Thousands of tiny pinpricks of light fill a dark sky on a cloudless night. For thousands of years, people have looked up at that star-filled, mysterious expanse and wondered what – or who – is out there. Do worlds like our own orbit other stars? Is life a common occurrence in the cosmos? Or, are we alone in the universe?
With the launch of a new mechanical “planet hunter” scheduled for this week after a delay, NASA will take the next step toward answering those ancient questions. TESS – the Transiting Exoplanet Survey Satellite – will scan almost the entire sky over the next two years, identifying planets orbiting stars (called exoplanets) in our own stellar neighborhood that may hold clues into the evolution of solar systems, Earth-like planets, and life.
But TESS isn’t the first orbiting telescope to search for exoplanets. NASA’s Kepler space telescope paved the way.
Since it first launched in 2009, Kepler has discovered more than 2,600 exoplanets and counting – nearly three-quarters of all known exoplanets. That flood of data, and the surprises hidden in it, have revolutionized how we see alien worlds and set scientists on a path to answering some of philosophers’ oldest and deepest questions about our place in the universe.
“Kepler gave us a true view of what’s out there,” says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in Ithaca, N.Y., and a member of the TESS science team. And, she says, “Nature seems to make planets wherever she can.”
Exoplanets are a relatively new discovery. Although philosophers and scientists had hypothesized that they were out there, it wasn’t until the 1990s that astronomers confirmed the first exoplanet discoveries. Before Kepler launched, a little more than 300 exoplanets had been detected using ground-based telescopes, and each was cause for celebration and many scientific publications.
But Kepler changed all that.
“It’s almost hard to remember how little we knew in 2009 when Kepler launched,” says Jessie Dotson, an astrophysicist at NASA’s Ames Research Center in Moffett Field, Calif., and a Kepler mission scientist.
From the ground, scientists discovered mostly giant exoplanets, thought to be like Jupiter. But Kepler data confirmed scientists’ suspicions: there are systems like ours out there – and ones that are wildly different.
A hunter's sight
By design, Kepler is a planet hunter. The mission was planned so that the space telescope could look at hundreds of thousands of stars at once and establish a sense of just how common exoplanets can be. To do that, it had to fit as many stars in its view as possible. So Kepler set its sights on a distant patch of sky in our galaxy, collecting data on stars’ brightness.
Using what’s called the “transit method,” scientists comb through that data looking for a dip in starlight, suggesting that a planet may be passing between its star and Kepler, momentarily blocking some of the light. Kepler was set to observe for at least three years in order to detect planets in an Earth-like orbit or tighter at least three times, to eradicate other explanations.
Focusing on an Earth-like orbit would enable Kepler to discover planets that might be able to host life. That’s because all life as we know it (read: life on Earth) requires liquid water. Earth’s distance from the sun makes it not too hot and not too cold for liquid water to be stable on the planet’s surface. That places it squarely in a band around the sun referred to as the habitable zone, or the Goldilocks Zone.
Kepler set out to determine if exoplanets are abundant in our galaxy, and they sure are. Scientists now say that there are more planets than stars.
“We expected that,” says Sara Seager, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, Mass., and deputy science director for the TESS mission. “Stars are born with disks of gas and dust. And just like those dust bunnies want to form in your living room, we think planets want to form out of that material.”
Still, Dr. Dotson says, “There’s a big difference between thinking something is possible and knowing it’s true.”
Stranger than (science) fiction
The most common planets in Kepler’s plentiful data were actually unfamiliar ones: planets between the size of Earth and Neptune. Sometimes described as “super Earths,” or “mini Neptunes,” these planets have no direct analogue in our solar system. Scientists also found that solar systems like ours are not the most common. Most planetary systems occur around the dim, reddish M-dwarf stars, not the yellowish G-type stars like our sun. These stars make up nearly three-quarters of the stars in our galaxy and are the main target for the TESS mission.
And some of Kepler’s discoveries defied imagination.
The mysterious “mini Neptunes” aren’t the only weird planets discovered in Kepler data. Scientists have found evidence of water worlds, hot Jupiters, lava worlds, and even circumbinary exoplanets, which, like the fictional planet Tatooine in “Star Wars,” orbit two stars.
“The diversity of planets is breathtaking,” Professor Kaltenegger says. “If we had just found planets like those in our solar system, it would’ve been nice, but kind of boring.”
Another astonishing exoplanet was spotted orbiting a white dwarf, the smoldering embers of an extremely old star near death. A closer look at the system revealed that the planet was being pulled apart and drawn into the dying star, illustrating one end of planetary evolution.
This discovery has the potential to expand where and how astrobiologists search for signs of life, too. “This told us that these have been around for a long time, and we shouldn’t think of things just in space, we should think about them in time, also,” says Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center who served as a Kepler mission scientist for a decade before joining the TESS mission as a support scientist.
“Maybe every system has a little slice of time where there’s life,” she adds.
Kepler has provided considerable data for astronomers to start to piece together models of planetary evolution. But, “like most great science missions,” Professor Seager says, “it created more questions than it answered.”
Hunting for habitability
Like Kepler, TESS is primarily a planet-finding mission, but the new space telescope will focus on identifying planets that may hold the answers. The satellite will scan almost the entire sky looking for planets close enough (within 300 light-years of Earth) and bright enough for future missions, like NASA’s James Webb Space Telescope, to investigate.
TESS’s eye will be largely on M-dwarf stars (those little reddish ones that are so plentiful, especially in our corner of the galaxy). As Kepler showed, those stars could host many Earth-like planets in that sweet spot for liquid water, which opens up new questions in the search for extraterrestrial life.
“It’s making me rethink what is meant by habitability,” Dotson says. “Now you start to ask the question, is liquid water enough for habitability?”
Kepler data has pushed scientists to think as expansively as possible, not limiting themselves to the models we know: our planet and our solar system.
“We have these four terrestrial planets, and these four giant planets, and a sunlike star,” Dr. Quintana says. But Kepler has shown that our system is by no means standard. “And what if actually the most common habitable planet is a circumbinary planet? We could imagine people on these circumbinary planets doing searches for life and only considering systems with multiple stars.”
Between Kepler, TESS, and other missions, scientists are amassing a growing catalogue of exoplanets for future generations of scientists to study and build upon.
Kepler laid the groundwork for TESS and other future missions, and it wasn’t easy. “Kepler basically plowed down everything in its way” to lead the way, Kaltenegger says. When the principal investigator, William Borucki, first proposed it in 1992, the Kepler mission was seen as too uncertain and risky. It took him five tries to finally get it approved in 2000.
And then, after four years of observation and exoplanet discoveries, the prime mission was brought to an abrupt halt in May 2013 by a mechanical failure. Two of the four reaction wheels that maintained the space telescope’s orientation broke, rendering Kepler ineffective as a planet hunter.
But scientists weren’t ready to be done with Kepler yet.
The scientific community quickly figured out that they could use the pressure of solar radiation to stabilize the telescope so that its field of view ran along the orbital plane of Earth, looking at a new section of sky every 83 or so days. NASA approved the second Kepler mission, dubbed K2, in 2014, and the exoplanet discoveries started flooding in again.
Now, the space telescope faces a certain death: it’s nearly out of fuel, and is expected to run out within the next few months. But that won’t mean the end of new Kepler-spotted exoplanets. It could take about a decade to sift through the treasure trove of data from both the Kepler and K2 missions, a process that will bring to light even more exoplanet discoveries.
Over the course of its two lifetimes, Kepler did more than accumulate vast amounts of scientific data. That data also changed our perspective of the universe – and our place in it.
As a graduate student in the 1990s, Dotson, who is now the K2 project scientist, remembers staring up at the night sky “and just being blown away by all those pinpricks of light. I was learning how they worked, and the scales of how far apart they all were, and all that. And I remember sitting there being blown away by that. But at the same time it felt very, very lonely, because it’s just so vast.”
“When I’m lucky enough to look at the night sky now,” she says, “I’m blown away from a different perspective. It no longer feels lonely; it now feels just like a whole realm of possibilities have opened up.”