Planet Formation Research Articles

Exploring the brown dwarf desert with precision radial velocities and Gaia DR3 astrometric orbits

Context. The observed scarcity of brown dwarfs in close orbits (within 10 au) around solar-type stars has posed significant questions about the origins of these substellar companions. These questions not only pertain to brown dwarfs but also impact our broader understanding of planetary formation processes. However, to resolve these formation mechanisms, accurate observational constraints are essential. Notably, most of the brown dwarfs have been discovered by radial velocity surveys, but this method introduces uncertainties due to its inability to determine the orbital inclination, leaving the true mass – and thus their true nature – unresolved. This highlights the crucial role of astrometric data, helping us distinguish between genuine brown dwarfs and stars. Aims. This study aims to refine the mass estimates of massive companions to solar-type stars, mostly discovered through radial velocity measurements and subsequently validated using Gαìα DR3 astrometry, to gain a clearer understanding of their true mass and occurrence rates. Methods. We selected a sample of 31 sources with substellar companion candidates validated by Gaia Data Release (DR3) and with available radial velocities. Using the Gaia DR3 solutions as prior information, we performed an MCMC fit with the available radial velocity measurements to integrate these two sources of data and thus obtain an estimate of their true mass. Results. Combining radial velocity measurements with Gaia DR3 data led to more precise mass estimations, leading us to reclassify several systems initially labeled as brown dwarfs as low-mass stars. Out of the 32 analyzed companions, 13 have been determined to be stars, 17 are substellar, and two have inconclusive results with the current data. Importantly, using these updated masses, we reevaluated the occurrence rate of brown dwarf companions (13–80 MJup) on close orbits (<10 au) in the CORALIE sample, determining a tentative occurrence rate of 0.8−0.2+0.3%.

XUE: Molecular Inventory in the Inner Region of an Extremely Irradiated Protoplanetary Disk

We present the first results of the eXtreme UV Environments (XUE) James Webb Space Telescope (JWST) program, which focuses on the characterization of planet-forming disks in massive star-forming regions. These regions are likely representative of the environment in which most planetary systems formed. Understanding the impact of environment on planet formation is critical in order to gain insights into the diversity of the observed exoplanet populations. XUE targets 15 disks in three areas of NGC 6357, which hosts numerous massive OB stars, including some of the most massive stars in our Galaxy. Thanks to JWST, we can, for the first time, study the effect of external irradiation on the inner (<10 au), terrestrial-planet-forming regions of protoplanetary disks. In this study, we report on the detection of abundant water, CO, 12CO2, HCN, and C2H2 in the inner few au of XUE 1, a highly irradiated disk in NGC 6357. In addition, small, partially crystalline silicate dust is present at the disk surface. The derived column densities, the oxygen-dominated gas-phase chemistry, and the presence of silicate dust are surprisingly similar to those found in inner disks located in nearby, relatively isolated low-mass star-forming regions. Our findings imply that the inner regions of highly irradiated disks can retain similar physical and chemical conditions to disks in low-mass star-forming regions, thus broadening the range of environments with similar conditions for inner disk rocky planet formation to the most extreme star-forming regions in our Galaxy.

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

Thermal Synthesis of Carbamic Acid and Its Dimer in Interstellar Ices: A Reservoir of Interstellar Amino Acids.

Reactions in interstellar ices are shown to be capable of producing key prebiotic molecules without energetic radiation that are necessary for the origins of life. When present in interstellar ices, carbamic acid (H2NCOOH) can serve as a condensed-phase source of the molecular building blocks for more complex proteinogenic amino acids. Here, Fourier transform infrared spectroscopy during heating of analogue interstellar ices composed of carbon dioxide and ammonia identifies the lower limit for thermal synthesis to be 62 ± 3 K for carbamic acid and 39 ± 4 K for its salt ammonium carbamate ([H2NCOO-][NH4+]). While solvation increases the rates of formation and decomposition of carbamic acid in ice, the absence of solvent effects after sublimation results in a significant barrier to dissociation and a stable gas-phase molecule. Photoionization reflectron time-of-flight mass spectrometry permits an unprecedented degree of sensitivity toward gaseous carbamic acid and demonstrates sublimation of carbamic acid from decomposition of ammonium carbamate and again at higher temperatures from carbamic acid dimers. Since the dimer is observed at temperatures up to 290 K, similar to the environment of a protoplanetary disk, this dimer is a promising reservoir of amino acids during the formation of stars and planets.

A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067.

Planets with radii between that of the Earth and Neptune (hereafter referred to as 'sub-Neptunes') are found in close-in orbits around more than half of all Sun-like stars1,2. However, their composition, formation and evolution remain poorly understood3. The study of multiplanetary systems offers an opportunity to investigate the outcomes of planet formation and evolution while controlling for initial conditions and environment. Those in resonance (with their orbital periods related by a ratio of small integers) are particularly valuable because they imply a system architecture practically unchanged since its birth. Here we present the observations of six transiting planets around the bright nearby star HD 110067. We find that the planets follow a chain of resonant orbits. A dynamical study of the innermost planet triplet allowed the prediction and later confirmation of the orbits of the rest of the planets in the system. The six planets are found to be sub-Neptunes with radii ranging from 1.94R⊕ to 2.85R⊕. Three of the planets have measured masses, yielding low bulk densities that suggest the presence of large hydrogen-dominated atmospheres.

Evidence for Low-level Dynamical Excitation in Near-resonant Exoplanet Systems* * This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

The geometries of near-resonant planetary systems offer a relatively pristine window into the initial conditions of exoplanet systems. Given that near-resonant systems have likely experienced minimal dynamical disruptions, the spin–orbit orientations of these systems inform the typical outcomes of quiescent planet formation, as well as the primordial stellar obliquity distribution. However, few measurements have been made to constrain the spin–orbit orientations of near-resonant systems. We present a Rossiter–McLaughlin measurement of the near-resonant warm Jupiter TOI-2202 b, obtained using the Carnegie Planet Finder Spectrograph on the 6.5 m Magellan Clay Telescope. This is the eighth result from the Stellar Obliquities in Long-period Exoplanet Systems survey. We derive a sky-projected 2D spin–orbit angle and a 3D spin–orbit angle , finding that TOI-2202 b—the most massive near-resonant exoplanet with a 3D spin–orbit constraint to date—likely deviates from exact alignment with the host star’s equator. Incorporating the full census of spin–orbit measurements for near-resonant systems, we demonstrate that the current set of near-resonant systems with period ratios P 2/P 1 ≲ 4 is generally consistent with a quiescent formation pathway, with some room for low-level (≲20°) protoplanetary disk misalignments or post-disk-dispersal spin–orbit excitation. Our result constitutes the first population-wide analysis of spin–orbit geometries for near-resonant planetary systems.

Constraints on sub-terrestrial free-floating planets from Subaru microlensing observations

ABSTRACT The abundance of protoplanetary bodies ejected from their parent star system is presently poorly constrained. With only two existing optical observations of interstellar objects in the 108–1010 kg mass range and a small number of robust microlensing observations of free-floating planets (FFPs) in the 1024–1025 kg mass range, there is a large range of masses for which there are no existing measurements of the unbound population. The three primary microlensing surveys currently searching for FFPs operate at a cadence greater than 15 min, which limits their ability to observe events associated with bodies with a mass much below an Earth mass. We demonstrate that existing high-cadence observations of M31 with the Subaru Hyper Suprime-Cam place constraints on the abundance of unbound objects at sub-terrestrial masses, with peak sensitivity at 10−4 M⊕ for Milky Way lenses and 10−1 M⊕ for lenses in M31. For a fiducial $\frac{dn}{dM}\propto M^{-2}$ mass distribution, we find that the abundance of unbound objects is constrained to $n_\text{unbound} \lt 1.4 \times 10^{7} ~\rm {pc}^{-3}$ for masses within 1 dex of 10−4 M⊕. Additionally, we compute limits on an artificial ‘monochromatic’ distribution of unbound objects and compare to existing literature, demonstrating that the assumed spatial distribution of lenses has very significant consequences for the sensitivity of microlensing surveys. While the observations ultimately do not probe abundances suggested by current models of planetary formation, our limits place direct observational constraints on the unbound population in the sub-terrestrial mass range and motivate new observational strategies for microlensing surveys.

Gas in distant galaxies: mixed or matched?

Gas is an essential ingredient of galaxies because it fuels star and planet formation. Studying this gas is important because it tells us how galaxies form, evolve, and interact with their environment. In this work we studied 64 small, distant galaxies and found that the gas is not well-mixed which tells us how it is moving in, out and around the galaxies.

Methane throughout the atmosphere of the warm exoplanet WASP-80b.

The abundances of main carbon- and oxygen-bearing gases in the atmospheres of giant exoplanets provide insights into atmospheric chemistry and planet formation processes1,2. Thermochemistry suggests that methane (CH4) should be the dominant carbon-bearing species below about 1,000 K over a range of plausible atmospheric compositions3; this is the case for the solar system planets4 and has been confirmed in the atmospheres of brown dwarfs and self-luminous, directly imaged exoplanets5. However, CH4 has not yet been definitively detected with space-based spectroscopy in the atmosphere of a transiting exoplanet6-11, but a few detections have been made with ground-based, high-resolution transit spectroscopy12,13 including a tentative detection for WASP-80b (ref. 14). Here we report transmission and emission spectra spanning 2.4-4.0 μm of the 825 K warm Jupiter WASP-80b taken with the NIRCam instrument of the JWST, both of which show strong evidence of CH4 at greater than 6σ significance. The derived CH4 abundances from both viewing geometries are consistent with each other and with solar to sub-solar C/O and around five times solar metallicity, which is consistent with theoretical predictions15-17.

The distribution of volatile elements during rocky planet formation

Core segregation and atmosphere formation are two of the major processes that redistribute the volatile elements—hydrogen (H), carbon (C), nitrogen (N), and sulfur (S)—in and around rocky planets during their formation. The volatile elements by definition accumulate in gaseous reservoirs and form atmospheres. However, under conditions of early planet formation, these elements can also behave as siderophiles (i.e., iron-loving) and become concentrated in core-forming metals. Current models of core formation suggest that metal-silicate reactions occurred over a wide pressure, temperature, and compositional space to ultimately impose the chemistries of the cores and silicate portions of rocky planets. Additionally, the solubilities of volatile elements in magmas determine their transfer between the planetary interiors and atmospheres, which has recently come into sharper focus in the context of highly irradiated, potentially molten exoplanets. Recently, there has been a significant push to experimentally investigate the metal-silicate and magma-gas exchange coefficients for volatile elements over a wide range of conditions relevant to rocky planet formation. Qualitatively, results from the metal-silicate partitioning studies suggest that cores of rocky planets could be major reservoirs of the volatile elements though significant amounts will remain in mantles. Results from solubility studies imply that under oxidizing conditions, most H and S are sequestered in the magma ocean, while most N is outgassed to the atmosphere, and C is nearly equally distributed between the atmosphere and the interior. Under reducing conditions, nearly all N dissolves in the magma ocean, the atmosphere becomes the dominant C reservoir, while H becomes more equally distributed between the interior and the atmosphere, and S remains dominantly in the interior. These chemical trends bear numerous implications for the chemical differentiation of rocky planets and the formation and longevity of secondary atmospheres in the early Solar System and exoplanetary systems. Further experimental and modeling efforts are required to understand the potential of chemical and physical disequilibria during core formation and magma ocean crystallization and to constrain the distributions of volatile elements in the interiors and atmospheres of rocky planets through their formation and long-term geologic evolution.

Mass and Angular Momentum Transport in a Gravitationally Unstable Protoplanetary Disk with Improved 3D Radiative Hydrodynamics

During early phases of a protoplanetary disk's life, gravitational instabilities (GIs) can produce significant mass transport, can dramatically alter disk structure, can mix and shock-process gas and solids, and may be instrumental in planet formation. We present a 3D grid-based radiative hydrodynamics study with varied resolutions of a 0.07 M ⊙ disk orbiting a 0.5 M ⊙ star as it settles over most of its radial extent into a quasi-steady asymptotic state that maintains approximate balance between heating produced by GIs and radiative cooling governed by realistic dust opacities. We assess disk stability criteria, thermodynamic properties, strengths of GIs, characteristics of density waves and torques produced by GIs, radial mass transport arising from these torques, and the level to which transport can be represented as local or nonlocal processes. Physical and thermal processes display distinct differences between inner optically thick and outer optically thin regions of the disk. In the inner region, gravitational torques are dominated by low-order Fourier components of the azimuthal mass distribution. These torques are strongly variable on the local dynamical time and are subject to rapid flaring presumably driven by recurrent swing amplification. In the outer region, m = 1 torques dominate. Ring-like structures exhibiting strong noncircular motions, and vortices develop near the inner edge between 8 and 14 au. We find that GI-induced spiral modes erupt in a chaotic manner over the whole low-Q part of the disk, with many spiral modes appearing and disappearing, producing gravitoturbulence, but dominated by fluctuating large-scale modes, very different from a simple α-disk.

Statistical analysis of terrestrial planets formation and observational bias

When investigating and researching into extrasolar planets, astronomers must deal with the inherent selection bias that arises because of limitations in technology or resources. Ideally, astronomers should aim to have a database with unbiased samples from across the universe. The ability for astronomers to efficiently use equipment will rely on statistical models employed to discover regions in which astronomical observations for extrasolar planets are lacking. Previous researchers have focused on the observation of Hot-Jupiters, Super Earths etc. This paper will be specific in exploring the selection bias involved with the detection of extrasolar terrestrials. In addition, the paper will investigate the probability of finding such terrestrial planets depending on the characteristics of exoplanet host stars. Ideally, the distribution of terrestrial planets should be random if the Earth is not considered special. The paper will also evaluate this hypothesis and determine whether there is any relationship between detection probability for exoplanets and their distance from Earth. Statistical methods such as logit regression and kernel estimations will be used in Stata to analyse data from the exoplanet encyclopedia. Hopefully, this paper will be able to provide more insight into the distribution of terrestrial planets and bias involved in detecting such planets.

A Lithium Depletion Age for the Carina Association

The dispersed remnants of stellar nurseries, stellar associations, provide unparalleled samples of coeval stars critical for studies of stellar and planetary formation and evolution. The Carina Stellar Association is one of the closest stellar associations to Earth, and yet measurements of its age have varied from 13 to 45 Myr. We aim to update the age of Carina using the lithium depletion boundary (LDB) method. We obtain new measurements of the Li 6708 Å absorption feature in likely members using optical spectra from the Goodman High Throughput Spectrograph on SOAR and NRES on LCO. We detect the depletion boundary at M K ≃ 6.8 (M5). This age is consistent within uncertainties across six different models, including those that account for magnetic fields and spots. We also estimate the age through analysis of the group’s overall variability, and by comparing the association members’ color–magnitude diagram to stellar evolutionary models using a Gaussian Mixture Model, recovering ages consistent with the LDB. Combining these age measures we obtain an age for the Carina association of Myr. The resulting age agrees with the older end of previous age measurements and is consistent with the lithium depletion age for the neighboring Tucana-Horologium moving group.

The Galactic Interstellar Object Population: A Framework for Prediction and Inference

The Milky Way is thought to host a huge population of interstellar objects (ISOs), numbering approximately 1015 pc−3 around the Sun, which are formed and shaped by a diverse set of processes ranging from planet formation to Galactic dynamics. We define a novel framework, first to predict the properties of this Galactic ISO population by combining models of processes across planetary and Galactic scales, and second to make inferences about the processes being modeled, by comparing the predicted population to what is observed. We predict the spatial and compositional distribution of the Galaxy’s population of ISOs by modeling the Galactic stellar population with data from the APOGEE survey and combining this with a protoplanetary disk chemistry model. Selecting the ISO water mass fraction as an example observable quantity, we evaluate its distribution both at the position of the Sun and averaged over the Galactic disk; our prediction for the solar neighborhood is compatible with the inferred water mass fraction of 2I/Borisov. We show that the well-studied Galactic stellar metallicity gradient has a corresponding ISO compositional gradient. We also demonstrate the inference part of the framework by using the current observed ISO composition distribution to constrain the parent star metallicity dependence of the ISO production rate. This constraint, and other inferences made with this framework, will improve dramatically as the Vera C. Rubin Observatory Legacy Survey of Space and Time progresses and more ISOs are observed. Finally, we explore generalizations of this framework to other Galactic populations, such as that of exoplanets.

Planets Across Space and Time (PAST). IV. The Occurrence and Architecture of Kepler Planetary Systems as a Function of Kinematic Age Revealed by the LAMOST–Gaia–Kepler Sample

One of the fundamental questions in astronomy is how planetary systems form and evolve. Measuring the planetary occurrence and architecture as a function of time directly addresses this question. In the fourth paper of the Planets Across Space and Time series, we investigate the occurrence and architecture of Kepler planetary systems as a function of kinematic age by using the LAMOST–Gaia–Kepler sample. To isolate the age effect, other stellar properties (e.g., metallicity) have been controlled. We find the following results. (1) The fraction of stars with Kepler-like planets (F Kep) is about 50% for all stars; no significant trend is found between F Kep and age. (2) The average planet multiplicity () exhibits a decreasing trend (∼2σ significance) with age. It decreases from ∼ 3 for stars younger than 1 Gyr to ∼ 1.8 for stars of about 8 Gyr. (3) The number of planets per star (η = F Kep× ) also shows a decreasing trend (∼2σ–3σ significance). It decreases from η ∼ 1.6–1.7 for young stars to η ∼ 1.0 for old stars. (4) The mutual orbital inclination of the planets (σ i,k ) increases from to as the stars age from 0.5 to 8 Gyr with a best fit of . Interestingly, the solar system also fits such a trend. The fact that F Kep remains relatively constant at approximately ∼ 50% across different ages suggests the robustness of planet formation throughout the history of the Galaxy. The age dependence of and σ i,k demonstrates that the planetary architecture is evolving, and planetary systems generally become dynamically hotter with fewer planets as they age.

WInDI: a Warp-Induced Dust Instability in protoplanetary discs

ABSTRACT We identify a new dust instability that occurs in warped discs. The instability is caused by the oscillatory gas motions induced by the warp in the bending wave regime. We first demonstrate the instability using a local 1D (vertical) toy model based on the warped shearing box coordinates and investigate the effects of the warp magnitude and dust Stokes number on the growth of the instability. We then run 3D smoothed particle hydrodynamics (SPH) simulations and show that the instability is manifested globally by producing unique dust structures that do not correspond to gas pressure maxima. The 1D and SPH analysis suggest that the instability grows on dynamical time-scales and hence is potentially significant for planet formation.

Phosphorus-bearing molecules PO and PN at the edge of the Galaxy

Despite its importance in planet formation and biology1, phosphorus has been identified only in the inner 12 kpc of the Galaxy2–19. The study of this element has been hindered in part by unfavourable atomic transitions2,4,20. Phosphorus is thought to be created by neutron capture on 29Si and 30Si in massive stars20,21, and released into the interstellar medium by Type II supernova explosions2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production23 to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre24. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System25. Supernovae are not present in the Outer Galaxy26, suggesting another source of phosphorus, such as ‘Galactic Fountains’, where supernova material is redistributed through the halo and circumgalactic medium27. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich28. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars29 which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon21, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.

Masses, revised radii, and a third planet candidate in the ‘Inverted’ planetary system around TOI-1266

ABSTRACT Is the population of close-in planets orbiting M dwarfs sculpted by thermally driven escape or is it a direct outcome of the planet formation process? A number of recent empirical results strongly suggest the latter. However, the unique architecture of the TOI-1266 system presents a challenge to models of planet formation and atmospheric escape given its seemingly ‘inverted’ architecture of a large sub-Neptune (Pb = 10.9 d, $R_{p,b}=2.62\pm 0.11\, \mathrm{R}_{\oplus }$) orbiting interior to that of the system’s smaller planet (Pc = 18.8 d, $R_{p,c}=2.13\pm 0.12\, \mathrm{R}_{\oplus }$). Here, we present revised planetary radii based on new TESS and diffuser-assisted ground-based transit observations, and characterize both planetary masses using a set of 145 radial velocity measurements from HARPS-N ($M_{p,b}=4.23\pm 0.69\, \mathrm{M}_{\oplus }, M_{p,c}=2.88\pm 0.80\, \mathrm{M}_{\oplus }$). Our analysis also reveals a third planet candidate (Pd = 32.3 d, $M_{p,d}\sin {i} = 4.59^{+0.96}_{-0.94}\, \mathrm{M}_{\oplus }$), which if real, would form a chain of near 5:3 period ratios, although the system is likely not in a mean motion resonance. Our results indicate that TOI-1266 b and c are among the lowest density sub-Neptunes around M dwarfs and likely exhibit distinct bulk compositions of a gas-enveloped terrestrial (Xenv,b = 5.5 ± 0.7 per cent) and a water-rich world (WMFc = 59 ± 14 per cent), which is supported by hydrodynamic escape models. If distinct bulk compositions are confirmed through atmospheric characterization, the system’s unique architecture would represent an interesting test case of inside-out sub-Neptune formation at pebble traps.

Statistical and Radio Analysis of Exoplanets and Their Host Stars

As of February 2022, over 4900 exoplanets have been confirmed. In this study, we conducted statistical analyses on both the exoplanets and their host stars’ parameters. Our findings suggest that the radius and true mass distribution of the exoplanets remain largely unchanged compared to prior research. However, we observed a correlation between the average eccentricity and the number of planets in a system, and fluctuations in the “size” of the planets may contribute to such variation. Moreover, we discovered that, among planets with precise measurements of radius, true mass, and semi-major axis, the true mass-radius relationship follows a power–law distribution. Interestingly, the power–law index tends to decrease from super-Earths to cold Jupiters, potentially due to atmospheric composition. We also revised the radius valley, and determined that M-type host stars with low mass and metal abundance exhibit high planetary ownership rates or harbor large-mass planets, suggesting a different planet formation mechanism than GK-type stars. Lastly, we assessed the possibility of detecting exoplanets using FAST and found that there are three planets in FAST sky that may be detected, namely CoRoT-3 b, GPX-1 b, and TOI-2109 b.

Binary planet formation through tides

ABSTRACT The search for satellites around exoplanets represents one of the greatest challenges in advancing the characterization of planetary systems. Currently, we can only detect massive satellites, which resemble additional planetary companions rather than rocky moons. It is not yet well understood whether such substellar pairs, known as binary planets, are common or how they form. In this study, we investigated the formation scenario for binary planets resulting from tidal dissipation during close encounters in the gravitational instability scenario. We conducted seven sets of simulations, varying the number of initial planets injected into the system from two to five, as well as the amount of energy lost due to tides. Our results demonstrate that this formation mechanism is quite efficient in producing binary planets, with an average occurrence rate for the simulated systems of 14.3 per cent. Additionally, we present the distribution of relevant physical parameters (semimajor axis, eccentricity, mass ratios, and formation time) for planet–planet pairs. We also provide comprehensive statistics for single planets and planet–planet pairs.