California-Kepler Survey. IX. Revisiting the Minimum-mass Extrasolar Nebula with Precise Stellar Parameters

Abstract

Previous works on Kepler multi-planet systems revealed a remarkable intra-system uniformity in planet radius/mass; moreover the average planet size increases with host mass/metallicity. This observation provides tantalizing evidence that the outcome of planet formation can be linked to the properties of the host star and its disk. In a simple in-situ formation scenario, the minimum-mass extrasolar nebula (MMEN) reconstructed from a planetary system reflect the properties of its natal disk.Specifically, one might expect a one-to-one correspondence between the solid surface density of the MMEN and the host star mass M and metallicity [Fe/H]. Leveraging on the precise host star properties from the California-Kepler-Survey (CKS), we found that Σ = 50 +33 −20 g cm −2 (a/AU) −1.75±0.07 (M /M) 1.04±0.22 10 0.22±0.05[Fe/H] for Kepler-like systems (1-4R ⊕ ; a <1AU). The strong M dependence is reminiscent of previous dust continuum results that the solid disk mass scales with M. The weaker [Fe/H] dependence shows that sub-Neptune planets, unlike giant planets, form readily in lower-metallicity environment. The innermost region (a < 0.1AU) of a MMEN maintains a smooth profile despite a steep decline of planet occurrence rate: a result that favors the truncation of disks by co-rotating magnetospheres with a range of rotation periods, rather than the sublimation of dusts. The Σ of Kepler multi-transiting systems shows a much stronger correlation with M and [Fe/H] than singles. This suggests that the dynamically hot evolution that produced single systems also partially removed the memory of formation in disks. Radial-velocity planets yielded a MMEN very similar to CKS planets; whereas transit-timing-variation planets' postulated convergent migration history is supported by their poorly constrained MMEN. We found that lower-mass stars have a higher efficiency of forming/retaining planets: for sun-like stars about 20% of the solid mass within ∼1AU are converted/preserved as sub-Neptunes, compared to 70% for late-K-early-M stars. This may be due to the lower binary fraction, lower giant-planet occurrence or the longer disk lifetime of lower-mass stars.

Ejection of close-in super-Earths around low-mass stars in the giant impact stage

“…We note that we focus on the distribution of massive super-Earths, which are observable around low-mass stars by ongoing and upcoming observational missions of RV measurements. The surface densities in the massive and less-massive series are similar to those estimated by RV planets and larger than those by the California-Kepler Survey (Dai et al 2020).…”

“…The surface density of protoplanets is given by the power-law disk model, Σ = Σ 1 (a/1 au) −3/2 , for which the power index was taken from the minimum-mass solar nebular model (Hayashi 1981). This power-law index is slightly larger than those derived from observed close-in super-Earths by Chiang & Laughlin (2013) (−1.6) and Dai et al (2020) (−1.75). The protoplanets are initially located at a = (0.05 au, 1 au), where a is the semimajor axis of a protoplanet.…”

“…The dependence of the surface density of protoplanets on the stellar mass (Σ 1 ∝ M h d * ) is not known. We adopt h d = 1 according to the recent study about the surface density of observed planets (Dai et al 2020, h d = 1.04). This dependence is also adopted in the fiducial case of Raymond et al (2007) and slightly shallower than that of the protoplanetary mass formed via pebble accretion (Liu et al 2019, h d = 4/3).…”

Why do more massive stars host larger planets?

“…Giant planets are scarce around M dwarfs (Bonfils et al 2013), and there is a higher occurrence of small planets around M dwarfs (Dressing & Charbonneau 2015;Gaidos et al 2016). The higher planetformation efficiency of low-mass stars (Dai et al 2020) may be related to the low giant planet occurrence or the longer disk lifetimes, or to other factors. The difference in planetary composition distribution is unclear so far.…”

Size evolution of close-in super-Earths through giant impacts and photoevaporation

“…We start the simulations with protoplanets around 1 solar mass star. Our protoplanet model is based on the recent minimum-mass extrasolar nebula (MMEN) disk profile for the multitransiting planets (Dai et al 2020).…”