How do you know something is there, even if you can't see it?

It's a tough problem, isn't it?

There are many things I've discussed in this show that we know, even though we've never

seen it.

We've never seen the inside of a supernova as it went

off in our backyard and let's hope we never do and yet we kind of sort of know how the inside of a

supernova works we've never seen a planet forming in real time and yet we kind of sort of know

how it works we can't see dark matter it's in the name and yet we have a pretty sort of know how it works. We can't see dark matter, it's in the name,

and yet we have a pretty decent idea of how it works.

But there is something special about seeing,

about direct observation,

about holding it in your hands and showing it to the class

that you want to be able to point to it and say,

yep, right there it exists.

But what if that's not an option? What if we can't directly observe? Science is all about inference,

which is a big fancy word, and that means educated guessing. We take the evidence and then we

build a case for that evidence. And if the evidence is

strong enough for what we're claiming, we're going to go ahead and believe it for now. But if new

evidence comes to light or the evidence weekends, we're going to discard what we believe. It's as

simple as that. And what I'm talking about today, when it comes to inference

and knowing something that we can't really see is about our solar system. And the planet's

up with the solar system. How many are there? Where are they? What are they like? And I'm not

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