By Susan Brown
Another Earth. The Goldilocks Planet.
Those were some of the international headlines earlier this year when the search for another world turned up a planet strikingly similar to our own.
“It has the right size and is at the right distance to have properties similar to our home planet,” said Elisa Quintana of the SETI Institute and NASA’s Ames Research Center. She told the press conference held in April that the planet was only a little larger than Earth and orbited its star at a distance thought to be just right for life.
Quintana, Revelle ’01, led the team that made the discovery using NASA’s Kepler Space Telescope. Fixing a steady gaze on a small patch of sky in the constellations Cygnus and Lyra for four years, the telescope measured the flickering light of 150,000 stars.
The planet Quintana and the team found is just 10 percent larger than Earth, small for a planet, which means it’s likely to have a rocky surface. They call it Kepler 186f.
The Kepler Mission’s main goal is to find planets orbiting stars within a habitable zone—close enough for water on their surfaces to be liquid, but far enough away so that it doesn’t boil off.
The process requires enormous patience. Researchers sift through the vast record of light patterns gathered by the telescope, looking for telling rhythms such as periodic dimming that could be the shadow of a planet passing between the satellite and a star.
That’s how Quintana and colleagues Jason Rowe and Thomas Barclay, all part of the larger Kepler Mission, first noticed a promising signal that revealed this intriguing planet.
“We’re kind of close-knit, in the corner,” Quintana says, referring to their office, a cramped, shared space. “The three of us found it together.”
This small band of planet hunters pounced upon each new batch of data as soon as it was released, looking for likely candidates.
“We would look at all the stars that we know have at least one planet, and try to see if we could pick up additional planets in the system,” Quintana says. “We play around with orbits in these systems all the time.”
“I took the lead with this candidate,” Quintana says. “So we do some physics. And then, think ‘oh, this is a small planet around a small star. This is interesting.’ That’s when it got really exciting, when we first saw it was Earth-sized.”
Quintana assembled a team that confirmed the additional planet along with its size and orbit—key attributes that suggest this distant place could support life.
“For the most part, I just do a lot of programming, playing with data, developing new algorithms. It’s not that glamorous,” Quintana says. However, friends who work in other fields thought she peered through the telescope and one even imagined her wearing a space suit.
Quintana says she just wasn’t the kind of kid who looked up at the stars and dreamed of being an astronaut. She reports uneven success in school—sometimes earning top grades but often rebellious. It was hard to see the point, she says, when she didn’t know what she wanted to be.
It wasn’t until she enrolled in Grossmont community college that Quintana found a knack for physics and mathematics. Her interest in astronomy began to emerge after she transferred to UC San Diego, where she majored in physics.
She remembers a course in cosmochemistry, taught by Jim Arnold who coordinated the Apollo mission’s retrieval of moon rocks for scientific study. Quintana then chose the former astronaut Sally Ride, a physics professor, as her advisor.
She worked on a project Ride had started called KidSat, now EarthKAM, which allows school children to request photographs of Earth to be taken from orbiting manned spacecraft. To help determine when the space shuttles would be in position to capture the requested images, Quintana calculated their orbits. “Out of all of physics, I like that part—orbital dynamics and collisions,” she says.
As a graduate student at the University of Michigan, Quintana applied for fellowships with NASA and landed a position at the Ames Research Center, where she finished her doctoral research, working out how planets form.
Planets form in disks of debris that collide and accrete around stars. Modeling the process means tracking the chaotic motion of hundreds of objects. To do this, astronomers use code called Keplerian, after the 17th-century mathematician and astronomer Johannes Kepler, whose lasting contribution was to figure out laws of planetary motion.
Most models of planet formation are based on our own solar system with its single star. But stars sometimes occur in pairs that orbit one another, called binary stars, adding complexity to the motion of orbiting debris.
“We used the exact same model as for our solar system, then put in another sun to perturb the whole thing. And tried to see how far away that second star would have to be to still form planets in the habitable zone,” Quintana says. “Now Kepler [the mission] has discovered planets around binaries.”
Early searches for life beyond our solar system focused not only on single stars, but also on bright yellow ones like our sun. Then, in 2006, Jill Tarter, director of research at the SETI Institute, convened a workshop to consider other suns, particularly a class of stars called M dwarfs. These stars are smaller, redder and cooler than our sun, and far more abundant.
“What if you have a small star,” Quintana points out. “You have to change the coordinates of everything. Planets are close, and the collisions are faster and harder. Can you form planets?”
We now know that you can. Kepler 186f and others, discovered by the mission, orbit M dwarf stars. M dwarfs account for more than 70 percent of the stars in the Milky Way galaxy. They’re also the longest lived, giving life billions of years to take hold. But M dwarfs also pose challenges for life. “Small stars are cooler and less luminous, so the habitable zone is closer in,” Quintana says. “Planets that close to the star can be vulnerable to giant flares that could scorch them.”
Closely orbiting planets can also be ‘tidally locked,’ meaning one side of the planet always faces the star—the way one side of the Moon always faces Earth—to roast while the other freezes in the dark chill of space. But little about Kepler 186f can be measured just from the oscillating light captured by the telescope.
More can be inferred, and imagined. For the cover of the journal that published the work, NASA asked artist Tim Pyle to create images of this other world.
“For fun, we thought: if it had an Earth-like composition and an Earth-like atmosphere, what would it look like from space?” Quintana says. Not as blue as Earth, they decided, even if it had a watery surface akin to our oceans. Blue wavelengths are largely missing from the light of its star, so the oceans in the image are rather grey and the clouds and polar ice a little dingy rather than bright white.
The green swaths you’d see across Earth’s continents from photosynthesizing plants might be different as well. Quintana consulted with an astrobiologist whose analysis—based on the spectrum of light from this star—suggested plants on Kepler 186f would be a dark yellowish color. However, the truth is that we’ll probably never know how this planet really looks because it’s 500 light-years away. But this particular discovery is just a beginning.
“You think about all the decades and decades of people wondering—are we alone, are there other Earths? I feel lucky to be working when we are finding these planets and finding these answers,” Quintana says.
A satellite called TESS scheduled to launch in 2017 will focus on stars in our galactic neighborhood, searching for Earth-like planets closer to us. The James Webb Space Telescope, scheduled to launch in 2018, will be able to detect biological signatures in their atmospheres.
“That might be the first time we detect life outside of Earth, by detecting chemical imprints that we think could only be attributed to life,” Quintana says. “That’s still in my lifetime, I hope.”
Susan Brown writes about the physical sciences for UC San Diego.