Planet Formation Models Research Articles

EDEN Survey: Small Transiting Planet Detection Limits and Constraints on the Occurrence Rates of Planets around Late-M Dwarfs within 15 pc

Earth-sized exoplanets that transit nearby, late-spectral-type red dwarfs will be prime targets for atmospheric characterization in the coming decade. Such systems, however, are difficult to find via widefield transit surveys like Kepler or TESS. Consequently, the presence of such transiting planets is unexplored and the occurrence rates of short-period Earth-sized planets around late-M dwarfs remain poorly constrained. Here, we present the deepest photometric monitoring campaign of 22 nearby late-M dwarf stars, using data from over 500 nights on seven 1–2 m class telescopes. Our survey includes all known single quiescent northern late-M dwarfs within 15 pc. We use transit injection-and-recovery tests to quantify the completeness of our survey, successfully identify most (>80%) transiting short-period (0.5–1 days) super-Earths (R >1.9 R ⊕), and are sensitive (∼50%) to transiting Earth-sized planets (1.0–1.2 R ⊕). Our high sensitivity to transits with a near-zero false-positive rate demonstrates an efficient survey strategy. Our survey does not yield a transiting planet detection, yet it provides the most sensitive upper limits on transiting planets orbiting our target stars. Finally, we explore multiple hypotheses about the occurrence rates of short-period planets (from Earth-sized planets to giant planets) around late-M dwarfs. We show, for example, that giant planets with short periods (<1 day) are uncommon around our target stars. Our data set provides some insight into the occurrence rates of short-period planets around TRAPPIST-1-like stars, and our results can help test planetary formation and system evolution models, as well as guide future observations of nearby late-M dwarfs.

The occurrence rate of giant planets orbiting low-mass stars withTESS

ABSTRACTWe present a systematic search for transiting giant planets ($0.6 \mbox{$R_{\rm J}$}\le \mbox{$R_{\rm P}$}\le 2.0 \mbox{$R_{\rm J}$}$) orbiting nearby low-mass stars ($\mbox{$M_{*}$}\le 0.71 \mbox{${\rm M}_{\odot }$}$). The formation of giant planets around low-mass stars is predicted to be rare by the core-accretion planet formation theory. We search 91 306 low-mass stars in the TESS 30 min cadence photometry detecting fifteen giant planet candidates, including seven new planet candidates which were not known planets or identified as TOIs prior to our search. Our candidates present an exciting opportunity to improve our knowledge of the giant planet population around the lowest mass stars. We perform planet injection-recovery simulations and find that our pipeline has a high detection efficiency across the majority of our targeted parameter space. We measure the occurrence rates of giant planets with host stars in different stellar mass ranges spanning our full sample. We find occurrence rates of 0.137 ± 0.097 per cent (0.088–0.26 M⊙), 0.108 ± 0.083 per cent (0.26–0.42 M⊙), and 0.29 ± 0.15 per cent (0.42–0.71 M⊙). For our full sample (0.088–0.71 M⊙), we find a giant planet occurrence rate of 0.194 ± 0.072 per cent. We have measured for the first time the occurrence rate for giant planets orbiting stars with $\mbox{$M_{*}$}\le 0.4\, \mbox{${\rm M}_{\odot }$}$ and we demonstrate this occurrence rate to be non-zero. This result contradicts currently accepted planet formation models and we discuss some possibilities for how these planets could have formed.

Anatomy of rocky planets formed by rapid pebble accretion

Volatile molecules containing hydrogen, carbon, and nitrogen are key components of planetary atmospheres. In the pebble accretion model for rocky planet formation, these volatile species are accreted during the main planetary formation phase. For this study, we modelled the partitioning of volatiles within a growing planet and the outgassing to the surface. The core stores more than 90% of the hydrogen and carbon budgets of Earth for realistic values of the partition coefficients of H and C between metal and silicate melts. The magma oceans of Earth and Venus are sufficiently deep to undergo oxidation of ferrous Fe2+to ferric Fe3+. This increased oxidation state leads to the outgassing of primarily CO2and H2O from the magma ocean of Earth. In contrast, the oxidation state of Mars’ mantle remains low and the main outgassed hydrogen carrier is H2. This hydrogen easily escapes the atmosphere due to the irradiation from the young Sun in XUV wavelengths, dragging with it the majority of the CO, CO2, H2O, and N2contents of the atmosphere. A small amount of surface water is maintained on Mars, in agreement with proposed ancient ocean shorelines, for moderately low values of the mantle oxidation. Nitrogen partitions relatively evenly between the core and the atmosphere due to its extremely low solubility in magma; the burial of large reservoirs of nitrogen in the core is thus not possible. The overall low N contents of Earth disagree with the high abundance of N in all chondrite classes and favours a volatile delivery by pebble snow. Our model of rapid rocky planet formation by pebble accretion displays broad consistency with the volatile contents of the Sun’s terrestrial planets. The diversity of the terrestrial planets can therefore be used as benchmark cases to calibrate models of extrasolar rocky planets and their atmospheres.

TOI-3235 b: A Transiting Giant Planet around an M4 Dwarf Star

We present the discovery of TOI-3235 b, a short-period Jupiter orbiting an M dwarf with a stellar mass close to the critical mass at which stars transition from partially to fully convective. TOI-3235 b was first identified as a candidate from TESS photometry and confirmed with radial velocities from ESPRESSO and ground-based photometry from HATSouth, MEarth-South, TRAPPIST-South, LCOGT, and ExTrA. We find that the planet has a mass of 0.665 ± 0.025 M J and a radius of 1.017 ± 0.044 R J. It orbits close to its host star, with an orbital period of 2.5926 days but has an equilibrium temperature of ≈ 604 K, well below the expected threshold for radius inflation of hot Jupiters. The host star has a mass of 0.3939 ± 0.0030 M ☉, a radius of 0.3697 ± 0.0018 R ☉, an effective temperature of 3389 K, and a J-band magnitude of 11.706 ± 0.025. Current planet formation models do not predict the existence of gas giants such as TOI-3235 b around such low-mass stars. With a high transmission spectroscopy metric, TOI-3235 b is one of the best-suited giants orbiting M dwarfs for atmospheric characterization.

Planetary Orbit Eccentricity Trends (POET). I. The Eccentricity–Metallicity Trend for Small Planets Revealed by the LAMOST–Gaia–Kepler Sample

Orbital eccentricity is one of the basic planetary properties, whose distribution may shed light on the history of planet formation and evolution. Here, in a series of works on Planetary Orbit Eccentricity Trends (dubbed POET), we study the distribution of planetary eccentricities and their dependence on stellar/planetary properties. In this paper, the first work of the POET series, we investigate whether and how the eccentricities of small planets depend on stellar metallicities (e.g., [Fe/H]). Previous studies on giant planets have found a significant correlation between planetary eccentricities and their host metallicities. Nevertheless, whether such a correlation exists for small planets (e.g., super-Earths and sub-Neptunes) remains unclear. Here, benefiting from the large and homogeneous LAMOST–Gaia–Kepler sample, we characterize the eccentricity distributions of 244 (286) small planets in single (multiple) transiting systems with the TDR method. We confirm the eccentricity–metallicity trend whereby the eccentricities of single small planets increase with stellar metallicities. Interestingly, a similar trend between eccentricity and metallicity is also found in the radial velocity sample. We also found that the mutual inclination of multiple transiting systems increases with metallicity, which predicts a moderate eccentricity–metallicity rising trend. Our results of the correlation between eccentricity (inclination) and metallicity for small planets support the core accretion model for planet formation, and they could be footprints of self (and/or external) excitation processes during the history of planet formation and evolution.

A dynamical mass for GJ 463 b: A massive super-Jupiter companion beyond the snow line of a nearby M dwarf

We determined the full orbital architecture and true mass of the recently Doppler-detected long-period giant planet GJ 463 b using the HIPPARCOS-Gaia proper motion anomaly in combination with the available radial velocities, constraints from the knowledge of the spectroscopic orbital parameters, and supplementary information from a sensitivity analysis of Gaia Data Release 3 astrometry. We determined an orbital inclination ib = 152−3+2 deg (for a prograde orbit) and a mass ratio q = 0.0070 ± 0.0007, corresponding to a true mass of the companion Mb = 3.6 ± 0.4 MJup. True mass determinations for a super-Jupiter companion at intermediate orbital separations beyond the snow line around low-mass stars (M⋆ ≤ 0.5 M⊙) are a rare occurrence. Its existence is possibly explained in the context of disk-instability models of planet formation.

Kepler K2Campaign 9 – II. First space-based discovery of an exoplanet using microlensing

ABSTRACTWe present K2-2016-BLG-0005Lb, a densely sampled, planetary binary caustic-crossing microlensing event found from a blind search of data gathered from Campaign 9 of the Kepler K2 mission (K2C9). K2-2016-BLG-0005Lb is the first bound microlensing exoplanet discovered from space-based data. The event has caustic entry and exit points that are resolved in the K2C9 data, enabling the lens-source relative proper motion to be measured. We have fitted a binary microlens model to the Kepler data and to simultaneous observations from multiple ground-based surveys. Whilst the ground-based data only sparsely sample the binary caustic, they provide a clear detection of parallax that allows us to break completely the microlensing mass-position-velocity degeneracy and measure the planet’s mass directly. We find a host mass of 0.58 ± 0.04 M⊙ and a planetary mass of 1.1 ± 0.1 MJ. The system lies at a distance of 5.2 ± 0.2 kpc from Earth towards the Galactic bulge, more than twice the distance of the previous most distant planet found by Kepler. The sky-projected separation of the planet from its host is found to be 4.2 ± 0.3 au which, for circular orbits, deprojects to a host separation $a = 4.4^{+1.9}_{-0.4}$ au and orbital period $P = 13^{+9}_{-2}$ yr. This makes K2-2016-BLG-0005Lb a close Jupiter analogue orbiting a low-mass host star. According to current planet formation models, this system is very close to the host mass threshold below which Jupiters are not expected to form. Upcoming space-based exoplanet microlensing surveys by NASA’s Nancy Grace Roman Space Telescope and, possibly, ESA’s Euclid mission, will provide demanding tests of current planet formation models.

Mercury’s formation within the early instability scenario

The inner solar system’s modern orbital architecture provides inferences into the epoch of terrestrial planet formation; a ∼100 Myr time period of planet growth via collisions with planetesimals and other proto-planets. While classic numerical simulations of this scenario adequately reproduced the correct number of terrestrial worlds, their semi-major axes and approximate formation timescales, they struggled to replicate the Earth–Mars and Venus–Mercury mass ratios (∼9 and 15, respectively). In a series of past independent investigations, we demonstrated that Mars’ mass is possibly the result of Jupiter and Saturn’s early orbital evolution, while Mercury’s diminutive size might be the consequence of a primordial mass deficit in the region (potentially the result of the growing Earth’s early outward migration). Here, we combine these ideas in a single modeled scenario designed to simultaneously reproduce the formation of all four terrestrial planets and the modern orbits of the giant planets in broad strokes. By evaluating our Mercury analogs’ core mass fractions, masses, and orbital offsets from Venus, we favor a scenario where Mercury forms through a series of violent erosive collisions between a number of ∼Mercury-mass embryos in the inner part of the terrestrial disk. We also compare cases where the gas giants begin the simulation locked in a compact 3:2 resonant configuration to a more relaxed 2:1 orientation and find the former to be more successful. In 2:1 cases, the entire Mercury-forming region is often depleted due to strong sweeping secular resonances that also tend to overly excite the orbits of Earth and Venus as they grow. While our model is quite successful at replicating Mercury’s massive core and dynamically isolated orbit, the planets’ low mass remains extremely challenging to match. Indeed, the majority of our Mercury analogs have masses that are 2–4 times that of the real planet. Finally, we discuss the merits and drawbacks of alternative evolutionary scenarios and initial disk conditions (specifically a narrow annulus of material between 0.7-1.0 au). We argue that the results of our N-body accretion models are not sufficient to break degeneracies between these different models, and implore future studies to apply further cosmochemical and dynamical constraints on terrestrial planet formation models.

Direct measurement of decimetre-sized rocky material in the Oort cloud

The Oort cloud is thought to be a reservoir of icy planetesimals and the source of long-period comets (LPCs) implanted from the outer Solar System during the time of giant planet formation. The abundance of rocky ice-free bodies is a key diagnostic of Solar System formation models as it can distinguish between ``massive" and ``depleted" proto-asteroid belt scenarios and thus disentangle competing planet formation models. Here we report a direct observation of a decimeter-sized ($\sim2$ kg) rocky meteoroid on a retrograde LPC orbit ($e \approx 1.0$, i = $121^{\circ}$). During its flight, it fragmented at dynamic pressures similar to fireballs dropping ordinary chondrite meteorites. A numerical ablation model fit produces bulk density and ablation properties also consistent with asteroidal meteoroids. We estimate the flux of rocky objects impacting Earth from the Oort cloud to be $1.08^{+2.81}_{-0.95} \mathrm{meteoroids/10^6 km^2/yr}$ to a mass limit of 10 g. This corresponds to an abundance of rocky meteoroids of $\sim6^{+13}_{-5}$\% of all objects originating in the Oort cloud and impacting Earth to these masses. Our result gives support to migration-based dynamical models of the formation of the Solar System which predict that significant rocky material is implanted in the Oort cloud, a result not explained by traditional Solar System formation models.

Photometric Properties of Jupiter Trojans Detected by the Dark Energy Survey

The Jupiter Trojans are a large group of asteroids that are coorbiting with Jupiter near its L4 and L5 Lagrange points. The study of Jupiter Trojans is crucial for testing different models of planet formation that are directly related to our understanding of solar system evolution. In this work, we select known Jupiter Trojans listed by the Minor Planet Center from the full six years data set (Y6) of the Dark Energy Survey (DES) to analyze their photometric properties. The DES data allow us to study Jupiter Trojans with a fainter magnitude limit than previous studies in a homogeneous survey with griz band measurements. We extract a final catalog of 573 unique Jupiter Trojans. Our sample include 547 asteroids belonging to L5. This is one of the largest analyzed samples for this group. By comparing with the data reported by other surveys we found that the color distribution of L5 Trojans is similar to that of L4 Trojans. We find that L5 Trojans’ g − i and g − r colors become less red with fainter absolute magnitudes, a trend also seen in L4 Trojans. Both the L4 and L5 clouds consistently show such a color–size correlation over an absolute magnitude range 11 < H < 18. We also use DES colors to perform taxonomic classifications. C- and P-type asteroids outnumber D-type asteroids in the L5 Trojans DES sample, which have diameters in the 5–20 km range. This is consistent with the color–size correlation.

The Perkins INfrared Exosatellite Survey (PINES). II. Transit Candidates and Implications for Planet Occurrence around L and T Dwarfs

We describe a new transit-detection algorithm designed to detect single-transit events in discontinuous Perkins INfrared Exosatellite Survey (PINES) observations of L and T dwarfs. We use this algorithm to search for transits in 131 PINES light curves and identify two transit candidates: 2MASS J18212815+1414010 (2MASS J1821+1414) and 2MASS J08350622+1953050 (2MASS J0835+1953). We disfavor 2MASS J1821+1414 as a genuine transit candidate due to the known variability properties of the source. We cannot rule out the planetary nature of 2MASS J0835+1953's candidate event and perform follow-up observations in an attempt to recover a second transit. A repeat event has yet to be observed, but these observations suggest that target variability is an unlikely cause of the candidate transit. We perform a Markov Chain Monte Carlo simulation of the light curve and estimate a planet radius ranging from to , depending on the host’s age. Finally, we perform an injection and recovery simulation on our light-curve sample. We inject planets into our data using measured M-dwarf planet occurrence rates and attempt to recover them using our transit-search algorithm. Our detection rates suggest that, assuming M-dwarf planet occurrence rates, we should have roughly a 1% chance of detecting a candidate that could cause the transit depth we observe for 2MASS J0835+1953. If 2MASS J0835+1953 b is confirmed, it would suggest an enhancement in the occurrence of short-period planets around L and T dwarfs in comparison to M dwarfs, which would challenge predictions from planet formation models.

SimAb: A simple, fast, and flexible model to assess the effects of planet formation on the atmospheric composition of gas giants

Context. The composition of exoplanet atmospheres provides us with vital insight into their formation scenario. Conversely, planet formation processes shape the composition of atmospheres and imprint their specific signatures. In this context, models of planet formation containing key formation processes help supply clues to how planets form. This includes constraints on the metallicity and carbon-to-oxygen ratio (C/O ratio) of the planetary atmospheres. Gas giants in particular are of great interest due to the amount of information we can obtain about their atmospheric composition from their spectra, and also due to their relative ease of observation. Aims. We present a basic, fast, and flexible planet formation model, called Simulating Abundances (SimAb), that forms giant planets and allows us to study their primary atmospheric composition soon after their formation. Methods. In SimAb we introduce parameters to simplify the assumptions about the complex physics involved in the formation of a planet. This approach allows us to trace and understand the influence of complex physical processes on the formed planets. In this study we focus on four different parameters and how they influence the composition of the planetary atmospheres: initial protoplanet mass, initial orbital distance of the protoplanet, planetesimal ratio in the disk, and dust grain fraction in the disk. Results. We focus on the C/O ratio and the metallicity of the planetary atmosphere as an indicator of their composition. We show that the initial protoplanet core mass does not influence the final composition of the planetary atmosphere in the context of our model. The initial orbital distance affects the C/O ratio due to the different C/O ratios in the gas phase and the solid phase at different orbital distances. Additionally, the initial orbital distance together with the amount of accreted planetesimals cause the planet to have subsolar or supersolar metallicity. Furthermore, the C/O ratio is affected by the dust grain fraction and the planetesimal ratio. Planets that accrete most of their heavy elements through dust grains will have a C/O ratio close to the solar C/O ratio, while planets that accrete most of their heavy elements from the planetesimals in the disk will end up with a C/O ratio closer to the C/O ratio in the solid phase of the disk. Conclusions. By using the C/O ratio and metallicity together we can put a lower and upper boundary on the initial orbital distance where supersolar metallicity planets are formed. We show that planetesimals are the main source for reaching supersolar metallicity planets. On the other hand, planets that mainly accrete dust grains will show a more solar composition. Supersolar metallicity planets that initiate their formation farther than the CO ice line have a C/O ratio closer to the solar value.

Global dynamics and architecture of the Kepler-444 system

Context. S-type planets, which orbit one component of multiple-star systems, place strong constraints on planet formation and evolution models. A notable case study is Kepler-444, a triple-star system whose primary is orbited by five planets smaller than Venus in a compact configuration, and for which the stellar binary companion revolves around the primary on a highly eccentric orbit. Aims. Several open questions remain about the formation and evolution of Kepler-444. Having access to the most precise up-to-date masses and orbital parameters is highly valuable when tackling those questions. We provide the first full dynamical exploration of this system, with the goal being to refine those parameters. Methods. We apply orbital stability arguments to refine the system parameters on models with and without the stellar binary companion in order to understand the origin of the dynamical constraints. This approach makes use of the numerical analysis of fundamental frequencies fast chaos indicator. We also explore potential two- and three-planet mean-motion resonances (MMRs) in the system. Prior to investigating the dynamics of a model that includes the binary companion, we update its orbital parameters and mass using new observational constraints from both HIRES radial velocity and Gaia astrometric data, as well as archival imaging of the system. Results. The planetary system does not appear in any of the low-order two- or three-planet MMRs. We provide the most precise up-to-date dynamical parameters for the planets and the stellar binary companion. The orbit of the latter is constrained by the new observations, and also by the stability analysis. This update further challenges the planets formation processes. We also test the dynamical plausibility of a sixth planet in the system, following hints found in HST data. We find that this putative planet could exist over a broad range of masses, and with an orbital period of between roughly 12 and 20 days. Conclusions. We note the overall good agreement of the system with short-term orbital stability. This suggests that a diverse range of planetary system architectures could be found in multiple-star systems, potentially challenging the planet formation models further.

Evidence of a signature of planet formation processes from solar neutrino fluxes

Solar evolutionary models are thus far unable to reproduce spectroscopic, helioseismic, and neutrino constraints consistently, resulting in the so-called solar modeling problem. In parallel, planet formation models predict that the evolving composition of the protosolar disk and, thus, of the gas accreted by the proto-Sun must have been variable. We show that solar evolutionary models that include a realistic planet formation scenario lead to an increased core metallicity of up to 5%, implying that accurate neutrino flux measurements are sensitive to the initial stages of the formation of the Solar System. Models with homogeneous accretion match neutrino constraints to no better than 2.7σ. In contrast, accretion with a variable composition due to planet formation processes, leading to metal-poor accretion of the last ∼4% of the young Sun’s total mass, yields solar models within 1.3σ of all neutrino constraints. We thus demonstrate that in addition to increased opacities at the base of the convective envelope, the formation history of the Solar System constitutes a key element in resolving the current crisis of solar models.

Toward a population synthesis of disks and planets

Context. Millimeter astronomy provides valuable information on the birthplaces of planetary systems. In order to compare theoretical models with observations, the dust component has to be carefully calculated. Aims. Here, we aim to study the effects of dust entrainment in photoevaporative winds, and the ejection and drag of dust due to the effects caused by radiation from the central star. Methods. We improved and extended the existing implementation of a two-population dust and pebble description in the global Bern/Heidelberg planet formation and evolution model. Modern prescriptions for photoevaporative winds were used and we accounted for settling and advection of dust when calculating entrainment rates. In order to prepare for future population studies with varying conditions, we explored a wide range of disk, photoevaporation, and dust parameters. Results. If dust can grow to pebble sizes, that is, if they are resistant to fragmentation or turbulence is weak, drift dominates and the entrained mass is small but larger than under the assumption of no vertical advection of grains with the gas flow. For the case of fragile dust shattering at velocities of 1m s−1 – as indicated in laboratory experiments –, an order of magnitude more dust is entrained, which becomes the main dust removal process. Radiation pressure effects disperse massive, dusty disks on timescales of a few hundred Myr. Conclusions. These results highlight the importance of dust entrainment in winds as a solid-mass removal process. Furthermore, this model extension lays the foundations for future statistical studies of the formation of planets in their birth environment.

A warm super-Neptune around the G-dwarf star TOI-1710 revealed with TESS, SOPHIE, and HARPS-N

We report the discovery and characterization of the transiting extrasolar planet TOI-1710 b. It was first identified as a promising candidate by the Transiting Exoplanet Survey Satellite. Its planetary nature was then established with SOPHIE and HARPS-N spectroscopic observations via the radial-velocity method. The stellar parameters for the host star are derived from the spectra and a joint Markov chain Monte-Carlo adjustment of the spectral energy distribution and evolutionary tracks of TOI-1710. A joint MCMC analysis of the TESS light curve and the radial-velocity evolution allows us to determine the planetary system properties. From our analysis, TOI-1710 b is found to be a massive warm super-Neptune (Mp = 28.3 ± 4.7 M⊕ and Rp = 5.34 ± 0.11 R⊕) orbiting a G5V dwarf star (Teff = 5665 ± 55 K) on a nearly circular 24.3-day orbit (e = 0.16 ± 0.08). The orbital period of this planet is close to the estimated rotation period of its host star Prot = 22.5 ± 2.0 days and it has a low Keplerian semi-amplitude K = 6.4 ± 1.0 m s−1; we thus performed additional analyses to show the robustness of the retrieved planetary parameters. With a low bulk density of 1.03 ± 0.23 g cm−3 and orbiting a bright host star (J = 8.3, V = 9.6), TOI-1710 b is one of the best targets in this mass-radius range (near the Neptunian desert) for atmospheric characterization via transmission spectroscopy, a key measurement in constraining planet formation and evolutionary models of sub-Jovian planets.

Exploring multiple generations of planetary embryos

Context. Global models of planet formation tend to begin with an initial set of planetary embryos for the sake of simplicity. While this approach gives valuable insights into the evolution of the initial embryos, the initial distribution itself is staked on a bold assumption. Limiting the study to an initial distribution may neglect essential physics that either precedes or follows such an initial distribution. Aims. We wish to investigate the effect of dynamic planetary embryo formation on the formation of planetary systems. Methods. The presented framework begins with an initial disk of gas, dust, and pebbles. The disk evolution, the formation of plan-etesimals and the formation of planetary embryos is modeled consistently. Embryos then grow by pebble accretion, followed by planetesimal and, eventually, gas accretion. Planet-disk interactions and N-body dynamics, along with a consideration of other simultaneously growing embryos, are included in the framework. Results. We show that the formation of planets can occur in multiple consecutive phases. Earlier generations grow massive by pebble accretion but are subject to fast type I migration and, thus, by accretion to the star. The later generations of embryos that form grow too much smaller masses by planetesimal accretion, as the amount of pebbles in the disk has vanished. Conclusions. The formation history of planetary systems may be far more complex than an initial distribution of embryos could reflect. The dynamic formation of planetary embryos needs to be considered in global models of planet formation to allow for a complete picture of the system’s evolution.

Rethinking the role of the giant planet instability in terrestrial planet formation models

Advances in computing power and numerical methodologies over the past several decades sparked a prolific output of dynamical investigations of the late stages of terrestrial planet formation. Among other peculiar inner solar system qualities, the ability of simulations to reproduce the small mass of Mars within the planets’ geochemically inferred accretion timescale of ≲10 Myr after the appearance of calcium aluminum-rich inclusions (CAIs) is arguably considered the gold standard for judging evolutionary hypotheses. At present, a number of independent models are capable of consistently generating Mars-like planets and simultaneously satisfying various important observational and geochemical constraints. However, all models must still account for the effects of the epoch of giant planet migration and orbital instability; an event which dynamical and cosmochemical constraints indicate occurred within the first 100 Myr after nebular gas dispersal. If the instability occurred in the first few Myr of this window, the disturbance might have affected the bulk of Mars’ growth. In this manuscript, we turn our attention to a scenario where the instability took place after t≃ 50 Myr. Specifically, we simulate the instability’s effects on three nearly-assembled terrestrial systems that were generated via previous embryo accretion models and contain three large proto-planets (i.e. Earth, Venus and Theia) with 0.5 <m<1.0M⊕ and orbits interior to a collection of ∼Mars-mass embryos (a>1.3 au and m<0.2M⊕) and debris. While the instability consistently triggers a Moon-forming impact and efficiently removes excessive material from the Mars-region in our models, we find that our final systems are too dynamically excited and devoid of Mars and Mercury analogs. Thus, we conclude that, while possible, our scenario is far more improbable than one where the instability either occurred earlier, or at a time where Earth and Venus’ orbits were far less dynamically excited than considered here.

How drifting and evaporating pebbles shape giant planets

Atmospheric abundances of exoplanets are thought to constrain the planet formation pathway because different species evaporate at different temperatures and therefor radii in the protoplanetary disk, leaving distinct signatures inside the accreted planetary atmosphere. In particular the planetary C/O ratio is thought to constrain the planet formation pathway because of the condensation sequence of H2O, CO2, CH4, and CO, resulting in an increase in the gas phase C/O ratio with increasing distance to the host star. Here we use a disk evolution model including pebble growth, drift, and evaporation coupled with a planet formation model that includes pebble and gas accretion as well as planet migration to compute the atmospheric compositions of giant planets. We compare our results to the recent observational constraints of the hot Jupiters WASP-77A b and τ Boötis b. WASP-77A b’s atmosphere features subsolar C/H, O/H, and H2O/H with slightly super-solar C/O, while τ Boötis b’s atmosphere features super-solar C/H, O/H, and C/O with subsolar H2O/H. Our simulations qualitatively reproduce these measurements and show that giants similar to WASP-77A b should start to form beyond the CO2 evaporation front, while giants similar to τ Boötis b should originate from beyond the water ice line. Our model allows for the formation of subsolar and super-solar atmospheric compositions within the same framework. On the other hand, simulations without pebble evaporation, as used in classical models, cannot reproduce the super-solar C/H and O/H ratios of τ Boötis b’s atmosphere without the additional accretion of solids. Furthermore, we identify the a viscosity parameter of the disk as a key ingredient regarding planetary composition because the viscosity drives the inward motion of volatile enriched vapor, which is responsible for the accretion of gaseous carbon and oxygen. Depending on the planet’s migration history through the disk across different evaporation fronts, order-of-magnitude differences in atmospheric carbon and oxygen abundances should be expected. Our simulations additionally predict super-solar N/H for τ Boötis b and solar N/H for WASP-77A b. We thus conclude that pebble evaporation is a key ingredient to explain the variety of exoplanet atmospheres because it can explain both subsolar and super-solar atmospheric abundances.

Interpreting the Atmospheric Composition of Exoplanets: Sensitivity to Planet Formation Assumptions

Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric compositions. We introduce a methodology that explores the effect of different formation model assumptions when interpreting atmospheric compositions. We apply this framework to the directly imaged planet HR 8799e. Based on its atmospheric composition, this planet may have migrated significantly during its formation. We show that including the chemical evolution of the protoplanetary disk leads to a reduced need for migration. Moreover, we find that pebble accretion can reproduce the planet’s composition, but some of our tested setups lead to too low atmospheric metallicities, even when considering that evaporating pebbles may enrich the disk gas. We conclude that the definitive inversion from atmospheric abundances to planet formation for a given planet may be challenging, but a qualitative understanding of the effects of different formation models is possible, opening up pathways for new investigations.