07/06/2018

In The Distant Past, A Massive Space Collision

12:16 minutes

two blue spheres each with rings, both resembling the planet uranus
A 2004 infrared composite image of the two hemispheres of Uranus obtained with Keck Telescope adaptive optics. Credit: Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory.

The icy planet Uranus is an odd place. It spins on an axis almost perpendicular to its orbit, with one pole pointed straight at the sun for much of the year. It’s also colder than expected and has an unusually-shaped magnetic field. One theory for how Uranus became such an oddball in our space neighborhood involves a massive impact strong enough to tip a young planet onto its side.

[Shine brightly, little neutron star.]

In research published this week in the Astrophysical Journal, a group of researchers ran the numbers on such a collision and simulated what the results might be if a planet one, two, or three times the size of the Earth were to strike Uranus in the early days of our solar system. Jacob Kegerreis, one of the authors of the report, joins the program to talk about the research and what it might tell us about planetary formation elsewhere in the universe.

View a simulation of the collision below.

an bright orange yellow blob resembling uranus with a purple tail end
The collision with Uranus of a massive object twice the size of Earth that caused the planet’s unusual spin, from a high-resolution simulation using over ten million particles, coloured by their internal energy. Credit: Jacob Kegerreis/Durham University

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Segment Guests

Jacob Kegerreis

Jacob Kegerreis is a researcher in the Institute for Computational Cosmology and the Department of Physics at Durham University in Durham, UK.

Segment Transcript

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. Ira Flatow is away. Later in the hour, we’ll be talking about efforts to save the endangered white rhino. Plus, science goes to the movies with a look at Jurassic World, Fallen Kingdom. And we want your questions about dinosaurs. You can call us at 844-724-8255. That’s 844-SCI-TALK, or you can tweet us @scifri.

But first, the icy planet Uranus is an odd place. It spins on an axis almost perpendicular to its orbit, with one pole pointed straight at the sun for much of the year. It’s also colder than expected, and has an unusually shaped magnetic field.

Now, one theory for how Uranus became such a standout in our space neighborhood involves a massive impact strong enough to tip the young planet onto its side. In research published this week in The Astrophysical Journal, a group of researchers runs the numbers on such a collision. Jacob Kegerreis is one of the authors of that report and a researcher in the Institute for Computational Cosmology and the Department of Physics at Durham University in the UK. Jacob, welcome to Science Friday.

JACOB KEGERREIS: Hi, thanks for having me.

JOHN DANKOSKY: So this idea of an impact causing the Uranus tilt isn’t new, is it?

JACOB KEGERREIS: No, that’s right. So it’s very hard to imagine, like you said, what could cause this odd tilt that Uranus has. And by far the most obvious idea is that something huge hit it and knocked it over.

So this is not a new idea, but what we’ve been able to do with this research is use large supercomputers and new modeling techniques to try and understand more of the details of how this happened and some of the other consequences that a huge violent event might have had on the planet.

JOHN DANKOSKY: Before we get into more details about that violent event, I said some of this upfront, but there’s a lot that’s weird about Uranus. Tell us what is unusual about this planet compared to the others in our solar system.

JACOB KEGERREIS: Yeah, exactly. One quick thing to say is also that we might not even know that much about how many weird things there are, because compared with any of the inner planets, Uranus and Neptune and Pluto and so on, they’re so far away. And specifically, Uranus has only been visited by the one probe, by Voyager 2 in the mid ’80s. So we know a lot of weird things about it. But there could well be even more that we haven’t had the close look at the planet to find out yet.

But anyway, to answer the question, obviously there is the tilt, which is odd, like you mentioned as well. It’s extremely cold. We expect that these giant planets, as they’re forming, have lots of heat in their center as all of this material is being squashed together as they formed from the early solar system. And the energy that we see coming out the surface of Uranus is practically exactly the same as the energy it receives from the sun. It’s as if there’s nothing coming out from the middle. So that’s very odd.

I think you also mentioned the magnetic field. And with something like the Earth and larger planets like Jupiter as well, you expect a magnetic field somewhat like a bar magnet, just that classic shape of a north pole and a south pole and a nice uniform map of the magnetic field lines. But around Uranus, and I think Neptune as well is somewhat similar, it’s this completely different chaotic shape. And there are loops going all over the place, and it doesn’t seem to either come from the center, and it’s not even aligned with the tilted spin axis either.

So those are a couple weird things. Some of the aspects of the moons, with small rings in the center, some normal-ish looking moons, and then weirder moons further out. So the list goes on.

JOHN DANKOSKY: So why is it so cold? I can I guess understand why an impact might knock a planet over, but what makes it so especially cold there?

JACOB KEGERREIS: Yeah, that’s a complicated question. Again, the short answer– and our research certainly doesn’t go all the way to explaining it– but a few years ago, what people found was that pretty much the only way they could find to explain how cold it is– because, like I mentioned, it’s not been long enough for Uranus to cool down in a normal way over time. The solar system isn’t old enough for all that heat to get out, we think.

So what they found was that if you had some boundary layer that stopped the heat from the middle getting to the outside, that that would be able to explain the cold outside that we see on the time scales that we think happened. So a potential aspect of this is one, that you have some physical obstruction that stops the heat from moving out and also more complicated considerations of the entropy and how the heat transfers.

But one of the simpler ideas that we can understand is that in big planets like this, the convection is a really important way of moving heat around. So actually moving blobs of hot stuff that can rise up just like air does in weather on Earth. Hot stuff rises, cold stuff sinks, and you can circulate things that way.

So one of the things we found in our simulations– which we weren’t exactly looking for, but we hoped we might see– was that for some especially grazing impacts, all of the material from this impact of this Earth or larger size thing that’s smashing in could end up almost coating around the outside of the young Uranus target it was hitting. And it’s possible that this is the kind of thing that could make a layer to trap the heat in or perhaps mix up the temperature gradients inside and stop this nice convection from happening.

JOHN DANKOSKY: So that’s potentially some of what happened with the impact. The debris caused this. What happened to all the other stuff that came off in this collision, both from the object that hit Uranus and also the planet itself?

JACOB KEGERREIS: That’s a really good question. So the first thing is, we expect most of it stayed in the planet. You can get an awful lot of stuff spread over a big area. But most of the mass, most of the meaty bits of the planet, we think stayed in. Part of the reason for that is that these distances from the sun, at this time we don’t expect an impact to be coming in at huge speed. It’s not going to be doing a hit and run collision and flying off into the distance.

And also, in order to explain the rate of spin that Uranus has, to knock it over and explain that most basic observation that we wanted to look at, you really need the impact to end up stuck inside the planet so it can transfer all of the angular momentum, all of this ability, to spin off the planet, transfer all of that to the end result. I can’t remember if that was exactly what you asked, but that’s a start.

JOHN DANKOSKY: Well no, that’s a good start. Actually gets me to my next question. This idea of an impact, I think I’m thinking of two cue balls in a pool table hitting and bouncing off of each other. But if two things that are roughly planet sized collide, it’s not going to be that quick. How long would the actual impact last as it drags across the surface of Uranus?

JOHN DANKOSKY: Good question. So not that long. You might expect– sometimes we talk about these astrophysical things, and they’re taking years or millennia to happen. This is going to be over a few hours, or maybe a few tens of hours, depending on when you decide to say that it’s over. But certainly, unlike two cue balls bouncing off each other, this is going to be an awful lot messier.

So planets this size, especially when they start hitting each other, everything’s going to be hot and molten. Bits are going to start flying off. Oh, that’s what you asked about before. Yeah, so as well as everything smushing and blobbing all over the place, you are going to scatter a lot of material out around in huge sprays. And that might be able to explain perhaps where some of Uranus’s moons came from. When these huge things collide, sure, lots might collect in the middle. You’re going to scatter debris over quite a wide area as well.

JOHN DANKOSKY: The object that you surmise hit it was about Earth sized?

JACOB KEGERREIS: Probably, if not actually quite a bit bigger. So we don’t know for certain, of course. So what we did is we tested a range of impacts and masses. We looked at one about the same mass as the Earth, and then twice as big, and three times as big. And in the early solar system, you expect to have probably quite a lot of this kind of object flying around before you have just the 10 or so big planet-y things that we have nowadays.

Earlier on, the solar system was a much more crowded and violent place. And so all of these small things flying around could be causing impacts like this, probably on all the planets. So we looked at this range of possible masses, and one of the things we found– again, perhaps as a surprise– is that almost all of these masses are plausible. They might be able to explain the spin. And so that’s partly why we looked at other things like the possible temperature effects and other complications to see if we can start narrowing down what the most likely impact was.

JOHN DANKOSKY: But you say that probably in the early solar system, there was a lot of stuff, maybe planet-sized, that was circling around, bouncing into each other. Not something that’s coming from outside at a very high rate of speed probably.

JACOB KEGERREIS: Yeah. I think that’s by far the most likely thing. So people have been talking a lot recently about [INAUDIBLE], if I’m pronouncing that right, this visitor that we had from a different solar system, from interstellar space. But my understanding is we currently know very little about that, is the short answer, whereas we have good reason to think that there were lots of these impactors that were homegrown, as it were. So that’s definitely where the finger usually points.

JOHN DANKOSKY: How exactly did you go about modeling this?

JACOB KEGERREIS: Good question. So the basic idea is not that complicated. In the same way as, say you wanted to program a video game. If you want your character in a game to be able to jump and then fall under gravity, you need to teach that computer how gravity works. You need to say, here’s the equation. This is how fast you accelerate towards whatever. It’s big.

So in our simulations, we need to tell it how gravity works, and we also need to tell it how these materials behave, how rock and ice and hydrogen atmospheres and so on behave when they’re squashed or heated. And that’s a difficult thing to do. But the idea is fairly simple. We want to test these hypotheses that we have, but we can’t just make a planet in a lab and hold it in our hands and smash it together and see what happens. So using computer simulations, we can put in the physics that we understand as best that we can and let it run to see the computer solve these equations, and tell us what happens next.

JOHN DANKOSKY: So in just a little bit of time we have left, why do we care about this? Obviously Uranus is interesting. It’s unusual. But what can we learn about everything else from what we learn about Uranus?

JACOB KEGERREIS: Yeah. So like you said, it’s always nice and exciting to answer the simple questions about– we look up at the solar system, at our planets around us, and how did they get to be the way they are? So that’s the immediate motivation. Like I mentioned, we think that giant impacts like this were common, both in our own solar system– so explaining things like where our moon came from, or you can go through a list of almost every planet in the solar system and something probably happened.

But also, we think they were common in all solar systems. So nowadays that we’re really starting to find so many of these exoplanets, planets around distant stars and other solar systems, not only do we think impacts were important for how all of them formed, but a lot of them seem to be really quite similar to Uranus and Neptune. So that immediately makes us keener to understand everything that we can about these close to home examples of this kind of ice giant planet so we can get a better handle on how those other ones that we start finding [INAUDIBLE].

JOHN DANKOSKY: And we’ve got to leave it there with Jacob Keggereis. Thank you so much.

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