Rocky Planet Formation: Quick and Neat

Abstract

We reconsider the commonly held assumption that warm debris disks are tracers of terrestrial planet formation. The high occurrence rate inferred for Earthmass planets around mature solar-type stars based on exoplanet surveys (∼ 20%) stands in stark contrast to the low incidence rate (≤ 2%-3%) of warm dusty debris around solar-type stars during the expected epoch of terrestrial planet assembly (∼ 10 Myr). If Earth-mass planets at AU distances are a common outcome of the planet formation process, this discrepancy suggests that rocky planet formation occurs more quickly and/or is much neater than traditionally believed, leaving behind little in the way of a dust signature. Alternatively, the incidence rate of terrestrial planets has been overestimated or some previously unrecognized physical mechanism removes warm dust efficiently from the terrestrial planet region. A promising removal mechanism is gas drag in a residual gaseous disk with a surface density 10 −5 of the minimum mass solar nebula.

Numerical Simulations of Collisional Cascades at the Roche Limits of White Dwarf Stars

“…As derived in Kenyon & Bromley (2016a), the collision time for a system of monodisperse particles orbiting within a ring is t 0 = rρP/12πΣ, where ρ is the mass density, P = 2π/Ω is the orbital period, and Σ = M d /2πa∆a is the surface density (see also Kenyon et al 2016, and references therein). This derivation assumes f g = 1.…”

Residual Gas and Dust around Transition Objects and Weak T Tauri Stars*

“…However, the same upper limit of 10 −4 of MMSN is not sensitive enough to weigh in on whether residual gas whisks away the dusty debris signature of terrestrial planet formation. As described by Kenyon et al (2016), the unexpected discrepancy between the high frequency of Earth-mass exoplanets within an AU of mature sun-like stars (∼ 20%) and low incidence rate (few %) of the warm debris that their formation is expected to produce could be explained as a consequence of a long-lived dilute reservoir of residual gas (at a level of 10 −5 of MMSN) that removes the debris through aerodynamic drag and radiation pressure. This explanation is perhaps more palatable than alternative explanations which include the possibility that rocky Earth-mass planets are not as common as currently estimated or that terrestrial planets form via a much different (neater) process than is currently imagined, producing much less debris.…”

“…As discussed by Kenyon et al (2016), if residual gas is present in the terrestrial planet region at a level 10 −5 of MMSN, the action of aerodynamic drag and radiation pressure can remove (or prevent the formation of) the debris particles that produce an observable excess.…”

“…The occurrence rate of Earth-mass exoplanets at orbital distances of 0.4-1 AU (∼ 20%) is found to be much larger than the incidence rate of warm debris around solar-type stars at ages 10 Myr (few %). As discussed by Kenyon et al (2016), a simple way to resolve this discrepancy is if a dilute reservoir of gas (10 −5 of MMSN) persists for ∼ 10 Myr and removes the debris produced by planet formation. More sensitive measures of residual gas than those obtained here, as well as thermal-chemical models tuned to the conditions of dissipating T Tauri disks, are needed to test this hypothesis.…”

“…A residual gas disk with a column density > 10 −5 of the minimum mass solar nebula (MMSN) that persists during the epoch of terrestrial planet assembly can quickly remove small grains from the terrestrial planet region or prevent their formation (Kenyon et al 2016). If either occurs, warm debris is not the reliable beacon of terrestrial planet formation that it is commonly regarded to be (Kenyon & Bromley 2004;Raymond et al 2011Raymond et al , 2012Leinhardt et al 2015).…”