 Welcome to part three of the Habitable Zone Unplugged, a four-part conversation about exoplanet science with Dr. Robert Hurt and Dr. Johanna Teske. I'm Calico Field. So in this series, of course, our explorers get to travel to these exoplanets and look down on them, and that's how they know what the compositions are. In 2018, we're a little more earthbound. So how do exoplanet scientists actually learn about the composition of planets now? Right. I would love to be able to go and travel to an exoplanet, but we can't do that at least not yet. And so one of the fundamental measurements that we used to guesstimate the composition of exoplanets is their density. And so the components of this are the mass of the planet and the radius. And with that, and assuming it's a sphere, you can get the bulk density of the planet and match those measurements to theoretical compositions, potential compositions of the planet and know whether it seems more to be iron-rich or is more like earth where it has a mix of rocks and iron. The hard part is that measuring the radii of small planets, we have gotten sort of good at that. But the masses are still really hard to measure, especially for stars that are farther away and thus fainter. And so right now, the population of planets that we have the measured bulk densities of is actually still pretty small. Actually, one of the things that they do in this show is because they do get to stand in the system and fly around, it's more like how we investigate our own solar system than the way that we have to be more creative in understanding what things are made of around other stars. But in the story, one of the things that they talk about is how once they get into orbit around the planet, the cast is able to tell you what the mass is. And so this is sort of getting at the idea that it's really hard to figure out what a mass of a planet is without seeing how it affects other objects around it. So if we just see a planet around another star, we might not be able to make that measurement easily or directly. But if you're actually there in the system, you can go in orbit, then that's how you would make that measurement. So they have a much easier time at figuring out what the planet density is. But what about looking to the future? What are you really looking forward to in terms of what you're gonna be able to learn about the composition of these planets and maybe their habitability? Absolutely, yeah. So one of the main missions that has helped detect a lot of exoplanets is the Kepler mission that was staring at one patch of the sky and looking for signs of planets around stars. But that type of measurement gives you the radius of the planet, whereas measuring the mass, the instruments that do that are all based on the ground. And so it's harder to get that type of measurement for a lot of planets. One thing I'm excited about actually is that there's a new NASA mission called the Transiting Exoplanet Survey Satellite or TESS and combined with that mission, which will be serving closer by stars with another instrument that NASA's building for one of these ground-based telescopes, then we should get a larger population of small planets that have radii and masses measured than we can know their densities. So water is something that comes up in the episode and that's obviously important for humans if we want to go live on a planet. Can exoplanet scientists detect water on exoplanets now? Right, so I talked a little bit about measuring the bulk density of a planet and from that you can say it might have this percentage water somewhere on it. But what we're really interested in is surface water or water maybe in the atmosphere. And so there are a couple of different ways that we can do that. One that's been used, at least so far, more on giant planets is what we call Transmission Spectroscopy, which is if you're the telescope, the star is here and the planet comes between you and the star. The light from the star filters through the exoplanet atmosphere and in that process a fingerprint of the composition of the planet's atmosphere is left on that light. And so that's one way that we can know whether there's water or methane or sulfur in the atmosphere of that planet. The other technique which future NASA telescopes will hopefully be able to do is measuring the light that comes off of the star and is reflected from the planet. Planets themselves don't radiate enough light for us to observe, but they get the light from their star and we can observe that and know then whether it might have surface liquid water. This actually came up with the script too because in a very early draft that I circulated I made reference to, I believe Kara made a comment that the telescopes were looking at the light from the planet and detected water but they didn't yet know that it was ice and one of our exoplanets scientists caught that and said, by the way, if you're looking at scattered light from the surface you would already know that that was in crystalline form because it would have a distinctive signature. So we changed the dialogue to reference looking at the atmosphere of the planet. They picked up the vapor in the atmosphere but they don't get to see it until they get up close where they actually find the crystalline form. We wrap things up in our next video by talking about the kinds of life scientists want to look for on other worlds. Spoiler alert, it's not just human life.