Characterization of exoplanets from their formation

Abstract

Context. This paper continues a series in which we predict the main observable characteristics of exoplanets based on their formation. In Paper I we described our global planet formation and evolution model that is based on the core accretion paradigm. In Paper II we studied the planetary mass-radius relationship with population syntheses. Aims. In this paper we present an extensive study of the statistics of planetary luminosities during both formation and evolution. Our results can be compared with individual directly imaged extrasolar (proto)planets and with statistical results from surveys. Methods. We calculated three populations of synthetic planets assuming different efficiencies of the accretional heating by gas and planetesimals during formation. We describe the temporal evolution of the planetary mass-luminosity relation. We investigate the relative importance of the shock and internal luminosity during formation, and predict a statistical version of the post-formation mass vs. entropy "tuning fork" diagram. Because the calculations now include deuterium burning we also update the planetary mass-radius relationship in time.Results. We find significant overlap between the high post-formation luminosities of planets forming with hot and cold gas accretion because of the core-mass effect. Variations in the individual formation histories of planets can still lead to a factor 5 to 20 spread in the post-formation luminosity at a given mass. However, if the gas accretional heating and planetesimal accretion rate during the runaway phase is unknown, the post-formation luminosity may exhibit a spread of as much as 2-3 orders of magnitude at a fixed mass. As a key result we predict a flat log-luminosity distribution for giant planets, and a steep increase towards lower luminosities due to the higher occurrence rate of low-mass (M 10-40 M ⊕ ) planets. Future surveys may detect this upturn. Conclusions. Our results indicate that during formation an estimation of the planetary mass may be possible for cold gas accretion if the planetary gas accretion rate can be estimated. If it is unknown whether the planet still accretes gas, the spread in total luminosity (internal+accretional) at a given mass may be as large as two orders of magnitude, therefore inhibiting the mass estimation. Due to the core-mass effect even planets which underwent cold accretion can have large post-formation entropies and luminosities, such that alternative formation scenarios such as gravitational instabilities do not need to be invoked. Once the number of self-luminous exoplanets with known ages and luminosities increases, the resulting luminosity distributions may be compared with our predictions.

Constraints on observing brightness asymmetries in protoplanetary disks at solar system scale

“…This ratio is in the order of what is expected for mid-to high-mass jovian planets found in latest planet formation simulations (e.g. Mordasini 2013;Szulágyi & Mordasini 2017;Mordasini et al 2017). -Within the assumed measurement accuracies closure phase measurements were found to be more suitable to detect the companions compared to visibility measurements.…”

“…Gas shocks due to accretion may extend to several hundred planetary radii resulting in an emission which is significantly brighter than the planetary radiation alone (Szulágyi & Mordasini 2017). Recent simulations show that the total luminosity of high-mass planets (10 M ), including the radiation originated from the planetary accretion and the shock, can be as high as several per cent of the solar luminosity (Mordasini 2013;Szulágyi & Mordasini 2017;Mordasini et al 2017). An unresolved circumplanetary disk may increase the observed luminosity even further.…”

The widest Hα survey of accreting protoplanets around nearby transition disks

“…The contrast curves that we obtained for the targets of the survey reflect the exquisite performance of the instrument, and they give strong constraints on the presence of accreting companions around the selected stars. If we compare our detection limits with the predictions from the population synthesis of Mordasini et al (2017), while also assuming complete cold accretion, we expect to be unable to spot low mass (< 1 M Jup ) medium Fig. 11: Same as Fig.…”

“…Detecting forming protoplanets is the cornerstone to understanding planet formation (Fortney et al 2008;Spiegel & Burrows 2012;Mordasini et al 2017). Approximately ∼30% of Herbig Ae/Be disks should host giant planets of ∼0.1 to 10 M Jup (Kama et al 2015).…”

Breaking the centrifugal barrier to giant planet contraction by magnetic disc braking

“…In the post-runaway phase, because of insufficient gas supply, the planet no longer fills the Bondi/Hill radius but detaches from the nebula to contract on the KH timescale (Bodenheimer et al 2000;Mordasini et al 2012Mordasini et al , 2017Ginzburg & Chiang 2019a). Previous one-dimensional models assumed the planet is free to shrink by orders of magnitude until its radius is comparable to that of Jupiter.…”