Evidence for the start of planet formation in a young circumstellar disk

TL;DR

This study provides evidence that grain growth and the formation of millimeter-sized dust grains occur early in the life of young protoplanetary disks, indicating that planet formation begins during the earliest stages of star development.

Contribution

It presents observational evidence that grain growth happens in very young disks, supporting early planet formation theories.

Findings

Lack of CO isotopologue emission within 15 au of the disk

Millimeter-sized grains cause shielding, explaining emission absence

Planet formation likely begins during embedded phases of star development

Abstract

The growth of dust grains in protoplanetary disks is a necessary first step towards planet formation. This growth has been inferred via observations of thermal dust emission towards mature protoplanetary systems (age >2 million years) with masses that are, on average, similar to Neptune3. In contrast, the majority of confirmed exoplanets are heavier than Neptune. Given that young protoplanetary disks are more massive than their mature counterparts, this suggests that planet formation starts early, but evidence for grain growth that is spatially and temporally coincident with a massive reservoir in young disks remains scarce. Here, we report observations on a lack of emission of carbon monoxide isotopologues within the inner ~15 au of a very young (age ~100,000 years) disk around the Solar-type protostar TMC1A. By using the absence of spatially resolved molecular line emission to infer the gas and dust content of the disk, we conclude that shielding by millimeter-size grains is responsible for the lack of emission. This suggests that grain growth and millimeter-size dust grains can be spatially and temporally coincident with a mass reservoir sufficient for giant planet formation. Hence, planet formation starts during the earliest, embedded phases in the life of young stars.