Planet Formation Theories Research Articles

Discovery and characterization of a dense sub-Saturn TOI-6651b

We report the discovery and characterization of a transiting sub-Saturn exoplanet TOI-6651b using PARAS-2 spectroscopic observations. The host, TOI-6651 (mV ≈ 10.2), is a sub-giant, metal-rich G-type star with [Fe / H] = 0.225−0.0450.044[Fe/H] = 0.225−0.045+0.044, Teff = 5940 ± 110 K, and log g = 4.087−0.032+0.035. Joint fitting of the radial velocities from PARAS-2 spectrograph and transit photometric data from Transiting Exoplanet Survey Satellite (TESS) reveals a planetary mass of 61.0−7.9+7.6 M⊕ and radius of 5.09−0.26+0.27 R⊕, in a 5.056973−0.000018+0.000016 day orbit with an eccentricity of 0.091−0.062+0.096. TOI-6651b has a bulk density of 2.52−0.44+0.52 g cm−3, positioning it among the select few known dense sub-Saturns and making it notably the densest detected with TESS. TOI-6651b is consistent with the positive correlation between planet mass and the host star’s metallicity. We find that a considerable portion ≈87% of the planet’s mass consists of dense materials such as rock and iron in the core, while the remaining mass comprises a low-density envelope of H/He. TOI-6651b lies at the edge of the Neptunian desert, which will be crucial for understanding the factors shaping the desert boundaries. The existence of TOI-6651b challenges conventional planet formation theories and could be a result of merging events or significant atmospheric mass loss through tidal heating, highlighting the complex interplay of dynamical processes and atmospheric evolution in the formation of massive dense sub-Saturns.

Formation of Close-in Neptunes around Low-mass Stars through Breaking Resonant Chains

Conventional planet formation theories predict a paucity of massive planets around small stars, especially very low-mass (0.1−0.3 M ⊙) mid-to-late M dwarfs. Such tiny stars are expected to form planets of terrestrial sizes but not much bigger. However, this expectation is challenged by the recent discovery of LHS 3154 b, a planet with period of 3.7 days and minimum mass of 13.2 M ⊕ orbiting a 0.11 M ⊙ star. Here, we propose that close-in Neptune-mass planets like LHS 3154 b formed through an anomalous series of mergers from a primordial compact system of super-Earths. We perform simulations within the context of the “breaking the chains” scenario, in which super-Earths initially form in tightly spaced chains of mean-motion resonances before experiencing dynamical instabilities and collisions. Planets as massive and close-in as LHS 3154 b (M p ∼ 12−20 M ⊕, P < 7 days) are produced in ∼1% of simulated systems, in broad agreement with their low observed occurrence. These results suggest that such planets do not require particularly unusual formation conditions but rather are an occasional by-product of a process that is already theorized to explain compact multiplanet systems. Interestingly, our simulated systems with LHS 3154 b-like planets also contain smaller planets at around ∼30 days, offering a possible test of this hypothesis.

Atmospheric Characterization of the Super-Jupiter HIP 99770 b with KPIC

Young, self-luminous super-Jovian companions discovered by direct imaging provide a challenging test for planet formation and evolution theories. By spectroscopically characterizing the atmospheric compositions of these super-Jupiters, we can constrain their formation histories. Here we present studies of the recently discovered HIP 99770 b, a 16 M Jup high-contrast companion on a 17 au orbit, using the fiber-fed high-resolution spectrograph KPIC ( R ∼ 35,000) on the Keck II telescope. Our K-band observations led to detections of H2O and CO in the atmosphere of HIP 99770 b. We carried out free retrieval analyses using petitRADTRANS to measure its chemical abundances, including the metallicity and C/O ratio, projected rotation velocity ( vsini ), and radial velocity (RV). We found that the companion’s atmosphere has C/O =0.55−0.04+0.06 and [M/H] =0.26−0.23+0.24 (1σ confidence intervals), values consistent with those of the Sun and with a companion formation via gravitational instability or core accretion. The projected rotation velocity vsin(i)<7.8 km s−1 is small relative to other directly imaged companions with similar masses and ages. This may imply a nearly pole-on orientation or effective magnetic braking by a circumplanetary disk. In addition, we added the companion-to-primary relative RV measurement to the orbital fitting and obtained updated constraints on orbital parameters. Detailed characterization of super-Jovian companions within 20 au like HIP 99770 b is critical for understanding the formation histories of this population.

Surviving in the Hot-Neptune Desert: The Discovery of the Ultrahot Neptune TOI-3261b

The recent discoveries of Neptune-sized ultra-short-period planets (USPs) challenge existing planet formation theories. It is unclear whether these residents of the Hot Neptune Desert have similar origins to smaller, rocky USPs, or if this discrete population is evidence of a different formation pathway altogether. We report the discovery of TOI-3261b, an ultrahot Neptune with an orbital period P = 0.88 day. The host star is a V = 13.2 mag, slightly supersolar metallicity ([Fe/H] ≃0.15), inactive K1.5 main-sequence star at d = 300 pc. Using data from the Transiting Exoplanet Survey Satellite and the Las Cumbres Observatory Global Telescope, we find that TOI-3261b has a radius of 3.82−0.35+0.42 R ⊕. Moreover, radial velocities from ESPRESSO and HARPS reveal a mass of 30.3−2.4+2.2 M ⊕, more than twice the median mass of Neptune-sized planets on longer orbits. We investigate multiple mechanisms of mass loss that can reproduce the current-day properties of TOI-3261b, simulating the evolution of the planet via tidal stripping and photoevaporation. Thermal evolution models suggest that TOI-3261b should retain an envelope potentially enriched with volatiles constituting ∼5% of its total mass. This is the second highest envelope mass fraction among ultrahot Neptunes discovered to date, making TOI-3261b an ideal candidate for atmospheric follow-up observations.

Planet Formation Regulated by Galactic-scale Interstellar Turbulence

Planet formation occurs over a few Myr within protoplanetary disks of dust and gas, which are often assumed to evolve in isolation. However, extended gaseous structures have been uncovered around many protoplanetary disks, suggestive of late-stage infall from the interstellar medium (ISM). To quantify the prevalence of late-stage infall, we apply an excursion set formalism to track the local density and relative velocity of the ISM over the disk lifetime. We then combine the theoretical Bondi–Hoyle–Lyttleton (BHL) accretion rate with a simple disk evolution model, anchoring stellar accretion timescales to observational constraints. Disk lifetimes, masses, stellar accretion rates, and gaseous outer radii as a function of stellar mass and age are remarkably well reproduced by our simple model that includes only ISM accretion. We estimate that 20%−70% of disks may be mostly composed of material accreted in the most recent half of their lifetime, suggesting that disk properties are not a direct test of isolated evolution models. Our calculations indicate that BHL accretion can also supply sufficient energy to drive turbulence in the outer regions of protoplanetary disks with viscous α SS ∼ 10−5 to 10−1, although we emphasize that angular momentum transport and particularly accretion onto the star may still be driven by internal processes. Our simple approach can be easily applied to semianalytic models. Our results represent a compelling case for regulation of planet formation by large-scale turbulence, with broad consequences for planet formation theory. This possibility urgently motivates deep observational surveys to confirm or refute our findings.

The First Evidence of a Host Star Metallicity Cutoff in the Formation of Super-Earth Planets

Planet formation is expected to be severely limited in disks of low metallicity, owing to both the small solid mass reservoir and the low-opacity accelerating the disk gas dissipation. While previous studies have found a weak correlation between the occurrence rates of small planets (≲4R ⊕) and stellar metallicity, so far no studies have probed below the metallicity limit beyond which planet formation is predicted to be suppressed. Here, we constructed a large catalog of ∼110,000 metal-poor stars observed by the TESS mission with spectroscopically derived metallicities, and systematically probed planet formation within the metal-poor regime ([Fe/H] ≤−0.5) for the first time. Extrapolating known higher-metallicity trends for small, short-period planets predicts the discovery of ∼68 super-Earths around these stars (∼85,000 stars) after accounting for survey completeness; however, we detect none. As a result, we have placed the most stringent upper limit on super-Earth occurrence rates around metal-poor stars (−0.75 < [Fe/H] ≤ −0.5) to date, ≤ 1.67%, a statistically significant (p-value = 0.000685) deviation from the prediction of metallicity trends derived with Kepler and K2. We find a clear host star metallicity cliff for super-Earths that could indicate the threshold below which planets are unable to grow beyond an Earth-mass at short orbital periods. This finding provides a crucial input to planet-formation theories, and has implications for the small planet inventory of the Galaxy and the galactic epoch at which the formation of small planets started.

The physical mechanism of the streaming instability

Abstract The main hurdle of planet formation theory is the metre-scale barrier. One of the most promising ways to overcome it is via the streaming instability (SI). Unfortunately, the mechanism responsible for the onset of this instability remains mysterious. It has recently been shown that the SI is a Resonant Drag Instability (RDI) involving inertial waves. We build on this insight and clarify the physical picture of how the SI develops, while bolstering this picture with transparent mathematics. Like all RDIs, the SI is built on a feedback loop: in the ‘forward action’, an inertial wave concentrates dust into clumps; in the ‘backward reaction’, those drifting dust clumps excite an inertial wave. Each process breaks into two mechanisms, a fast one and a slow one. At resonance, each forward mechanism can couple with a backward mechanism to close a feedback loop. Unfortunately, the fast-fast loop is stable, so the SI uses the fast-slow and slow-fast loops. Despite this last layer of complexity, we hope that our explanation will help understand how the SI works, in which conditions it can grow, how it manifests itself, and how it saturates.

Formation of Giant Planets by Gas Disk Gravitational Instability on Wide Orbits around Protostars with Varied Masses. II. Quadrupled Spatial Resolution and Beta Cooling

Exoplanet demographics are sufficiently advanced to provide important constraints on theories of planet formation. While core and pebble accretion are preferred for rocky and icy planets, there appears to be a need for gas disk gravitational instability (GDGI) to play a role in the formation of M-dwarf gas giants and those orbiting at large distances. Here we present GDGI models that go beyond those presented by Boss (2011) dealing with the formation of wide-orbit gas giants. The new models use quadrupled spatial resolution, in both the radial and azimuthal directions, to reduce the effects of finite spatial resolution. The new models also employ the β cooling approximation, instead of the diffusion approximation used by Boss (2011), in order to push the models further in time. As in Boss (2011), the central protostars have masses of 0.1, 0.5, 1.0, 1.5, or 2.0 M ⊙, surrounded by disks with masses ranging from 0.019 M ⊙ to 0.21 M ⊙. For each case, two models are computed, one with an initial minimum Toomre Q stability value ranging from 1.1 to 1.7, and one with a higher initial disk temperature, resulting in the initial minimum Q ranging from 2.2 to 3.4. These new models continue to show that GDGI can explain the formation of gas giants at distances of ∼30 to ∼50 au on eccentric orbits (e less than ∼0.2), though the number formed drops to 0 as the protostar mass decreases to 0.1 M ⊙.

No Signature of the Birth Environment of Exoplanets from Their Host Stars’ Mahalanobis Phase Space

The architectures of extrasolar planetary systems often deviate considerably from the “standard” model for planet formation, which is largely based on our own solar system. In particular, gas giants on close orbits are not predicted by planet formation theory and so some processes are thought to move the planets closer to their host stars. Recent research has suggested that hot-Jupiter host stars display a different phase space compared to stars that do not host hot Jupiters. This has been attributed to these stars forming in star-forming regions of high stellar density, where dynamical interactions with passing stars have perturbed the planets. We test this hypothesis by quantifying the phase space of planet-hosting stars in dynamical N-body simulations of star-forming regions. We find that stars that retain their planets have a higher phase space than nonhosts, regardless of their initial physical density. This is because an imprint of the kinematic substructure from the regions birth is retained, as these stars have experienced fewer and less disruptive encounters than stars whose planets have been liberated and become free-floating. However, host stars whose planets remain bound but have had their orbits significantly altered by dynamical encounters are also primarily found in high phase space regimes. We therefore corroborate other research in this area that has suggested the high phase space of hot-Jupiter host stars is not caused by dynamical encounters or stellar clustering, but rather reflects an age bias in that these stars are (kinematically) younger than other exoplanet host stars.

Stellar obliquity measurements of six gas giants

One can infer the orbital alignment of exoplanets with respect to the spin of their host stars using the Rossiter-McLaughlin effect, thereby giving us the chance to test planet formation and migration theories and improve our understanding of the currently observed population. We analyzed archival HARPS and HARPS-N spectroscopic transit time series of six gas giant exoplanets on short orbits, namely WASP-77 Ab, WASP-101b, WASP-103b, WASP-105b, WASP-120b, and WASP-131b. We find a moderately misaligned orbit for WASP-101b (λ = 34° ± 3) and a highly misaligned orbit for WASP-131b (λ = 161° ± 5), while the four remaining exoplanets appear to be aligned: WASP-77 Ab (λ = −8°−18+19), WASP-103b (λ = −2°−36+35), WASP-105b (λ = −14°−24+28), and WASP-120b (λ = −2° ± 4). For WASP-77 Ab, we are able to infer its true orbital obliquity (Ψ = 48°−21+22). We additionally performed transmission spectroscopy of the targets in search of strong atomic absorbers in the exoatmospheres, but were unable to detect any features, most likely due to the presence of high-altitude clouds or Rayleigh scattering muting the strength of the features. Finally, we comment on future perspectives on studying these planets with upcoming space missions to investigate their evolution and migration histories.

A fading radius valley towards M dwarfs, a persistent density valley across stellar types

The radius valley separating super-Earths from mini-Neptunes is a fundamental benchmark for theories of planet formation and evolution. Observations show that the location of the radius valley decreases with decreasing stellar mass and with increasing orbital period. Here, we build on our previous pebble-based formation model. Combined with photoevaporation after disc dispersal, it has allowed us to unveil the radius valley as a separator between rocky and water-worlds. In this study, we expand our model for a range of stellar masses spanning from 0.1 to 1.5 M⊙. We find that the location of the radius valley is well described by a power-law in stellar mass as Rvalley = 1.8197 M⋆0.14(+0.02/−0.01), which is in excellent agreement with observations. We also find very good agreement with the dependence of the radius valley on orbital period, both for FGK and M dwarfs. Additionally, we note that the radius valley gets filled towards low stellar masses, particularly at 0.1–0.4 M⊙, yielding a rather flat slope in Rvalley − Porb. This is the result of orbital migration occurring at lower planet mass for less massive stars, which allows for low-mass water-worlds to reach the inner regions of the system, blurring the separation in mass (and size) between rocky and water worlds. Furthermore, we find that for planetary equilibrium temperatures above 400 K, the water in the volatile layer exists fully in the form of steam, puffing the planet radius up (as compared to the radii of condensed-water worlds). This produces an increase in planet radii of ∼30% at 1 M⊕ and of ∼15% at 5 M⊕ compared to condensed-water worlds. As with Sun-like stars, we find that pebble accretion leaves its imprint on the overall exoplanet population as a depletion of planets with intermediate compositions (i.e. water mass fractions of ∼0 − 20%), carving an planet-depleted diagonal band in the mass-radius (MR) diagrams. This band is better visualised when plotting the planet’s mean density in terms of an Earth-like composition. This change in coordinates causes the valley to emerge for all the stellar mass cases.

Accretion of primordial H–He atmospheres in mini-Neptunes: The importance of envelope enrichment

Context. Out of the more than 5000 detected exoplanets, a considerable number belong to a category called “mini-Neptunes”. Interior models of these planets suggest that they have primordial H–He-dominated atmospheres. As this type of planet is not found in the Solar System, understanding their formation is a key challenge in planet formation theory. Unfortunately, quantifying how much H–He planets have, based on their observed mass and radius, is impossible due to the degeneracy of interior models. Aims. Another approach to estimating the range of possible primordial envelope masses is to use formation theory. As different assumptions in planet formation can heavily influence the nebular gas accretion rate of small planets, it is unclear how large the envelope of a protoplanet should be. We explore the effects that different assumptions regarding planet formation have on the nebular gas accretion rate, particularly by exploring the way in which solid material interacts with the envelope. This allows us to estimate the range of possible post-formation primordial envelopes. Thereby, we demonstrate the impact of envelope enrichment on the initial primordial envelope, which can be used in evolution models. Methods. We applied formation models that include different solid accretion rate prescriptions. Our assumption is that mini-Neptunes form beyond the ice line and migrate inward after formation; thus, we formed planets in situ at 3 and 5 au. We considered that the envelope can be enriched by the accreted solids in the form of water. We studied how different assumptions and parameters influence the ratio between the planet’s total mass and the fraction of primordial gas. Results. The primordial envelope fractions for low- and intermediate-mass planets (total mass below 15 M⊕) can range from 0.1% to 50%. Envelope enrichment can lead to higher primordial mass fractions. We find that the solid accretion rate timescale has the largest influence on the primordial envelope size. Conclusions. Rates of primordial gas accretion onto small planets can span many orders of magnitude. Planet formation models need to use a self-consistent gas accretion prescription.

Discovery of two warm mini-Neptunes with contrasting densities orbiting the young K3V star TOI-815

We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K–M binary system. Analysis of its spectra and rotation period reveal the star to be young, with an age of 200−200+400 Myr. TOI-8l5b has a 11.2-day period and a radius of 2.94 ± 0.05 R⊕ with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-8l5c, has a radius of 2.62 ± 0.10 R⊕, based on observations of three nonconsecutive transits with TESS; targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6 ± 1.5 M⊕ (ρP = 1.64−0.31+0.33 g cm−3) and 23.5 ± 2.4 M⊕ (ρP = 7.2−1.0+1.1 g cm−3), respectively. Thus, the planets have very different masses, which is unusual for compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those undergoing strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere that constitutes a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3σ level. This emphasizes the peculiarity of the system’s orbital architecture, and probably hints at an eventful dynamical history.

The Statistical Analysis of Exoplanet and Host Stars Based on Multi-Satellite Data Observations

In recent years, the rapid development of exoplanet research has provided us with an opportunity to better understand planetary systems in the universe and to search for signs of life. In order to further investigate the prevalence of habitable exoplanets and to validate planetary formation theories, as well as to comprehend planetary evolution, we have utilized confirmed exoplanet data obtained from the NASA Exoplanet Archive database, including data released by telescopes such as Kepler and TESS. By analyzing these data, we have selected a sample of planets around F, G, K, and M-type stars within a radius range of 1 to 20 R⊕ and with orbital periods ranging from 0.4 days to 400 days. Using the IDEM method based on these data, we calculated the overall formation rate, which is estimated to be 2.02%. Then, we use these data to analyze the relationship among planet formation rates, stellar metallicity, and stellar gravitational acceleration (logg). We firstly find that the formation rate of giant planets is higher around metal-rich stellars, but it inhibits the formation of gas giants when logg &gt; 4.5, yet the stellar metallicity seems to have no effect on the formation rate of smaller planets. Secondly, the host stellar gravitational acceleration affects the relationship between planet formation rate and orbital period. Thirdly, there is a robust power-law relationship between the orbital period of smaller planets and their formation rate. Finally, we find that, for a given orbital period, there is a positive correlation between the planet formation rate and the logg.

LHS 475 b: A Potential Venus Analog Orbiting a Nearby M Dwarf

Based on photometric observations by TESS, we present the discovery of a potential Venus analog transiting LHS 475, an M3 dwarf located 12.5 pc from the Sun. The mass of the star is 0.274 ± 0.015 M ☉. The planet, originally reported as TOI 910.01, has an orbital period of 2.0291010 ± 0.0000017 days and an estimated radius of 0.975 ± 0.058 R ⊕. We confirm the validity and source of the transit signal with MEarth and Las Cumbres Observatory Global Telescope ground-based follow-up photometry. We present radial velocity data from CHIRON that rule out massive companions. In accordance with the observed mass–radius distribution of exoplanets as well as planet formation theory, we expect this planetary companion to be terrestrial, with an estimated radial velocity semiamplitude of 1.1 m s−1. LHS 475 b is likely too hot to be habitable but is a suitable candidate for emission and transmission spectroscopy.

The Fuzzy Cores of Jupiter and Saturn

AbstractNew interior models of Jupiter and Saturn suggest that both planets have “fuzzy cores.” These cores should be viewed as central regions that are enriched with heavy elements but are not distinct from the rest of the deep interior. These cores may contain large amounts of hydrogen and helium though small pure heavy‐element cores may also exist. New measurements along with advanced planetary modeling have revolutionized the way we think about the interiors of giant planets and provide important constraints for planet formation and evolution theories. These developments are also relevant for the characterization of giant exoplanets.

A Review to the Observational and Theoretical Studies of Planetary Systems around Red Dwarfs

Red dwarfs are one of the smallest and dimmest main-sequence stars. With the improvement of observation technology, a rich population of planets have been discovered around them, which exhibits distinct distribution characteristics from the planets around Solar-like stars. Earth-like planets have a higher occurrence rate around red dwarfs, and the planetary systems are tightly compact. The closer habitable zone makes it easier to search for habitable planets. The distribution of the radius valley between super-Earths and sub-Neptunes also differs from that around solar-like stars. Moreover, the discovery of giant planets around these low-mass stars can be hard to explain by current leading theories of planet formation. As the mass of solid material in the protoplanetary disk decreases with decreasing stellar masses, the formation of giant planets remains challenging. The observations from telescopes such as Transiting Exoplanet Survey Satellite (TESS), James Webb, and Atacama Large Millimeter/submillimeter Array (ALMA) have provided invaluable insights and opportunities for the study of planetary formation. In this paper, we review the observations of different populations of planets and summarize the up-to-date understanding of planetary formation around red dwarfs.

Searching for Giant Exoplanets around M-dwarf Stars (GEMS) I: Survey Motivation

Recent discoveries of transiting giant exoplanets around M-dwarf stars (GEMS), aided by the all-sky coverage of TESS, are starting to stretch theories of planet formation through the core-accretion scenario. Recent upper limits on their occurrence suggest that they decrease with lower stellar masses, with fewer GEMS around lower-mass stars compared to solar-type. In this paper, we discuss existing GEMS both through confirmed planets, as well as protoplanetary disk observations, and a combination of tests to reconcile these with theoretical predictions. We then introduce the Searching for GEMS survey, where we utilize multidimensional nonparameteric statistics to simulate hypothetical survey scenarios to predict the required sample size of transiting GEMS with mass measurements to robustly compare their bulk-density with canonical hot Jupiters orbiting FGK stars. Our Monte Carlo simulations predict that a robust comparison requires about 40 transiting GEMS (compared to the existing sample of ∼15) with 5σ mass measurements. Furthermore, we discuss the limitations of existing occurrence estimates for GEMS and provide a brief description of our planned systematic search to improve the occurrence rate estimates for GEMS.

TOI-4860 b, a short-period giant planet transiting an M3.5 dwarf

We report the discovery and characterisation of a giant transiting planet orbiting a nearby M3.5V dwarf (d = 80.4pc, G = 15.1 mag, K=11.2mag, R* = 0.358 ± 0.015 R⊙, M* = 0.340 ± 0.009 M⊙). Using the photometric time series from TESS sectors 10, 36, 46, and 63 and near-infrared spectrophotometry from ExTrA, we measured a planetary radius of 0.77 ± 0.03 RJ and an orbital period of 1.52 days. With high-resolution spectroscopy taken by the CFHT/SPIRou and ESO/ESPRESSO spectrographs, we refined the host star parameters ([Fe/H] = 0.27 ± 0.12) and measured the mass of the planet (0.273 ± 0.006 MJ). Based on these measurements, TOI-4860 b joins the small set of massive planets (&gt;80 ME) found around mid to late M dwarfs (&lt;0.4 R⊙), providing both an interesting challenge to planet formation theory and a favourable target for further atmospheric studies with transmission spectroscopy. We identified an additional signal in the radial velocity data that we attribute to an eccentric planet candidate (e = 0.66 ± 0.09) with an orbital period of 427 ± 7 days and a minimum mass of 1.66 ± 0.26 MJ, but additional data would be needed to confirm this.

Planet formation around intermediate-mass stars

Context. The study of protoplanetary disc evolution and theories of planet formation has predominantly concentrated on solar- (and low-) mass stars since they host the majority of confirmed exoplanets. Nevertheless, the confirmation of numerous planets orbiting stars more massive than the Sun (up to ~3 M⊙) has sparked considerable interest in understanding the mechanisms involved in their formation, and thus in the evolution of their hosting protoplanetary discs. Aims. We aim to improve our knowledge of the evolution of the gaseous component of protoplanetary discs around intermediate-mass stars and to set the stage for future studies of planet formation around them. Methods. We study the long-term evolution of protoplanetary discs affected by viscous accretion and photoevaporation by X-ray and far-ultraviolet (FUV) photons from the central star around stars in the range of 1–3 M⊙, considering the effects of stellar evolution and solving the vertical structure equations of the disc. We explore the effect of different values of the viscosity parameter and the initial mass of the disc. Results. We find that the evolutionary pathway of protoplanetary disc dispersal due to photoevaporation depends on the stellar mass. Our simulations reveal four distinct evolutionary pathways for the gas component not reported before that are a consequence of stellar evolution and that likely have a substantial impact on the dust evolution, and thus on planet formation. As the stellar mass increases from one solar mass to ~1.5–2 M⊙, the evolution of the disc changes from the conventional inside-out clearing, in which X-ray photoevaporation generates inner holes, to a homogeneous disc evolution scenario where both inner and outer discs formed after a gap is opened by photoevaporation vanish over a similar timescale. As the stellar mass continues to increase, reaching ~2–3 M⊙, we identify a distinct pathway that we refer to as revenant disc evolution. In this scenario, the inner and outer discs reconnect after the gap opened. For the largest masses, we observe outside-in disc dispersal, in which the outer disc dissipates first due to a stronger FUV photoevaporation rate. Revenant disc evolution stands out as it is capable of extending the disc lifespan. Otherwise, the disc dispersal timescale decreases with increasing stellar mass except for low-viscosity discs.