State-of-the-art, Project Objectives and Results

1. State-of-the-art

Understanding the demographics of substellar companions in the transition mass regime between giant planets (GPs) and brown dwarfs (BDs) is today one of the key open questions in exoplanetary science. Broadly speaking, the fundamental goal is to measure the frequency/occurrence rate fp of such companions as a function of 1) their parameters (mass, radius, orbital elements), 2) host star properties (mass, age, and metallicity), and 3) environment (multiplicity, birthplace) [1,2]. These distribution functions represent the ultimate empirical data set (the “ground truth”), as they retain imprints of the dominant physical processes driving their different formation and evolutionary histories [3,4].

Substellar objects are classified according to their masses. Massive (giant) planets typically are defined as having M > 0.3 Jupiter masses (MJ; [5]). BDs are substellar objects in the 13–75 MJ mass range and occupy the domain between massive planets and stars (Fig. 1). The mass limit that separates BDs from massive planets is conventionally the minimum mass required to fuse deuterium in the core of the substellar object, that is 13 MJ [6,7], while the limit between BDs and stars comes from the minimum mass for hydrogen fusion in the core [8]. Both boundaries depend to a small extent on the metallicity of the object [9]. Despite the discovery of thousands of GPs and BDs, the statistics on their occurrence and properties are still very incomplete. Most statistical studies of substellar companions are for close-in (a < 0.5-1 AU) hot and warm (super-)Jupiters [11,12], while cold (a > 1 AU) GPs and BDs, which are intrinsically more abundant than hot Jupiters, are poorly characterized as a population [5]. Like our Solar System’s Jupiter and Saturn, they are dynamically dominant in their system and influence the formation and evolution of any interior planets, including habitable worlds [3,4,13,14].

The are two leading planet formation models: the core accretion model [15,16,17,18,19] predicts giant planets with masses up to 30-40 MJ within 20-30 au from the central star. The second path is based on fragmentation of a self-gravitating disk which is most likely to occur only in the outer part of the protoplanetary disk in its initial stages of evolution [20,21]. This mechanism occurs preferentially around higher mass stars [22] while it is inefficient for M type stars, even if this aspect is still debated [23]. It is therefore possible that low-mass BDs might form like massive planets, through the disk gravitational instability scenario. High-mass BDs, by contrast, are likely to form like stellar binary systems, through molecular cloud fragmentation [24]. Brown dwarfs are interesting as the transition between the formation mechanisms of GPs and stars runs through their population. One interesting characteristic of the BD population is that few are detected at short separations (a < 3 AU), a feature known as the BD desert [25,26,27]. New detections have shrunk this desert in recent years [28,29,30], but there is still a detection deficit for orbital periods under 100 days and masses between 30 and 60 MJ [24,29,31]. At wider separations (≈ 20−1000 AU) BD occurrence rises to 2 − 3% [24]. While the existence of the BD desert is well-established, its actual boundaries and shape are still a matter of debate.

Discriminating between the shape of the mass function of BDs and massive planets, and their possible dependence on orbital and stellar host properties, would provide the critical observational evidence to distinguish between the two possible formation channels, and to empirically and robustly constrain for the first time the value of the transition mass between the two regimes. Major improvements in our understanding of occurrence rates of massive planets and BDs in the intermediate- and wide-separation regime ought to come from the effective combination of data from different techniques, and from the development of joint and conditional occurrence rates. However, the comparison, or combination, of statistical constraints from different surveys with different detection techniques is hindered by a) lack of data with sufficient sensitivity and b) unaccounted for or unknown effects of biases and systematic errors sources (uncertainty in detection efficiency, orbital eccentricities, planet multiplicity, stellar parameters, reliability of weak planet candidates). As a consequence, many major questions about the demographics of these two samples remain open: What is the transition mass between GP and BD companions? Are their occurrence rates fundamentally different? What is the relationship between intermediate- and wide-separation massive planets and BDs and inner low-mass planets? How do systems architectures depend on the host stars’ properties and environment (e.g., binarity)?

2. Project Objectives

We propose to combine, analyze, model and interpret Gaia DR3 high-precision absolute astrometry information with the best-available (archival and new) ground-based high-contrast imaging (HCI) data, high-precision RV measurements, and transit photometry results gathered with state-of-the-art and next-generation cutting-edge instrumentation. Members of our team have preferential access for the next three years to the top facilities in the world in both hemispheres for HCI (SPHERE@VLT, SHARK-VIS/NIR@LBT) and Doppler spectroscopy (HARPS@ESO-3.6m, ESPRESSO@VLT, HARPS-N@TNG), and to space-based high-precision transit photometry (CHEOPS) through conspicuous GTO and competitively approved long-term/large programs. On our team are stellar and exoplanetary scientists, observers and theoreticians, instrument builders and software experts. Table 1 summarizes our team’s expertise and project involvement. We will carry out a research program with a broad, challenging, scientific objective that can be seen as structured in 4 major, interconnected goals: 

Goal 1 – Characterization GP and BD Orbits and Masses – We will effectively combine Gaia DR3 data products with existing RVs, transit light-curves (LCs), and HCI data to derive improved masses and orbital architectures of known systems containing intermediate- and wide-separation GPs and BDs, and detect (or place limits on the presence of) new short- and long-period substellar companions.

Goal 2 – Demographics of the GP and BD Population – We will perform the first-ever systematic study of the demographics of GPs and BDs over a broad range of separations using the first sample of true dynamical masses derived by Gaia astrometry as the centerpiece. We will derive occurrence rates, study trends and correlations of the population taking into account the complex parameter space (planetary parameters, multiplicity, stellar properties such as mass, age, chemical composition, binarity) and the global Gaia survey sensitivity (noise properties, inhomogeneous coverage, and false positives). We will obtain the first detailed characterization of the transition mass regime between planets and brown dwarfs. Finally, we will connect our findings with other statistical inferences from other surveys in the overlapping region of parameter space, and compare our results with the outcome of existing studies of the demographics of close-in exoplanets.

Goal 3 - GP and BD Formation and Evolution Pathways – We will make detailed comparisons between our occurrence rate calculations and expectations from state-of-the-art formation models of GPs and BDs [4,19]. We will reproduce the orbital properties of individual objects in terms of outcomes of competing models of orbital evolution (e.g., high eccentricity migration induced by scattering events, smooth disk migration), as additional probes of their origin. We will investigate the dynamical evolution of systems in which GPs and BDs are accompanied by other planetary companions, and verify the existence of stable regions at habitable zone distances where terrestrial planets might be found. We will outline dynamical similarities among the different systems, which may be related to similar evolutionary paths, and search for evidence of possible correlations between order of multiplicity, hierarchy in mass, and orbital properties.

Goal 4 - Host Characterization – We will combine the spectroscopic information from high-resolution spectra (effective temperature, surface gravity, iron abundance) with photometric information on the spectral energy distribution (SED), and the revised Gaia DR3 parallaxes to determine, using constraints from stellar evolutionary tracks, accurate and precise fundamental physical parameters of the host stars (age, mass, radius), which will be utilized (Goal 1) to derive improved values of the giant planetary and brown dwarf companion masses (and radii, and hence densities, for transiting systems). Homogeneous iron abundance measurements of the hosts will be used in the detailed investigations of trends and correlations in occurrence rates (Goal 2), while elemental abundance of refractory and volatiles elements will be used as diagnostics to provide constraints of the GP/BD formation and migration history (Goal 3).

3. Results

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