The Kepler space telescope was launched into orbit around the sun in 2009. Its charge: Take a census of a small slice of the Milky Way in an effort to understand the “demographics” of our galaxy. How many stars are like our sun? How many of those host planets? How many planets orbit in the habitable zone? Is there anyplace else in this vast universe that living beings might call home?
In its first four years, Kepler surveyed just .025 percent of the sky. And for every potential planet detected, NASA estimates that 100 to 200 lurk beyond the telescope's reach. Given a little time and some sophisticated models, scientists will use the Kepler catalogue to estimate how many stars in our galaxy could host an “Earth 2.0.”
Based on how many habitable-zone planets have already been identified, Caltech astrophysicist Courtney Dressing thinks that number could be sizable.
“I, for one, am ecstatic,” she said at a news conference Monday.
“The important thing for us is, are we alone?” added Kepler Program Scientist Mario Perez. “Kepler today tells us, indirectly … that we are probably not alone.”
This is the eighth update of the Kepler planet catalogue and the most thorough survey of the space telescope's data. Of the 4,034 candidates, more than half have been confirmed as exoplanets and not the result of miscalculations or false signals. Kepler research scientist Susan Thompson, the lead author of the catalogue study, said her team is confident about all 10 of the new “Earth-like” planets found in their stars' habitable zones.
Several of these planets orbit G dwarfs — the same species of star as our own sun. And one, dubbed KOI 7711 (for Kepler Object of Interest), is a possible “Earth twin,” a rocky world just 30 percent bigger than our own and about the same distance from its star.
It's too soon to say whether KOI 7711 truly merits the label “Earth-like,” Thompson cautioned. Kepler is incapable of determining whether an exoplanet bears an atmosphere or liquid water. If aliens were observing our solar system using a similar instrument, they might think it contained three rocky, potentially habitable worlds — Venus, Earth and Mars. “But I'd only want to live on one of them,” Thompson said.
A second research group combined the Kepler data with measurements from ground-based telescopes to calculate the approximate sizes and compositions of 2,000 exoplanets. They found that smaller worlds, the kind Kepler was designed to detect, fall into two distinct groups: rocky planets that could be up to 1.75 times the size of our own, called “super-Earths,” and gaseous “mini-Neptunes,” which lack a solid surface and are two to three times bigger than Earth. Nearly every star surveyed hosted a planet in one of these two categories. But, curiously, no planets straddled the divide. Each world was either smaller and rocky, or larger and gassy.
Benjamin Fulton, an astronomer at Caltech and the University of Hawaii at Manoa, compared the new categories with species of animals.
“Finding two distinct groups of exoplanets is like discovering mammals and lizards make up distinct branches of a family tree,” he told reporters Monday. And just as discovering distinctions between species helps us understand evolution, this revelation could help astronomers determine how planets take shape.
Fulton and his colleagues think that the sharp distinction between “super-Earths” and “mini-Neptunes” may be because of how much hydrogen and helium contributed to their formation. These elements are extremely light and exist as gas at all but the lowest temperatures. Rocky worlds such as Earth, with thin atmospheres and nice, firm surfaces, contain relatively little of these elements. Perhaps they started off with less, or perhaps the light elements were burned or blown away.
But if a planet can hold onto just a bit more of these gases, it “puffs up” like a balloon, Fulton said. Hydrogen and helium form vast, thick atmospheres around mini-Neptunes, making these worlds much bigger than their rocky counterparts.
It's difficult to know for sure, because our own sun doesn't host a mini-Neptune — unless you count the hypothesized “Planet Nine” that some scientists think lurks at the outer edge of the solar system. (For the record, Fulton doesn't — not yet.) But researchers are bent on figuring out what leads a world to become rocky rather than gassy, because as far as we're aware life can only take shape on solid ground.
Kepler's original mission ended in 2013 when one of the wheels that helped to keep the spacecraft pointed toward the Cygnus constellation failed, so it could no longer scan the same small slice of sky. But by using pressure from light particles from the sun to stay oriented, the telescope has been refashioned for a second exoplanet search project called K2. NASA estimates the telescope has enough fuel to remain active into 2018.
By then, the space agency hopes to be ready to launch the Transiting Exoplanet Survey Satellite, which will search for small planets around the brightest stars in the sky, and the James Webb Space Telescope, which is designed to detect atmospheres on other planets. The results from Kepler, that new satellite and the Webb will inform the next generation of telescopes — ones that can actually take pictures of planets in motion around distant stars.
“It feels a bit like the end of an era,” Thompson said, “but actually I see it as a new beginning. It's amazing the things that Kepler has found. It has shown us these terrestrial worlds, and we still have all this work to do to really understand how common Earths are in the galaxy.”
Correction: A previous version of this article misstated the size of exoplanet KOB 7711. It is 30 percent bigger than Earth, not 30 times as big.