Hosts
Derek Thompson
Derek ThompsonAbout the episode
Last week, a team of astrophysicists from the University of Cambridge announced that they had discovered the “strongest indication” ever of extraterrestrial life. The source did not come from Mars or Venus or any nearby moon. It came from K2-18b, a massive planet some 120 light-years from Earth.
If this finding checks out, it is, without question, one of the most important discoveries in the history of science. But many scientists think that … well, it might not check out at all.
Today’s guest is Sara Seager, a celebrated astrophysicist at MIT. Seager is a pioneer in the study of exoplanets and their atmospheres. She has done as much as practically anybody to develop the science of interpreting light from faraway stars to make inferences about planets. In today’s show, Seager and I slowly worked our way up to last week’s announcement by building a foundation of the basic science at play. What are exoplanets? How do we know that they’re there? How do we have any idea about the chemicals present on that planet if we can’t send probes to test their air? What does the K2-18b finding really tell us? And what larger philosophical questions about life and aliens are raised by this new science of exoplanet atmospheres?
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Summary
In the following excerpt, Derek talks to Sara Seager about exoplanets and how we’ve been able to detect them.
Derek Thompson: What are exoplanets, and how do we know they’re out there?
Sara Seager: Well, exoplanets are planets outside of our solar system. Usually, we like to define them as a planet orbiting a star other than the sun, because all those stars are suns out there. And if our sun has planets—Mercury, Venus, Earth, Mars, et cetera—it makes sense that other stars, these other suns, have planets also. And they do.
Thompson: And how do we know that those exoplanets exist? Can we see them directly with telescopes, or are we somehow detecting them indirectly?
Seager: Both, actually. I mean, honestly, we have many techniques to find exoplanets. The most productive ones so far are looking at them indirectly. So, for example, the most popular one today is we’ll see a planet that is orbiting in a lucky way, like it’s orbiting just so that it goes in front of the star as seen from our viewpoint, over and over again every orbit. And while we don’t spatially resolve the star, we only see the star as a point of light, we can monitor the brightness of that star, and you know it’ll dim by the tiniest amount when the planet goes in front of the star. And when the planet is finished going in front of the star, the star returns to its normal brightness.
Thompson: There is a lot packed into that answer that I want to slow down for. You mentioned that we don’t see many exoplanets directly. We pick up their presence indirectly, and I want to get to that in a second. But first I’ve read that the science of detecting exoplanets is very, very new, barely 30 years old. In the 1990s, when you were starting your career, many people were skeptical that we could discover exoplanets. Why?
Seager: Without question, everyone understood that planets exist because we never just find one thing in science, like the fact that our sun has planets. But the evidence that there should be other planets out there were stars that are being born. They are all, without question, accompanied by a disk of leftover garbage, if you will, like dust and gas. And we saw those for a very long time, and they have to be forming planets. It’s like you have to have a dust bunny under your couch at some point in your life. It wants to form. They just want to form. So it wasn’t that people doubted there were other planets around other stars. It was just that the level of number of decimal places you would have to go, like the precision of a measurement you’d have to make to find them, just seemed laughable. So that’s why it was so crazy.
And then people were searching for solar system copies. In science, it’s never good when you only have one example. We had our solar system; we’ve known about that for millennia. But, wow, searching for Jupiter, it is far from our star. It is very hard to find. It has an incredibly weak signal. So the reason for all the skepticism was when the first exoplanets were found, they were found orbiting sunlike stars, but they were the easy ones to find: big planets very close to their star, Jupiter-mass planets, many times closer to their star than Mercury is to our sun.
And remember when I told you we’ve seen disks of gas and dust that surely are forming planets? Well, that close to the star, there’s not enough material to form a massive planet. So people couldn’t accept it. They couldn’t accept these planets were real and tried to explain it away with other phenomena having to do with the star itself. So there was definitely a lot of skepticism early on, and that skepticism continued. I mean, first you had people saying, “Well, we can’t detect them.” Then you had people saying, “Well, we’ve detected something. It’s probably not a planet.” Then you had people saying, “OK, fine, we’ve detected planets, but it’s going to peter out as we find the easy ones and dead-end before we get to the harder ones to find.”
Thompson: Is there a certain type of exoplanet that is most extreme, that would seem most alien to earthlings? Like if we managed to get a rolling camera shot of several types of exoplanets, what planet type would be the most strikingly bizarre or sublime if you could set up a GoPro there and just capture hours and hours of footage provided? Of course, the GoPro didn’t melt immediately upon contact with high temperatures or sulfuric acid or whatever, but how crazy do we think some of these exoplanets are compared to the familiar neighborhood of solar system planets that we have here near to Planet Earth?
Seager: Honestly, they are probably more crazy than we could possibly imagine. But so far, believe it or not, we don’t know if all the planets out there will fall into neat categories. Let’s say we fast-forward 20 or 30 years and, hey, there’s 30 categories of planets. Or is there truly a continuum where there are literally thousands of different kinds of planet types? We don’t know.
And the other point I want to get across before answering your question is it’s probably the opposite, that they’re all crazy and exotic, I mean, compared to our planet. Because what’s amazing is that we find exoplanets in all masses, all sizes, all orbits within the laws of physics. There’s literally a continuum of planet types. It’s like, we have adults and we’re all a certain size and there’s a distribution, and maybe there are definitely outliers, but it seems more extreme than that. There’s not this average. There’s just a continuum of options. But I could still pick a couple for you—
Thompson: Please.
Seager: There are planets so close to the star. There are some, we call them super-Earths. They’re planets about Earth’s size or larger, and they are so close to their star, based on Kepler’s third law, they also orbit the star very quickly. So their year, the time it takes to go around the star, is less than half of an Earth day. And these planets are so hot, we think their surfaces are melted. So you’d have lakes or oceans of liquid rock, not from volcanoes, but just because of the heat from the star.
This excerpt has been edited and condensed.
Host: Derek Thompson
Guest: Sara Seager
Producer: Devon Baroldi