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Register a new userAn unusually low density ultra-short period super-Earth and three mini-Neptunes around the old star TOI-561
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Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterisation of the planetary system orbiting the late G dwarf TOI-561. After the identification of three transiting candidates by TESS, we discovered two additional external planets from RV analysis. RVs cannot confirm the outer TESS transiting candidate, which would also make the system dynamically unstable. We demonstrate that the two transits initially associated with this candidate are instead due to single transits of the two planets discovered using RVs. The four planets orbiting TOI-561 include an ultra-short period (USP) super-Earth (TOI-561 b) with period $P_{rm b} = 0.45$ d, mass $M_{rm b} =1.59 pm 0.36$ M$_oplus$ and radius $R_{rm b}=1.42 pm 0.07$ R$_oplus$, and three mini-Neptunes: TOI-561 c, with $P_{rm c} = 10.78$ d, $M_{rm c} = 5.40 pm 0.98$ M$_oplus$, $R_{rm c}= 2.88 pm 0.09$ R$_oplus$; TOI-561 d, with $P_{rm d} = 25.6$ d, $M_{rm d} = 11.9 pm 1.3$ M$_oplus$, $R_{rm d} = 2.53 pm 0.13$ R$_oplus$; and TOI-561 e, with $P_{rm e} = 77.2$ d, $M_{rm e} = 16.0 pm 2.3$ M$_oplus$, $R_{rm e} = 2.67 pm 0.11$ R$_oplus$. Having a density of $3.0 pm 0.8$ g cm$^{-3}$, TOI-561 b is the lowest density USP planet known to date. Our N-body simulations confirm the stability of the system and predict a strong, anti-correlated, long-term transit time variation signal between planets d and e. The unusual density of the inner super-Earth and the dynamical interactions between the outer planets make TOI-561 an interesting follow-up target.
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Ultra-short period (USP) planets are a class of exoplanets with periods shorter than one day. The origin of this sub-population of planets is still unclear, with different formation scenarios highly dependent on the composition of the USP planets. A better understanding of this class of exoplanets will, therefore, require an increase in the sample of such planets that have accurate and precise masses and radii, which also includes estimates of the level of irradiation and information about possible companions. Here we report a detailed characterization of a USP planet around the solar-type star HD 80653 $equiv$ EP 251279430 using the K2 light curve and 108 precise radial velocities obtained with the HARPS-N spectrograph, installed on the Telescopio Nazionale Galileo. From the K2 C16 data, we found one super-Earth planet ($R_{b}=1.613pm0.071 R_{oplus}$) transiting the star on a short-period orbit ($P_{rm b}=0.719573pm0.000021$ d). From our radial velocity measurements, we constrained the mass of HD 80653 b to $M_{b}=5.60pm0.43 M_{oplus}$. We also detected a clear long-term trend in the radial velocity data. We derived the fundamental stellar parameters and determined a radius of $R_{star}=1.22pm0.01 R_{odot}$ and mass of $M_{star}=1.18pm0.04 M_{odot}$, suggesting that HD 80653, has an age of $2.7pm1.2$ Gyr. The bulk density ($rho_{b} = 7.4 pm 1.1$ g cm$^{-3}$) of the planet is consistent with an Earth-like composition of rock and iron with no thick atmosphere. Our analysis of the K2 photometry also suggests hints of a shallow secondary eclipse with a depth of 8.1$pm$3.7 ppm. Flux variations along the orbital phase are consistent with zero. The most important contribution might come from the day-side thermal emission from the surface of the planet at $Tsim3480$ K.
Dynamical histories of planetary systems, as well as atmospheric evolution of highly irradiated planets, can be studied by characterizing the ultra-short-period planet population, which the TESS mission is particularly well suited to discover. Here, we report on the follow-up of a transit signal detected in the TESS sector 19 photometric time series of the M3.0 V star TOI-1685 (2MASS J04342248+4302148). We confirm the planetary nature of the transit signal, which has a period of P_b=0.6691403+0.0000023-0.0000021 d, using precise radial velocity measurements taken with the CARMENES spectrograph. From the joint photometry and radial velocity analysis, we estimate the following parameters for TOI-1685 b: a mass of M_b=3.78+/-0.63 M_Earth, a radius of R_b=1.70+/-0.07 R_Earth, which together result in a bulk density of rho_b=4.21+0.95-0.82 g/cm3, and an equilibrium temperature of Teq_b=1069+/-16 K. TOI-1685 b is the least dense ultra-short period planet around an M dwarf known to date. TOI-1685 b is also one of the hottest transiting Earth-size planets with accurate dynamical mass measurements, which makes it a particularly attractive target for thermal emission spectroscopy. Additionally, we report a further non-transiting planet candidate in the system, TOI-1685[c], with an orbital period of P_[c]=9.02+0.10-0.12 d.
The Transiting Exoplanet Survey Satellite, TESS, is currently carrying out an all-sky search for small planets transiting bright stars. In the first year of the TESS survey, steady progress was made in achieving the missions primary science goal of establishing bulk densities for 50 planets smaller than Neptune. During that year, TESSs observations were focused on the southern ecliptic hemisphere, resulting in the discovery of three mini-Neptunes orbiting the star TOI-125, a V=11.0 K0 dwarf. We present intensive HARPS radial velocity observations, yielding precise mass measurements for TOI-125b, TOI-125c and TOI-125d. TOI-125b has an orbital period of 4.65 days, a radius of $2.726 pm 0.075 ~mathrm{R_{rm E}}$, a mass of $ 9.50 pm 0.88 ~mathrm{M_{rm E}}$ and is near the 2:1 mean motion resonance with TOI-125c at 9.15 days. TOI-125c has a similar radius of $2.759 pm 0.10 ~mathrm{R_{rm E}}$ and a mass of $ 6.63 pm 0.99 ~mathrm{M_{rm E}}$, being the puffiest of the three planets. TOI-125d, has an orbital period of 19.98 days and a radius of $2.93 pm 0.17~mathrm{R_{rm E}}$ and mass $13.6 pm 1.2 ~mathrm{M_{rm E}}$. For TOI-125b and TOI-125d we find unusual high eccentricities of $0.19pm 0.04$ and $0.17^{+0.08}_{-0.06}$, respectively. Our analysis also provides upper mass limits for the two low-SNR planet candidates in the system; for TOI-125.04 ($R_P=1.36 ~mathrm{R_{rm E}}$, $P=$0.53 days) we find a $2sigma$ upper mass limit of $1.6~mathrm{M_{rm E}}$, whereas TOI-125.05 ( $R_P=4.2^{+2.4}_{-1.4} ~mathrm{R_{rm E}}$, $P=$ 13.28 days) is unlikely a viable planet candidate with upper mass limit $2.7~mathrm{M_{rm E}}$. We discuss the internal structure of the three confirmed planets, as well as dynamical stability and system architecture for this intriguing exoplanet system.
The star EPIC 210894022 has been identified from a light curve acquired through the K2 space mission as possibly orbited by a transiting planet. Our aim is to confirm the planetary nature of the object and derive its fundamental parameters. We combine the K2 photometry with reconnaissance spectroscopy and radial velocity (RV) measurements obtained using three separate telescope and spectrograph combinations. The spectroscopic synthesis package SME has been used to derive the stellar photospheric parameters that were used as input to various stellar evolutionary tracks in order to derive the parameters of the system. The planetary transit was also validated to occur on the assumed host star through adaptive imaging and statistical analysis. The star is found to be located in the background of the Hyades cluster at a distance at least 4 times further away from Earth than the cluster itself. The spectrum and the space velocities of EPIC 210894022 strongly suggest it to be a member of the thick disk population. We find that the star is a metal poor ([Fe/H]=-0.53+/-0.05 dex) and alpha-rich somewhat evolved solar-like object of spectral type G3 with Teff=5730+/-50 K, logg=4.15+/-0.1 (cgs), radius of 1.3+/-0.1 R_Sun, and mass of 0.88+/-0.02 M_Sun. The RV detection together with the imaging confirms with a high level of significance that the transit signature is caused by a super-Earth orbiting the star EPIC 210894022. We measure a mass of 8.6+/-3.9 M_Earth and a radius of 1.9+/-0.2 R_Earth. A second more massive object with a period longer than about 120 days is indicated by a long term linear acceleration. With an age of > 10 Gyrs this system is one of the oldest where planets is hitherto detected. Further studies of this planetary system is important since it contains information about the planetary formation process during a very early epoch of the history of our Galaxy.
We present the discovery and characterization of two sub-Neptunes in close orbits, as well as a tentative outer planet of a similar size, orbiting TOI-1260 - a low metallicity K6V dwarf star. Photometry from TESS yields radii of $R_{rm b} = 2.33 pm 0.10$ $R_{oplus}$ and $R_{rm c} = 2.82 pm 0.15$ $R_{oplus}$, and periods of 3.13 and 7.49 days for TOI-1260b and TOI-1260c, respectively. We combined the TESS data with a series of ground-based follow-up observations to characterize the planetary system. From HARPS-N high-precision radial velocities we obtain $M_{rm b} = 8.61_{ - 1.46 } ^ { + 1.36 }$ $M_{oplus}$ and $M_{rm c} = 11.84_{ - 3.23 } ^ { + 3.38 }$ $M_{oplus}$. The star is moderately active with a complex activity pattern, which necessitated the use of Gaussian process regression for both the light curve detrending and the radial velocity modelling, in the latter case guided by suitable activity indicators. We successfully disentangle the stellar-induced signal from the planetary signals, underlining the importance and usefulness of the Gaussian Process approach. We test the systems stability against atmospheric photoevaporation and find that the TOI-1260 planets are classic examples of the structure and composition ambiguity typical for the $2-3$ $R_{oplus}$ range.
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