Planetary bonanza

Last month delivered a bumper crop to planet hunters, with  NASA’s Kepler mission announcing the discovery of 715 new planets on 26th February.  All of the planets are in multiple systems orbiting a total of 305 stars; 94% of the planets discovered are smaller than Neptune; and four of the planets are less than 2.5 times the size of Earth and orbit their star’s habitable zone. The habitable zone is a region around a star where liquid water could exist on the surface of solid planets.  This new dataset also shows that small planets in multiple systems appear to have flat and circular orbits that are similar to the planets in our inner Solar System.

Kepler’s multiple-transiting planet systems. For edge-on planetary system, the planets eclipse or ‘transit’ their host star as seen from the vantage point of the observer. (Credit: NASA)

The Kepler spacecraft was launched in 2009, and the mission’s goal has been to search for terrestrial planets in their host star’s habitable zone using the transit method.  The telescope has 1.4-m primary mirror and one instrument: a 0.95-m photometer which comprises an array of 42 CCDs that measure stellar brightness variations over a wide, 15-degree field of view. The primary mission, which started collecting data in May 2009 and ended in November 2012, targeted almost 150,00 stars in a single star field in Cygnus-Lyra region. Each star was continuously monitored over the 3.5 year period.  With the failure of a second of its four gyroscope-like reaction wheel in May 2013, Kepler ceased its terrestrial exoplanet search phase.

However, the Kepler dataset continues to provide fantastic discoveries. The latest batch of 715 new planet discoveries are based on two years of data form May 2009 to March 2011. Of the ~150,000 stars that Kepler monitored for brightness variations during those two years, there are over 2,500 exoplanet candidates, and of these about 450 are potentially multiple planetary systems (meaning more than one planet per star).  Most exoplanet candidates are confirmed planet-by-planet, and three observed transits are generally required for a confirmation of an exoplanet.  This is generally a very time-consuming process.

Jack Lissauer from NASA’s Ames Research Center and collaborators used a statistical technique termed ‘verification by multiplicity’, a method that only works for multiple planet systems, to discover the 715 new planets.  The verification by multiplicity method uses the logic of probability.  The multiplicity method relies on the principal that planets are often clustered in multi-planet configurations, while ‘false positives’ occur randomly.  False positives are mistaken identifications for a potential planet, such as eclipsing binary stars and chance alignments in the Kepler sample. If false positives are random, then they are less likely to occur near the same star. Given this probability, if it appears that a there are several planet ‘signals’ near one star, it is unlikely that they are all false positives. Using this method, the team confirmed in a second paper, led by Jason Rowe from the SETI Institute, that 851 planet candidates in multiple planetary systems are valid planets.

Histogram of the number of planets by size for all known exoplanets. The blue bars represent the exoplanets population prior to the 26 Feb 2014 announcement, and the gold bars add Kepler’s newly-verified planets. (Credit: NASA Ames/W Stenzel)

The Kepler mission now accounts for 57% of all confirmed planets, bringing the total of confirmed exoplanets to 1,690. It is expected that hundreds more planets will be discovered as another two years of Kepler data is processes. The discovery of so many multi-planet systems, and lower mass planet, is extremely valuable for demographic studies and will significantly add to our understanding of the formation of solar systems that have Earth-like planets potentially capable of supporting life.

For more information, see

P.S. Happy birthday Kepler!  5 years strong

[Sheridan Lacey & Sarah Maddison]

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