ID03: Planet Formation

The 1 mm continuum emission from the protoplanetary disk around the nearby star TW Hydrae from the Submillimeter Array. The image is 200 au on a side; the resolution is shown as a white ellipse in the lower left corner. At the time, this was the highest spatial resolution image of such a disk (18 au). (Andrews et al. 2012, ApJ, 744, 162)

The 1 mm continuum emission from the same disk, but now at 30 milliarcsecond resolution from ALMA. The factor of 10 resolution improvement reveals the disk is rife with small-scale ring and gap features. (Andrews et al. 2016, ApJL, 820, L40; Huang et al. 2018, ApJ, 852, 122)

Beyond the existential desire to understand our origins, the processes involved in the formation and early evolution of planetary systems have profound impacts on the interpretation of the observed properties of Solar System bodies and the vast population of known exoplanets.  The masses, orbital architectures, and compositions of planets as they are observed today were dramatically modified over a short interval during their formation epoch, primarily through complicated dynamical interactions with their birth environments, namely the disks of gas and dust that orbit young stars.  Measurements of the properties of these disks provide crucial information for developing a more nuanced theory of how planets form and evolve. New opportunities for more detailed, sensitive observations of these disks are now available in the especially useful millimetre wavelength regime, thanks to the ALMA observatory.

One of the foundational issues in planet formation theories is the assembly of disk solids into planetesimals – planetary building blocks.  The small dust grains incorporated into the disk as it forms need to grow at least 10 orders of magnitude on roughly million-year timescales if terrestrial planets or giant planet cores are to be formed before the disk dissipates.  There are some key bottlenecks in growth toward planetesimals when pebble sizes (mm / cm) are reached. For the standard assumption of a smooth gas disk, these pebbles feel a drag force that sends them streaming toward the star on very short timescales, depleting the solid mass reservoir needed to make larger bodies.  The hypothesised solution to this long-standing problem is that the gas disk must not be smooth. Instead, it should be riddled with small density (pressure) modulations that can slow or stop the migration of solids, making local regions of amplified solid densities that facilitate the rapid growth of planetesimals. These local concentrations of pebbles should manifest themselves observationally as fine-scale “substructures” in the thermal continuum morphologies emitted by disks at mm wavelengths.  An example of the rapid progress toward that observational goal is shown in the first image at right.

The Disk Substructures at High Angular Resolution Project (DSHARP), one of the first ALMA Large Programs, is designed to search for and characterise the basic demographic properties of such substructures, using a deep 1.3 mm continuum survey of 20 nearby disks at an angular resolution of 35 milliarcseconds.  The ALMA data from DSHARP reveal that these disk substructures are ubiquitous, though they vary substantially in their forms, scales, amplitudes, spatial distributions, symmetry, and connections to their stellar host properties. These features are generally consistent with the conditions necessary to facilitate rapid planetesimal formation.  In some cases, the substructure properties are in line with the expected dynamical perturbations from unseen planetary companions (which would imply that planetesimal formation occurs much earlier than is normally expected).

These preliminary results from DSHARP herald a new beginning – enabled by the high angular resolution of the ALMA observatory, the subject of planet formation now has a clear observational counterpart to decades of theoretical focus.  The prospects are good for extensive new observational programs to link the formation and early evolution of planetary systems with their birth environments, and thereby to provide essential context for a better understanding of the properties of the exoplanet population and our own Solar System.

Sean M. Andrews is an astrophysicist on the staff of the Smithsonian Institution’s Astrophysical Observatory (SAO), part of the Harvard-Smithsonian Center for Astrophysics (CfA), and a Lecturer in the Harvard University Department of Astronomy.