On Monday, earthlings watched as Mercury passed between our planet and the sun, all three celestial bodies lining up in just the right way for Mercury to appear as a small black dot creeping over our bright host star. That phenomenon — one planet passing in front of its star, from the visual perspective of another planet — is known as a “transit.” And that’s how Kepler finds new, alien worlds.
Kepler (which is technically broken, but still finds new planets in its second life as “K2”) tracks the subtle dimming of distant stars to detect possible planets that orbit them. It’s our best method for detecting exoplanets, even though it can only hunt down worlds that are set up to “transit” from Earth’s perspective.
Even though the data collection of the K1 mission is over, scientists are still working on parsing out the primary mission’s data. They have to weed out false positives from the thousands of potential planets — star dimming actually caused by mischievous companion stars or other objects:
In a paper published Tuesday in the Astrophysical Journal, a team led by Princeton University’s Timothy Morton presents a new statistical method for calculating the likelihood that a given candidate is, in fact, a planet. Their analysis yielded 1,284 confirmations. Another 1,327 planets from the Kepler catalogue are almost certainly planets, according to the researchers, but these worlds don't reach the 99 percent probability threshold — so more study will be needed to adequately confirm their existence. The other 707 potential worlds are likely nonexistent, according to the analysis.
Of the newly confirmed planets, nine are thought to be rocky planets (like Earth, as opposed to gas giants or tiny worlds made of ice) in the habitable zone — meaning that they’re the right distance from their host stars to potentially host liquid water, a necessary ingredient for life as we know it.
“Planet candidates can be thought of like bread crumbs,” Morton said in a statement. “If you drop a few large crumbs on the floor, you can pick them up one by one. But, if you spill a whole bag of tiny crumbs, you’re going to need a broom. This statistical analysis is our broom.”
The statistical method could allow scientists to decide which “planets” to set aside as false positives or doggedly pursue as potential sites for alien life using less time and fewer resources.
“They say not to count your chickens before they’re hatched, but Tim’s numbers allow us to do exactly that,” Natalie Batalha, Kepler mission scientist at NASA’s Ames Research Center, said during Tuesday’s news conference. “This is going to be very important for Kepler’s most valuable planet discoveries, those small planets found orbiting in the habitable zone.”
As of today, NASA knows of 21 exoplanets that it considers likely to be rocky, potentially wet worlds. And based on Kepler data, Batalha said, our galaxy probably has more than 10 billion rocky planets that live in the habitable zones of their stars.
That’s a lot of Earths.
Before Kepler, we had no idea of how common these kinds of planets might be. The fact that they seem to be run-of-the-mill is great news in the search for life: The less special we are, the more likely we are to have company somewhere in the galaxy or beyond.
Scientists could further explore the habitability of these worlds by measuring the way their host stars’ light changes as it passes through planetary atmospheres. The molecular signatures analyzed using this method could reveal the presence of water and other life-giving molecules, showing us which worlds are closest to Earth on the planetary family tree.