Formation of rocky super-earths from a narrow ring of planetesimals

Abstract

The formation of super-Earths, the most abundant planets in the Galaxy, remains elusive. These planets have masses that typically exceed that of the Earth by a factor of a few; appear to be predominantly rocky, although often surrounded by H/He atmospheres; and frequently occur in multiples. Moreover, planets that encircle the same star tend to have similar masses and radii, whereas those belonging to different systems exhibit remarkable overall diversity. Here, we advance a theoretical picture for rocky planet formation that satisfies the aforementioned constraints: building upon recent work -which demonstrates that planetesimals can form rapidly at discrete locations in the disk -we propose that super-Earths originate inside rings of silicate-rich planetesimals at approximately ∼1AU. Within the context of this picture, we show that planets grow primarily through pairwise collisions among rocky planetesimals, until they achieve terminal masses that are regulated by isolation and orbital migration. We quantify our model with numerical simulations and demonstrate that our synthetic planetary systems bear a close resemblance to compact, multi-resonant progenitors of the observed population of short-period extrasolar planets. Our results thus indicate that the absence of short-period super-Earths within the solar system can simply be attributed to the comparatively low mass of the primordial planetesimal ring within the protosolar nebula.

Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

“…This calls into question the correspondence shown in Figure 1. However, other models argue for "in situ" formation (e.g., Lee et al 2014;Batygin & Morbidelli 2023), and as we discuss below, at earlier stages the nebular gas is warmer with a more distant soot line. Observational tests of these competing ideas are thus strongly desired.…”

Inner Planetary System Gap Complexity is a Predictor of Outer Giant Planets

“…On the other hand, recent simulations show that OGs can also induce a secular resonance sweeping propagating inward through the planet-forming disk, leading to an enhancement of planetesimal rings in the inner regions from which super-Earths can form (Best et al 2023). One pathway that has been proposed to broadly explain the observed architectures of inner multiplanet systems is the so-called "inside-out planet formation (IOPF)" whereby planets form in successive rings of material building up at the magnetorotational instability (MRI) boundary starting at ∼0.1 au (Chatterjee & Tan 2014Hu et al 2016;see Tan et al 2016 for a summary; see also Batygin & Morbidelli 2023 for the formation of multiple small planets from a single narrow ring). As one planet forms and carves a gap in the gaseous disk, the MRI boundary retreats farther out, producing another ring and repeating the process.…”

Dissipative Capture of Planets into First-order Mean-motion Resonances

“…To this end, numerous lines of evidence suggest that the outer solar system itself originated in a compact multiresonant configuration before becoming temporarily unstable and eventually settling in its current state by way of dynamical friction (Batygin & Brown 2010;Nesvorný & Morbidelli 2012). Such a sequence of events may in fact constitute a relatively typical postnebular evolutionary path of planetary systems, and recent modeling has shown that both the period ratio distribution, as well as the degree of intrasystem uniformity of short-period super-Earths, can be satisfactorily reproduced if the majority of systems originate as resonant chains that subsequently relax toward more-widely spaced orbits through dynamical instabilities (Izidoro et al 2017(Izidoro et al , 2021Batygin & Morbidelli 2023).…”