Planet Formation Process Research Articles

Low-energy cosmic rays: regulators of the dense interstellar medium

Low energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low energy cosmic rays in the Galaxy.

Chemical Diversity in Protoplanetary Disks and Its Impact on the Formation History of Giant Planets

Giant planets can interact with multiple and chemically diverse environments in protoplanetary disks while they form and migrate to their final orbits. The way this interaction affects the accretion of gas and solids shapes the chemical composition of the planets and of their atmospheres. Here we investigate the effects of different chemical structures of the host protoplanetary disk on the planetary composition. We consider both scenarios of molecular (inheritance from the prestellar cloud) and atomic (complete chemical reset) initial abundances in the disk. We focus on four elemental tracers of different volatility: C, O, N, and S. We explore the entire extension of possible formation regions suggested by observations by coupling the disk chemical scenarios with N-body simulations of forming and migrating giant planets. The planet formation process produces giant planets with chemical compositions significantly deviating from that of the host disk. We find that the C/N, N/O, and S/N ratios follow monotonic trends with the extent of migration. The C/O ratio shows a more complex behavior, dependent on the planet accretion history and on the chemical structure of the formation environment. The comparison between S/N* and C/N* (where * indicates normalization to the stellar value), constrains the relative contribution of gas and solids to the total metallicity. Giant planets whose metallicity is dominated by the contribution of the gas are characterized by N/O* > C/O* > C/N* and allow to constrain the disk chemical scenario. When the planetary metallicity is instead dominated by the contribution of the solids we find that C/N* > C/O* > N/O*.

A Novel Method of Transit Detection Using Parallel Processing and Machine Learning

Ultra-short-period (USP) planets are rare Earth-sized planets with the shortest possible orbital periods of all known planets. The study of this group is important for investigating planet formation and evolution processes. To date, only slightly over 100 USPs have been detected in Kepler photometry data of nearby FGKM dwarfs. However, traditional methods used in detecting planets and transit-like events are often biased, inefficient and time-consuming. For the first time, we introduce a GPU fast phase folding technique coupled with a Deep Convolutional Neural Network (DCNN) specifically used for searching for USP planets. The DCNN is trained on a set of 2,000,000 synthetic USP samples and performs exceedingly well in identifying both true and false positive transit signals, with a 99.5% validation accuracy over the given training set. With the computational power provided by GPU fast phase folding, our method, compared to the traditional Box Least Squares method, has shown to be ~1000 times faster in searching for transit signals in a photometric light curve with the same or better precision and recall rate. Furthermore, our method can also be applied to other interesting planet populations beyond USP planets. We used this method to search through all available KIDs in the Kepler database and were able to reproduce all existing USP planets with a 100% recovery rate. We also discuss how further adjustments can be made, making this system even more efficient and powerful, as well as how it can be applied to broader planet populations.

The GAPS Programme at TNG

Context. Great strides have been made in recent years in the understanding of the mechanisms involved in the formation and evolution of planetary systems. Despite this, many observational findings have not yet been corroborated by astrophysical explanations. A fine contribution to the study of planetary formation processes comes from the study of young, low-mass planets, with short orbital periods (<100 days). In the last three years, the NASA/TESS satellite has identified many planets of this kind and their characterization is clearly necessary in order to understand how they formed and evolved. Aims. Within the framework of the Global Architecture of Planetary System (GAPS) project, we performed a validation and characterization (radius and mass) of the ultra-short period planet TOI-1807 b, which orbits its young host star BD+39 2643 (~300 Myr) in only 13 h. This is the youngest ultra-short period planet discovered so far. Methods. Thanks to a joint modeling of the stellar activity and planetary signals in the TESS light curve and in new HARPS-N radial-velocity measurements, combined with accurate estimation of stellar parameters, we validated the planetary nature of TOI-1807 b and measured its orbital and physical parameters. Results. By using astrometric, photometric, and spectroscopic observations, we found that BD+39 2643 is a young, active K dwarf star and a member of a 300 ± 80 Myr old moving group. Furthermore, it rotates in Prot = 8.8 ± 0.1 days. This star hosts an ultra-short period planet, exhibiting an orbital period of only Pb = 0.54937 ± 0.00001 days. Thanks to the exquisite photometric and spectroscopic series, along with the accurate information on its stellar activity, we measured both the radius and the mass of TOI-1807 b with high precision, obtaining PP,b = 1.37 ± 0.09 R⊕ and MP,b = 2.57 ± 0.50 M⊕. These planet parameters correspond to a rocky planet with an Earth-like density (ρb = 1.0 ± 0.3 ρ⊕) and no extended H/He envelope. From the analysis of the age-RP distribution for planets with well measured ages, we inferred that TOI-1807 b may have already lost a large part of its atmosphere over the course of its 300 Myr lifetime.

Analytic Approach to the Late Stages of Giant Planet Formation

This paper constructs an analytic description for the late stages of giant planet formation. During this phase of evolution, the planet gains the majority of its final mass through gas accretion at a rapid rate. This work determines the density and velocity fields for material falling onto the central planet and its circumplanetary disk, and finds the corresponding column density of this infalling envelope. We derive a steady-state solution for the surface density of the disk as a function of its viscosity (including the limiting case where no disk accretion occurs). Planetary magnetic fields truncate the inner edge of the disk and determine the boundary conditions for mass accretion onto the planet from both direct infall and from the disk. The properties of the forming planet and its circumplanetary disk are determined, including the luminosity contributions from infall onto the planet and disk surfaces, and from disk viscosity. The radiative signature of the planet formation process is explored using a quasi-spherical treatment of the emergent spectral energy distributions. The analytic solutions developed herein show how the protoplanet properties (envelope density distribution, velocity field, column density, disk surface density, luminosity, and radiative signatures) vary with input parameters (instantaneous mass, orbital location, accretion rate, and planetary magnetic field strength).

Interstellar objects

ABSTRACT Since 2017, two macroscopic interstellar objects have been discovered in the inner Solar System, both of which are distinct in nature. The first interstellar object, 1I/‘Oumuamua, passed within lunar distances of the Earth, appeared asteroidal lacking detectable levels of gas or dust loss, yet exhibited a nongravitational acceleration. 1I/‘Oumuamua's brief visit left open questions regarding its provenance which has given rise to many theoretical hypotheses, including an icy comet lacking a dust coma, an elongated fragment of a planet or planetesimal that was tidally disrupted, and an ultra-porous fractal aggregate. The second interstellar object, 2I/Borisov, was distinct from 1I/‘Oumuamua in terms of its bulk physical properties and displayed a definitive cometary tail. We review the discoveries of these objects, the subsequent observations and characterisations, and the theoretical hypotheses regarding their origins. We describe 1I/‘Oumuamua and 2I/Borisov in the context of active asteroids and comets in the Solar System. The discovery of these two objects implies a galactic-wide population of similar bodies. Forthcoming observatories should detect many more interstellar planetesimals which may offer new insights into how planetary formation processes vary throughout the Galaxy.

Direct Formation of Planetary Embryos in Self-gravitating Disks

Giant planets have been discovered at large separations from the central star. Moreover, a striking number of young circumstellar disks have gas and/or dust gaps at large orbital separations, potentially driven by embedded planetary objects. To form massive planets at large orbital separations through core accretion within the disk lifetime, however, an early solid body to seed pebble and gas accretion is desirable. Young protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks may efficiently concentrate solid material at the midplane driven by spiral waves. We run 3D local hydrodynamical simulations of gravitoturbulent disks with Lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies, seeding later planet formation. When self-gravity between dust particles is included, solids of size St = 0.1–1 concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps up to several M ⊕ in mass at wide orbits. Simulations with dust that drift most efficiently, St = 1, form the most massive clouds of particles, while simulations with smaller dust particles, St = 0.1, have clumps with masses an order of magnitude lower. When the effect of dust backreaction onto the gas is included, dust clumps become smaller by a factor of a few but more numerous. The existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets distant from the central stars and young protoplanetary disks with substructures.

Combined effects of disc winds and turbulence-driven accretion on planet populations

ABSTRACT Recent surveys show that protoplanetary discs have lower levels of turbulence than expected based on their observed accretion rates. A viable solution to this is that magnetized disc winds dominate angular momentum transport. This has several important implications for planet formation processes. We compute the physical and chemical evolution of discs and the formation and migration of planets under the combined effects of angular momentum transport by turbulent viscosity and disc winds. We take into account the critical role of planet traps to limit Type I migration in all of these models and compute thousands of planet evolution tracks for single planets drawn from a distribution of initial disc properties and turbulence strengths. We do not consider multiplanet models nor include N-body planet–planet interactions. Within this physical framework we find that populations with a constant value disc turbulence and winds strength produce mass–semimajor axis distributions in the M–a diagram with insufficient scatter to compare reasonably with observations. However, populations produced as a consequence of sampling discs with a distribution of the relative strengths of disc turbulence and winds fit much better. Such models give rise to a substantial super Earth population at orbital radii 0.03–2 au, as well as a clear separation between the produced hot Jupiter and warm Jupiter populations. Additionally, this model results in a good comparison with the exoplanetary mass–radius distribution in the M–R diagram after post-disc atmospheric photoevaporation is accounted for.

The Ariel Target List: The Impact of TESS and the Potential for Characterizing Multiple Planets within a System

The ESA Ariel mission has been adopted for launch in 2029 and will conduct a survey of around 1000 exoplanetary atmospheres during its primary mission life. By providing homogeneous data sets with a high signal-to-noise ratio and wide wavelength coverage, Ariel will unveil the atmospheric demographics of these faraway worlds, helping to constrain planet formation and evolution processes on a galactic scale. Ariel seeks to undertake a statistical survey of a diverse population of planets; therefore, the sample of planets from which this selection can be made is of the utmost importance. While many suitable targets have already been found, hundreds more will be discovered before the mission is operational. Previous studies have used predictions of exoplanet detections to forecast the available planet population by the launch date of Ariel, with the most recent noting that the Transiting Exoplanet Survey Satellite (TESS) alone should provide over 1000 potential targets. In this work, we consider the planet candidates found to date by TESS to show that, with the addition of already confirmed planets, Ariel will already have a more than sufficient sample to choose its target list from once these candidates are validated. We showcase the breadth of this population, as well as exploring, for the first time, the ability of Ariel to characterize multiple planets within a single system. Comparative planetology of worlds orbiting the same star, as well as across the wider population, will undoubtedly revolutionize our understanding of planet formation and evolution.

The Quest For Water

Water played a key role in shaping the Solar System—from the formation of early solids to the processes of planetary and moon formation. The presence of water in molecular clouds influences the initial abundance and distribution of water in the circumsolar disk, which, in turn, affected the water budget of the terrestrial planets and, therefore, their geological activity and habitability. On Earth, surficial and deep-water cycles have largely governed the planet’s geodynamical and geochemical evolution. This issue focuses on the past and present distribution of water within the Solar System and how this important molecule affects astrophysical and geological processes.

Photoionized Herbig–Haro objects in the Orion Nebula through deep high-spectral resolution spectroscopy – III. HH 514

ABSTRACT We analyse the physical conditions and chemical composition of the photoionized Herbig–Haro object HH 514, which emerges from the protoplanetary disc (proplyd) 170-337 in the core of the Orion Nebula. We use high-spectral resolution spectroscopy from Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope and IFU-spectra from MEGARA at the Gran Telescopio de Canarias. We observe two components of HH 514, the jet base and a knot, with $n_{\rm e}= (2.3 \pm 0.1) \times 10^5 \rm{cm}^{-3}$ and $n_{\rm e}= (7 \pm 1) \times 10^4 \rm{ cm}^{-3}$, respectively, both with Te ≈ 9000 K. We show that the chemical composition of HH 514 is consistent with that of the Orion Nebula, except for Fe, Ni, and S, which show higher abundances. The enhanced abundances of Fe and Ni observed in HH objects compared with the general interstellar medium are usually interpreted as destruction of dust grains. The observed sulphur overabundance (more than two times solar) is challenging to explain since the proplyd photoevaporation flow from the same disc shows normal sulphur abundance. If the aforementioned S-overabundance is due to dust destruction, the formation of sulfides and/or other S-bearing dust reservoirs may be linked to planet formation processes in proplyds, which filter large sulfide dust grains during the accretion of matter from the disc to the central star. We also show that published kinematics of molecular emission close to the central star are not consistent with either a disc perpendicular to the optical jet, or with an outflow that is aligned with it.

Possible solution to the triple alpha fine-tuning problem: Spallation reactions during planet formation

Carbon is produced during the helium burning phase of sufficiently massive stars through the triple alpha process. The 0+ energy level of the carbon nucleus allows for resonant nuclear reactions, which act to greatly increase the carbon yields compared to the non-resonant case. Many authors have argued that small changes to the energy level of this resonance would lead to a significantly lower carbon abundance in the universe, and this sensitivity is often considered an example of fine-tuning. By considering spallation reactions occurring during the process of planet formation, this paper presents a partial solution to this triple alpha fine-tuning problem. Young stellar objects generate substantial luminosities of particle radiation (cosmic rays) that can drive nuclear reactions through spallation. If the standard triple alpha process is inoperative, stars tend to synthesize oxygen (and other alpha elements) rather than carbon. Cosmic rays can interact with oxygen nuclei to produce carbon while planets are forming. The resulting carbon abundances are significant, but much smaller than those observed in our universe. However, for a range of conditions – as delineated herein – spallation reactions can result in carbon-to-oxygen ratios roughly comparable to those found on Earth and thereby obviate the triple alpha fine-tuning problem.

H2CS deuteration maps towards the pre-stellar core L1544

Context. Deuteration is a crucial tool for understanding the complexity of interstellar chemical processes, especially when they involve the interplay of gas-phase and grain-surface chemistry. In the case of multiple deuteration, comparing observation with the results of chemical modelling is particularly effective to study how molecules are inherited in the different stages within the process of star and planet formation. Aims. We aim to study the D/H ratio in H2CS across the prototypical pre-stellar core L1544. This study allows us to test current gas–dust chemical models involving sulfur in dense cores. Methods. Here, we present single-dish observations of H2CS, HDCS and D2CS with the IRAM 30 m telescope. We analysed their column densities and distributions and compared these observations with gas–grain chemical models. The deuteration maps of H2CS in L1544 were compared with the deuteration maps of methanol, H2CO, N2H+, and HCO+ towards the same source. Furthermore, we compared the single and double deuteration of H2CS towards the dust peak of L1544 with H2CO and c-C3H2. The difference between the deuteration of these molecules in L1544 is discussed and compared with the prediction of chemical models. Results. The maximum deuterium fractionation for the first deuteration of H2CS is N(HDCS)/N(H2CS) ~ 30% and is located towards the north-east at a distance of about 10000 AU from the dust peak. While for c-C3H2 the first and second deuteration have a similar efficiency, for H2CS and H2CO the second deuteration is more efficient, leading to D2CX/HDCX ~ 100% (with X = O or S). Conclusions. Our results imply that the large deuteration of H2CO and H2CS observed in protostellar cores as well as in comets is likely inherited from the pre-stellar phase. However, comparison with state-of-the-art chemical models suggests that the reaction network for the formation of the doubly deuterated H2CS and H2CO it is not complete yet.

Classification of Hot Jupiter Population through Statistical Framework

The recent trend in the discovery of a range of exoplanets opens up a door to evaluate their origin and classification under the light of different planetary attributes. This paper enthusiastically focused on a typical branch of exoplanets, hot Jupiter, and several planetary characteristics were observed to frame the population into substantive categories. In this paper, a statistical framework was also established to understand different planetary formation processes for hot Jupiters. Finally, the relevance of hot Jupiters in search of habitable planets is also discussed briefly.

Architectures of Compact Super-Earth Systems Shaped by Instabilities

Abstract Compact nonresonant systems of sub-Jovian planets are the most common outcome of the planet formation process. Despite exhibiting broad overall diversity, these planets also display dramatic signatures of intrasystem uniformity in their masses, radii, and orbital spacings. Although the details of their formation and early evolution are poorly known, sub-Jovian planets are expected to emerge from their natal nebulae as multiresonant chains, owing to planet–disk interactions. Within the context of this scenario, the architectures of observed exoplanet systems can be broadly replicated if resonances are disrupted through postnebular dynamical instabilities. Here, we generate an ad hoc sample of resonant chains and use a suite of N-body simulations to show that instabilities can not only reproduce the observed period ratio distribution, but that the resulting collisions also modify the mass uniformity in a way that is consistent with the data. Furthermore, we demonstrate that primordial mass uniformity, motivated by the sample of resonant chains coupled with dynamical sculpting, naturally generates uniformity in orbital period spacing similar to what is observed. Finally, we find that almost all collisions lead to perfect mergers, but some form of postinstability damping is likely needed to fully account for the present-day dynamically cold architectures of sub-Jovian exoplanets.

Nature versus Nurture: Investigating the Effects of Measurement Uncertainties in the Assessment of Potential Trends between Planetary and Stellar Properties

Abstract Correlations between planetary and stellar properties, particularly age, can provide insight on planetary formation and evolution processes. However, the underlying source of such trends can be unclear, and measurement uncertainties and small sample sizes can leave doubt as to whether an observed trend truly exists. We use a Bayesian framework to examine how uncertainties in measured parameters influence the odds ratios of competing hypotheses for the source of an observed trend. We analyze three reported trends from the literature. In each application, while uncertainties do affect the numerical value of the odds ratios, our conclusions remain the same whether or not uncertainties are taken into account: hot Jupiter eccentricities are circularized over time, obliquities of hot Jupiter hosts are driven by stellar temperature, and there is not enough evidence to favor a trend of 2:1 orbital resonances with age over a chance relation. Updated samples for the 2:1 resonance and obliquity cases do not change the original conclusions. Simulated 2:1 resonance data show that sample size may be more important than measurement precision for drawing a firm conclusion. However, if 2:1 resonances get disrupted on a wide range of timescales, an age trend will be inherently difficult to confirm over a chance relation, even with a large sample. For some applications, full incorporation of measurement uncertainties may be too computationally expensive, making it preferable to use the framework without uncertainties and perform additional tests to examine the effects of highly uncertain measurements.

The role of density perturbation on planet formation by pebble accretion

ABSTRACT Protoplanetary discs exhibit a diversity of gaps and rings of dust material, believed to be a manifestation of pressure maxima commonly associated with an ongoing planet formation and several other physical processes. Hydrodynamic disc simulations further suggest that multiple dust ring-like structures may be ubiquitous in discs. In the recent past, it has been shown that dust rings may provide a suitable avenue for planet formation. We study how a globally perturbed disc affects dust evolution and core growth by pebble accretion. We performed global disc simulations featuring a Gaussian pressure profile, in tandem with global perturbations of the gas density, mimicking wave-like structures, and simulated planetary core formation at pressure minima and maxima. With Gaussian pressure profiles, grains in the inside disc regions were extremely depleted in the first 0.1 Myr of disc lifetime. The global pressure bumps confined dust material for several million years, depending on the strength of perturbations. A variety of cores formed in bumpy discs, with massive cores at locations where core growth was not feasible in a smooth disc, and small cores at locations where massive cores could form in a smooth disc. We conclude that pressure bumps generated by a planet and/or other physical phenomena can completely thwart planet formation from the inside parts of the disc. While inner disc parts are most favourable for pebble accretion in a smooth disc, multiple wave-like pressure bumps can promote rapid planet formation by pebble accretion in broad areas of the disc.

The hot Neptune WASP-166 b with ESPRESSO II: confirmation of atmospheric sodium

ABSTRACT The hot Neptune desert, a distinct lack of highly irradiated planets in the size range of Neptune, remains one of the most intriguing results of exoplanet population studies. A deeper understanding of the atmosphere of exoplanets sitting at the edge or even within the Neptune desert will allow us to better understand if planetary formation or evolution processes are at the origin of the desert. A detection of sodium in WASP-166 b was presented previously with tentative line broadening at the $3.4\, \sigma$ with the HARPS spectrograph. We update this result with two transits observed with the ESPRESSO spectrograph, confirming the detection in each night and the broadened character of the line. This result marks the first confirmed resolved sodium detection within the Neptune desert. In this work, we additionally highlight the importance of treating low-SNR spectral regions particularly where absorption lines of stellar sodium and planetary sodium overlap at mid transit – an important caveat for future observations of the system.

Habitable Planet Formation around Low-mass Stars: Rapid Accretion, Rapid Debris Removal, and the Essential Contribution of External Giants

Abstract In recent years, a paradigm shift has occurred in exoplanet science, wherein low-mass stars are increasingly viewed as a foundational pillar of the search for potentially habitable worlds in the solar neighborhood. However, the formation processes of this rapidly accumulating sample of planet systems are still poorly understood. Moreover, it is unclear whether tenuous primordial atmospheres around these Earth analogs could have survived the intense epoch of heightened stellar activity that is typical for low-mass stars. We present new simulations of in situ planet formation across the M-dwarf mass spectrum, and derive leftover debris populations of small bodies that might source delayed volatile delivery. We then follow the evolution of this debris with high-resolution models of real systems of habitable zone planets around low-mass stars such as TRAPPIST-1, Proxima Centauri, and TOI-700. While debris in the radial vicinity of the habitable zone planets is removed rapidly, thus making delayed volatile delivery highly unlikely, we find that material ubiquitously scattered into an exo-asteroid belt region during the planet-formation process represents a potentially lucrative reservoir of icy small bodies. Thus, the presence of external approximately Neptune–Saturn mass planets capable of dynamically perturbing these asteroids would be a sign that habitable zone worlds around low-mass stars might have avoided complete desiccation. However, we also find that such giant planets significantly limit the efficiency of asteroidal implantation during the planet-formation process. In the coming decade, long-baseline radial velocity studies and Roman Space Telescope microlensing observations will undoubtedly further constrain this process.

Constraining giant planet formation with synthetic ALMA images of the Solar System’s natal protoplanetary disk

New ALMA observations of protoplanetary disks allow us to probe planet formation in other planetary systems, giving us new constraints on planet formation processes. Meanwhile, studies of our own Solar System rely on constraints derived in a completely different way. However, it is still unclear what features the Solar System protoplanetary disk could have produced during its gas phase. By running 2D isothermal hydro-simulations used as inputs for a dust evolution model, we derive synthetic images at millimeter wavelengths using the radiative transfer code RADMC3D. We find that the embedded multiple giant planets strongly perturb the radial gas velocities of the disk. These velocity perturbations create traffic jams in the dust, producing over-densities different from the ones created by pressure traps and located away from the planets’ positions in the disk. By deriving the images atλ= 1.3 mm from these dust distributions, we show that very high resolution observations are needed to distinguish the most important features expected in the inner part (<15 AU) of the disk. The traffic jams, observable with a high resolution, further blur the link between the number of gaps and rings in disks and the number of embedded planets. We additionally show that a system capable of producing eccentric planets by scattering events that match the eccentricity distributions in observed exoplanets does not automatically produce bright outer rings at large radii in the disk. This means that high resolution observations of disks of various sizes are needed to distinguish between different giant planet formation scenarios during the disk phase, where the giants form either in the outer regions of the disks or in the inner regions. In the second scenario, the disks do not present planet-related features at large radii. Finally, we find that, even when the dust temperature is determined self-consistently, the dust masses derived observationally might be off by up to a factor of ten compared to the dust contained in our simulations due to the creation of optically thick regions. Our study clearly shows that in addition to the constraints from exoplanets and the Solar System, ALMA has the power to constrain different stages of planet formation already during the first few million years, which corresponds to the gas disk phase.