Next week, NASA is launching its new exoplanet hunter: a satellite that will stare out at the cosmos searching for never-before-seen worlds. Dubbed TESS, the spacecraft is tasked with looking for planets circling around stars outside of our Solar System to help scientists figure out what these planets are made of and if any might be able to support life.
TESS is launching on April 16th, just as NASA’s old exoplanet hunter is about to hang up its hat. The agency’s Kepler spacecraft, which launched in 2009, will run out of fuel sometime in the next several months. But TESS has a different mission than its predecessor. Kepler’s goal was to simply find as many exoplanets as possible; TESS is going to be pickier, searching for planets around the closest stars to Earth. These worlds will be much easier to study since their stars will be brighter, according to NASA.
“Kepler was all about doing a census: How common are planets in general? What is the size distribution of planets like? Are Earth-sized planets common?” Stephen Rinehart, the project scientist for TESS at NASA, tells The Verge. “TESS is really optimized for knocking on doors in the neighborhood and saying, ‘Hi, how are you? What is this planet actually like?’”
And one type of planet in particular is on TESS’s wish list: rocky, Earth-sized worlds that are in the right orbit around their stars, where liquid water can pool. If such a planet was also found to have an atmosphere similar to Earth’s, it would open up the possibility that life could survive on that world, too.
Why brighter is better
Kepler only stared at a few small patches of sky at a time, looking at upwards of 100,000 stars. TESS will be observing a field of view that’s 400 times bigger. And it will be able to see as many as 200,000 stars, perhaps millions. “Having TESS in the fold is just fantastic,” Jessie Dotson, an astrophysicist at NASA’s Ames Research Center and the project scientist for the Kepler spacecraft, tells The Verge. “They’re going to find planets in parts of the sky we can’t look at.”
TESS will use the same technique as Kepler to find planets. It will look for worlds as they pass in front of their host stars, in what’s known as a transit. Whenever a planet transits, it slightly dims the light of its parent star enough for an orbiting telescope to measure. But TESS will target much closer stars than Kepler saw. They’ll only be tens to hundreds of light-years away, as opposed to thousands of light-years away, making them 30 to 100 times brighter in the sky. That will make it easier for astronomers to learn more about the planets around them.
To really know what a planet is made of — whether it’s rocky like Earth or a gas giant like Jupiter — you need to know its density. And the best way to measure density is to observe how the planet tugs on its host star. Even though a planet is relatively small, it still has a gravitational influence over its star, causing the celestial object to wobble slightly. The extent of this wobble speaks to how massive a planet is.
Brighter stars make it easier to measure this wobble fairly quickly. With distant and faint stars, astronomers don’t collect as much light, so it takes longer to pick up how the star is shaking. For the kinds of stars Kepler observed, it could take weeks or months to figure out a star’s wobble and, and as a result, the composition of a nearby exoplanet. But for the bright stars that TESS will study, it could take just a few hours.
With Kepler, astronomers were only allowed to measure a few stars, says Rinehart. “With TESS, it’s going to be the opposite problem. It’s going to be ‘I can do any of these targets. Which one do I want to do?’”
The exoplanets TESS will find
With its four cameras, TESS will stare at the same patch of sky for just 27 days at a time before moving on to the next patch. In comparison, Kepler’s observations spanned months to years. So TESS will mostly be looking for planets with super short orbits, ones that take less than a month to complete. And that does limit the types of worlds the spacecraft can find.
For one thing, TESS won’t find Earth’s exact twin. The types of habitable Earth-sized worlds TESS will find are those orbiting small, faint stars known as red dwarfs. These types of stars put out much less energy than our Sun. So planets have to hug these stars tightly to be considered habitable. They have to be close to get enough heat to have a chance of hosting liquid water. And that means a year for these planets could last just a dozen or so days.
Astronomers have been particularly excited about finding planets around red dwarfs because these stars are abundant throughout the Universe. But just being in the so-called “habitable zone” of such a star may not mean these worlds can host life. Planets orbiting close to a red dwarf usually have one side that is in constant daylight, while the other experiences constant nighttime. And red dwarfs tend to flare up a lot, showering nearby planets with energetic particles. “They’re not going to be Earth-like, but what’s so interesting is that we don’t really know what we’re going to find,” Sara Seager, a planetary scientist at MIT and deputy science director for TESS, tells The Verge.
When NASA’s next big space observatory, the James Webb Space Telescope, comes online, astronomers will be able to peer into the atmospheres of the small Earth-sized planets that TESS finds. However, they’ll have to wait a bit to know what’s around them. James Webb isn’t supposed to launch until 2020 at the earliest, so it’ll be a few years until we learn if these worlds have atmospheres that can support life.
In the meantime, Seager says she’s excited about finding planets that are about two to three times the size of Earth, known as super-Earths or mini-Neptunes. Kepler discovered that these weird in-between planets are actually the most common worlds out there. But we know very little about what they’re made of and where they came from. When TESS finds these mini-Neptunes and their densities, astronomers will be able to do follow-up observations with telescopes on the ground to see what is in their atmospheres. “Are they all the same? Are they all different?” she asks. “It will help us understand how they formed.”
Getting to orbit
Before all this happens, however, TESS needs to get to orbit. The probe is slated to launch at 6:32PM ET from Cape Canaveral, Florida, on top of a SpaceX Falcon 9 rocket next week. If all goes well, the spacecraft should be collecting data as early as June, which is just around the time that Kepler starts to go offline. As Kepler runs out of fuel, it won’t be able to redirect its antenna to Earth to send its data. It also won’t be able to use its engines to stay in position, as it’s getting pushed off by solar wind.
TESS won’t have this fuel problem. The spacecraft will be in a super elliptical orbit around Earth that will bring the probe as far out as the distance of the Moon. This basically means TESS will be in a weird gravitational dance with our planet and the Moon. The orbit, which will last 13.7 days, has never been used for a spacecraft before, but it’s incredibly stable. So TESS doesn’t need very much fuel. “If you get a little bit off, the Moon tends to kick you back into the orbit you’re supposed to be on,” says Rinehart. “This orbit could be stable for more than 100 years.”
Getting to this type of orbit is tricky, though. It requires a lot of energy. But TESS has a big advantage: its small size. Weighing just shy of 800 pounds (362 kilograms), it’s much smaller than many bus-sized satellites that can weigh many thousands of pounds. The spacecraft will take up very little room on its Falcon 9, and that will make it easier for the rocket to get this spacecraft where it needs to go. “It’s mostly empty space in there,” says Rinehart, referring to the nose cone on top of the rocket. “TESS is a tiny little thing in this big fairing. We’re light and we’re small.”
TESS is slated to do science for two years when in space, but engineers are already planning how to extend the spacecraft’s mission. If so, this tiny probe could create a new comprehensive index of exoplanets that astronomers will be studying for years to come. “People love planets... and the fact that they seem to be coming in all sizes and masses and orbits,” says Seager. “Now, after TESS, we’ll have a whole catalog of these planets in an order of priority for follow-up.”