In the hunt for extraterrestrial life, scientists started by searching for a world orbiting a star just like the sun. After all, the steady warmth of that glowing yellow ball in the sky makes life on Earth possible.
But as astronomers continue to discover thousands of planets, they're realizing that if (or when) we find signs of extraterrestrial life, chances are good that those aliens will orbit a star quite different from the sun-one that's redder, cooler, and at a fraction of the sun's size and mass. So in the quest for otherworldly life, many astronomers have set their sights on these small stars, known as red dwarfs or M dwarfs.
At first, planet-hunting astronomers didn't care so much about M dwarfs. After the first planet outside the solar system was discovered in 1995, scientists began hunting for a true Earth twin: a rocky planet like Earth with an orbit like ours around a sun-like star. Indeed, the search for that kind of system drove astronomers through most of the 2000s, says astronomer Phil Muirhead of Boston University.
But then astronomers realized that it might be technically easier to find planets around M dwarfs. Detecting another planet is really hard, and scientists rely on two main methods. In the first, they look for a drop in a star's brightness when a planet passes in front of it. In the second, astronomers measure the slight wobble of a star, caused by the gentle gravitational tug of an orbiting planet. With both of these techniques, the signal is stronger and easier to detect for a planet orbiting an M dwarf. A planet around an M dwarf also orbits more frequently, increasing the chances that astronomers will spot it.
M dwarfs got a big boost from the Kepler space telescope, which launched in 2008. By staring at small patch of the sky, the telescope searches for suddenly dimming stars when a planet passes in front of them. In doing so, the spacecraft discovered a glut of planets-more than 1,000 at the latest count-it found a lot of planets around M dwarfs. "Kepler changed everything," Muirhead said. Because M-dwarf systems are easier to find, the bounty of such planets is at least partly due to a selection effect. But, as Muirhead points out, Kepler is also designed to find Earth-sized planets around sun-like stars, and the numbers so far suggest that M-dwarfs may offer the best odds for finding life.
"By sheer luck you would be more likely to find a potentially habitable planet around an M dwarf than a star like the sun," said astronomer Courtney Dressing of Harvard. She led an analysis to estimate how many Earth-sized planets-which she defined as those with radii ranging from one to one-and-a-half times Earth's radius-orbit M dwarfs in the habitable zone, the region around the star where liquid water can exist on the planet's surface. According to her latest calculations, one in four M dwarfs hosts such a planet.
That's higher than the estimated number of Earth-sized planets around a sun-like star, she says. For example, an analysis by astronomer Erik Petigura of UC Berkeley suggests that fewer than 10 percent of sun-like stars have a planet with a radius between one and two times that of Earth's.
This illustration shows Kepler-186f, the first rocky planet found in a star's habitable zone. Its star is an M dwarf. NASA Ames/SETI Institute/JPL-Caltech
M dwarfs have another thing going for them. They're the most common star in the galaxy, comprising an estimated 75 percent of the Milky Way's hundreds of billions of stars. If Dressing's estimates are right, then our galaxy could be teeming with 100 billion Earth-sized planets in their stars' habitable zones.
To be sure, these estimates have lots of limitations. They depend on what you mean by the habitable zone, which isn't well defined. Generally, the habitable zone is where it's not too hot or too cold for liquid water to exist. But there are countless considerations, such as how well a planet's atmosphere can retain water. With a more generous definition that widens the habitable zone, Petigura's numbers for Earth-sized planets around a sun-like star go up to 22 percent or more. Likewise, Dressing's numbers could also go up.
Astronomers were initially skeptical of M-dwarf systems because they thought a planet couldn't be habitable near this kind of star. For one, M dwarfs are more active, especially during within the first billion years of its life. They may bombard a planet with life-killing ultraviolet radiation. They can spew powerful stellar flares that would strip a planet of its atmosphere.
And because a planet will tend to orbit close to an M dwarf, the star's gravity can alter the planet's rotation around its axis. When such a planet is tidally locked, as such a scenario is called, part of the planet may see eternal daylight while another part sees eternal night. The bright side would be fried while the dark side would freeze-hardly a hospitable situation for life.
But none of these are settled issues, and some studies suggest they may not be as big of a problem as previously thought, says astronomer Aomawa Shields of UCLA. For example, habitability may depend on specific types and frequency of flares, which aren't well understood yet. Computer models have also shown that an atmosphere can help distribute heat, preventing the dark side of a planet from freezing over.
