A conversation with Steve Howell, NASA Senior Research Scientist on astrophysics. For more information on the TRAPPIST-1 discovery, visit https://www.nasa.gov/feature/ames/kepler/know-thy-star-know-thy-planet

Transcript

Matthew C. Buffington (Host): Welcome to NASA in Silicon Valley, episode 9. Today’s episode is with Steve Howell, a Senior NASA Research Scientist who recently published a paper on TRAPPIST-1, a nearby star home to three Earth-sized planets, one of which is in the habitable zone. At just 40 light years away, the TRAPPIST-1 system is a hop, skip, and a jump away, cosmically speaking, of course. We discuss how Steve joined NASA, the various methods to hunt for exoplanets, and go into detail on his latest discovery. Here is Steve Howell.

[Music]

Host:Steve, welcome.

Steve Howell: Hi! Thanks.

Host: We always like to start it off—tell us a little about yourself and what brought you to Silicon Valley, what brought you to NASA.

Steve Howell: So, I’ve always wanted to be an astronomer since I was very young. I always looked up at the night sky and was interested in that: amateur astronomer as a teenager, built telescopes in my backyard, had the neighbors over, joined clubs, all that kind of stuff. I knew I was going to college right away. I didn’t really know what an astronomer was; I grew up in a tiny town with a pretty poor school district.

Host: I was going to say, did you grow up here in California?

Steve Howell: No, I grew up in western Pennsylvania in a coal mining, train town of 400 people.

Host: Yeah, I’m from Ohio, so I’m familiar. It’s Midwestern-ish.

Steve Howell: Midwestern-ish, yeah. And so I always just wanted to do astronomy and I eventually got through school, got my PhD at the University of Amsterdam in Europe.

Host: Wow, that’s a nice jump from Pennsylvania.

Steve Howell: Yeah, it was great. I couldn’t wait to get out. And I worked in lots of places, spent a lot of my time in Tucson, Arizona, working for the national observatories. I got involved with Kepler before it was Kepler, way back when it was called FRESIP, before the first proposal got accepted, and moved out here in 2009 and became the project scientist for Kepler.

Host: Really? So, did you just find like a posting online to apply for it, or was it through connections from other astronomers who were like “Hey, Steve, you gotta check out this.”

Steve Howell: I mean, we all know Kepler. Kepler does a lot of things, but it finds planets, in particular. That was its goal.

Host: It’s an exoplanet hunter.

Steve Howell: An exoplanet hunter and it does that by very precise measurements of the light. And, so, in the early days when Bill Borucki, the Ames PI, was putting together the FRESIP mission—”Frequency of Earth-Sized Inner Planets”, I think is what it stood for…

Host: There’s always fun acronyms. And “PI”: Bill Borucki, PI, stands for…

Steve Howell: “Principal investigator”, right, of the Kepler mission, who had the idea way back. He called me up one day, because I’ve been doing some very precise measurements of light, but with telescopes on the ground, with instruments that I had built, and said, “Hey, why don’t you come out and talk to a bunch of us at a meeting about how we can do really precise photometry.” And I came out and learned about Kepler and said, “Wow, this is going to be the coolest thing ever. Let’s go!”

Host: Oh, so you just packed up from Arizona, came out here…

Steve Howell: Well, I worked sort of half my time for about six or seven years and then finally got a job here at NASA to be the project scientist of the mission. It was great.

Host: So, how many people are working just on that, that work with you?

Steve Howell: Kepler, I think, in its heyday, like when we first launched and we were really actively getting data with the original Kepler mission, there was maybe 60 people, 80 people here at Ames and people outside of Ames that ran the spacecraft and knew about it at Ball Aerospace, for example, and people that communicated with it every week. Yeah, so it’s quite a group. Now, the K2 mission has been running now for two years. That’s a much smaller operation we have less, sort of, science to do—the community does most of our science—so maybe now it’s 25 people.

Host: How’s that split up? Obviously, everybody’s kind of got their own widget, their only aspect of what they’re looking at. So what’s your core thing?

Steve Howell: Yeah, so, until recently I was what’s called the project scientist. So, it’s kind of the person who looks at the big picture of science for the mission; doesn’t really direct all the scientists who work for you but kind of, you know, lays out a future plan and what goals we should reach and makes sure the community is happy with our data and we’re doing the right science. Things like that.

Host: Yeah, it’s an interesting community. You’re feeding info to the astronomy community and then they feed you guys back: “This is what we’re finding.”

Steve Howell: Yeah, they do a lot of science, they write papers, they make us famous. So it’s great.

Host: Excellent. There’s so much data, it’s almost too much for anybody to take in all at once.

Steve Howell: It is.

Host: Even long after a group of data has been acquired, people are still writing stuff.

Steve Howell: Oh yeah, it’ll be decades. We put everything in, every piece of information we take goes into an archive and the public—mostly the professional community, but not entirely—can get that data and do whatever they want with it. Some people find it exciting to work on one star in great detail and other people find it exciting to work on “fifty thousand stars in some big sample”-type of science.

Host: This is great, you being there from the beginning of it or early on when it first launched. What were your expectations going into it? A lot of excitement, but were there things that surprised you?

Steve Howell: Well, you know, there were a lot of things. I mean, it was certainly very exciting. The launch was great. We were all very excited. The fact that we were going to do something that no other mission had done, that’s very great. And the first data came in and you’re kind of scared. You’re like “Yay, it worked!”, but then you’re kind of scared because you were using simulated data and that always works. And then you get real data and it’s like, “Huh. It’s not quite what we expected.” A lot of great software programmers that we had working for us did a lot of hard work to figure out how to turn the real data into stuff that worked like the simulated data. Then you start finding dips that you think might be planets and you validate those and all of a sudden you’re excited that “Wow! The mission’s really doing stuff! It’s really doing what we thought it would do!”

Host: And then, of all the different ways to find exoplanets—I remember going through that with some of your colleagues—what is the main focus for Kepler? Is it mainly the transit method?

Steve Howell: It’s the transit method. You wait for the planet that’s orbiting the star. It passes directly in front of the star, along your line of sight…

Host: That’s if we’re fortunate enough!

Steve Howell: That’s right, if we’re fortunate enough. It’s roughly one percent of all the possible orbits that will do that. If you find one planet, there’s 99 you’ve missed that will be in other orbits. Or they could have really long orbital periods and the time you look at that star, they just haven’t made it around once.

Host: Yeah, because you figure Earth takes 365 days to get around. And that’s thinking that they may be on our orbital plane. Because maybe there’s a planet, but it’s going around it, up and down, and we could never see it.

Steve Howell: That’s right and we certainly believe that’s true. There’s no reason to expect everything’s lined up with us.

Host: Ok, and then going through it, from when you guys find and can confirm this is an exoplanet, how do you then find out what are those properties of those planets, or get estimates of whether they’re a big hot Jupiter or are they rocky like Earth? How do you figure that out?

Steve Howell: So, the direct information we can get is we can measure the size of the planet. It’s pretty simple: if the little dip you see in the light is deeper, the planet’s bigger. If the dip is shallower, the planet’s smaller. It’s pretty straightforward. And then we can tell how often we see the dip, so we assume that’s every time it orbits the star. So, we can tell the orbital period. We can use Kepler’s Laws. Kepler, the old Kepler guy, not the new Kepler mission!

Host: And not the big telescope!

Steve Howell: Right, he did the laws of planetary motion, which is why we named it after him, and you can use those to tell how far away the planet orbits from its star. And then you make assumptions about how much energy that planet might receive on its surface. For example, in our solar system we have a very interesting pair of planets: we have the Earth and Venus. They’re roughly the same size, they’re roughly the same mass, but they’re very different. Venus has this thick carbon dioxide atmosphere. It wouldn’t be very good for us to live on at all. In fact, we’d all die. It’s very hot, high pressure; yet the Earth is a beautiful place. And, so, just because you see this planet and you know its size doesn’t mean you can tell if it’s Earth or Venus. You can only make an assumption.

Host:…or Mars, for that matter.

Steve Howell: Or Mars, that’s right. You don’t know anything about its atmosphere, you just say, you know, if it’s this distance and it has an atmosphere, it might have this temperature.

Host: And I know you’re working on something called TRAPPIST. Tell me a little about that.

Steve Howell: So, TRAPPIST-1 was a recently—

Host: And TRAPPIST is an acronym, right?

Steve Howell: Of course it’s an acronym. It’s NASA. Actually, TRAPPIST wasn’t developed by NASA, but everybody in astronomy loves acronyms. It’s the name of instrumentation a telescope in South America that the Europeans built. So, the very first star that they looked at where they discovered planets is now called TRAPPIST-1, the first one. And it turns out it’s a very tiny, faint star that has three planets known, so far, to orbit around it. All the planets are very small—the size of the earth—and one of those planets, the farthest one out, orbits in the so-called habitable zone.

Host: The Goldilocks zone.

Steve Howell: That’s right. So, that’s very exciting for us. This star also is very close by, in astronomical terms. It’s only about 30 light-years away. In astronomy, that’s pretty close. That’s your neighbor in space.

Host: Not quite as close as Alpha Centauri.

Steve Howell: Right, not as close as Alpha Centauri.

Host: It’s not the closest, but compared to all the other stars we can see…

Steve Howell: Right, it’s very, very close. So, the big advantage, while it’s not as close as Proxima Centauri, is that the planets transit. That means they pass in front of their star periodically, so they’re very good candidates to measure atmospheres of the planets.

Host: So, how do you go about finding out if it has an atmosphere, from that transit method?

Steve Howell: From the transit method. Again, everything we do in astrophysics is simple, it just sounds complicated. You learn big words and you sound fancy.

Host: Impress people at parties.

Steve Howell: That’s right. So, you look at the star when the planet is not in front of it, and then you look at the star when the planet is in front of it. And the difference between those two measurements is light that actually passes through the planet’s atmosphere on its way to you. And the planet’s atmosphere will absorb some of that light. If you can measure that tiny amount that it absorbs, you can tell what’s in the atmosphere of that planet.

Host: Just walk us through what exactly do we know that we didn’t know before?

Steve Howell: Sure, so TRAPPIST-1 is, I think probably of the most interesting exoplanet host star that has been found.

Host: Not that you’re biased.

Steve Howell: Not that I’m biased, yes, not at all. And the “1” means it is the first exoplanet they found. The first star with exoplanet they found in their survey of very low mass stars. And the point of looking at low mass stars, low mass stars are small and if you have a small star you can find smaller planets easier by the transit method.

So, this is a very interesting object, it is only twelve parsecs away, 36 light years away. In astronomy that is next door, so not as close as Proxima Centari, which is just four light-years. But here, this system has three planets, they are all roughly the size of the Earth and they transit the star, unlike the Proxima Centari planet.

Host: So the telescope basically sensed three different dips?

Steve Howell: Yeah, three different dips.

Host: And how can you tell that they are all Earth like planets?

Steve Howell: You can tell… be careful, Earth sized.

Host: Oh, Earth sized planets. There is a big difference there.

Steve Howell: That’s right, but one of them is in the habitable zone, and therefore it could be Earth like.

Host: You can’t rule it out. I have not yet visited, so… we don’t know.

Steve Howell: Yeah, we are going next week, by the way. Don’t tell anybody. Anyways, so, you can tell by the dip. How deep it is tells you the size of the planet and they know that these planets are all about the size of the Earth. And one of them, as I mentioned, is orbiting in the habitable zone with a very short orbital period, like seventeen days. So the star is very low in luminosity and so the planets can be very close to it and still be in the habitable zone.

Host: Did people already think that there were exoplanets around this star, is this like a follow-up thing…

Steve Howell: So the discovery was looking… they are just looking at a bunch of these very small, low mass stars to see if they can find planets. And they found one. So what we did with our observations is… we know from work we’ve done for the past few years that many stars have binary star companions, have another star companion. So if you have two stars, instead of just one, then the amount of light you are getting is really more than you think you are getting because it is from two stars instead of one star. And the little dip you see from the transit tells you the radius of the planet, but the radius is then diluted and so the planets are bigger than you really think they are.

So if this object had been a binary, these planets instead of being Earth sized, might have been super Earths or Neptune size planets. And then they are still interesting but not as interesting. And so our research has shown that there is nothing else in this system, it is clearly a single star.

Host: With this discovery, what are the next steps? Are there more follow ups that have to happen?

Steve Howell: So the next step, this system will get a lot of study by people I think because of the small planets in the habitable zone, very close. So the next steps for us are going to be that now that we are sort of interested in the very low luminosity stars that might have small planets orbiting them, and we know people are looking for them. We are going to start making a concerted effort to go out and getting really high resolution images to see how many of them are binaries and how many aren’t, because we don’t know what the population is.

Host: The coolest part about the work is you are taking telescopes, land telescopes, all of this data. Combining it, and learning things that you never knew before, but how did this all kind of line up into your trajectory?

Steve Howell: Some of it we can thank, is just brilliant planning, but it also could just be we were really lucky, so this…

Host: A little bit of both.

Steve Howell: A little bit of both. We had observing time on the telescope in South America, the Gemini South Telescope, we were going down there anyways for a 15 night run of observing host stars or planets. And a few weeks or months before we went down, the TRAPPIST group announced this discovery, and I said, “Hey we can look at that star. We are there, let’s do it”. So we added it to our list of stars and it turned out to be a great result.

Host: I mean I’m guessing your work is all exoplanets, all day, every day. Are there other NASA things that kind of like pique your interest?

Steve Howell: Sure! I actually came into Kepler with a non-exoplanet background. I worked on building instruments for telescopes and did a lot of work on variable stars, but exoplanets were very exciting and I do a lot of work on that, as you might imagine. And we have the greatest dataset ever – why wouldn’t you? And when we moved on to the K2 mission—this is the mission after Kepler had a few reaction wheel failures…

Host: Yeah, this is one of the best stories of NASA ingenuity, in figuring out “Well, we have a million-dollar thing already in space. How can we make it work?”

Steve Howell: So, we now look at the ecliptic; we look at young stars and old stars; we look at galaxies; we look at comets in the solar system; we look at clusters of stars…

All of that and, in addition, we also are looking for planets, as well. And so we’re very successful at all of those, but I get interested in all sorts of science. It’s kind of: What fun stuff can you do with these great datasets?

Host: Yeah, I remember—tell me if I’m correct—for Kepler, it’s almost like if you hold your hand out into the sky, your hand covers roughly about the patch of sky that Kepler was looking at.

Steve Howell: Yeah, that’s right.

Host: It’s just doing a survey, just gathering as much information from that patch.

Steve Howell: It stared at one patch of sky, looked at about a hundred and sixty thousand stars for four years. Never blinking, basically. And now K2 can only look for about 80 days at a patch of the sky, still that same size, but we look at many, many patches of the sky.

Host: Cause it is almost like going through a straight line through the sky instead of focusing on just one. We talked about that transit method, and I know some of the ground-based telescopes are looking for that Doppler effect or the wobble. The one that I find completely fascinating is the microlensing. So, tell people who may not be familiar with microlensing a little bit how that works.

Steve Howell: Sure. Microlensing is certainly really cool to talk about, because it involves relativity. So, it’s kind of a simple idea—and, again, everything we do is simple! It has lensing involved in it, so it’s kind of like a magnifying glass. If you have a faraway star that you can’t see, but it turns out that between you, on the Earth, and that faraway star, another star passes directly along the line of sight—right in front of it, but they’re not anywhere near each other in space and you can’t see that star either, but the background star will get a lot brighter for a very brief period of time. It will be magnified by the mass of that in-between star.

Host: The gravity literally warps that…

Steve Howell: It warps space and it magnifies the star.

Host: So, looking and studying that warp…

Steve Howell: Right so if you stare at a field like the center of our galaxy that has millions and millions and millions of stars in a small region of the sky, all overlapping each other, your chances of seeing stars in the foreground pass in front of stars in the background is higher. So, everybody on the ground that looks for microlensing events stares at the center of the galaxy and they find microlens events quite often, actually.

Some of those events are just a single star passing in front of another single star, but some of those events are: A star that has a planet orbiting it passes in front of another star and you get a magnification of not only the star, but also the planet. And, so, in this measurement of the light output, you can detect planets – large planets in long-period orbits, like Jupiter and Saturn – that orbit stars that you could never detect in any other way. So, it’s really a great thing and it works very well and it also samples the outer parts of solar systems, compared to Kepler that sampled the inner parts of solar systems. So, it’s a nice way to know about big planets in outer solar systems.

Host: Oh, awesome.

Steve Howell: We just finished a campaign with K2 where we, the telescope, stared at the center of the galaxy. Twenty telescopes on the earth stared at the center of the galaxy. They were looking at the same microlensing events at the same time. They each recorded a slightly different brightening pattern, which was to be expected. And then when you combine those two, you can not only tell where the object was and the planet orbiting it—how far away it was—you can tell the mass of the planet by combining these two datasets. So, it’s really pretty cool. So, that just happened in the last few months and those results will be coming out probably this fall.

Host: Excellent. So, for people who are super into exoplanets I’m guessing the best place to go is nasa.gov.

Steve Howell: Go to nasa.gov. Look for press releases that come out. I might go to the exoplanet archive and look for planets that are listed there. Go to the K2 Science Center website. You’ll find all the information.

Host: Excellent, and so for anybody else who’s listening, if you have any questions for Steve, we can go back to him. We are @NASAAmes. You can also check out @NASAKepler. And we’re using the hashtag #NASASilicon Valley. Thanks so much for coming over, Steve.

Steve Howell: Sure! Thanks a bunch. It’s been fun.