To the list of planets orbiting distant stars, add another 715. That's the number of planets, strewn among 305 planetary systems, popping out of the observational data from NASA's Kepler Space Telescope, according to two new scientific papers released Wednesday by NASA.
"We've almost doubled today the number of planets known to humanity," said Jack Lissauer, a NASA planetary scientist, during a teleconference with reporters.
This new haul of distant worlds is making astronomers rethink the very nature of multi-planet systems.
Most multi-planet systems don't look like our solar system. Here, Earth and the other planets are spaced relatively far apart, at significant distances from the sun. But it appears that the norm elsewhere in our galaxy is for planets to be crammed close to the star, as if seeking warmth.
For example, Kepler has found a multi-planet system in which five of the six planets are closer to the star than Mercury is to our sun. Given what is known about how planets form in the disks of dust and gas that form stars, scientists doubt that these close-in planets materialized in their current location. Instead, they probably migrated over time.
"It's starting to look like planets further out are rare," said Sara Seager, an M.I.T. astrophysicist and planetary scientist who was not among the authors of the new Kepler papers, which have been accepted for publication in the Astrophysical Journal.
The Kepler team has previously announced several thousand possible "exoplanets" outside our solar system, but most have yet to be confirmed by subsequent observation. This new batch of planets has been verified -- or "validated to substantially better than the 99 percent confidence level," as one of the new papers puts it -- through a statistical technique that calculates the number of "false positives." The key realization is that there aren't many false positives in the multi-planet systems, and thus the "candidate" planets in those systems are almost always bona fide planets.
In the nearly two decades since the first discovery of an exoplanet, astronomers have found hundreds of planets through several different methods of scrutinizing starlight. But no instrument has been as adept at finding large batches of planets as the $600 million Kepler telescope, which is about to celebrate the fifth anniversary of its March 7, 2009 launch.
These new planets are not seen directly, and little about them is known other than how long it takes them to orbit their parent star and, in some cases, roughly how big they are.
The search for exoplanets is intrinsically difficult because they're very far away -- many tens of trillions of miles at least -- and the light reflecting off the planet is overwhelmed by the brilliance of the parent star. Someday a new generation of space telescopes may be able to occult the starlight and see only the planet, but for now, planet hunters look for quirks in the starlight itself that might reveal a planetary companion.
If the starlight wobbles, showing a Doppler shift in its wavelength, that can be the signature of the gravitational tug of a planet. If the starlight dims in a regular pattern, that can be from a planet passing in front of the star as seen from the telescope. The latter technique is the one that Kepler employs.
Adding another layer of complexity to the search is the natural variability of stars: They don't always shine steadily and predictably, but can brighten and then become dimmer due to astrophysical processes that have nothing to do with nearby planets.
The Kepler telescope suffered a malfunction in its pointing system last year, hampering its observational abilities. But scientists continue to examine the massive amount of data accumulated over the first four years of operation.
In November, scientists working with Kepler's data announced that, extrapolating from what has been seen in one small patch of the sky, they would infer that one out of five sun-like stars has a roughly Earth-sized planet in a "habitable zone" orbit, at a distance from the star that could allow water to be in a liquid state at the surface. That would suggest there are tens of billions of potentially habitable, roughly Earth-sized worlds in this galaxy alone.