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Register a new userHot planets around cool stars -- two short-period mini-Neptunes transiting the late K-dwarf TOI-1260
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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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We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from MEarth and LCOGT telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a radius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet, TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of 5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.
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.
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.
We report the discovery of TOI-1444b, a 1.4-$R_oplus$ super-Earth on a 0.47-day orbit around a Sun-like star discovered by {it TESS}. Precise radial velocities from Keck/HIRES confirmed the planet and constrained the mass to be $3.87 pm 0.71 M_oplus$. The RV dataset also indicates a possible non-transiting, 16-day planet ($11.8pm2.9M_oplus$). We report a tentative detection of phase curve variation and secondary eclipse of TOI-1444b in the {it TESS} bandpass. TOI-1444b joins the growing sample of 17 ultra-short-period planets with well-measured masses and sizes, most of which are compatible with an Earth-like composition. We take this opportunity to examine the expanding sample of ultra-short-period planets ($<2R_oplus$) and contrast them with the newly discovered sub-day ultra-hot Neptunes ($>3R_oplus$, $>2000F_oplus$ TOI-849 b, LTT9779 b and K2-100). We find that 1) USPs have predominately Earth-like compositions with inferred iron core mass fractions of 0.32$pm$0.04; and have masses below the threshold of runaway accretion ($sim 10M_oplus$), while ultra-hot Neptunes are above the threshold and have H/He or other volatile envelope. 2) USPs are almost always found in multi-planet system consistent with a secular interaction formation scenario; ultra-hot Neptunes ($P_{rm orb} lesssim$1 day) tend to be ``lonely similar to longer-period hot Neptunes($P_{rm orb}$1-10 days) and hot Jupiters. 3) USPs occur around solar-metallicity stars while hot Neptunes prefer higher metallicity hosts. 4) In all these respects, the ultra-hot Neptunes show more resemblance to hot Jupiters than the smaller USP planets, although ultra-hot Neptunes are rarer than both USP and hot Jupiters by 1-2 orders of magnitude.
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.
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