KEPLER-445,KEPLER-446 AND THE OCCURRENCE OF COMPACT MULTIPLES ORBITING MID-M DWARF STARS

Abstract

We confirm and characterize the exoplanetary systems Kepler-445 and Kepler-446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler-445 is a metal-rich ([Fe/H]=+0.25 ± 0.10) M4 dwarf with three transiting planets, and Kepler-446 is a metal-poor ([Fe/H]=-0.30 ± 0.10) M4 dwarf also with three transiting planets. Kepler-445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler-446 system is similar to the Kepler-42 system: both are metal-poor with large galactic space velocities and three shortperiod, likely-rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for shortduration transits. Combining Kepler-445, Kepler-446 and Kepler-42, and isolating all mid-M dwarf stars observed by Kepler with the precision necessary to detect similar systems, we calculate that 21 +7−5 % of mid-M dwarf stars host compact multiples (multiple planets with periods of less than 10 days) for a wide range of metallicities. We suggest that the inferred planet masses for these systems support highly efficient accretion of protoplanetary disk metals by mid-M dwarf protoplanets.The same calculation for the Sun results in only 5% of disk metals contributing to rocky planets (Earth, Venus, Mars and Mercury), with significantly more contributing to the cores of the solar system's gas giant planets.The preference for metals to contribute to rocky planets rather than gas-giant cores would be strong evidence for the planet-formation scenario suggested by Laughlin et al. (2004), in which gas-giant-core embryos form in the protoplanetary disks around M dwarf stars; however, the gas in the disk dissipates before those embryos grow large enough to accrete and are cut-off as terrestrial planets. The scenario is already supported by the relative scarcity of gas-giant exoplanets found to orbit M dwarf stars. Using radial velocity observations, Johnson et al. (2010) found a statistical decrease in giant planet planet occurrence with decreasing host star mass, including M dwarfs in the radial velocity sample. However, Gaidos & Mann (2014) do not find strong support for a statistical deficiency of gas giant planets orbiting M dwarfs, though they cannot statistically rule out a deficiency. Regardless, the presence of failed embryos in some consistent proportion to the amount of available metals in the protoplanetary disk would provide support for the cut-off accretion scenario.

THEK2-ESPRINT PROJECT III: A CLOSE-IN SUPER-EARTH AROUND A METAL-RICH MID-M DWARF

“…Muirhead et al (2015) showed that a significant fraction (21 % 5 7 -+ ) of mid-M dwarfs host multiple planets within 10 days. Concerning K2-28, we could not find evidence of another transiting planet in our BLS analysis.…”

“…While K2-28 is faint in the optical, it is relatively bright in the NIR (m H =11.03±0.02 mag), and thus is likely within the reach of existing and planned NIR radial velocity instruments (e.g., IRD, CARMENES, SPIrou, HPF; Artigau et al 2014;Kotani et al 2014;Mahadevan et al 2014;Quirrenbach et al 2014), which could also reveal any non-transiting planets. K. Planets in single and multiple systems are shown in blue squares and red circles, respectively (Charbonneau et al 2009;Muirhead et al 2012Muirhead et al , 2015Berta-Thompson et al 2015). "Kepler-" is abbreviated as "K-" in this plot.…”

“…GJ 1214b is one of the most intensely observed exoplanets, and the first detailed atmospheric characterization of this intermediate-sized planet between Earth-and Neptune-like planets was enabled by the large transit depth and brightness of its host star (e.g., Charbonneau et al 2009;Bean et al 2010;Kreidberg et al 2014). However, both the census of small planets around mid-to-late M dwarfs (stars with effective temperatures of T 3400 eff  K) and the atmospheric characterization of these objects are still in their infancy; only five transiting systems (GJ 1214, GJ 1132, Kepler-42, Kepler-445, andKepler-446) have been reported to date around mid-to-late M dwarfs (Muirhead et al 2012(Muirhead et al , 2015Berta-Thompson et al 2015), and the latter three are too faint or the transit depths too shallow to permit intensive follow-up studies.…”

A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100-hr Spitzer Campaign

“…Muirhead et al (2015) estimated that 21 % 5 7 -+ of mid-M dwarfs host multiple planets, all with orbital periods less than 10 days. Assuming a disk mass-tostellar mass ratio of 1%, these planets would account for nearly all of the non-hydrogen and helium disk mass, and Muirhead et al (2015) suggest that this may indicate a very high efficiency in the planet formation mechanism around M dwarfs and a lack of planets at large orbital periods around mid-M dwarfs. A study by Ballard & Johnson (2016) generated synthetic populations of exoplanets described by a range of planet multiplicity and mutual inclination for comparison to the Kepler M dwarf sample.…”

First limits on the occurrence rate of short-period planets orbiting brown dwarfs

“…We therefore conclude that the occurence is in the region of 20-30%. We extend our simulations to investigate whether we could detect Kepler-42 (Muirhead et al 2012) and TRAPPIST-1 (Gillon et al 2016). Both systems involve three Earth-sized planets at varying orbital periods:…”