A deep radius valley revealed by Kepler short cadence observations

Abstract

The characteristics of the radius valley, i.e. an observed lack of planets between 1.5-2 Earth radii at periods shorter than about 100 days, provide insights into the formation and evolution of close-in planets. We present a novel view of the radius valley by refitting the transits of 431 planets using Kepler 1-minute short cadence observations, the vast majority of which have not been previously analysed in this way. In some cases, the updated planetary parameters differ significantly from previous studies, resulting in a deeper radius valley than previously observed. This suggests that planets are likely to have a more homogeneous core composition at formation. Furthermore, using support-vector machines, we find that the radius valley location strongly depends on orbital period and stellar mass and weakly depends on stellar age, with $\partial \log {\left(R_{p, \text{valley}} \right)}/ \partial \log {P} = -0.096_{-0.027}^{+0.023}$, $\partial \log {\left(R_{p, \text{valley}} \right)}/ \partial \log {M_{\star }} = 0.231_{-0.064}^{+0.053}$, and $\partial \log {\left(R_{p, \text{valley}} \right)}/ \partial \log {\left( \text{age} \right)} = 0.033_{-0.025}^{+0.017}$. These findings favour thermally-driven mass loss models such as photoevaporation and core-powered mass loss, with a slight preference for the latter scenario. Finally, this work highlights the value of transit observations with short photometric cadence to precisely determine planet radii, and we provide an updated list of precisely and homogeneously determined parameters for the planets in our sample.

A Unified Treatment of Kepler Occurrence to Trace Planet Evolution. I. Methodology

“…The positive correlation between planet size and stellar mass for the sub-Neptunes implies that larger stars produce larger planet cores, agreeing with Wu (2019). Ho & Van Eylen (2023) use Kepler short cadence observations to find a slope of a = -+ 0.231 0.06 0.053 . It should be noted that these analyses are in the observed density of planets, and not in the occurrence of planets.…”

Conclusive Evidence for a Population of Water Worlds around M Dwarfs Remains Elusive

“…On this note, we highlight the work of Lopez (2017) that demonstrates that the escape of steam atmospheres is not efficient enough to reproduce the exoplanet demographics. We also stress that the observed lack of planets in the radius gap (e.g., Van Eylen et al 2018;Ho & Van Eylen 2023) places strong constraints on the composition spread allowed in such models, i.e., if one allows too much spread in water-mass fraction, then one cannot reproduce the emptiness of the radius gap. We conclude that while water is likely present in many, if not all sub-Neptunes to some degree, clear evidence for a population of water worlds with large ice-mass fractions (such as 1:1 silicate-to-ice ratios) still remains elusive.…”

Scaling K2. VI. Reduced Small-planet Occurrence in High-galactic-amplitude Stars

“…These stellar proximity dependent mechanisms separate the sub-Neptunes from the super-Earths. We chose to use a physically motivated bound for these planet classes and employed the empirically derived radius valley period function in Ho & Van Eylen (2023) 24 as our divider. The separation between the other classes of planets is less clear; thus, we used 4-8R ⊕ for sub-Saturns and 8-20R ⊕ for Jupiters.…”

“…We used the two-dimensional support-vector machine model fromHo & Van Eylen (2023), which was derived using the Kepler sample alone. Thus, we assumed the valley would be consistent within the K2 sample, as shown byHardegree-Ullman et al (2020).…”