The newborn planet population emerging from ring-like structures in discs

Abstract

ALMA has observed a plethora of ring-like structures in planet forming discs at distances of 10-100 au from their host star. Although several mechanisms have been invoked to explain the origin of such rings, a common explanation is that they trace new-born planets. Under the planetary hypothesis, a natural question is how to reconcile the apparently high frequency of gap-carving planets at 10-100 au with the paucity of Jupiter mass planets observed around main sequence stars at those separations. Here, we provide an analysis of the new-born planet population emerging from observations of gaps in discs, under the assumption that the observed gaps are due to planets. We use a simple estimate of the planet mass based on the gap morphology, and apply it to a sample of gaps recently obtained by us in a survey of Taurus with ALMA. We also include additional data from recent published surveys, thus analysing the largest gap sample to date, for a total of 48 gaps. The properties of the purported planets occupy a distinctively different region of parameter space with respect to the known exo-planet population, currently not accessible through planet finding methods. Thus, no discrepancy in the mass and radius distribution of the two populations can be claimed at this stage. We show that the mass of the inferred planets conforms to the theoretically expected trend for the minimum planet mass needed to carve a dust gap. Finally, we estimate the separation and mass of the putative planets after accounting for migration and accretion, for a range of evolutionary times, finding a good match with the distribution of cold Jupiters.

The paradox of youth for ALMA planet candidates

“…This calculation shows that planet migration paradox for ALMA planet candidates exists independently of whether the planet accretes gas or not. The calculation also agrees with the results of Lodato et al (2019), predicting that the planets would end up as massive planets in the future. However the challenge is that this happens all too quickly, begging the question of how can it be that we are so lucky to observe the planets in their apparently short-lived state.…”

“…Vigan et al 2017). The planets could then be migrating inward of 10 AU rapidly, as suggested by Lodato et al (2019). However, in that case we need a train of ∼ t * /t mig gas giants per ALMA disc, that is, as many as 10 − 1000 planets per disc.…”

“…To predict how these candidate planets should evolve we need to know the gas disc surface density Σ at the planet location. Lodato et al (2019); Nayakshin et al (2019) picked 'reasonable' protoplanetary disc parameters for these systems given our current understanding of protoplanetary disc evolution (e.g., Alexander et al 2014) and performed population synthesis, e.g., a statistical analysis of the problem. Here we instead use the observed stellar accretion rates to constrain Σ more directly for each system.…”

“…1 and 2 in Mordasini et al 2012). Nayakshin et al (2019) showed that ALMA data require the runaway gas accretion rates to be reduced from the theoretically expected ones by about an order of magnitude since otherwise the embedded ALMA candidate planets should rapidly become gas giants with masses significantly above Jupiter for these wide separations (see also Lodato et al 2019;Ndugu et al 2019). Further, Manara et al (2019) have shown how observed gas accretion rates onto the stars can be used to check population synthesis models of planet formation.…”

ALMA reveals a large structured disk and nested rotating outflows in DG Tauri B

“…Following the method outlined in Long et al (2018), we estimate a gap center of R g =53 au with a width of ∆=9 au, which is typical of those recently observed in Class II disks (Long et al 2018;Andrews et al 2018). Assuming that ∆ is 5.5 times the planet's Hill radius (Lodato et al 2019), the mass of the planet that could carve the inner gap in DG Tau B would be 0.1 M J . .…”