Ů��С��Ƶ

It’s the biggest batch of new planets yet. On 10 May, scientists from NASA’s Kepler mission announced that they have more than doubled the number of confirmed planets the spacecraft has found around other stars.

The announcement bumps 1284 planets from “candidate” to “confirmed” status, adding to the previous 1041 confirmed worlds Kepler has discovered. Combined with planets discovered through other methods, there are now more than 3200 confirmed worlds outside our solar system.

These distinctions matter, because Kepler doesn’t see the planets themselves. It catches planets by watching how stars dip in brightness when planets cross in front of them. Other astronomical objects, such as distant binary stars, can cause similar dimming, meaning any world Kepler discovers could disappear under closer scrutiny.

The surest way to verify a Kepler planet is to observe its star at length with ground-based telescopes and catch an independent hint of the planet – but that’s a costly proposition. And small planets, which may be both the most exciting and the hardest to study, are the most difficult to check.

“Imagine planet candidates as breadcrumbs. If you drop a few large pieces of bread on the ground, you can pick them up individually, one by one,” said of Princeton University in New Jersey in a press conference. “But if you spill a whole bucketful of small crumbs you’re going to need a broom to clean them up.”

Morton’s chosen broom is an algorithm that checks each Kepler discovery against the other astronomical phenomena that could mimic the same signal. From the Kepler team’s perspective, anything that the algorithm gives a more than 99 per cent likelihood of being a planet has now been granted planethood.

Hall of fame

From 2009 until one of its steering wheels broke in 2013, Kepler stared at one patch of sky in an attempt to calculate just how common planets like Earth are in the Milky Way.

All of the new confirmed planets come from that original run. Although the mission has continued in an altered form, and survived a scare last month, our best information about the frequency Earth-sized planets still comes from analysing the old data.

The new planets include 9 worlds around the size of Earth that may be in the right range of temperatures for liquid water, and therefore life. That brings the number of confirmed, potentially habitable Kepler planets up to 21.

This list, and especially the handful of small, possibly Earth-like planets, is the key to Kepler’s ultimate legacy.  “These are the select few; this is the exoplanet hall of fame; these are the things that are really hard to find,” said of NASA’s Ames Research Center in California.

“Doing the math, you’re talking about tens of billons of potentially habitable earth-size planets out there in the galaxy,” she said. An updated figure is expected before October 2017.

As Kepler prepares for retirement, other missions, like NASA’s TESS spacecraft due to launch in 2017, will aim to find closer worlds that are easier to study in detail. Like Kepler, TESS will detect planets by watching their stars for periodic dips in brightness, so Morton’s broom should apply to its planet candidates as well.

“We’re getting ready to pass the baton to future missions,” Batalha said.