Abstract
The relative rarity of giant planets around low-mass stars compared with solar-type stars is a key prediction from the core-accretion planet formation theory. In this paper we report on the ...discovery of four gas giant planets that transit low-mass late K and early M dwarfs. The planets HATS-74Ab (TOI 737b), HATS-75b (TOI 552b), HATS-76b (TOI 555b), and HATS-77b (TOI 730b) were all discovered from the HATSouth photometric survey and follow-up using TESS and other photometric facilities. We use the new ESPRESSO facility at the VLT to confirm systems and measure their masses. We find that these planets have masses of 1.46 ± 0.14
M
J, 0.491 ± 0.039
M
J, 2.629 ± 0.089
M
J, and
1.374
−
0.074
+
0.100
M
J, respectively, and radii of 1.032 ± 0.021
R
J, 0.884 ± 0.013
R
J, 1.079 ± 0.031
R
J, and 1.165 ± 0.021
R
J, respectively. The planets all orbit close to their host stars with orbital periods ranging from 1.7319 days to 3.0876 days. With further work, we aim to test core-accretion theory by using these and further discoveries to quantify the occurrence rate of giant planets around low-mass host stars.
We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (
K
= 8.8), quiet, and nearby (37 pc) M3V dwarf TOI-1266. We validate the planetary nature of ...TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Mártir (México). We also include additional ground-based follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner, larger planet has a radius of
R
= 2.37
−0.12
+0.16
R
⊕
and an orbital period of 10.9 days. The outer, smaller planet has a radius of
R
= 1.56
−0.13
+0.15
R
⊕
on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of
M
p
= 13.5
−9.0
+11.0
M
⊕
(<36.8
M
⊕
at 2-
σ
) for TOI-1266 b and 2.2
−1.5
+2.0
M
⊕
(<5.7
M
⊕
at 2-
σ
) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09
−0.05
+0.06
(<0.21 at 2-
σ
) for TOI-1266 b and 0.04 ± 0.03 (< 0.10 at 2-
σ
) for TOI-1266 c. The equilibrium temperatures of both planets are of 413 ± 20 and 344 ± 16 K, respectively, assuming a null Bond albedo and uniform heat redistribution from the day-side to the night-side hemisphere. The host brightness and negligible activity combined with the planetary system architecture and favourable planet-to-star radii ratios makes TOI-1266 an exquisite system for a detailed characterisation.
Context.
Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these ...planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST.
Aims.
Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star’s low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models.
Methods.
We first infer properties of the host star by analysing the star’s near-infrared spectrum, spectral energy distribution, and
Gaia
parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data.
Results.
We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties
T
eff
= 3100 ± 75 K,
M
*
= 0.162 ± 0.008
M
⊙
,
R
*
= 0.202 ± 0.011
R
⊙
, and Fe∕H = −0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with
R
p
= 2.94 ± 0.17
R
⊕
and
P
= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3
σ
, prompting questions about the dynamical history of the system.
Conclusions.
This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet’s mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.
We present the discovery and validation of two TESS exoplanets orbiting nearby M dwarfs: TOI-2084 b, and TOI-4184b. We characterized the host stars by combining spectra from
Shane
/Kast and
Magellan
.../FIRE, spectral energy distribution analysis, and stellar evolutionary models. In addition, we used Gemini-South/Zorro & -North/Alopeke high-resolution imaging, archival science images, and statistical validation packages to support the planetary interpretation. We performed a global analysis of multi-colour photometric data from TESS and ground-based facilities in order to derive the stellar and planetary physical parameters for each system. We find that TOI-2084 band TOI-4184 bare sub-Neptune-sized planets with radii of
R
p
= 2.47 ± 0.13
R
⊕
and
R
p
= 2.43 ± 0.21
R
⊕
, respectively. TOI-2084 b completes an orbit around its host star every 6.08 days, has an equilibrium temperature of
T
eq
= 527 ± 8 K and an irradiation of
S
p
= 12.8 ± 0.8
S
⊕
. Its host star is a dwarf of spectral M2.0 ± 0.5 at a distance of 114 pc with an effective temperature of
T
eff
= 3550 ± 50 K, and has a wide, co-moving M8 companion at a projected separation of 1400 au. TOI-4184 b orbits around an M5.0 ± 0.5 type dwarf star (
K
mag
= 11.87) each 4.9 days, and has an equilibrium temperature of
T
eq
= 412 ± 8 K and an irradiation of
S
p
= 4.8 ± 0.4
S
⊕
. TOI-4184 is a metal poor star (Fe/H = −0.27 ± 0.09 dex) at a distance of 69 pc with an effective temperature of
T
eff
= 3225 ± 75 K. Both planets are located at the edge of the sub-Jovian desert in the radius-period plane. The combination of the small size and the large infrared brightness of their host stars make these new planets promising targets for future atmospheric exploration with JWST.
ABSTRACT
We present the characterization of an inner mini-Neptune in a 9.2292005 ± 0.0000063 d orbit and an outer mono-transiting sub-Saturn planet in a 95.50$^{+0.36}_{-0.25}$ d orbit around the ...moderately active, bright (mv = 8.9 mag) K5V star TOI-2134. Based on our analysis of five sectors of TESS data, we determine the radii of TOI-2134b and c to be 2.69 ± 0.16 R⊕ for the inner planet and 7.27 ± 0.42 R⊕ for the outer one. We acquired 111 radial-velocity (RV) spectra with HARPS-N and 108 RV spectra with SOPHIE. After careful periodogram analysis, we derive masses for both planets via Gaussian Process regression: 9.13$^{+0.78}_{-0.76}$ M⊕ for TOI-2134b and 41.89$^{+7.69}_{-7.83}$ M⊕ for TOI-2134c. We analysed the photometric and RV data first separately, then jointly. The inner planet is a mini-Neptune with density consistent with either a water-world or a rocky core planet with a low-mass H/He envelope. The outer planet has a bulk density similar to Saturn’s. The outer planet is derived to have a significant eccentricity of 0.67$^{+0.05}_{-0.06}$ from a combination of photometry and RVs. We compute the irradiation of TOI-2134c as 1.45 ± 0.10 times the bolometric flux received by Earth, positioning it for part of its orbit in the habitable zone of its system. We recommend further RV observations to fully constrain the orbit of TOI-2134c. With an expected Rossiter–McLaughlin (RM) effect amplitude of 7.2 ± 1.3 $\rm m\, s^{-1}$, we recommend TOI-2134c for follow-up RM analysis to study the spin–orbit architecture of the system. We calculate the Transmission Spectroscopy Metric, and both planets are suitable for bright-mode Near Infrared Camera (NIRCam) atmospheric characterization.
Context.
Thanks to the relative ease of finding and characterizing small planets around M-dwarf stars, these objects have become cornerstones in the field of exoplanet studies. The current paucity of ...planets in long-period orbits around M dwarfs makes such objects particularly compelling as they provide clues about the formation and evolution of these systems.
Aims.
In this study we present the discovery of TOI-2257 b (TIC 198485881), a long-period (35 d) sub-Neptune orbiting an M3 star at 57.8 pc. Its transit depth is about 0.4%, large enough to be detected with medium-size, ground-based telescopes. The long transit duration suggests the planet is in a highly eccentric orbit (
e
~ 0.5), which would make it the most eccentric planet known to be transiting an M-dwarf star.
Methods.
We combined TESS and ground-based data obtained with the 1.0-meter SAINT-EX, 0.60-meter TRAPPIST-North, and 1.2-meter FLWO telescopes to find a planetary size of 2.2
R
⊕
and an orbital period of 35.19 days. In addition, we make use of archival data, high-resolution imaging, and vetting packages to support our planetary interpretation.
Results.
With its long period and high eccentricity, TOI-2257 b falls into a novel slice of parameter space. Despite the planet’s low equilibrium temperature (~256 K), its host star’s small size (
R
*
= 0.311 ± 0.015) and relative infrared brightness (
K
mag
= 10.7) make it a suitable candidate for atmospheric exploration via transmission spectroscopy.
We report the discovery by the TESS mission of a super-Earth on a 4.8-days orbit around an inactive M4.5 dwarf (TOI-1680), validated by ground-based facilities. The host star is located 37.14 pc ...away, with a radius of 0.2100 ± 0.0064
R
⊙
, mass of 0.1800 ± 0.0044
M
⊙
, and an effective temperature of 3211 ±100 K. We validated and characterized the planet using TESS data, ground-based multi-wavelength photometry from TRAPPIST, SPECULOOS, and LCO, as well as high-resolution AO observations from Keck/NIRC2 and
Shane.
Our analyses have determined the following parameters for the planet: a radius of 1.466
−0.049
+0.063
R
⊕
and an equilibrium temperature of 404 ± 14 K, assuming no albedo and perfect heat redistribution. Assuming a mass based on mass-radius relations, this planet is a promising target for atmospheric characterization with the
James Webb
Space Telescope (JWST).
Context.
Several planetary formation models have been proposed to explain the observed abundance and variety of compositions of super-Earths and mini-Neptunes. In this context, multitransiting ...systems orbiting low-mass stars whose planets are close to the radius valley are benchmark systems, which help to elucidate which formation model dominates.
Aims.
We report the discovery, validation, and initial characterization of one such system, TOI-2096 (TIC 142748283), a two-planet system composed of a super-Earth and a mini-Neptune hosted by a mid-type M dwarf located 48 pc away.
Methods.
We characterized the host star by combining optical spectra, analyzing its broadband spectral energy distribution, and using evolutionary models for low-mass stars. Then, we derived the planetary properties by modeling the photometric data from TESS and ground-based facilities. In addition, we used archival data, high-resolution imaging, and statistical validation to support our planetary interpretation.
Results.
We found that the stellar properties of TOI-2096 correspond to a dwarf star of spectral type M4±0.5. It harbors a super-Earth (
R
= 1.24 ± 0.07
R
⊕
) and a mini-Neptune (
R
= 1.90 ± 0.09
R
⊕
) in likely slightly eccentric orbits with orbital periods of 3.12 d and 6.39 d, respectively. These orbital periods are close to the first-order 2:1 mean-motion resonance (MMR), a configuration that may lead to measurable transit timing variations (TTVs). We computed the expected TTVs amplitude for each planet and found that they might be measurable with high-precision photometry delivering mid-transit times with accuracies of ≲2 min. Moreover, we conclude that measuring the planetary masses via radial velocities (RVs) could also be possible. Lastly, we found that these planets are among the best in their class to conduct atmospheric studies using the NIRSpec/Prism onboard the
James Webb
Space Telescope (JWST).
Conclusions.
The properties of this system make it a suitable candidate for further studies, particularly for mass determination using RVs and/or TTVs, decreasing the scarcity of systems that can be used to test planetary formation models around low-mass stars.
Abstract Sub-Neptunes with radii of 2–3 R ⊕ are intermediate in size between rocky planets and Neptune-sized planets. The orbital properties and bulk compositions of transiting sub-Neptunes provide ...clues to the formation and evolution of close-in small planets. In this paper, we present the discovery and follow-up of four sub-Neptunes orbiting M dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406), three of which were newly validated by ground-based follow-up observations and statistical analyses. TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b have radii of R p = 2.740 − 0.079 + 0.082 R ⊕ , 2.769 − 0.068 + 0.073 R ⊕ , 2.120 ± 0.067 R ⊕ , and 2.830 − 0.066 + 0.068 R ⊕ and orbital periods of P = 8.02, 8.11, 5.80, and 3.08 days, respectively. Doppler monitoring with the Subaru/InfraRed Doppler instrument led to 2 σ upper limits on the masses of <19.1 M ⊕ , <19.5 M ⊕ , <6.8 M ⊕ , and <15.6 M ⊕ for TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b, respectively. The mass–radius relationship of these four sub-Neptunes testifies to the existence of volatile material in their interiors. These four sub-Neptunes, which are located above the so-called “radius valley,” are likely to retain a significant atmosphere and/or an icy mantle on the core, such as a water world. We find that at least three of the four sub-Neptunes (TOI-782 b, TOI-2120 b, and TOI-2406 b), orbiting M dwarfs older than 1 Gyr, are likely to have eccentricities of e ∼ 0.2–0.3. The fact that tidal circularization of their orbits is not achieved over 1 Gyr suggests inefficient tidal dissipation in their interiors.
Context.
Exoplanets with orbital periods of less than one day are known as ultra-short period (USP) planets. They are relatively rare products of planetary formation and evolution processes, but ...especially favourable for characterisation with current planet detection methods. At the time of writing, 125 USP planets have already been confirmed.
Aims.
Our aim is to validate the planetary nature of two new transiting planet candidates around M dwarfs announced by the NASA Transiting Exoplanet Survey Satellite (TESS), registered as TESS Objects of Interest (TOIs) TOI-1442.01 and TOI-2445.01.
Methods.
We used TESS data, ground-based photometric light curves, and Subaru/IRD spectrograph radial velocity (RV) measurements to validate both planetary candidates and to establish their physical properties.
Results.
TOI-1442 bis a validated exoplanet with an orbital period of
P
= 0.4090682 ± 0.0000004 day, a radius of
R
p
= 1.15 ± 0.06
R
⊕
, and equilibrium temperature of
T
p,eq
= 1357
−42
+49
K. TOI-2445 b is also validated with an orbital period of
P
= 0.3711286 ± 0.0000004 day, a radius of
R
p
= 1.33 ± 0.09
R
⊕
, and equilibrium temperature of
T
p,eq
= 1330
−56
+61
K. Their physical properties align with current empirical trends and formation theories of USP planets. Based on the RV measurements, we set 3
σ
upper mass limits of 8
M
⊕
and 20
M
⊕
, thus confirming the non-stellar, sub-Jovian nature of both transiting objects. More RV measurements will be needed to constrain the planetary masses and mean densities, and the predicted presence of outer planetary companions. These targets extend the small sample of USP planets orbiting around M dwarfs up to 21 members. They are also among the 20 most suitable terrestrial planets for atmospheric characterisation via secondary eclipse with the
James Webb
Space Telescope, according to a widespread emission spectroscopy metric.
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