TESS: Searching Closer to Home

IN THE ONGOING HUNT FOR PLANETS beyond our own solar system, spacecraft in coming years will focus their telescopes on the nearest stars – those in our immediate neighborhood of the Milky Way galaxy.

Studying the closest stars offers the best chances for studying Earth-size planets – simply because these smaller planets are so difficult to detect and characterize in the planetary systems of faraway stars.

The Transiting Exoplanet Survey Satellite, or TESS, could be a successor to today’s Kepler mission, which is compiling a statistical view of extrasolar systems by surveying 156,000 distant stars between 600 and 3,000 light years from Earth – all of which are roughly in the direction of the constellation, Cygnus.

TESS, by contrast, is being designed at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MKI) to conduct an all-sky survey of stars much closer to Earth. It would survey 400 times as much of the sky as any previous mission – looking for telltale dips in starlight as planets transit, or cross, in front of their home star as they orbit. TESS would have the ability to study the light from a variety of stars, including M stars, which are smaller and cooler than solar-type stars and much more abundant in the galaxy.

This Fall, NASA announced that the TESS project was one of five proposals selected as a potential “Explorer Mission,” and as a result it will receive $1 million to conduct an 11-month mission concept study. Low-cost Explorer missions are capped at $200 million each, excluding the launch vehicle. In 2013, NASA plans to select up to two Explorer Mission proposals for flight, and if selected TESS could launch as early as 2016.

Shortly after the announcement this Fall, The Kavli Foundation spoke with George Ricker, a senior research scientist at MKI, about how TESS would fit into the larger effort to detect and characterize exoplanets, its strategy for finding small Earth-like planets, and how the proposed mission is being designed to fly in tough budgetary times.

THE KAVLI FOUNDATION (TKF): The TESS mission, which will look for transiting planets orbiting stars beyond the solar system, will build on discoveries by today’s Kepler mission and pave the way for studies by the James Webb Space Telescope (JWST), which will have the capability to spectroscopically analyze the atmospheres of exoplanets. Can you explain broadly how TESS will fulfill this pioneering role, after Kepler and before JWST, to advance exoplanet studies?

GEORGE RICKER: The role of TESS will be to scout the sky for extrasolar planets that will be the best targets for detailed study in the future. And it will do that by searching the entire sky and by concentrating on the brightest stars. TESS’s search area will be about 400 times greater than the area Kepler is covering, and it will be able to seek out targets that are optimally placed for follow-up and also optimally bright for focused follow-up using optical and infrared telescopes, especially those that are being developed to do detailed spectroscopy and those optimized for long-term monitoring. The importance of finding the very best targets, for which the results will be most conclusive, is going to be extremely important because the follow-up observations are difficult and require a lot of telescope time.

TKF: So astronomers studying extrasolar planets will build on what TESS discovers by strategically following up the best targets. But how will TESS benefit from what Kepler is discovering?

RICKER: One of the things we've learned from Kepler is that multi-planetary systems like our solar system appear to be very common. A quarter of all the stars that we can see in the Kepler field could have planetary systems associated with them. So, you need to take that into account in any kind of overall search strategy. Now it turns out that one of the best ways of fully revealing multi-planet systems is to find situations in which a couple of the planets in the system actually transit, or cross in front of, their host star. As those planets orbit their host star, they interact gravitationally, pushing and pulling on one another so that they accelerate slightly or decelerate slightly as they orbit. Even smaller planets we cannot directly detect will alter the transit time of a larger planet very slightly. These subtle variations can be measured.

Measuring transit time variations is particularly important because if you find a large Neptune- or Saturn-sized planet that appears to be tugged a little bit, you can make an estimate of how massive the planet is that is actually doing the tugging. Measuring transit time variations, which is a technique enabled by Kepler data, is a bit like successively unnesting a Russian matryoshka doll – you go a few layers down and you can find smaller and smaller planets in the system. That's one of the things that doing transit time variations, during the TESS mission, will enable us to do.

TKF: So, TESS will be looking at a much closer population of stars than the Kepler mission. But it will also keep its eyes open for these kinds of multiplanet systems that Kepler has discovered are much more common than previously believed?

RICKER: That's right. So this methodology, which is called TTV, or Transit Time Variation, is a powerful new tool that astronomers have for studying extrasolar planets. In addition, by looking at very bright stars, TESS will greatly facilitate observations from ground-based telescopes to study these transit time variations. Ground-based telescopes so far have not been able to do this because the planets that Kepler has studied have largely been associated with stars that are too faint.

TKF: Astronomers have discovered only a few exoplanets that are anywhere close to the Earth in size and composition. Your team has estimated that TESS could detect as many as 2,700 planets, including several hundred Earth-size planets. What particular capabilities will the TESS mission have that will lead to such a huge cache of discoveries?

RICKER: There are a number of things that TESS will be able to do to exploit this treasure trove of Earth-size planets that we envision. First, TESS will be able to detect very subtle changes in the intensity of a star’s light as an Earth-sized planet passes in front of the star. These changes in light intensity will be incredibly small. Just to give you an idea of how small these effects are: the Earth passing in front of the sun, viewed from outside the solar system, would cause a drop in the light of the sun of about 85 parts in a million. TESS is being designed to detect a drop in light intensity that is even smaller – in the range of approximately 40 parts per million.

The other thing that TESS will do is that it will be particularly sensitive to stars that are smaller and cooler than the sun. The sun is a typical star, but in some sense it's not quite average. The sun is a little bit warmer and larger than many stars in the galaxy. Most stars are significantly smaller than the Sun, typically with radii only half to two thirds that of the sun. These are called M stars, and it turns out that they are 2 to 3 times as abundant as solar-type stars. Because these M stars are cooler, they emit a lot of light in the near infrared part of the spectrum.

TKF: TESS began in 2007 when NASA announced its Small Explorer (SMEX) satellite program. At a time when spaceflight budgets are as tight as they’ve ever been, small-to-mid-sized missions that squeeze the most research out of every dollar make a lot of sense. What are some of the design considerations your team is making to keep the cost of the spacecraft as low as possible while maximizing the science return?

RICKER: First, a little history about TESS. We actually started thinking about the mission as a kind of a follow-on to an MIT Kavli Institute for Astrophysics mission called HETE-2. The High Energy Transient Explorer studied gamma ray bursts and was completed in 2006. That was a modest mission that focused on a single problem, and it was carried out by a small group of dedicated scientists and engineers.

TESS also was developed around the idea of a small mission focused on a sharply defined problem. In 2006, we were fortunate to have funding from The Kavli Foundation and Google to begin developing the TESS concept. At the time, before the financial crash of 2008, we thought that we could actually do this as a private initiative. We came up with a concept, but it turned out that the resources that we required were slightly above what was plausible for private funding. And then NASA came out with the Small Explorer solicitation in 2007, and that looked like it was an appropriate target for us.

The philosophy of our group at MIT is to basically take ideas that already have been worked on in the commercial semi-conductor industry to make things very compact, lightweight and very, very simple. We then modify them so they can be used in space.

The CCD's themselves – those we actually manufacture here at the MIT Lincoln Lab. They are designed to be very efficient for photon detection, all the way up to a wavelength of about a micron. These detectors are actually a derivative of silicon CCDs that my group had developed previously for X-ray use. It turns out that the same thing that makes it possible for the detectors to be very effective at detecting near infrared also makes them good X-ray detectors. We've been flying these detectors on satellite missions since the early 1990s, and they are now flying on the Chandra X-ray Observatory, as well as a number of missions that we have flown in collaboration with our Japanese colleagues.

"The role of TESS will be to scout the sky for extrasolar planets that will be the best targets for detailed study in the future. And it will do that by searching the entire sky and by concentrating on the brightest stars." — George Ricker

We’ve tried as much as possible to utilize high-quality components that have been developed with a commercial interest in mind, and we've modified them for space use. This is actually somewhat different from the normal philosophy for developing space instruments, where often the technology is developed in-house at great expense. Our philosophy has been more about taking very reliable semiconductors, testing them, and then modifying them as needed to fit the space environment – rather than trying to duplicate all the billions of dollars of effort that went into making devices commercially.

The other thing we’ve done to contain costs is to have a core group of a half dozen or so key scientists involved from the very beginning, from when you actually conceive the mission to when you build the instrument, fly it, and then analyze the data. That way, there's a lot better communication within the team, and it dramatically keeps costs down and reduces the management overhead. We've been very successful with that approach. And again that fits in with this philosophy of the NASA Explorer program, because the idea there is you take a highly focused group, you concentrate on a high-value problem, and you try to solve that specific problem.

TKF: When TESS has completed its mission, what would you consider to be success? What will that look like and what will we know then, in broad terms, that we don’t know now?

RICKER: What we envision is that after two years we will complete a census of all the transiting extrasolar planets in the sky that have a period of less than about a month. This will be particularly valuable for studying the small M stars that are so abundant in our galaxy. We will have a good sample of planet periods extending out to about a year, and this will cover a wide variety of solar types – from stars that are hotter than the sun to stars that are a lot colder than the sun, and for which the habitable zones range from small distances from the stars to large distances from the stars.

TESS will have pinpointed an ample number of Earth- and super-Earth-sized planets to keep JWST busy. It's likely that these same planets will also be studied by future space telescopes – and even possibly giant ground-based telescopes – that will take direct images of extrasolar planets. All those missions are going to require carefully-chosen planetary targets, and the role of TESS is to scout the sky and find the premier targets – those orbiting the best and brightest stars – for future study by humankind.