Takeout and Delivery: Erasing the Dusty Signature of Late-stage Terrestrial Planet Formation

Abstract

The formation of planets like Earth is expected to conclude with a series of late-stage giant impacts that generate warm dusty debris, the most anticipated visible signpost of terrestrial planet formation in progress. While there is now evidence that Earth-sized terrestrial planets orbit a significant fraction of solar-type stars, the anticipated dusty debris signature of their formation is rarely detected. Here we discuss several ways in which our current ideas about terrestrial planet formation imply transport mechanisms capable of erasing the anticipated debris signature. A tenuous gas disk may be regenerated via takeout (i.e., the liberation of planetary atmospheres in giant impacts) or delivery (i.e., by asteroids and comets flung into the terrestrial planet region) at a level sufficient to remove the warm debris. The powerful stellar wind from a young star can also act, its delivered wind momentum producing a drag that removes warm debris. If such processes are efficient, terrestrial planets may assemble inconspicuously, with little publicity and hoopla accompanying their birth. Alternatively, the rarity of warm excesses may imply that terrestrial planets typically form very early, emerging fully formed from the nebular phase without undergoing late-stage giant impacts. In either case, the observable signposts of terrestrial planet formation appear more challenging to detect than previously assumed. We discuss observational tests of these ideas.

RZ Piscium Hosts a Compact and Highly Perturbed Debris Disk

“…Grains 1 μm in radius will be ejected via radiation pressure force (Arnold et al 2019), exhibiting rapid drops in the infrared flux (Su et al 2019). Grains larger than the radiation blowout size would also drift outward radially with the aid of tenuous gas (Kenyon & Bromley 2016;Najita & Kenyon 2023). Larger grains will experience drag forces from the stellar wind (particularly important for young solar-like systems) and Poynting-Robertson drag.…”

“…The gas could also originate from outgassing comets scattered toward the star, collisions of volatile-rich asteroids, or stripped atmospheres from giant impacts between protoplanets. Najita & Kenyon (2023) recently reviewed these processes and concluded that all of them are capable of generating 10 −3 -10 −2 M ⊕ of secondary gas for stars younger than a few hundred megayears. If the gas is secondary in origin, the ALMA CO gas limit (<10 −5 M ⊕ ) strongly disfavors a giant impact origin, as the estimated mass of the stripped atmosphere is on the order of 10 −4 M ⊕ for each giant impact (Najita & Kenyon 2023).…”

“…Najita & Kenyon (2023) recently reviewed these processes and concluded that all of them are capable of generating 10 −3 -10 −2 M ⊕ of secondary gas for stars younger than a few hundred megayears. If the gas is secondary in origin, the ALMA CO gas limit (<10 −5 M ⊕ ) strongly disfavors a giant impact origin, as the estimated mass of the stripped atmosphere is on the order of 10 −4 M ⊕ for each giant impact (Najita & Kenyon 2023). It is difficult to differentiate the remaining two mechanisms because both asteroids and comets could store volatile material when they formed and would later release it in the inner region.…”

Giant Impacts and Debris Disk Morphology

“…Giant impacts may also explain the spatially unresolved, timevariable infrared excesses in ID8 (Meng et al 2014) and V488 Persei (Rieke et al 2021). Detecting catastrophic collisions occurring <10 au from the star was anticipated from simulations of terrestrial planet formation (Kenyon & Bromley2005; but see also Najita & Kenyon 2023 for ways to erase the signature of warm dusty debris).…”

Giant Impacts and Debris Disk Morphology

“…Giant impacts may also explain the spatially unresolved, time-variable infrared excesses in ID8 (Meng et al 2014) and V488 Persei (Rieke et al 2021). Detecting catastrophic collisions occurring < 10 au from the star was anticipated from simulations of terrestrial planet formation (Kenyon & Bromley 2005; but see also Najita & Kenyon 2023 for ways to erase the signature of warm dusty debris).…”