Universe of Learning 2021-03-11 Exploring Exoplanets Part 2 of 2 Page 1
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Universe of Learning 2021-03-11 Exploring Exoplanets Part 2 of 2 page 1 Dr. Quyen Hart 0:03 Thank you for walking us through all the challenges and difficulties in finding Earth, mass planets and Earth sized planets around Sun like stars. And we're gonna hold off questions until after the last speaker. So, let's welcome our last speaker, Dr. Nikole Lewis. Dr. Lewis is an assistant professor of astronomy at Cornell University and Deputy Director of the Carl Sagan Institute. Dr. Lewis is involved with dozens of observational campaigns with the former Spitzer, current Hubble and future James Webb Space Telescope, that aim to determine the nature of exoplanet atmospheres. She developed atmospheric models that are necessary for the interpretation of current exoplanet observations and for planning exoplanet observations with future facilities. Before joining the faculty at Cornell, she served as the James Webb Space Telescope Project Scientist at the Space Telescope Science Institute, and was a Carl Sagan Postdoc Fellow at MIT. Dr. Lewis, take it away whenever you're ready. Dr. Nikole Lewis 1:05 Thank you, thank you, Dr. Dressing and Dr. Ciardi for really setting this up for me. So now you have all the details you need on how we find planets. And then that's where I come in. So, I am not a planet Hunter. I ask the question, "What is in the air of the planets that we find, or do they have air?" As mentioned in my introduction, I primarily do that with NASA facilities. The Spitzer and the Hubble Space Telescopes have really been the workhorse for studying the atmospheres of exoplanets to date. And it's important to remember that neither of those telescopes were actually designed with that type of science in mind. They were designed before we even really knew that exoplanets existed. So, it's been amazing that we've been able to study planets beyond our solar system in detail with these facilities. And now we're looking to the future with the James Webb Space Telescope to do even more. Next slide, please. Dr. Nikole Lewis 2:01 Alright, so I'm going to focus mostly here on the primary way in which the Spitzer and Hubble Space Telescopes study the atmospheres of transiting exoplanets. There's also a primary way in which the James Webb Space Telescope will study the atmospheres of exoplanets. And that actually leverages one of the methods that was previously talked about, and that's the transit method. And so, when planets are detected via the transit method, you see a dip in their light as they pass in front of their host star, which tells you something about the size of that planet. But it turns out that during that process, the planet, if it has an atmosphere, it's actually filtering the starlight through that atmosphere. And so, what we're able to do is to look at how that planet's atmosphere affects the starlight. Next slide, please. Dr. Nikole Lewis 2:58 What this looks like in process is that we actually observe the planet at multiple wavelengths of light. And so, if you take a star light and you spread it out over a rainbow. So, you look at the sun and you use a prism, say, to spread that light over a rainbow of colors. If you were just looking at the star, you see basically just a plain old rainbow. But because we have that planet, and it has an atmosphere in front of Universe of Learning 2021-03-11 Exploring Exoplanets Part 2 of 2 page 2 it, what we'll actually see is that the planet appears larger at certain wavelengths of light. So, in this case, the diagram that we have here is a planet that has both sodium and potassium in its atmosphere. And so, at the green and yellow wavelengths of light, the planet actually appears bigger, because it's actually absorbing more light at those wavelengths, because those particular chemical species have basically absorbed light leaving their fingerprints on that spectrum. And so, what we do is instead of just focusing on measuring the rays of the planet over a broad range of wavelengths, so maybe over the entire spectrum of the rainbow, we will get measuring the radius of the planet as a function of wavelength. And that tells us something about the composition of the planetary atmosphere. Next slide, please. Dr. Nikole Lewis 4:07 And so, using both Hubble and Spitzer to date, we've been able to actually learn a huge amount about what the air is like in planets beyond our solar system. Now we've primarily done this for what we call hot Jupiters. So, these are Jupiter sized planets that orbit very, very closely to their host stars, just like 51 Peg b, the first exoplanet around a Sun-like star was discovered to do. And so usually they're going around their stars every few days. They're pretty hot. We expect that we'll be able to feel gases in their atmospheres. And in fact, using Hubble and Spitzer we've been able to see water. We've been able to measure things like carbon monoxide, titanium oxide, which is in a lot of people's favorite sunscreens. Methane, helium, potassium, sodium and a whole host of other species. We've also been able to see signatures of both clouds, but not like the water clouds that we have here on Earth. No, these clouds are made of common rock species, things like iron and silicates and other materials that we might expect, say, in a lava-type field. We've also seen indications of things like hazes, or just small particles that might be formed through different processes such as photochemistry. Next slide, please. Dr. Nikole Lewis 5:24 Now, that was what we've seen, mostly for these giant planets. Again, they're fairly easy to detect. Much like how we saw on the other talks, it's pretty easy to detect the Jupiter sized planet, either using a variety of techniques. And so those tend to be the lowest hanging fruit. But we've also used both Hubble and Spitzer to look at some of the first Earth-sized worlds in the quote unquote, "habitable zones" of their stars. So, because planets actually orbit smaller stars, their habitable zones are closer. And perhaps the most famous one you've already heard about is the TRAPPIST-1 system. And Hubble has actually taken a look at the atmospheres of all the planets in the TRAPPIST-1 system. And what we found from those observations is that we know that they don't have a substantial amount of gases like helium and hydrogen in their atmosphere. So, we know that they must, in fact, have what we call secondary atmospheres, atmospheres that formed well after the planet initially formed, just like the terrestrial planets and our own system. We've also looked at worlds like K(epler) 218b, which is sort of a, what we call a sub Neptune, a planet slightly smaller than Neptune, but it is also in the habitable zone of its system. And we've been able to measure water in the atmosphere of this planet, and also measure the presence of clouds. And so, these are really, really important for stuff as we think about trying to find the signatures of other planets out there that look a lot like Earth, especially in terms of their climate. Next slide, please. Universe of Learning 2021-03-11 Exploring Exoplanets Part 2 of 2 page 3 Dr. Nikole Lewis 6:57 Now, most of that work was done, where we're watching the planet pass in front of the host star and watching that starlight get filtered through the planetary atmosphere. But that's not the only way that you can use facilities like Spitzer or Hubble and Webb, to look at exoplanet atmospheres. One thing you can do is to actually directly probe light coming from those planets as a function of time. And the way that this works is that for most of these planets, they tend to orbit in these few day long orbits, which means they're very, very close. And so, we can consider them to be what we call tidally locked, just like the moon is tidally locked to the earth, that means that they're always presenting the same space to their star. And so, what we can do is if we watch a planet through the entirety of its orbit, we can measure its phases, and see how the brightness of, say, the day side of the planet compares to the brightness of the night side of the planet. And what you're seeing on that diagram, in the slide on the left, is that the way that we measure this is when we're looking at a system, we're actually looking at light both from the planet and the star. But we assume that the starlight is fairly constant throughout the planetary orbit. And we look for variations in the overall light of the system to determine how bright the planet is as a function of time. And so, by making these types of measurements - go on to the movie - we're actually able to determine what the atmospheric circulation patterns or weather looks like on those planets. And so, we've actually been able to determine that the weather on exoplanets is pretty crazy, they have winds on order of kilometers per second.