NASA has announced the discovery of 715 new planets found with the Kepler Space Telescope. This is the largest number of planets reported in a single go, bringing the running total to 1690. The findings confirm the belief that multiplanetary systems are the norm, and provide a feast of opportunities for analysis of patterns in planetary formation.
From 1988, when the first planet outside the solar system was discovered, new announcements came in ones and twos. However, Kepler's 2009 launch changed the game. Where early planetary discoveries were made by observing the wobble of stars caused by the gravity of their planets over long periods of time, Kepler detects the tiny blip as a planet passes in front of its parent star.
Only a tiny portion of planets – those whose orbit brings them directly between their star and Earth – can be detected this way. Nevertheless, planets turn out to be so common Kepler has far outpaced all previous efforts.
Many of those that have been discovered in this way are likely to remain obscure, useful only as fodder for statistical analysis establishing the most common planetary sizes and locations. However, some are interesting in their own right. Planets have been found orbiting around tightly bunched star pairs before, but Kepler has found a binary system where one star is orbited by two planets and the other star by one.
Since gas giants block out more light than rocky Earth-like worlds most of the new finds are of this sort, but they're no longer the super-Jupiters that dominated the first exoplanet discoveries. Almost 95% are smaller than Neptune, dramatically changing the known distribution of planet sizes; four are not only less than 2.5 times the size of the Earth but orbiting at a distance where liquid water might exist. There are also systems where the planets are so closely bunched they are noticeably affecting each others' orbits, giving us a chance to learn about their masses.
"Of course we have every type of planetary system in our validated set that people can think of, except the perfect Earth analogue,” says Jason Rowe of NASA Ames Research Center at Moffett Field, California. The findings are to be published in The Astrophysical Journal.
In four years Kepler found 3500 candidate planets. Researchers are now picking through the data to see which ones can be confirmed. "We studied just over 1,200 systems, and from there we were able to validate 719 planets," says Rowe, although four of these have since been dropped. Nevertheless, the newly detected planets have not been checked using the gravitational wobble method, which remains the gold standard for exoplanet confirmation.
"From this study we learn planets in these multi-systems are small and their orbits are flat and circular," says Rowe, similar to our own solar system. This is helpful, making it likely that if one planet in a system transits, so will others, increasing our confidence we are seeing something real.
Early exoplanet hunters found multiplanetary systems a problem, as the differing cycles created confusion. Rowe's team saw a way to turn the signs of multiple planets to their advantage. Systems with only a single planet – or at least only one that passes between the star and our telescopes – can be hard to tell apart from partially eclipsing binaries, where one star sometimes blocks the edge of another.
However, when multiple planets are seen crossing in front of the star with different orbital periods astronomers become more confident. Triple star systems, let alone those with even more stars, are unstable and therefore rare, so if several objects are passing in front of the star the odds are that at least one is a planet. “We started to get into the statistics to see if we can quantify and see how many we can pull out and say with very good confidence that they are validated planets,” says Rowe.
Kepler failed last year, but the new discoveries can be expected to keep coming; Rowe's team only looked at candidates found before March 2011, leaving two years of data still to study. Rowe says, "The more we explore the more we find familiar traces of ourselves amongst the stars that remind us of home."
