There’s a rule of thumb that can come in handy when you hear about new planet discoveries from the Kepler space mission. If they’re talking about a handful of new worlds orbiting distant stars, it’s an actual discovery. If they’re talking about hundreds, it’s not actual planets but “planet candidates,” which haven’t yet been independently verified — a key step before scientists can claim they’ve genuinely found something.
But the latest announcement from the Kepler science team has just turned that reliable rule on its head. As of yesterday, astronomers knew of about a thousand verified planets beyond our solar system, the majority of them found with telescopes other than Kepler. Today, Kepler added an unprecedented 715 new ones to the list, nearly doubling the count of bona fide exoplanets known to science. “For years, I’ve trained myself to say ‘planet candidate’ rather than ‘planet’ most of the time,” said SETI Institute and Kepler scientist Jason Rowe in a press conference. “I have to change that.”
Of these new worlds, none is what NASA scientist Jack Lissauer calls “Earth 2.0,” meaning an Earth-size planet in an Earth-like orbit around a sun-like star. Just four of the planets, in fact, sit in their stars’ so-called habitable zones, where water could plausibly exist in liquid form on the surface. And in these cases, the stars are smaller and dimmer than the sun, while the planets are bulkier than Earth. But most of the new planets are between Neptune and Earth in size, confirming a trend of moderate-size planets that astronomers have been seeing for several years now — and giving them good reason to think that true twins of Earth aren’t at all uncommon.
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The reason for such a huge leap in planet verifications is based on another trend astronomers have noticed: most of the planets they’re finding come not singly but in solar systems containing two, three, four or more planets. That’s not a huge surprise, given our solar system's eight (formerly nine).
Lissauer and other theorists realized, however, that that fact gave them a way to vet planet candidates in wholesale fashion. The reason you need a double check at all is that Kepler finds new worlds by looking for a slight dimming as a planet passes in front of its star, blocking a tiny bit of the star’s light. Other astronomical phenomena can mimic that effect, though — most notably a pair of stars that orbit each other, with one passing in front of the other. S o astronomers confirm candidates using other techniques — looking for the gravitational wobble the planet imposes on the star, for example — which have to be done one candidate at a time.
It would be much less likely, however, for something to mimic a swarm of planets passing in front of a star and dimming it, each with its own rhythm based on orbital distance. The risk of a false positive, in other words, is exceedingly low. All of the planets just announced are indeed multiples, with 715 worlds distributed among just 305 stars.
Verifying a huge number of new planets all in one shot is a big enough deal, but what makes it especially interesting is that so many of them are clustered very tightly near their stars — multiple worlds bigger than Earth but huddled in packs with orbits smaller than Venus’ or even Mercury’s. “So why isn’t Earth crammed close to the sun?” asked MIT astronomer Sara Seager, who joined the press conference as an independent commenter. One possible answer: these planets formed from a primordial disk of gas and dust much denser than the one from which we emerged. But that’s just speculation at this point. What’s clear, she said, “This just reminds us that planetary systems can be very different from ours."
The fact that so many exoplanets are like mini-Neptunes in size is also a surprise that astronomers have been coming to grips with over the past few years. “We have none of these in our solar system,” said Seager, “so we don’t really know for sure what they’re made of.”
Much of that could become clearer with a new generation of space telescopes scheduled to go into orbit later in the decade. One is the Transiting Exoplanet Survey Satellite, or TESS, which will look for planet-induced dimming in bright stars, mostly close to Earth; another is the James Webb Space Telescope, whose huge mirror and powerful infrared-sensitive cameras will look for chemical signatures of life in the atmospheres of nearby exoplanets.
Kepler, meanwhile, will keep finding planets — not so much through new observations, since a malfunction last spring has left it crippled, but via the backlog of observations that still haven’t been analyzed. The new planets, said Lissauer, come from the first two years’ worth of Kepler data. That means a lot of planets are yet to come, including some with longer, more Earth-like orbits.
“Kepler,” said Seager, “is the gift that keeps on giving.”