 10,000 years even though it probably is.
Fraser: Well, this is completely unrelated but I think I get the impression there is this kind of psychological feeling that it would be very exciting, very ground-breaking; very frontier discovery.
I think it would just be hard and you’d wish there was just air and rocks and trees and the Sun and oceans [Laughter]. We just want all the stuff that makes living possible as opposed to scrambling for everything that you need to just not die.
Pamela: Yeah. [Laughter]
Fraser: I think as you said that is the best case of a terrible inevitable scenario. We’re kind of out of time now, but you know what the funny part is, I think this is the happy show. [Laughter]
Pamela: Yes. The next one gets oh so much worse.
Fraser: The next one gets really sad. Thank you very much for taking us down this week’s road and I think we’ll talk next week about how the Universe itself ends.
Pamela: Okay, well I will see you on that depressing topic again next week Fraser.
This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.