Astronomers using data from the Kepler spacecraft’s K2 mission have discovered two mini-Neptune exoplanets orbiting a star in an open star cluster called the Beehive Cluster. The discovery will be detailed in two papers in the Astronomical Journal and the Monthly Notices of the Royal Astronomical Society.
An artist’s impression of the EPIC 211964830 planetary system. Image credit: Sci-News.com.
The newly-discovered exoplanets, called EPIC 211964830b and c, are both mini-Neptunes — gaseous planets smaller than our Solar System’s Uranus and Neptune.
The inner planet has a radius of around 2.3 times that of Earth, and the outer planet has a radius of 2.8 Earth radii. They orbit their parent star extremely closely, with periods of 5.8 and 19.7 days.
Designated EPIC 211964830, the star is an M2.5 dwarf, with a radius and a mass of about 47% that of the Sun.
It lies on the periphery of the central core of the Beehive Cluster (also known as Messier 44, NGC 2632 and Praesepe), a tightly packed collection of about 1,000 stars located approximately 550 light-years away in the constellation Cancer.
The Beehive stars were born out of the same cloud and have remained together for the past 650 million years.
EPIC 211964830b and c were discovered via the transit method used by NASA’s Kepler space telescope, i.e. by detecting a small dip in the flux of the star, caused by the planet passing in front of it and blocking a small fraction of the stellar light.
The EPIC 211964830 system is currently the second known multi-transit system in open clusters younger than one billion years.
“There are now several detected transiting planets in young open clusters and associations observed by K2, though EPIC 211964830 is one of only two multiple-planet systems, the other being K2-136, a three transiting-planet system in the Hyades open cluster,” said Dr. Aaron Rizzuto from the University of Texas at Austin and colleagues.
“As the second known system of multiple planets transiting a star in a cluster, EPIC 211964830 should be valuable for testing theories of photoevaporation in systems of multiple planets,” added Dr. John Livingston of MIT and the University of Tokyo and co-authors.
“Follow-up observations with current near-infrared spectrographs could yield planet mass measurements, which would provide information about the mean densities and compositions of small planets soon after photoevaporation is expected to have finished.”
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Aaron C. Rizzuto et al. 2018. Zodiacal Exoplanets in Time (ZEIT) VIII: A Two Planet System in Praesepe from K2 Campaign 16. AJ, in press; arXiv: 1808.07068
John H. Livingston et al. 2018. EPIC 211964830: A transiting multi-planet system in the Praesepe open cluster. MNRAS, in press; arXiv: 1809.01968
