Project “Statistical comparison with observations”: The New Generation Planetary Population Synthesis (NGPPS)

Based on the newest version of the Bern global planet formation and evolution model, we have conducted a new generation of large-scale planetary population synthesis calculations. The new Generation III of the Bern model couples a fast N-body integrator (which allows the inclusion of 100 or more planetary seed per disc) with the long-term thermodynamical evolution of the planets. Thanks to this, the Generation III Bern model predicts directly the observable quantities of all major observational techniques (mass, orbit, radius, luminosity, magnitudes). This makes it possible to statistically compare the theoretical results with very different sets of observational constraints. This major project is conducted in a collaboration including several partners inside (Projects 2.1, 2.5), and outside of the NCCR (MPIA Heidelberg; Univ. of Arizona, Tucson). These new NGPPS simulations may be among the most comprehensive simulations of planetary system formation and evolution ever made.

The animation shows the formation of 978 planetary systems around solar-like stars in population J36. There are initially 100 lunar-mass seeds in each disk. The colors show the planetary bulk composition (green=iron+silicates, blue=with ice, red=more than 50% hydrogen+helium in mass). The black horizontal lines extend from the periastron to the apoastron. The planets of the solar system are also shown with black crosses. Gray dots show protoplanets that were lost along the formation process through collisions, ejections, or by falling into the host star.

Project “Luminosities at early times”

Except for Beta Pictoris b, the mass-luminosity relation of young giant planets is today solely based on theoretical considerations. This theoretical relation is highly uncertain as it depends on assumptions about the post-formation entropy of the planetary interior: for a given planet mass, a hot start (high entropy) associated with a high luminosity results if the accretional energy is transported into the planet during formation. This scenario is traditionally associated with giant planet formation via gravitational instability. A cold start (low luminosity and entropy) in contrast results if the accretion energy is radiated away at an accretion shock. This is often associated with core accretion. In this project, we have conducted (Marleau et al. 2017, Marleau et al. 2019) detailed radiation-hydrodynamic simulations of the planetary accretion shock to obtain a grid of post shock entropies and radiative shock efficiencies. We have found rather high post-shock entropies which may point toward warm starts.

Physical loss efficiency of the radiative accretion shock as a function of shock Mach number. The limit of an efficiency = 0 corresponds to all incoming energy being absorbed into the planet, while an efficiency = 100% means that the kinetic energy of the gas is entirely radiated away. Note that even a high efficiency which is close to, but not exactly equal 100 % can already be a very efficient heating source for the planet. Figure from Marleau et al. (2017).

Project “The impact of stellar proximity: evaporation and bloating”

The analysis of the Kepler data shows that there is a depleted region in the distance-radius plane separating larger Sub-Neptune planets from smaller Super-Earth planets (Fulton et al. 2017). This valley can be explained as the consequence of the evaporative loss of primordial H/He envelopes, making it an “evaporation valley”. This feature was theoretically predicted before its discovery independently by several theoretical models, including ours. With the Bern evolution model COMPLETO21 (which models the cooling and contraction of planets including the effect of atmospheric escape), we have shown (Jin & Mordasini 2018) that the observed gap not only agrees with the theoretical predictions, but that its locus even allows to constraint the bulk composition of the planets inside of a few 0.1 AU with radii less than about 1.6 Earth radii. It is found that the observed valley agrees with a predominantly Earth-like (silicate&iron) composition, but rules out a mainly ice-dominated composition. These results put key constraints on formation models: they show that first, these planets have formed while the protoplanetary gas disk was still present, and second that the planets accrete only inside of the iceline with orbital migration restricted to the inner system.

Comparison of the distribution of planets in the plane of orbital distance versus planetary radius in two numerical simulations (colored and black dots) with the observed frequency (yellow-brown contours, with dark colors indicating high occurrence, from Fulton et al. 2017). The diagonal evaporation valley is visible. Its locus agrees with rocky, Earth-like cores (left panel), but not with icy cores (right panel). Figure from Jin & Mordasini (2018).

Project “Thermodynamical and compositional evolution of planets” and Bern Exoplanet Tracks (BEX)

Future observational facilities like the JWST and the ELT will be able to directly image young planets of lower masses than current instruments. In a collaboration with Projects 2.3 and 3.1, we (Linder et al. 2019) have therefore extended the classical cooling models (like Burrows et al. 1997 or Baraffe et al. 2003) to lower masses, and assessed the detectability of the planets. We have found that planets of 20, 100, and 1000 Earth masses are detectable with the JWST in the background-limited regime to ages of about 10, 100, and 1000 Myr, respectively.

In future work, we will generalize these Bern Exoplanet Tracks (BEX) together with Project 3.1 to a wider mass range, range of post-formation entropies, atmospheric and bulk compositions, and couple them to new atmospheric models developed in Project 3.1. The results will be compared with observations from Projects 2.3 and 3.3.

Spectra of a 20 Earth-mass self-luminous planet as a function of time for cloud-free solar metallicity atmospheres together with the associated blackbody spectrum. The age is given in colors. The temperature of the blackbodies corresponds to the temperature of the planet at the given age. The grey dots show the background sensitivity limits of JWST/NIRCam, the black dots those of JWST/MIRI. Figure from Linder et al. (2019).

Project “Disk Instability Population synthesis” (DIPSY)

The objective of this project is to develop a new population synthesis model in the framework of the gravitational instability/direct collapse model for giant planet formation. This activity is partially supported by the NCCR PlanetS phase-1 new initiative “DIPSY” and is conducted in collaboration with Project 2.4. This project is also linked to observations of young giant exoplanets by direct imagining (Projects 2.3 and 3.3) and hydrodynamical simulations of protoplanetary disks (Project 1.2).

Figure caption: Surface density (color coded) as a function of time and distance from the star in an infalling and evolving circumstellar disk. The disk fragments six times, and the forming gas clump migrates into the star. From Schib et al. in prep.