“This announcement more than doubles the number of confirmed planets from Kepler,” said Ellen Stofan, chief scientist at NASA Headquarters in Washington. “This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth.”
NASA scientists estimate about 550 of these planets could be rocky ones like Earth. And of these potentially rocky worlds, nine of them sit inside their star’s habitable zones. That brings the tally of worlds where surface temperatures could allow liquid water to pool to 21 planets.
Here’s an image showing all 21 planets and where they sit within the habitable zones.
Yellow circles represent planets discovered in the latest Kepler survey. The different shades of green show the conservative and optimistic ranges of the habitable zone. Earth clearly sits inside the conservative area. 11 of the 21 planets join Earth and Mars in this potentially habitable region.
We know orbiting within this region doesn’t guarantee life. Just look at Mars. Scientists believe seas once covered the now desolate planet. But without a substantial atmosphere, Mars turned into a dry, dusty world.
Kepler’s tiny reach
150,000 stars. Sounds like a lot, doesn’t it? It is, but compared to the rest of the sky – Kepler’s reach is tiny. Here’s the patch of sky Kepler stared at for four years.
Its instruments kept constant tabs on thousands of stars at once looking for telltale signs of dimming. The short video below shows the change in brightness scientists see when an object passes in front of a star.
But not every object is a planet. When scientists first see the brightness drop, the object is placed in a ‘candidate’ pool. Low-mass stars or grazing stellar binaries can also be responsible for the changing brightness of a star.
Once Kepler finds a candidate, ground-based telescopes get to work.
Confirming each candidate isn’t a walk in the park. Time and resources are spent going through each planet candidate and confirming if they are the real deal or a star masquerading as a planet. Another issue is that the planets discovered by Kepler are often fainter than planets discovered by ground-based telescopes. This makes imaging them much tougher in follow-up observations from the ground.
But scientists have come up with a way to confirm a planet is a planet. At least, with a high degree of reliability. Using a new statistical validation technique, scientists take simulations of transit signals caused by planets and other objects (stars) and simulations of how common for the candidates to be stars in the Milky Way. They combine the two to form a reliability score between zero and one. Candidates that reach 99% or greater become ‘validated planets.’
Of the 4,302 candidate planets from Kepler’s July 2015 planet candidate catalog, 1,284 were newly validated. 984 planets were reconfirmed. 1,327 are classified as ‘more likely planet,’ but failed to hit the 99% threshold. These planets warrant additional observations. 707 planets are probably some other object such as a star.
Kepler keeps on searching
After a scare last month, Kepler is back in action and continuing the search for exoplanets. Today, Kepler is busy with its latest observing campaign called Campaign 9.
While Kepler is used to searching for a drop in brightness, Campaign 9 will be on the lookout for spikes in brightness. One of Campaign 9’s focuses is free-floating exoplanets. Planets without a home star. Here’s a short animation showing how gravitational microlensing could reveal planets we would otherwise never see.
Scientists hope Kepler will see around 100 of these lensing events. And maybe 10 exoplanets sitting in the dark regions of space.
“We are seizing the opportunity to use Kepler’s uniquely sensitive camera to sniff for planets in a different way,” said Geert Barentsen, a research scientist at Ames.