Understanding the formation and evolution of giant planets (≥1 MJup) at wide orbital separation (≥5 AU) is one of the goals of direct imaging. Over the past 15 yr, many surveys have placed strong ...constraints on the occurrence rate of wide-orbit giants, mostly based on non-detections, but very few have tried to make a direct link with planet formation theories. In the present work, we combine the results of our previously published VLT/NaCo large program with the results of 12 past imaging surveys to constitute a statistical sample of 199 FGK stars within 100 pc, including three stars with sub-stellar companions. Using Monte Carlo simulations and assuming linear flat distributions for the mass and semi-major axis of planets, we estimate the sub-stellar companion frequency to be within 0.75–5.70% at the 68% confidence level (CL) within 20–300 AU and 0.5–75 MJup, which is compatible with previously published results. We also compare our results with the predictions of state-of-the-art population synthesis models based on the gravitational instability (GI) formation scenario with and without scattering. We estimate that in both the scattered and non-scattered populations, we would be able to detect more than 30% of companions in the 1–75 MJup range (95% CL). With the threesub-stellar detections in our sample, we estimate the fraction of stars that host a planetary system formed by GI to be within 1.0–8.6% (95% CL). We also conclude that even though GI is not common, it predicts a mass distribution of wide-orbit massive companions that is much closer to what is observed than what the core accretion scenario predicts. Finally, we associate the present paper with the release of the Direct Imaging Virtual Archive (DIVA), a public database that aims at gathering the results of past, present, and future direct imaging surveys.
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.
Several recent studies have found that planet migration in adiabatic disks differs significantly from migration in isothermal disks. Depending on the thermodynamic conditions, that is, the ...effectiveness of radiative cooling, and on the radial surface density profile, planets migrate inward or outward. Clearly, this will influence the semimajor-axis-to-mass distribution of planets predicted by population-synthesis simulations. Our goal is to study the global effects of radiative cooling, viscous torque desaturation, gap opening, and stellar irradiation on the tidal migration of a synthetic planet population. Outward migration in parts of a disk helps some planets to survive long enough to become massive. The convergence zones lead to potentially observable accumulations of low-mass planets at certain semimajor axes. Our results indicate that more studies of the mass at which the corotation torque saturates are needed since its value has a main impact on the properties of planet populations.
Direct imaging has only begun to inventory the population of gas giant planets on wide orbits around young stars in the solar neighborhood. Following this approach, we carried out a deep imaging ...survey in the near-infrared using VLT/NaCo to search for substellar companions. Here we report the discovery of a probable companion orbiting the young (10-17 Myr), dusty, early-type (A8) star HD 95086 at 56 AU in L' (3.8 mu m) images. This discovery is based on observations with more than a year time lapse. Our first epoch clearly revealed the source at Asymptotically = to10sigma, while our second epoch lacks good observing conditions, yielding a Asymptotically = to3sigma detection. Various tests were thus made to rule out possible artifacts. This recovery is consistent with the signal at the first epoch but requires cleaner confirmation. Nevertheless, our astrometric precision suggests that the companion is comoving with the star with a 3sigma confidence level. The planetary nature of the source is reinforced by a non-detection in the K s-band (2.18 mu m) images according to its possible extremely red K s-L' color. Conversely, background contamination is rejected with good confidence level. The luminosity yields a predicted mass of about 4-5 M sub(Jup) (at 10-17 Myr) using "hot-start" evolutionary models, making HD 95086 b the exoplanet with the lowest mass ever imaged around a star.
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 R valley = 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 R valley − P orb . 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.
Context. The assessment of the frequency of planetary systems reproducing the solar system’s architecture is still an open problem in exoplanetary science. Detailed study of multiplicity and ...architecture is generally hampered by limitations in quality, temporal extension and observing strategy, causing difficulties in detecting low-mass inner planets in the presence of outer giant planets. Aims. We present the results of high-cadence and high-precision HARPS observations on 20 solar-type stars known to host a single long-period giant planet in order to search for additional inner companions and estimate the occurence rate fp of scaled solar system analogues – in other words, systems featuring lower-mass inner planets in the presence of long-period giant planets. Methods. We carried out combined fits of our HARPS data with literature radial velocities using differential evolution MCMC to refine the literature orbital solutions and search for additional inner planets. We then derived the survey detection limits to provide preliminary estimates of fp. Results. We generally find better constrained orbital parameters for the known planets than those found in the literature; significant updates can be especially appreciated on half of the selected planetary systems. While no additional inner planet is detected, we find evidence for previously unreported long-period massive companions in systems HD 50499 and HD 73267. We finally estimate the frequency of inner low mass (10–30 M⊕) planets in the presence of outer giant planets as fp < 9.84% for P < 150 days. Conclusions. Our preliminary estimate of fp is significantly lower than the literature values for similarly defined mass and period ranges; the lack of inner candidate planets found in our sample can also be seen as evidence corroborating the inwards-migration formation model for super-Earths and mini-Neptunes. Our results also underline the need for high-cadence and high-precision followup observations as the key to precisely determine the occurence of solar system analogues.
Context.
Accreting planetary-mass objects have been detected at H
α
, but targeted searches have mainly resulted in non-detections. Accretion tracers in the planetary-mass regime could originate from ...the shock itself, making them particularly susceptible to extinction by the accreting material. High-resolution (
R
> 50 000) spectrographs operating at H
α
should soon enable one to study how the incoming material shapes the line profile.
Aims.
We calculate how much the gas and dust accreting onto a planet reduce the H
α
flux from the shock at the planetary surface and how they affect the line shape. We also study the absorption-modified relationship between the H
α
luminosity and accretion rate.
Methods.
We computed the high-resolution radiative transfer of the H
α
line using a one-dimensional velocity–density–temperature structure for the inflowing matter in three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explored the wide relevant ranges of the accretion rate and planet mass. We used detailed gas opacities and carefully estimated possible dust opacities.
Results.
At accretion rates of
Ṁ
≲ 3 × 10
−6
M
J
yr
−1
, gas extinction is negligible for spherical or polar inflow and at most
A
H
α
≲ 0.5 mag for magnetospheric accretion. Up to
Ṁ
≈ 3 × 10
−4
M
J
yr
−1
, the gas contributes
A
H
α
≲ 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at H
α
to be
κ
~ 0.01–10 cm
2
g
−1
, which is 10–10
4
times lower than in the interstellar medium. Extinction flattens the
L
H
α
–
Ṁ
relationship, which becomes non-monotonic with a maximum luminosity
L
H
α
~ 10
−4
L
⊙
towards
Ṁ
≈ 10
−4
M
J
yr
−1
for a planet mass ~10
M
J
. In magnetospheric accretion, the gas can introduce features in the line profile, while the velocity gradient smears them out in other geometries.
Conclusions.
For a wide part of parameter space, extinction by the accreting matter should be negligible, simplifying the interpretation of observations, especially for planets in gaps. At high
Ṁ
, strong absorption reduces the H
α
flux, and some measurements can be interpreted as two
Ṁ
values. Highly resolved line profiles (
R
~ 10
5
) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.
Aims. We explore the relations between physical and orbital properties of planets and properties of their host stars to identify the main observable signatures of the formation and evolution ...processes of planetary systems. Methods. We used a large sample of FGK dwarf planet-hosting stars with stellar parameters derived in a homogeneous way from the SWEET-Cat database to study the relation between stellar metallicity and position of planets in the period-mass diagram. We then used all the radial-velocity-detected planets orbiting FGK stars to explore the role of planet-disk and planet-planet interaction on the evolution of orbital properties of planets with masses above 1 MJup. Results. Using a large sample of FGK dwarf hosts we show that planets orbiting metal-poor stars have longer periods than those in metal-rich systems. This trend is valid for masses at least from ≈10 M⊕ to ≈4 MJup. Earth-like planets orbiting metal-rich stars always show shorter periods (fewer than 20 days) than those orbiting metal-poor stars. However, in the short-period regime there are a similar number of planets orbiting metal-poor stars. We also found statistically significant evidence that very high mass giants (with a mass higher than 4 MJup) have on average more eccentric orbits than giant planets with lower mass. Finally, we show that the eccentricity of planets with masses higher than 4 MJup tends to be lower for planets with shorter periods. Conclusions. Our results suggest that the planets in the P − MP diagram are evolving differently because of a mechanism that operates over a wide range of planetary masses. This mechanism is stronger or weaker, depending on the metallicity of the respective system. One possibility is that planets in metal-poor disks form farther out from their central star and/or they form later and do not have time to migrate as far as the planets in metal-rich systems. The trends and dependencies obtained for very high mass planetary systems suggest that planet-disk interaction is a very important and orbit-shaping mechanism for planets in the high-mass domain.
Over the past decade, direct imaging has confirmed the existence of substellar companions on wide orbits from their parent stars. To understand the formation and evolution mechanisms of these ...companions, their individual as well as the full population properties must be characterized. We aim at detecting giant planet and/or brown dwarf companions around young, nearby, and dusty stars. Our goal is also to provide statistics on the population of giant planets at wide-orbits and discuss planet formation models. The observations were conducted with the ground-based adaptive optics system VLT/NaCo at L'-band (3.8 mu m). We resolve and characterize new visual binaries and do not detect any new substellar companion. The survey's median detection performance reaches contrasts of 10 mag at 0.5" and 11.5 mag at 1.0" . We will have to wait for the next generation of planet finders to start exploring that population, and even for the extremely large telescopes for a more complete overlap with other planet-hunting techniques.
Combining high-contrast imaging with medium-resolution spectroscopy has been shown to significantly boost the direct detection of exoplanets. HARMONI, one of the first-light instruments to be mounted ...on ESO’s future extremely large telescope (ELT), will be equipped with a single-conjugated adaptive optics system to reach the diffraction limit of the ELT in the
H
and
K
bands, a high-contrast module dedicated to exoplanet imaging, and a medium-resolution (up to
R
= 17 000) optical and near-infrared integral field spectrograph. When combined, these systems will provide unprecedented contrast limits at separations between 50 and 400 mas. This paper is aimed at estimating the capabilities of the HARMONI high-contrast module for the direct detection of young giant exoplanets. We use an end-to-end model of the instrument to simulate high-contrast observations performed with HARMONI, based on realistic observing scenarios and conditions. We then analyze these data with the so-called “molecule mapping” technique combined with a matched-filter approach in order to disentangle companions from the host star and tellurics and to increase the signal-to-noise ratio (S/N) of the planetary signal. We detected planets above 5
σ
at contrasts up to 16 mag and separations down to 75 mas in several spectral configurations of the instrument. We show that molecule mapping allows for the detection of companions up to 2.5 mag fainter compared to state-of-the-art high-contrast imaging techniques based on angular differential imaging. We also demonstrate that the performance is not strongly affected by the spectral type of the host star and we show that we are able to reach close sensitivities for the best three quartiles of observing conditions at Armazones, which means that HARMONI could be used in near-critical observations during 60 to 70% of telescope time at the ELT. Finally, we simulated planets from population synthesis models to further explore the parameter space that HARMONI and its high-contrast module will open up and compare this to the current high-contrast instrumentation.
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