The Earth looks like a perfect sphere, but down here on the surface we see that there are mountains, rivers, oceans, glaciers, all kinds of features with different densities and shapes. Scientists can map this produce a highly detailed gravity map of our planet. And it turns out, this is very useful for other worlds too.

502: No Touching: Determining Composition of Worlds Remotely Astronomy Cast 502: No Touching: Determining Composition of Worlds Remotely by Fraser Cain & Dr. Pamela Gay

How do you know what something is made of if you can't reach out and touch it? How do we know what planets lights years away have in their atmosphere? What about the rocks all around Curiosity? Or the geysers coming out of Europa and Enceladus? Scientists have a few handy tricks.

501: Water Worlds Revisited Astronomy Cast 501: Water Worlds Revisited by Fraser Cain & Dr. Pamela Gay

We're not learning that the vast majority of potentially habitable worlds out there are actually icy moons like Europa and Enceladus. Good news, there are hundreds, if not thousands of times more of them than worlds like Earth. Bad news, they're locked in ice. What have we learned about water worlds and their potential for habitability?

500: Live Celebration! Astronomy Cast 500: Live Celebration! by Fraser Cain & Dr. Pamela Gay

Welcome to episode 500 of Astronomy Cast. To celebrate this momentous occasion, we’re going to look back 500 years into the past to see what we learned about the Universe. And then we’re going to look 500 years into the future.

499: What is the proposed Hubble-Lemaitre Law? Astronomy Cast 499: What is the proposed Hubble-Lemaitre Law? by Fraser Cain & Dr. Pamela Gay

We started out Astronomy Cast with the controversal decision to de-planet Pluto. And here we are, more than a decade later, at the brink of recording our 500th episode when another big decision is coming down from the IAU: whose name goes on the concept that our Universe is expanding: Hubble or Lemaître? It's a big deal and Pamela knows all about it.

Astronomy Cast will be on hiatus until September, 2018.  When we return, we'll be gearing up for the 500th Episode!