Planet Hunting and the Origins of Life

GEORGE RICKER: My name is George Ricker. I was an undergraduate at MIT, and then I did a master's degree in astronomy at Yale. And then, I came back to MIT and finished my PhD work in a group in high energy astrophysics here. Currently, I'm a senior research scientist in MIT's Kavli Institute for Astrophysics and Space Research.

I've always been interested in astronomy, from the time that I was in the third grade. I've built my own telescope at home. I grew up in Florida, and so there were lots of clear nights, and people who are also interested in observing the stars. And so it was a natural transition, that when I did end up at MIT, that I took a lot of courses. Not only in physics, but I also took some courses in astronomy, and ended up saying, well, this is really what I want to do for the rest of my life.

One of the things that characterizes MKI's work, as we call him at the Kavli Institute, is the way in which students, both undergraduates, grad students, and also postdoctoral students, are involved in research. And for example, among the undergraduates, there's a special program called UROP-- the Undergraduate Research Opportunities Program-- in which students can actually either work for credit, or actually receive stipends for doing research in our lab.

Many students are involved in this. This summer there's something like 25 or 30 such students. I actually serve as the coordinator for the UROP program at MKI, and that's one of the most rewarding aspects of the work that I do here.

The other way in which outreach occurs is that, there's a large number of students who come to MIT from other universities and other countries. In our research group, currently about a third of the participants are from countries outside the US. But also in our group, more than half of the participants in the research programs involved in looking for exoplanets are actually women students, and they're both undergraduates, graduate students, and postdocs.

GEORGE RICKER: The MIT Kavli Institute was actually founded back in the 1960s, and the person who was actually responsible for this building being constructed was actually James Webb-- which the telescope is named after as well. And the idea from the very beginning was that, what was going to be unique for MIT-- this was back when the space race was on, and there was an opportunity to think about doing astronomy and astrophysics, and solar system astronomy for that matter, in space.

And a lot of that involved new types of telescopes and detectors, that there was a lot of interest at MIT in developing. And so, when I actually did my senior thesis in the group that-- at that time the name was different. It was called the Center for Space Research. And the idea was to actually develop instruments that could be perfected in the laboratories here at MIT, and then launched by NASA.

And then small satellites. And that was one of the things that our group did first. The small satellites were things that I was involved in fairly early on, in the 1990s, when we did some collaborative programs with the Institute for Space and Astronautical Science in Japan. My research group flew the very first photon-counting imaging X-ray telescope in space.

And then we went on to build increasingly sophisticated and more sensitive detectors that were developed for other space missions, including the Chandra X-ray Observatory, which came out of our research group here. And then, we also built a very small satellite. It was only about a little over 100 kilograms in mass, that we actually fabricated entirely in our lab, in this very building.

And then launched it into an equatorial orbit from the Marshall Islands in the Western Pacific. And then used that to establish the locations and brightness of some of the very first detectable short gamma ray bursts, which are a-- which we later learned were the type of object that's also responsible for gravitational radiation events-- that MIT's LIGO program has actually been very successful, and is now world famous for the results that they've had on gravitational radiation detection.

And these objects, it turned out, were pairs of neutron stars. And when the neutron stars would coalesce, as they would after a few billion years, the end result would be that they would-- when they would fall together, the energy that came out would be a flash of electromagnetic radiation in the X-ray band and the optical band, but also gravitational radiation. And that was-- and little did we know, when we started detecting these objects in their X-ray emission and gamma ray emission back in the early 2000s, that in 2017 they would turn out to be the precursors for gravitational radiation. And that was established by the LIGO group here at MIT.

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GEORGE RICKER: The basic idea for the test satellite, the Transiting Exoplanet Survey Satellite, came about in the period from around 2006 to 2008. And at that time, there were only a handful of exoplanets that were known. And we thought, in our group, about, how would you go about building an entire survey of the whole sky and find all the ones that are relatively close by?

Because the studies that had been done at that time, going back to the mid-1990s, had found a handful of what are called hot Jupiters. These are very large planets-- Jupiter size-- that were orbiting very close to their host star. And that discovery was the subject of a Nobel Prize later on, as the first planets being discovered.

And so one of the things that NASA had done, at that point, was, they had provided support for something called the Kepler satellite. And the Kepler satellite would actually stare at one small part of the sky for three or four years and make a census for that small portion of the sky. So the portion of the sky that they would look at would be about 1% of the sky. That's about right. And that was the plan that was made. And that satellite was supposed to launch around 2010 or 2011, at that point.

So the idea that we had, in our group, is, we had had experience building very wide-field cameras for this small satellite called [INAUDIBLE] that we had developed in the mid-1990s. And the basic idea there was, in order to determine what the origin of gamma-ray bursts were-- and these were-- later turned out, as I mentioned before, that they were coalescing neutron star systems-- it was necessary to look, basically, at the whole sky.

Well, the whole sky is about 40,000 square degrees. And so the challenge was, how do you do that? How do you manage to look at a big piece of sky? Because for the goals that we had for that small satellite, which weighed only about 100 kilograms, we had to figure out ways to pack cameras in it that would look at a variety of different wavelengths and then also provide localization so that follow-up of those events could occur.

Well, the thing that we knew is that if we could figure out a way to do this for extrasolar planets, and we basically were to look around the entire sky, looking at 40,000 square degrees, that would mean that we could find many thousands of such extrasolar planets, because we had an inkling that there probably-- because from the small number of stars that had been searched, we thought that exoplanets were really quite common. That was our hunch, and we wanted to actually prove that that was the case.

The other thing that we also knew was that if these planets were as common as we thought they would be, and we looked around and looked at all of the bright stars in the solar neighborhood, that would mean that, in general, the planets that we would be seeing would be closer, because the brightest stars that you see in the night sky are generally closer than the faint stars are. It's just as you would expect.

So anyway, we worked out a method that would allow us to-- every time we would look at the sky, we would be able to take images of about 5% of the entire night sky. And then we would basically figure out a way to mosaic the sky in a relatively short time. This was the idea. And initially, we were going to use some of the same instrumentation that we had used on this earlier satellite, [INAUDIBLE].

But as we got into the project and thought about it for the next three or four years, we had support from the Kavli Foundation for this work, which we were very appreciative of. And then there were also some other private support that we got from, among other organizations, Google. So they were very interested in the problem, just in general terms, mainly out of just curiosity. But they provided some funding that helped us in this area.

GEORGE RICKER: TESS has an incredibly wide field of view. It actually covers about 2,300 square degrees of sky every time it takes an image. This is a band that's about 24 degrees wide by 96 degrees tall. And the fields of view are broken up into four cameras, each of which are 24 degrees on the side. So it's an absolutely enormous chunk of sky that we view at any given time.

And then because of the way the TESS orbit was configured, we basically step around the sky and do the sky as if it were slices from an orange. And the number of slices that are in this configuration of sky mapping that we come up to is 13 per year. And that's because there are 13 lunar months per year. And that relates to the way the satellite is synchronized with the moon.

So with that information, we can cover the entire sky, but first, the Northern hemisphere and then the Southern hemisphere in two years time. And then as a result of that, and the two full sky surveys that we've done to date, we've been able to observe more than a hundred million stars for the possibility that they have planets. And so-- and thus far, we've detected almost 6,000 candidates for planets, of which approximately 500 have had their masses established by spectroscopic measurements that are largely made on the ground, and also from space.

So that's the major yield of the satellite so far. And because every-- for every year and every revisit that we do to the sky, we're finding periods that are longer and longer because we've gotten more samples. We find many more stars that have planets and then as a result of that, we expect the number of planets per year, new planets that we find, to increase by about 1,500 per year as the mission goes on.

So it's 1,500 per year. So for the new mission, the new three year period, we're talking about another 6,000. And then if we continue operating from 2025 to 2028, that's another 6,000. And the satellite will actually continue at this rate probably for more than a decade. So it really is a census of the sky in a unique way that has never been possible before.

And then what we find from these planets is basically they are-- they provide the targets for missions like the Webb telescope that was just launched who can study either the masses of those new planets that we've seen. Because the transit observation only tells you how big the planet is in terms of its diameter. It does not tell you what its mass is. It does not tell you about its atmosphere.

But if you know where the planets actually are, and you know approximately where they are relative to their host star, then you can make in some cases theoretical estimates, but in many cases, definitive estimates of the properties of them, you can tell whether it's a water-- what we call a water world, or it's a rocky planet. And you can also as you look at different types of stars, you can see whether there are multiple planets around those stars.

For example, we found some systems that have as many as six planets orbiting around them. And basically, the search goes on for the systems that we've already found. And so this is one-- this is one of the key results from the TESS satellite. And it's been essential that the instruments be high reliability, and that they operate in a very stable way for years at a time. And that's one of the things that we're most proud of in the way that we built and operate the satellite.

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GEORGE RICKER: So for our group, the next new thing is something that we've been excited to start thinking about and putting together a team to explore in more detail. We have four cameras that are flying on the test satellite, but it turns out that we had to build a flight spare. And the flight spare is quite a nice camera, just as good as the ones that we're actually flying.

So one idea that we've been working with is to put this camera, along with another relatively small telescope-- the telescope itself would be about 45 centimeters in diameter, which is about the span that I'm showing with my hands-- and put these into a satellite that we would position at a different location in the solar system. TESS is in this orbit that takes it out to about 400,000 kilometers away from Earth.

There's another location in the Earth's orbit that-- it's called a Lagrangian point. Lagrange was a mathematician who worked out orbits and properties of orbits in the 1700s. And this point is called-- is the fifth point that he was able to actually establish. So it's called Earth-Sun L5. This location actually forms an equilateral right triangle with the Earth, the sun, and this location. So that means it's 150 million kilometers away from Earth, and it's also 150 million kilometers away from the sun.

But the thing that's really interesting about this point-- it's distant, but why would you want to go there? The reason that you would want to go there is that it's the darkest place that you could be in the solar system. And what that means is that, there are no sources-- there are no bright planets nearby. You're well away from the sun. The sun, of course, would illuminate the satellite, but without other reflecting surfaces-- you could always put a sunshade in place, and then that would provide the darkest place in which to view. In fact, the sky there is about-- it's more than-- it's about a million times-- between 100,000 and a million times darker than it is in the TESS [INAUDIBLE]

So, why this why is this interesting? This means that-- this is interesting not only because it's dark, and dark skies are always important for astronomy, but the other thing that it provides is that-- with the baseline between a small satellite at that location and TESS, you can look at objects that have very short durations, and basically do what physicists call coincidence measurements. If it's a flash from a supernova or some other event like that in the sky that lasts for a relatively brief time, and it's very faint, if you see it in two satellites at the same time, you know it's real. If you see a flash of some sort from just one satellite, it could be some sort of an instrumental anomaly, or cosmic ray, or something like that.

The other thing that this gives you, is it gives you a chance to basically do what astronomers call a parallax measurement for the outer solar system. And that is, if you think about what's happening as the Earth revolves around the sun, in one year's time, this location with this small satellite will also take a year to move around. But, the difference is there's going to be a baseline between the two of 150 million kilometers.

So if you look to the outer solar system, that means that you can actually, carefully, map out all of what are called trans-Neptunian objects. Which are remnants from the formation of the solar system. Of which we've discovered a few hundred, but we know that there are thousands of these objects that are out there-- and what their composition is, and their size distribution. A lot of these things we have no idea.

But more to the point, you can also look even further than those objects, which are typically a few hundred to about 1,000 times the distance from the Earth to the sun, you can really look out 10,000 times further than that to the outer solar system. And if you do that, you can see what we refer to as the Oort cloud, which is the realm of planets-- Or, sorry, the realm of comets, from which they come. And you can map out that area as well, or at least the inner portion of it.

So, those are two of the scientific areas that the satellite would allow us to operate with. And the satellite would be significantly smaller than TESS, so that's the idea that we're working on.

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