We are answering more of our listeners’ questions in this episode. Jake wonders: how come all the planets in the solar system orbit almost on the same plane? And the answer has a culinary analogy!

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Chris
Today’s question comes from Jake, or @J7urn on Twitter, who I believe it’s his birthday today. So Happy birthday, Jake from the future… No wait!

Alfredo
We are definitely keeping that in. Because you don’t understand how the linear progression of time works.

Chris
Okay, happy birthday from the past, I guess. Yes. Sorry about that normal service. Well, this is normal service. Anyway, Jake’s question: Why do all of our planets orbit the sun at the same horizontal axis? It seems like that’s a lot of wasted…. space!

Alfredo
Taking a deep breath to deal with the pun. No that is a very, very good question. We don’t call it an horizontal axis, we call it the horizontal plane.

Chris
Did he not say plane? Because I… No, you’re right, he said axis and that confused me because I thought he was talking about the the axis of the individual planets. And I immediately came back with no look at Uranus, because Uranus is at a complete tilt. Yes, and but that’s not what he meant. He meant like all of the planets, they’re all flat and I didn’t think they were flat.

Alfredo
Okay, let’s explain a little bit better rather than saying that all of the planets: they’re all flat. That is extreme Flat Earthing

Chris
[laughs] Flat Galaxy! [laughs more]

Alfredo
Well, the galaxy is actually quite flat…

Chris
Okay, this is a tangent.

Alfredo
When are we not going on a tangent? Okay, now Okay, let’s stop. Let’s stop. Let’s stop. Let’s go back to the discussion.

Chris
So I though that. I’ve seen gifs of the solar system where the Sun is in the middle. And then the sun is going in one direction. Like, let’s say, for the sake of this, the sun is going upwards. Okay. And all of the other planets in the solar system, they’re obviously going with the sun. But they were all like all over the place. There was some up if we’re thinking of this as like a 2d thing. There were some in the northwest, there were some in the southeast, there was another planet up to the right, there was one down to the left. But in actuality, all of the planets in the solar system are on the same plane.

Alfredo
Yes. Sorry. I’m hoping you’re listening that you’re enjoying how much this episode is visual for all of you. But yes, the orbits of every planet in the solar system is very close to the equator of the sun. I think that is fairly…

Chris
Uuuuuh… is this something to do with… What’s the word? What’s the word? Like…

Alfredo
Platypus?

Chris
No.

Alfredo
I’ve really confused Chris, sorry

Chris
[laughs] The momentum?

Alfredo
Yes, yes….

Chris
I always remember the analogy of when a ballet dancer spins and

Alfredo
That is very good. What is that?

Chris
I can’t remember. It’s like spinning momentum… something momentum?

Alfredo
something momentum!

What is it spanning?

Chris
circular momentum? Angular momentum!

Alfredo
Angular momentum, correct!

Who’s getting a gold star today? so what is that called?

Chris
Angular moment?

Alfredo
Yeah, no, but the important physical law behind this? what is happening to the angular momentum?

Chris
No, Einstein’s law relativity, you know,

Alfredo
We’re just gonna throw stuff

Chris
I sound drunk.

Alfredo
Yes, he’s not even drunk today. Just wait a few seconds and he’s gonna say quantum Angular momentum

Chris
I wouldn’t say quantum No, it’s not magic. Sorry for all those who don’t know, Alf just scowled at me.

Alfredo
All of them don’t know. Again, it’s not a visual medium. Why are we so visual today? Anyway! what’s happened to the angular momentum?

Chris
It’s spinning.

Alfredo
The angular momentum itself is not spinning is the property of an object that the spins. From 10 minutes ago to now how has the angular momentum changed?

Chris
On the planet or of the Sun or f me. No, not me. I’m not spinning?

Alfredo
You’re not spinning. Oh my god.

Chris
So: the planets

Alfredo
Of the planets! Of the sun!

Chris
I guess they’ve moved a bit.

Alfredo
Okay, there’s an important thing about angular momentum. Okay, what happens? So why the spinning rate of which the ballerina or the ice skater changes when they have hands out and hands close to the body? Why does it change?

Chris
Not to do with the point of gravity, no?

Alfredo
no. So what’s happening to the angular momentum?

Chris
Is it something to do with it getting narrower and wider?,

Alfredo
Yes, but what’s happening to the angular momentum?

Chris
Nothing?

Alfredo
Nothing correct. So we say that the angular momentum is…

Chris
Unchanged!

Alfredo
A better word than that?

Chris
Stable? Constant?

Alfredo
You were so close… Con..

Chris
Continuing? Concentrated

Alfredo
The law of concentratred of the angular momentum

Chris
Conservation!

Alfredo
YES!

Chris
Yes! Got there in the end, bloody hell!

Alfredo
So great, the angular momentum is conserved. Why does that matter?

Chris
Because otherwise the planets would just fly off wherever they want. No, because no, that’s not going in the episode… I don’t know.

Alfredo
Okay, that’s very good. I’m here for it. So what is happening: we need to go back to the very beginning of the solar system. So we just have a big gas cloud. And from the gas cloud, at the core of this cloud, or in a place that has a little bit more density, what happens gravity, so gravity starts pulling things together. Eventually, it pulls enough mass to collapse into a star and start nuclear fusion, and that is how the sun very, very quickly was born. As things fall on the reaction of gravity, they start moving and they move because of how gravity works, along curve lines and they acquire angular momentum. And so, the proto-Sun starts spinning… And that is now in the sort of system where I think 98/99% of the whole mass of the solar system is

Chris
That much? 98 % ?

Alfredo
Yes, so it has a huge, huge angular momentum compared to everything else. So as it starts spinning, what happens? The cloud around, start moving, and it flattens out. The best description that I can think of in my head, is if you ever seen a professional pizza maker spinning a pizza on their hands in the air, that is what happens. It’s the fact that as it spins, because there are things that had more or less angular motion, things move out. And this is because of conservation of angular momentum, which is made by the velocity and the distance from the axis of rotation. And the axis of rotation of the solar system is inside the sun. This is what happens. And so you say “Okay, oh, that’s very nice and easy. So all the planets are in a nice simple plane” it is not exactly correct anymore, because there are interactions between the planets. There probably been a lot of chaos in the very beginning of the solar system. So there are things around the plane of the solar system, but not exactly sort of the same level of the equator. What is interesting is we only just started measuring these properties for other star systems. And we found others that are like ours. But we also found things are completely different from what we see like there’s one that orbits its star on a polar plane. So it means that goes around at 90 degrees from the equator. And we’re not sure how those kinds of things evolve. So is it other stars? is other planets in the in the very beginning of the formation? And what we see in our solar system is certainly not the exception, but it’s also definitely not the rule. We know that a simple plane is how things start. But there is more to it. And we have a lot of doubts on how exactly planetary formation happens.

Chris
So I have another question that ties into this. So you have conservation of angular momentum, you have all of the planets within the solar system roughly on the same plane. They’re spinning around the Sun, orbiting to use the scientific term. They’re spinning around and around and around. And gravity is keeping them within the pull of the field of the sun. Is the gravity that the sun is holding on to these planets with strong enough to fight against what in my mind seems like the inevitability of them spinning so much that they eventually break free of the sun’s orbit?

Alfredo
Interesting question. So you are picturing it by picturing it like

Chris
A pizza being okay. If the toppings are already on the pizza and spin fast enough, all of the toppings are just going to go all over the walls.

Alfredo
Yeah, but Okay, first of all, when you’re making pizza, the spinning part is is just the dough. But also you have not something that makes the toppings stick to the pizza towards the center. I think it’s easier to visualize it. Imagine if you have a string and a cup at the end and then you start spinning that cup over your head with the with the string. There is a tension between the cup and the string. And although it’s spinning around, the tension stays there. So the cup goes in a circle.

Chris
So the planets are safe in that regard.

Alfredo
Yeah, because gravity is there. They cannot just escape the pull of the Sun. And actually, it is a very interesting question and maybe you will realize something. In a previous episode, we had Dr. David Williams, who works on the Solar Orbiter, and he mentioned how difficult it is to get to the sun. And last season, we add another guest, Josh Laurie, who mentioned after you suggested that we just shoot space junk into the sun. how difficult that is. Why do you think is so difficult? You think like, oh, the sun, it’s like the most massive things around here. It’s just going downhill. Why it is so difficult for us to throw something of the Sun?

Chris
because of angular momentum again?

Alfredo
Because of the speed that our planet is going around the sun.

Chris
Is it implying anything we threw off our planet? It was launched into space it would carry on along the trajectory of Earth.

Alfredo
Absolutely. It has that speed and it’s still affected by the sun’s gravity. So what happens is that you need to have a way for, for example, spacecraft or your space junk, garbage lorry towards the sun to slow down. But to do is with chemical rockets, it’s such a huge amount of energy that is not worth it. So for mission like solar orbiter, for missions like Bepi-Colombo that is going to visit Mercury, for NASA Parker Solar Probe. what it’s done is using the internal planets to sort do clever tricks with the rockets and the planets’ gravity and angular momentum, to slow it down, change trajectory, and in a way it gets closer and closer to Sun. So it is interesting seeing things. The earth that cannot just escape the gravitational pull of the sun and it will not automatically fall in. It’s just moving too fast. Or is moving at the right speed to be in the orbit that we are in.

Chris
I have one more question links to this subject. And it’s again, it’s just me trying to break the laws of physics as I’m prone to do. When the sun eventually goes white dwarf, it’s going to slightly collapse on itself a little bit.

Alfredo
It’s just going to be the core of it left.

Chris
Has the mass of the Sun changed?

Alfredo
Yes.

Chris
So therefore the the amount of gravity it exudes on our solar system would also have changed

Alfredo
Yes.

Chris
Would then our planets have enough force to just drift away from what is left of our Sun?

Alfredo
Still no, just because of sizes.Like for example,

Chris
it would be a lot less gravity. But still enough to keep everything where it is.

Alfredo
And not exactly where it is things will change. And before the sun goes white dwarf, it will evolve to Red Giant, so few planets, maybe even Earth, might be gone by then. And turning into a red giant as a whole lot of issue in terms of gravity because obviously, the sun will become massive in terms of size, its density will go way low, and the mass distribution will change. For some planets things get complicated when is when you cannot just say “Oh, it’s going to be a single point that it’s far away”. Maybe an example that is more interesting in terms of the complexity of mixing, angular momentum, and gravity and how orbits might change. Something we can consider is the Earth-Moon system. So the earth and the Moon are very unusual. The Earth and the Moon sizes like the sun and all the rest of the planets, which is mostly Jupiter and Saturn in terms of mass combined, because the moon is about 1.2% the mass of the Earth.

Chris
I thought, it would be bigger.

Alfredo
Yeah, well, it’s it’s quite big in terms of like radius. I think the diameter is 3700 and something kilometers, and Earth’s diameter is 12,000. But obviously when you scale it with volume, things get cubed. So the moon is still huge compared to the outer planets to satellite ratio. To see something even close to it, so we need to go to the dwarf planet Pluto and its moon Charon. But the effects of the moon are sizable. The tide slow down the spin of the earth. So, Earth days are getting longer, the Earth’s rotation has been slow down a tiny amount takes about 50,000 years to slow down the rotation of just one second.

Chris
Wow.

Alfredo
And at the same time, you say: “Oh, Alfredo but angular momentum should be conserved! It’s slowing down”. Yes, but the system, the total angular momentum of the system is conserved. So as the Sarth slow down, so it spins diminish the distance between the Earth and the Moon increases, and what will happen at some point is that okay, at some point, the moon will be too far away to be affected by that at some point is just going to be a this is done, the moon could escape, like, you can imagine that that eventually will run away but it’s never going to get to that point. It will never get to a point in which the gravity of the Earth is not big enough to get the Moon. What will happen is a certain point, like the moon is tidally locked. So we only see one face of the moon, the near side. What happens is that at some point the earth will also be tidally locked with the Moon. And by at some point, I mean, wow, billions and billions of years from now

Chris
So it will just always appear in the same point in the sky. Likely you’ll be able to see it from like Australia or something

Alfredo
Non like Australia… It’s like one vertical side of the planet.

Chris
Okay.

Alfredo
But yeah, it’s it is in such a further future that most likely other things will happen to the earth in the meantime. So not to worry about the moon will be poetically in the sky for a long time coming.

Chris
Well, a few things that blew my mind and I learnt a lot from this one.

Alfredo
Wonderful.

Chris
And please don’t hate me for probably forgetting conservation of angular momentum at some points in the future yet again.

Alfredo
I look forward to just making you guess, the more conservation once again.

Chris
Thanks, Jake for sending in your question.

Alfredo
Thank you Jake. It was really fun to answer and have a Happy Birthday for next year and then it’s going to be from the past.

Chris
Okay, thank you very much.

And this is it for another episode of The Astroholic Explains. Follow us on social media @Astroholic and feel free to send in more questions.

Image Credit (where nothing is in proportion): NASA