How do we know Saturn is not made of anti-matter?

Sure there would be things like much bigger explosions from asteroids hitting it, but just from looking at it, is it possible to know if it, or any other planet we have not “touched”, is made of anti-matter?

It’s late at night so I’ll jump in here with both feet. Saturn, earth, the Sun, etc. are several generations removed from the primordeal stuff. Any anti-matter that was present at the beginning would already have reacted with matter leaving us with an all matter, no anti-matter universe.

Wild guess: photons would not be interacting with it in the normal manner if it were made of antimatter.

I’d think it would have some sort of visible reaction with the solar wind.

“Positive matter” + Anti-matter = Atomic Explosion.

The constant rain of “positive matter” asteroids would create quite sizable nuclear explosions on a daily basis, all over Saturn.
Very visible, through a telescope.

Indeed. The composition of the solar wind is reasonably well understood and we know it’s matter rather than antimatter. When it reaches Saturn, some of it is drawn in by the planet’s magnetic field to the poles, where it collides with the atmosphere producing aurora. If these were due to matter-antimatter annhilation, then they’d have very distinct signatures in their spectra.

Actually, because they’re their own antiparticles, photons don’t distinguish between matter and antimatter in their interactions with them.

And it’s important to note that for precisely this reason, you can’t tell matter from anti-matter by “just looking at it”, if by “just looking at it” you mean studying its interactions with light. An atom of anti-hydrogen (one positron orbiting an antiproton) will, for example, absorb & emit photons of the exact same frequencies of light as will an atom of hydrogen. If you want to be able to tell the difference, you have to send in some conventional matter (e.g. asteroids, solar wind, interplanetary probes) to interact with it.

If Saturn were made of antimatter, the planet would be a source of 0.511 MeV gamma rays generated by the annihilation of protons in the solar wind. It’s not.

I’ll chime in that as well as the solar wind and asteroids, we’d probably be able to detect matter-antimatter annihalation signatures from interplanetary dust and gas colliding with Saturn, if it were contraterrene in nature. (Or is there no interplanetary gas that doesn’t get caught up in the solar wind and become part of it??)

To add a new element to the OP: What, then, is the smallest system that we have already discovered that MIGHT be antimatter?? I’ve heard of the possibility of antimatter galaxies – is it theoretically possible that one of the galaxies we’ve discovered could be antimatter?? There’s so much emptiness between the galaxies that there probably wouldn’t be risk of much annihalation there.

If so, then can we push it any further? (I kinda doubt this.) An antimatter star, or several of them, hidden here in our galaxy?? An antimatter glob cluster??

Hold on there, chief. There’s plenty of antimatter around from various nuclear decay processes and energetic collisions, as well as our own efforts.

More importantly, there’s no reason to assume that there was more of one than the other in the primordial universe, which by your logic would lead to everything annihilating and leaving only radiation. The catch is that you’re implicitly assuming perfect mixing here, which is completely unjustified. More likely is that (as yet unexplained) fluctuations led to certain areas being a bit more matter and other areas being a bit more antimatter. As the universe expanded these blew up along with it, and once the matter and antimatter annihilated some regions were left matter-heavy while others were left antimatter-heavy.

Now, where you’re on the right track is this: Saturn coalesced from the same cloud of dust and such that formed the sun and all the other planets, so it’s from the same kind of matter.

I’m not an astrophysicist, but I think it is fairly sure that the entire solar system took its origin in one huge cloud of matter that began contracting, forming the sun and the planets, and rotating. This would imply that the sun, as well as all the planets in our system, are made of the same sort of matter.

which is exactly what I said in the last paragraph of my post…

Nitpick: That’s the electron mass. Protons are about 938 Mev, if I remember correctly.

Why yes it is! Oops.

Not neccesarily. A large number of galaxies live in galaxy groups and clusters. That is, gravitationally bound ensembles of galaxies, with varying amounts of hot gas not only in between galaxies (the intra group/cluster medium) but in between stars in individual galaxies (the intra-galactic medium). The gas and galaxies do interact, in many cases, quite strongly. You’d need an entire group or cluster to be made of anti-matter in that case, and then when that group/cluster encountered another galaxy or group or cluster, that’d have to be made of anti-matter to to prevent everything from simply going kaboom rather spectacularly.

Again, unlikely. Our galaxy is quite dusty (its what comes of being a young spiral thing, it hasn’t got its head round this cleaning thing yet). As we’re made of matter, and our local patch of galaxy is also, we’d get some pretty major explosions, if there were anti-matter stars in our own galaxy.

Thinking about things some more, its possible that radio galaxies, such as these may be at least partly anti-matter. Radio astronomers know that they consist of a plasma, confined by magnetic fields. What we don’t know, and can’t glean from either observations or simulations, is what the plasma consists of. Our two best hypotheses are either a proton-electron plasma (i.e. pure matter), or a positron-electron plasma (i.e. a matter/anti-matter plasma).

I thought physics showed that there was some sort of inherent asymmetry that led to the creation of very slightly more matter than antimatter, and the different accounts for the matter currently existing. Is this not something necessarily agreed upon?

No, actually it leads to annihilation reactions, which release gamma rays and don’t cause atomic explosions.

As far as the imbalance of matter/antimatter in the primordial universe, everything I’ve read has assumed a basic imbalance in the begining, leading to the annihilation of most of the antimatter and leaving mostly matter.

No, you’re not misremembering. This old thread describes a bit about how a universe with equal amounts of matter and antimatter can wind up in the current situation. Basically, in the simplest picture, a small known and observed asymmetry in electroweak interactions preferentially turned antimatter into matter during the early stages of the Big Bang.

I presume it’s the comments by myself and MikeS that have confused you. But note that we were both careful to restrict ourselves to talking about photons. The electroweak asymmetry involved can never show up in interactions that just involve photons. To see it you have to be exchanging the closely related W[sup]+[/sup] and W[sup]-[/sup] particles. The reactions in the early universe do involve these, so there’s no problem there.
Before anybody asks, the big problem with shining a beam of W-particles at Saturn is that they have very, very, very short half-lives and will all have decayed away into other particles by the time they’d get there.

Oh, I understand that photons interact with antimatter the same way they interact with matter. I was responding to Mathochist’s supposition that there still might be equal amounts of antimatter and matter in the universe.