In this episode we tackle two more questions from our lovely listeners. The first is about what determines the rotation of planets and the second one focuses on the matter at hand: are we really made from stardust?

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Chris
Welcome to the Astroholic explains. I’m your host, Chris

Alfredo
And I’m Alfredo, The Astroholic.

Chris
This is a podcast where I throw some cosmic conundrums your way,

Alfredo
And I try my best to answer them. Let’s get started!

Chris
This question comes from Richard Lewis, who asks, with respect to the rotation of a planet, what determines its speed? How come Mercury has a longer day than Earth despite being smaller? And then he also adds “likely a stupid question, but I’m curious”

Alfredo
No, not a stupid question at all! Let’s do it!

Chris
So what is it?

Alfredo
It’s about the conservation of angular momentum. So, this is a property of the universal property of physics in which things that are rotating, if there is no friction or anything stopping them, they will continue to rotate with the same rotational speed. So what happens? For example, you can think of ice skaters. When they want to speed up rotating on themselves, they put their arms closer together. When they want to slow down, they stretch out. So on that, you might think “okay, so smaller things should be rotating faster then!” but that is not exactly the case. Because although there is no friction in space, there are gravitational forces that interfere with planets rotation, etc, etc. It’s important to consider how a planet formed and thus gain angular momentum

Chris
Like when it started spinning the first time?

Alfredo
Yeah

Chris
So was it always spinning right from the very first second or do something make it spin?

Alfredo
It’s always spinning right from the very first second. Think of that…

Chris
Is that because of the gravity well? Is that how something like that happens?

Alfredo
Yeah. So how the Solar System form you have a district with a big cloud of hydrogen and dust of other elements. For a star to form, the core of this cloud needs to be cold, which is a bit counterintuitive.

Chris
Yeah, I’d thought it would be hot

Alfredo
It will have to become hot for the star to go nuclear

Chris
But it starts off cold.

Alfredo
Yeah, because you need the gas cloud to condense

Chris
it like it’s freezing. Like it’s freezing together!

Alfredo
in a way or just think that for the gas to sort of collapse on itself, it needs to cool down because if the molecules are too hot, they’re shooting all about and you cannot have them condense and then collapse.

Chris
That makes sense. In terms of temperature, what how cold we talking?

Alfredo
I’m not sure but I think…

Chris
like close to zero?

Alfredo
Close to absolute zero!

Chris
Oooh, it’s a lot lower.

Alfredo
Yeah, I’m not sure I but I’m thinking that these molecular clouds which are the stellar nursery needs to be over 100 degrees below zero Celsius, if not more. I don’t remember… the number -170 comes to mind, but I have literally no knowledge attached to it. Why is that number? I don’t know, I probably just pulled it out of my head with no meaning. [ -263 to -223 degrees Celsius is the correct temperature]

So we have a cooling-down nebula and the core condenses. Then gravity takes over and the core collapses on itself and this additional thing ignites nuclear fusion. Really you have a star, a proto-star at the center and the heat starts dissipating the gas cloud. And what you get is the formation of some more complex molecules or breaking down complex molecules and you have two very distinct areas. One is the internal area in which the materials dry and one is, what we call beyond the snowline, where the material is wet in a way, but mostly is you have ice and you have a lot of pebbles.

And this is one of the ideas of planetary formation that you start from small fragments that start hitting each other and merging together, and getting bigger, bigger. And the bigger you are the more of these pebbles your attract.

Chris
What about gas planets? They’ve just accrued all of the gas around the area and hold it all together in one place.

Alfredo
Yeah. Then so for example, we think that after Jupiter is made mostly of hydrogen but at the center has a rocky core, a small rocky core or small relatively…

What we also need to consider is that these pebbles don’t just like jump up on the planets without having their own trajectory and force. And so they start hitting the planets in a certain direction and this and the natural movement of original pebbles that creates the angular momentum. And having this angular momentum doesn’t mean that it needs to be unchanging. You have, for example, Venus, which rotates in the other direction compared to every other planet in the solar system.

Chris
I found out about that a while ago and it weirded me out, like why is that one so special?

Alfredo
Probably an impact. Well, you have reasons to be weirded out, although is Venus the weirdest? I don’t know. We need to consider that Uranus rotates on its side pretty much.

Chris
Slander!

Alfredo
We think that Venus goes the other way round because of an impact, and also an impact changed the rotational speed of the Earth, although more indirectly as far as we can tell…

Chris
Would that be the impact that created the moon?

Alfredo
Yes, there we go.

Chris
Something massive hit the Earth. A large chunk broke off and made the moon we also that impact also sent the world spinning the other direction on its axis?

Alfredo
No, no, no. The Earth spins in agreement with all the other planets and the Sun

Chris
Unless you’re Venus because it’s weird!

Alfredo
Unless you’re Venus cause it’s weird. But what’s interesting is that the way the moon formed, the moon is so big that it’s slowing down the Earth’s rotation because of tides

Chris
Water tides?

Alfredo
Yes, well anything that is fluid on earth is affected by.. well, everything is affected by the gravity of the Moon on Earth. It’s just that solid things don’t move about.

Chris
So when the moon’s gravity has a pull on the tides it’s like it’s holding the world back from going faster.

Alfredo
It’s friction.

Chris
Okay, so it’s not like someone’s trying to run away and you’re holding their clothes.

Alfredo
No, it’s… Actually, I don’t know. It’s like somebody is trying to spin and you slowly brushing against them. That’s probably a closer interpretation because they continue to spin, but they are slowly losing energy.

Chris
Effectively, the world is spinning, but it doesn’t want to spin too fast in case it hurts the moon.

Alfredo
No, the moon is slowly… Actually not just the moon, it’s the moon and the sun, the tides are created by both. They’re just slowly slowing down Earth.

Chris
Okay, all right, cool. Okay, I think we’ll move on from this question now. Thank you very much, Richard.

The next question comes from Lewis Fieldwater on Instagram, who asks, “Are we really made of stardust?”

Alfredo
We are mostly made of stardust, apart from hydrogen, so the hydrogen in our body is the only thing that is not made from stardust.

Chris
Explain it. Why stardust? And where is hydrogen in us?

Alfredo
How much water do you contain?

Chris
True? Quite a lot, I guess. 90 something percent?

Alfredo
90 percent??? What are you, a cucumber? In humans, it’s about 60 percent.

Chris
I thought it was higher. I thought we were like 98 percent water or something ridiculous.

Alfredo
Sorry about the cucumber comment. Are you a jellyfish? 98 percent water? Come on! You know that we have bones!

Chris
Yeah, I just thought that the rest of us was a lot more.. and bones are brittle and they contain a lot of air. No?

Alfredo
Nooo!

Okay, moving swiftly on from this excursion. All the elements that make us apart from hydrogen were created in stars or in cataclysmic stellar events.

Chris
I’m going to jump in because we’re formed originally by a sperm and an egg. But those sperm and an egg are generated within us. They grow from other things mixing together inside us. If they’re generated inside us, how can they be made of stardust? Is it because the things inside us are all made of stardust? So anything that comes from us has to be made of stardust?

Alfredo
Not only everything that is inside of us, but everything we see and touch and just everything on the planet is made of stardust.

Chris
Wait, so is there absolutely nothing that isn’t made of stardust? There’s not a single thing?

Alfredo
Hydrogen and helium and a bit of lithium

Chris
There’s no single physical object or anything that I don’t know NASA, or some government agency, have made that is not made of stardust; that was originally created and completely synthetic?

Alfredo
You can create synthetic elements but since they are bigger and bigger… Ooh, this is a very interesting question about let’s finish on the stardust and then we go on what is synthetic. During the Big Bang, the elements that were created were 75%, hydrogen 25%, helium and a little bit of lithium. So, some of the lithium in your batteries, a small tiny percentage was there since the beginning of the universe, As with most of the hydrogen, as with a good chunk of helium. Every other element was created in stars. And it could have been released during the late-stages of a star, it could have been released during a supernova, or sometimes some of the elements are created by the completion of two neutron stars.

Chris
So anything that was ever formed for the solar system, for the universe, is stardust

Alfredo
It is stardust. But you said very interestingly, “what about synthetic elements?” So let’s think of the big radioactive elements that we can create in the lab. Those are created by smashing together simpler elements are not so simple elements

Chris
Aren’t those elements also stardust?

Alfredo
Yeah. So the synthetic stuff is formed by components that are from stars. Now, something that I think it’s more for a philosophical argument is particles that we create from energy or during the collision of proton beams or stuff like that. So, proton beams are just the nucleus of hydrogen, although they are not obtained from hydrogen. The nucleus of regular hydrogen is made by one proton. If you ionise hydrogen, so you make it lose its electron, you just have the proton.

Chris
That’s what the beam is made of?

Alfredo
Yes. That’s not how we obtain proton beams. I think we make them from the decay of some heavier element. But let’s assume that we just take hydrogen, we ionize it and we just shoot it at other protons and now we create new particles.

Chris
So this is all happens at CERN.

Alfredo
Yes. In that case, I think the particle so physically because you’re switching hydrogen into something else you’d be creating stuff not from stardust. But since all your tech around is made stardust, are you really not using stardust to create the particles? So, we are in this sort of weird philosophical territory of what is natural and what is artificial and if everything is made of stardust, but hydrogen, helium and a little bit of lithium, what is synthetic and what is not.

Chris
I like as well the concept of using elements of stardust, we have created objects such as telescopes that we use to observe everything around us in the solar system. It’s like we use Stardust to look at stardust. Really cool.

Alfredo
I think it’s the Carl Sagan quote, that we are the universe knowing itself.

Top Image Credit: ESO/M. Kornmesser + Chris Carpineti