A bright K-type dwarf star called Gliese 9827 hosts one of the most massive (and dense) super-Earths detected to date, according to a study to be published in the Astronomical Journal.
A system of three super-Earth exoplanets. Image credit: NASA / JPL-Caltech.
Gliese 9827, also known as GJ 9827 or HIP 115752, is located 99 light-years away in the constellation of Pisces.
The star hosts a trio of transiting super-Earths, detected recently by NASA’s Kepler/K2 mission.
“Intriguingly, no planets of this size exist in our Solar System. This makes us curious about the conditions under which they form and evolve,” said Dr. Johanna Teske of the Carnegie Institution for Science and co-authors.
“One important key to understanding a planet’s history is to determine its composition. Are these super-Earths rocky like our own planet? Or do they have solid cores surrounded by large, gassy atmospheres?”
“To try to understand what an exoplanet is made of, we need to measure both its mass and its radius, which allows them to determine its bulk density.”
When quantifying planets in this way, astronomers have noticed a trend. It turns out that exoplanets with radii greater than about 1.7 times that of Earth are have a gassy envelope, like Neptune, and those with radii smaller than this are rocky, like our planet.
Some scientists have proposed that this difference is caused by photoevaporation, which strips planets of their surrounding envelope of so-called volatiles — substances like water and carbon dioxide that have low boiling points — creating smaller-radius planets. But more information is needed to truly test this theory.
This is why planets Gliese 9827b, c and d are so special — with radii of 1.64, 1.29, and 2.08, respectively, they span this dividing line between super-Earth (rocky) and sub-Neptune (somewhat gassy) planets.
Luckily, Dr. Teske and colleagues have been monitoring Gliese 9827 with their Planet Finding Spectrograph (PFS), an instrument on the Magellan Clay Telescopes at Carnegie’s Las Campanas Observatory, so they were able to constrain the masses of the three planets with data in hand, rather than having to scramble to get many new observations of the star.
“Usually, if a transiting planet is detected, it takes months if not a year or more to gather enough observations to measure its mass,” Dr. Teske said.
“Because Gliese 9827 is a bright star, we happened to have it in the catalog of stars that Carnegie astronomers been monitoring for planets since 2010.”
The PFS observations indicate that Gliese 9827b is roughly 8 times the mass of Earth, which would make it one of the most-massive and dense super-Earths yet discovered.
The masses for planets Gliese 9827c and d are estimated to be between 2.5 and 4 times that of Earth respectively, although the uncertainty in these two determinations is very high.
This information suggests that Gliese 9827d has a significant volatile envelope, and leaves open the question of whether Gliese 9827c has a volatile envelope or not.
But the better constraint on the mass of Gliese 9827b suggests that that it is roughly 50% iron.
“More observations are needed to pin down the compositions of these three planets,” said co-author Dr. Sharon Wang, also from the Carnegie Institution for Science.
“But they do seem like some of the best candidates to test our ideas about how super-Earths form and evolve, potentially using the future NASA/ESA/CSA James Webb Space Telescope.”
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Johanna K. Teske et al. 2018. Magellan/PFS Radial Velocities of GJ 9827, a late K dwarf at 30 pc with Three Transiting Super-Earths. AJ, in press; arXiv: 1711.01359
