Of all the big discoveries the Kepler mission has notched in its over three illustrious years of service, there’s one that excites Sarah Ballard more than the rest.
“What’s inspired me most is that exoplanet size seems to be a function of star size. In other words, small stars are twice as likely to have Earth-size planets as ones the size of our sun. This is something brand new -- we’ve literally only known it for a few months.”
A NASA Sagan Fellow at the University of Washington, Ballard’s quest is to study and characterize exoplanets that orbit red dwarf (also known as M-dwarf) stars, which are smaller and cooler than the sun. “Nature just seems to make more little planets around these stars,” she said. “Chances are that where you find a small, Earth-size planet, it orbits a small star.”
It’s a breakthrough discovery for astronomers looking for habitable worlds that are near our solar system. Our own stellar neighborhood is about 75 percent composed of red dwarf stars.
Planets orbiting close enough to a dwarf star for liquid water to exist on their surfaces would be tidally locked, with one side always facing towards the star.
Because red dwarf stars are so much cooler than the sun, any habitable planet that orbits one would have to be much, much closer to its star than the Earth. And that’s when things get weird.
“It’s really interesting to conceive of life on a planet orbiting a red dwarf star,” Ballard said. “The planet would most likely be tidally locked, which means that it’s parked so that one half always faces the star, like how the moon always faces the same way towards the Earth. You’d have a day side, a night side, and areas with eternal sunset. Or it might rotate, but very slowly - a single day could take an entire lifetime.”
A habitable exoplanet would have to be so close to its host M-dwarf that it would loom huge in the sky. Two or more suns in the sky are also a possibility, as many small stars exist in binary configurations.
Creatures that evolved on the surface of these worlds would most likely look very different from what we’re used to. “Most of the light that small stars put out is in the infrared spectrum,” Ballard said. “Human eyes have evolved to the wavelengths that our sun emits, but a creature living on a planet orbiting a red dwarf would be sensitive to infrared light. Table salt shines like a mirror in the infrared and ice looks black ... their reality would be completely different from ours, totally a function of the wavelengths they can see.”
Red dwarf stars may be the best place to look for potentially habitable worlds, but they are far from the easiest. “They are incredibly hard to characterize,” Ballard said. “With sun-like stars, we have accurate models. We can generate an accurate fake spectrum and compare it to the spectrum of a real star to pick out which light is actually coming from the planet. It doesn’t work for low-mass stars ... their atmospheres are full of grainy material and very hard to model.”
To make up for these shortcomings, Ballard is looking to nearby red dwarfs that have been extensively studied by other astronomers. “By comparing the spectra of well-known low-mass stars with ones that Kepler has found that are similar, we can try to pick out planets in the Kepler data.”
Already Ballard’s research has turned up one major discovery: the Kepler-19 system, with a pair of planets – one just over twice the size of Earth – orbiting an star slightly smaller than the sun. And with Kepler continuing to bring in mountains of data, Ballard hopes that she’ll be able to get even deeper into the world of small planets orbiting small stars.
Now that astronomers know red dwarfs are a great place to look for small, potentially habitable planets, Ballard hopes that future missions will enable her to hunt for these worlds in our own backyard. “Kepler’s data has been beyond my wildest dreams -- we have incredible data on low-mass stars and the planets that orbit them. The next step is to look for small planets around the nearest M-dwarf stars. In the future, possibly with the James Webb Space Telescope, we could analyze their atmospheres for signs of organic life.”
“I realize that I’m probably never going to visit the planets we find,” Ballard said. “But there’s something amazing about realize that these are real places, where you could walk around. It’s amazing and evocative to find another world, and I find it really inspiring.”