Person:

Latham, David

Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of five transiting planets, all packed within the orbital distance of Mercury in our own solar system. A transition from rocky to gaseous planets with a planetary transition radius of ~1.6 ${R}{\oplus }$ has recently been proposed by several articles in the literature. Kepler-20b (${R}{p}$ ~ 1.9 ${R}{\oplus }$) has a size beyond this transition radius; however, previous mass measurements were not sufficiently precise to allow definite conclusions to be drawn regarding its composition. We present new mass measurements of three of the planets in the Kepler-20 system that are facilitated by 104 radial velocity measurements from the HARPS-N spectrograph and 30 archival Keck/HIRES observations, as well as an updated photometric analysis of the Kepler data and an asteroseismic analysis of the host star (${M}{\star }$ = $0.948\pm 0.051$ ${M}{\odot }$ and ${R}{\star }$ = $0.964\pm 0.018$ ${R}{\odot }$). Kepler-20b is a ${1.868}{-0.034}^{+0.066}$ ${R}{\oplus }$ planet in a 3.7 day period with a mass of ${9.70}{-1.44}^{+1.41}$ ${M}{\oplus }$, resulting in a mean density of ${8.2}{-1.3}^{+1.5}$ ${\rm{g}},{\mathrm{cm}}^{-3}$, indicating a rocky composition with an iron-to-silicate ratio consistent with that of the Earth. This makes Kepler-20b the most massive planet with a rocky composition found to date. Furthermore, we report the discovery of an additional non-transiting planet with a minimum mass of ${19.96}{-3.61}^{+3.08}$ ${M}{\oplus }$ and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).

Kepler-93b is a 1.478 ± 0.019 R⊕ planet with a 4.7 day period around a bright (V = 10.2), astroseismically characterized host star with a mass of 0.911 ± 0.033 M and a radius of 0.919 ± 0.011 R. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02±0.68 M⊕. The corresponding high density of 6.88±1.18 g cm−3 is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1 and 6 M⊕, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 M⊕. All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1–6 M⊕ planets.

We present the discovery of the Kepler-20 planetary system, which we initially identified through the detection of five distinct periodic transit signals in the Kepler light curve of the host star 2MASS J19104752+4220194. From high-resolution spectroscopy of the star, we find a stellar effective temperature T eff = 5455 ± 100 K, a metallicity of [Fe/H] = 0.01 ± 0.04, and a surface gravity of log g = 4.4 ± 0.1. We combine these estimates with an estimate of the stellar density derived from the transit light curves to deduce a stellar mass of M sstarf = 0.912 ± 0.034 M ☉ and a stellar radius of R sstarf = 0.944+0.060 –0.095 R ☉. For three of the transit signals, we demonstrate that our results strongly disfavor the possibility that these result from astrophysical false positives. We accomplish this by first identifying the subset of stellar blends that reproduce the precise shape of the light curve and then using the constraints on the presence of additional stars from high angular resolution imaging, photometric colors, and the absence of a secondary component in our spectroscopic observations. We conclude that the planetary scenario is more likely than that of an astrophysical false positive by a factor of 2 × 105 (Kepler-20b), 1 × 105 (Kepler-20c), and 1.1 × 103 (Kepler-20d), sufficient to validate these objects as planetary companions. For Kepler-20c and Kepler-20d, the blend scenario is independently disfavored by the achromaticity of the transit: from Spitzer data gathered at 4.5 μm, we infer a ratio of the planetary to stellar radii of 0.075 ± 0.015 (Kepler-20c) and 0.065 ± 0.011 (Kepler-20d), consistent with each of the depths measured in the Kepler optical bandpass. We determine the orbital periods and physical radii of the three confirmed planets to be 3.70 days and 1.91+0.12 –0.21 R ⊕ for Kepler-20b, 10.85 days and 3.07+0.20 –0.31 R ⊕ for Kepler-20c, and 77.61 days and 2.75+0.17 –0.30 R ⊕ for Kepler-20d. From multi-epoch radial velocities, we determine the masses of Kepler-20b and Kepler-20c to be 8.7 ± 2.2 M ⊕ and 16.1 ± 3.5 M ⊕, respectively, and we place an upper limit on the mass of Kepler-20d of 20.1 M ⊕ (2σ).

Kepler-10b was the first rocky planet detected by the Kepler satellite and confirmed with radial velocity follow-up observations from Keck-HIRES. The mass of the planet was measured with a precision of around 30%, which was insufficient to constrain models of its internal structure and composition in detail. In addition to Kepler-10b, a second planet transiting the same star with a period of 45 days was statistically validated, but the radial velocities were only good enough to set an upper limit of 20 M ⊕ for the mass of Kepler-10c. To improve the precision on the mass for planet b, the HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N spectrograph on the Telescopio Nazionale Galileo on La Palma. In total, 148 high-quality radial-velocity measurements were obtained over two observing seasons. These new data allow us to improve the precision of the mass determination for Kepler-10b to 15%. With a mass of 3.33 ± 0.49 M ⊕ and an updated radius of $1.47^{+0.03}{-0.02}$ R ⊕, Kepler-10b has a density of 5.8 ± 0.8 g cm–3, very close to the value predicted by models with the same internal structure and composition as the Earth. We were also able to determine a mass for the 45-day period planet Kepler-10c, with an even better precision of 11%. With a mass of 17.2 ± 1.9 M ⊕ and radius of $2.35^{+0.09}{-0.04}$ R ⊕, Kepler-10c has a density of 7.1 ± 1.0 g cm–3. Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods.