Context.
Planet formation is sensitive to the conditions in protoplanetary disks, for which scaling laws as a function of stellar mass are known.
Aims.
We aim to test whether the observed population ...of planets around low-mass stars can be explained by these trends, or if separate formation channels are needed.
Methods.
We address this question by confronting a state-of-the-art planet population synthesis model with a sample of planets around M dwarfs observed by the HARPS and CARMENES radial velocity (RV) surveys. To account for detection biases, we performed injection and retrieval experiments on the actual RV data to produce synthetic observations of planets that we simulated following the core accretion paradigm.
Results.
These simulations robustly yield the previously reported high occurrence of rocky planets around M dwarfs and generally agree with their planetary mass function. In contrast, our simulations cannot reproduce a population of giant planets around stars less massive than 0.5 solar masses. This potentially indicates an alternative formation channel for giant planets around the least massive stars that cannot be explained with current core accretion theories. We further find a stellar mass dependency in the detection rate of short-period planets. A lack of close-in planets around the earlier-type stars (
M
*
> 0.4
M
⊙
) in our sample remains unexplained by our model and indicates dissimilar planet migration barriers in disks of different spectral subtypes.
Conclusions.
Both discrepancies can be attributed to gaps in our understanding of planet migration in nascent M dwarf systems. They underline the different conditions around young stars of different spectral subtypes, and the importance of taking these differences into account when studying planet formation.
Context. State-of-the-art planet formation models are now capable of accounting for the full spectrum of known planet types. This comes at the cost of an increasing complexity of the models, which ...calls into question whether established links between their initial conditions and the calculated planetary observables are preserved. Aims. In this paper, we take a data-driven approach to investigate the relations between clusters of synthetic planets with similar properties and their formation history. Methods. We trained a Gaussian mixture model on typical exoplanet observables computed by a global model of planet formation to identify clusters of similar planets. We then traced back the formation histories of the planets associated with them and pinpointed their differences. Using the cluster affiliation as labels, we trained a random forest classifier to predict planet species from properties of the originating protoplanetary disk. Results. Without presupposing any planet types, we identified four distinct classes in our synthetic population. They roughly correspond to the observed populations of (sub-)Neptunes, giant planets, and (super-)Earths, plus an additional unobserved class we denote as “icy cores”. These groups emerge already within the first 0.1 Myr of the formation phase and are predicted from disk properties with an overall accuracy of >90%. The most reliable predictors are the initial orbital distance of planetary nuclei and the total planetesimal mass available. Giant planets form only in a particular region of this parameter space that is in agreement with purely analytical predictions. Including N-body interactions between the planets decreases the predictability, especially for sub-Neptunes that frequently undergo giant collisions and turn into super-Earths. Conclusions. The processes covered by current core accretion models of planet formation are largely predictable and reproduce the known demographic features in the exoplanet population. The impact of gravitational interactions highlights the need for N-body integrators for realistic predictions of systems of low-mass planets.
Context. Recent observational findings have suggested a positive correlation between the occurrence rates of inner super-Earths and outer giant planets. These results raise the question of whether ...this trend can be reproduced and explained by planet formation theory. Aims. Here, we investigate the properties of inner super-Earths and outer giant planets that form according to a core accretion scenario. We study the mutual relations between these planet species in synthetic planetary systems and compare them to the observed exoplanet population. Methods. We invoked the Generation 3 Bern model of planet formation and evolution to simulate 1000 multi-planet systems. We then confronted these synthetic systems with the observed sample, taking into account the detection bias that distorts the observed demographics. Results. The formation of warm super-Earths and cold Jupiters in the same system is enhanced compared to the individual appearances, although it is weaker than what has been proposed through observations. We attribute the discrepancy to warm and dynamically active giant planets that frequently disrupt the inner systems, particularly in high-metallicity environments. In general, a joint occurrence of the two planet types requires intermediate solid reservoirs in the originating protoplanetary disk. Furthermore, we find differences in the volatile content of planets in different system architectures and predict that high-density super-Earths are more likely to host an outer giant. This correlation can be tested observationally.
Context. Previous theoretical works on planet formation around low-mass stars have often been limited to large planets and individual systems. As current surveys routinely detect planets down to ...terrestrial size in these systems, models have shifted toward a more holistic approach that reflects their diverse architectures. Aims. Here, we investigate planet formation around low-mass stars and identify differences in the statistical distribution of modeled planets. We compare the synthetic planet populations to observed exoplanets and we discuss the identified trends. Methods. We used the Generation III Bern global model of planet formation and evolution to calculate synthetic populations, while varying the central star from Solar-like stars to ultra-late M dwarfs. This model includes planetary migration, N-body interactions between embryos, accretion of planetesimals and gas, and the long-term contraction and loss of the gaseous atmospheres. Results. We find that temperate, Earth-sized planets are most frequent around early M dwarfs (0.3 M⊙–0.5 M⊙) and that they are more rare for Solar-type stars and late M dwarfs. The planetary mass distribution does not linearly scale with the disk mass. The reason behind this is attributed to the emergence of giant planets for M⋆ ≥ 0.5 M⊙, which leads to the ejection of smaller planets. Given a linear scaling of the disk mass with stellar mass, the formation of Earth-like planets is limited by the available amount of solids for ultra-late M dwarfs. For M⋆ ≥ 0.3 M⊙, however, there is sufficient mass in the majority of systems, leading to a similar amount of Exo-Earths going from M to G dwarfs. In contrast, the number of super-Earths and larger planets increases monotonically with stellar mass. We further identify a regime of disk parameters that reproduces observed M-dwarf systems such as TRAPPIST-1. However, giant planets around late M dwarfs, such as GJ 3512b, only form when type I migration is substantially reduced. Conclusions. We are able to quantify the stellar mass dependence of multi-planet systems using global simulations of planet formation and evolution. The results fare well in comparison to current observational data and predict trends that can be tested with future observations.
Context.
Previous theoretical works on planet formation around low-mass stars have often been limited to large planets and individual systems. As current surveys routinely detect planets down to ...terrestrial size in these systems, models have shifted toward a more holistic approach that reflects their diverse architectures.
Aims.
Here, we investigate planet formation around low-mass stars and identify differences in the statistical distribution of modeled planets. We compare the synthetic planet populations to observed exoplanets and we discuss the identified trends.
Methods.
We used the Generation III Bern global model of planet formation and evolution to calculate synthetic populations, while varying the central star from Solar-like stars to ultra-late M dwarfs. This model includes planetary migration,
N
-body interactions between embryos, accretion of planetesimals and gas, and the long-term contraction and loss of the gaseous atmospheres.
Results.
We find that temperate, Earth-sized planets are most frequent around early M dwarfs (0.3
M
⊙
–0.5
M
⊙
) and that they are more rare for Solar-type stars and late M dwarfs. The planetary mass distribution does not linearly scale with the disk mass. The reason behind this is attributed to the emergence of giant planets for
M
⋆
≥ 0.5
M
⊙
, which leads to the ejection of smaller planets. Given a linear scaling of the disk mass with stellar mass, the formation of Earth-like planets is limited by the available amount of solids for ultra-late M dwarfs. For
M
⋆
≥ 0.3
M
⊙
, however, there is sufficient mass in the majority of systems, leading to a similar amount of Exo-Earths going from M to G dwarfs. In contrast, the number of super-Earths and larger planets increases monotonically with stellar mass. We further identify a regime of disk parameters that reproduces observed M-dwarf systems such as TRAPPIST-1. However, giant planets around late M dwarfs, such as GJ 3512b, only form when type I migration is substantially reduced.
Conclusions.
We are able to quantify the stellar mass dependence of multi-planet systems using global simulations of planet formation and evolution. The results fare well in comparison to current observational data and predict trends that can be tested with future observations.
Context.
Recent observational findings have suggested a positive correlation between the occurrence rates of inner super-Earths and outer giant planets. These results raise the question of whether ...this trend can be reproduced and explained by planet formation theory.
Aims.
Here, we investigate the properties of inner super-Earths and outer giant planets that form according to a core accretion scenario. We study the mutual relations between these planet species in synthetic planetary systems and compare them to the observed exoplanet population.
Methods.
We invoked the Generation 3 Bern model of planet formation and evolution to simulate 1000 multi-planet systems. We then confronted these synthetic systems with the observed sample, taking into account the detection bias that distorts the observed demographics.
Results.
The formation of warm super-Earths and cold Jupiters in the same system is enhanced compared to the individual appearances, although it is weaker than what has been proposed through observations. We attribute the discrepancy to warm and dynamically active giant planets that frequently disrupt the inner systems, particularly in high-metallicity environments. In general, a joint occurrence of the two planet types requires intermediate solid reservoirs in the originating protoplanetary disk. Furthermore, we find differences in the volatile content of planets in different system architectures and predict that high-density super-Earths are more likely to host an outer giant. This correlation can be tested observationally.
Context.
State-of-the-art planet formation models are now capable of accounting for the full spectrum of known planet types. This comes at the cost of an increasing complexity of the models, which ...calls into question whether established links between their initial conditions and the calculated planetary observables are preserved.
Aims.
In this paper, we take a data-driven approach to investigate the relations between clusters of synthetic planets with similar properties and their formation history.
Methods.
We trained a Gaussian mixture model on typical exoplanet observables computed by a global model of planet formation to identify clusters of similar planets. We then traced back the formation histories of the planets associated with them and pinpointed their differences. Using the cluster affiliation as labels, we trained a random forest classifier to predict planet species from properties of the originating protoplanetary disk.
Results.
Without presupposing any planet types, we identified four distinct classes in our synthetic population. They roughly correspond to the observed populations of (sub-)Neptunes, giant planets, and (super-)Earths, plus an additional unobserved class we denote as “icy cores”. These groups emerge already within the first 0.1 Myr of the formation phase and are predicted from disk properties with an overall accuracy of >90%. The most reliable predictors are the initial orbital distance of planetary nuclei and the total planetesimal mass available. Giant planets form only in a particular region of this parameter space that is in agreement with purely analytical predictions. Including
N
-body interactions between the planets decreases the predictability, especially for sub-Neptunes that frequently undergo giant collisions and turn into super-Earths.
Conclusions.
The processes covered by current core accretion models of planet formation are largely predictable and reproduce the known demographic features in the exoplanet population. The impact of gravitational interactions highlights the need for
N
-body integrators for realistic predictions of systems of low-mass planets.
Context.
The CARMENES exoplanet survey of M dwarfs has obtained more than 18 000 spectra of 329 nearby M dwarfs over the past five years as part of its guaranteed time observations (GTO) program.
...Aims.
We determine planet occurrence rates with the 71 stars from the GTO program for which we have more than 50 observations.
Methods.
We use injection-and-retrieval experiments on the radial-velocity time series to measure detection probabilities. We include 27 planets in 21 planetary systems in our analysis.
Results.
We find 0.06
−0.03
+0.04
giant planets (100
M
⊕
<
M
pl
sin
i
< 1000
M
⊕
) per star in periods of up to 1000 d, but due to a selection bias this number could be up to a factor of five lower in the whole 329-star sample. The upper limit for hot Jupiters (orbital period of less than 10 d) is 0.03 planets per star, while the occurrence rate of planets with intermediate masses (10
M
⊕
<
M
pl
sin
i
< 100
M
⊕
) is 0.18
−0.05
+0.07
planets per star. Less massive planets with 1
M
⊕
<
M
pl
sin
i
< 10
M
⊕
are very abundant, with an estimated rate of 1.32
−0.31
+0.33
planets per star for periods of up to 100 d. When considering only late M dwarfs with masses
M
⋆
< 0.34
M
⊙
, planets more massive than 10
M
⊕
become rare. Instead, low-mass planets with periods shorter than 10 d are significantly overabundant.
Conclusions.
For orbital periods shorter than 100 d, our results confirm the known stellar mass dependences from the
Kepler
survey: M dwarfs host fewer giant planets and at least two times more planets with
M
pl
sin
i
< 10
M
⊕
than G-type stars. In contrast to previous results, planets around our sample of very low-mass stars have a higher occurrence rate in short-period orbits of less than 10 d. Our results demonstrate the need to take into account host star masses in planet formation models.
We announce the discovery of two planets orbiting the M dwarfs GJ 251 (0.360 ± 0.015
M
⊙
) and HD 238090 (0.578 ± 0.021
M
⊙
) based on CARMENES radial velocity (RV) data. In addition, we ...independently confirm with CARMENES data the existence of Lalande 21185 b, a planet that has recently been discovered with the SOPHIE spectrograph. All three planets belong to the class of warm or temperate super-Earths and share similar properties. The orbital periods are 14.24 d, 13.67 d, and 12.95 d and the minimum masses are 4.0 ± 0.4
M
⊕
, 6.9 ± 0.9
M
⊕
, and 2.7 ± 0.3
M
⊕
for GJ 251 b, HD 238090 b, and Lalande 21185 b, respectively. Based on the orbital and stellar properties, we estimate equilibrium temperatures of 351.0 ± 1.4 K for GJ 251 b, 469.6 ± 2.6 K for HD 238090 b, and 370.1 ± 6.8 K for Lalande 21185 b. For the latter we resolve the daily aliases that were present in the SOPHIE data and that hindered an unambiguous determination of the orbital period. We find no significant signals in any of our spectral activity indicators at the planetary periods. The RV observations were accompanied by contemporaneous photometric observations. We derive stellar rotation periods of 122.1 ± 2.2 d and 96.7 ± 3.7 d for GJ 251 and HD 238090, respectively. The RV data of all three stars exhibit significant signals at the rotational period or its first harmonic. For GJ 251 and Lalande 21185, we also find long-period signals around 600 d, and 2900 d, respectively, which we tentatively attribute to long-term magnetic cycles. We apply a Bayesian approach to carefully model the Keplerian signals simultaneously with the stellar activity using Gaussian process regression models and extensively search for additional significant planetary signals hidden behind the stellar activity. Current planet formation theories suggest that the three systems represent a common architecture, consistent with formation following the core accretion paradigm.
We present the discovery of a transiting mini-Neptune around TOI-1201, a relatively bright and moderately young early M dwarf (
J
≈ 9.5 mag, ~600–800 Myr) in an equal-mass ~8 arcsecond-wide binary ...system, using data from the Transiting Exoplanet Survey Satellite, along with follow-up transit observations. With an orbital period of 2.49 d, TOI-1201 b is a warm mini-Neptune with a radius of
R
b
= 2.415 ± 0.090
R
⊕
. This signal is also present in the precise radial velocity measurements from CARMENES, confirming the existence of the planet and providing a planetary mass of
M
b
= 6.28 ± 0.88
M
⊕
and, thus, an estimated bulk density of 2.45
−0.42
+0.48
g cm
−3
. The spectroscopic observations additionally show evidence of a signal with a period of 19 d and a long periodic variation of undetermined origin. In combination with ground-based photometric monitoring from WASP-South and ASAS-SN, we attribute the 19 d signal to the stellar rotation period (
P
rot
= 19–23 d), although we cannot rule out that the variation seen in photometry belongs to the visually close binary companion. We calculate precise stellar parameters for both TOI-1201 and its companion. The transiting planet is anexcellent target for atmosphere characterization (the transmission spectroscopy metric is 97
−16
+21
) with the upcoming
James Webb
Space Telescope. It is also feasible to measure its spin-orbit alignment via the Rossiter-McLaughlin effect using current state-of-the-art spectrographs with submeter per second radial velocity precision.
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