An international team of astronomers has announced the discovery of twelve planets orbiting nine low-mass stars. The discovery is based on data from the second mission of NASA’s planet-hunting Kepler Space Telescope, known as K2.
K2-151b, one of the 12 newly-discovered planets, orbits a metal-poor M-type dwarf star every 3.84 days. The size of K2-151b — 1.35 Earth radii — suggests that it is likely a rocky planet. Image credit: NASA / Ames / JPL-Caltech.
Due to the loss of two of its four reaction wheels, NASA’s Kepler Space Telescope can no longer maintain the pointing stability required to observe its original field of view.
The telescope was re-purposed for a new series of observations under the name K2 and now scans a much larger portion of our Milky Way Galaxy.
As with the original Kepler mission, K2 discovers planets by measuring the tiny decrease in a star’s brightness caused by a planet passing in front of the star during its orbit.
The newly-discovered planets — dubbed K2-146b; K2-147b; K2-148b, c and d; K2-149b; K2-150b; K2-151b; K2-152b; K2-153b; K2-154b and c — are from K2’s 5th, 8th and 10th fields in the sky.
All of them orbit low-mass stars called red dwarfs.
“Red dwarfs are of enormous interest for studies of planetary formation and evolution,” said Tokyo Institute of Technology researcher Teruyuki Hirano and co-authors.
“A key outcome from the current study was that planets orbiting red dwarfs may have remarkably similar characteristics to planets orbiting Sun-like stars.”
“It’s important to note that the number of planets around red dwarfs is much smaller than the number around Sun-like stars,” Dr. Hirano said.
“Red dwarf systems, especially coolest red dwarfs, are just beginning to be investigated, so they are very exciting targets for future exoplanet research.”
For example, while the so-called radius gap of planets around Sun-like stars has been reported previously, this is the first time that astronomers have shown a similar gap in planets around red dwarfs.
“This is a unique finding, and many theoretical astronomers are now investigating what causes this gap,” Dr. Hirano said.
“The most likely explanation for the lack of large planets in the proximity of host stars is photoevaporation, which can strip away the envelope of the planetary atmosphere.”
The team also investigated the relationship between planet radius and metallicity of the host star.
“Large planets are only discovered around metal-rich stars, and what we found was consistent with our predictions,” Dr. Hirano said.
“The few planets with a radius about three times that of Earth were found orbiting the most metal-rich red dwarfs.”
The discovery is detailed in a scientific paper published in the March 2018 issue of the Astronomical Journal (arXiv.org preprint).
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Teruyuki Hirano et al. 2018. Exoplanets around Low-mass Stars Unveiled by K2. AJ 155, 127; doi: 10.3847/1538-3881/aaa9c1
