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تسجيل مستخدم جديدThe L 98-59 System: Three Transiting, Terrestrial-Sized Planets Orbiting a Nearby M-dwarf
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We report the Transiting Exoplanet Survey Satellite (TESS) discovery of three terrestrial-sized planets transiting L 98-59 (TOI-175, TIC 307210830) -- a bright M dwarf at a distance of 10.6 pc. Using the Gaia-measured distance and broad-band photometry we find that the host star is an M3 dwarf. Combined with the TESS transits from three sectors, the corresponding stellar parameters yield planet radii ranging from 0.8REarth to 1.6REarth. All three planets have short orbital periods, ranging from 2.25 to 7.45 days with the outer pair just wide of a 2:1 period resonance. Diagnostic tests produced by the TESS Data Validation Report and the vetting package DAVE rule out common false positive sources. These analyses, along with dedicated follow-up and the multiplicity of the system, lend confidence that the observed signals are caused by planets transiting L 98-59 and are not associated with other sources in the field. The L 98-59 system is interesting for a number of reasons: the host star is bright (V = 11.7 mag, K = 7.1 mag) and the planets are prime targets for further follow-up observations including precision radial-velocity mass measurements and future transit spectroscopy with the James Webb Space Telescope; the near resonant configuration makes the system a laboratory to study planetary system dynamical evolution; and three planets of relatively similar size in the same system present an opportunity to study terrestrial planets where other variables (age, metallicity, etc.) can be held constant. L 98-59 will be observed in 4 more TESS sectors, which will provide a wealth of information on the three currently known planets and have the potential to reveal additional planets in the system.
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L 98-59 is an M3V dwarf star that hosts three small (R < 1.6 Earth radii) planets. The host star is bright (K = 7.1) and nearby (10.6 pc), making the system a prime target for follow-up characterization with the Hubble Space Telescope (HST) and the u
pcoming James Webb Space Telescope (JWST). Herein, we use simulated transmission spectroscopy to evaluate the detectability of spectral features with HST and JWST assuming diverse atmospheric scenarios (e.g., atmospheres dominated by H2, H2O, CO2, or O2). We find that H2O and CH4 present in a low mean-molecular weight atmosphere could be detected with HST in 1 transit for the two outermost planets, while H2O in a clear steam atmosphere could be detected in 6 transits or fewer with HST for all three planets. We predict that observations using JWST/NIRISS would be capable of detecting a clear steam atmosphere in 1 transit for each planet, and H2O absorption in a hazy steam atmosphere in 2 transits or less. In a clear, desiccated atmosphere, O2 absorption may be detectable for all three planets with NIRISS. If the L 98-59 planets possess a clear, Venus-like atmosphere, NIRSpec could detect CO2 within 26 transits for each planet, but the presence of H2SO4 clouds would significantly suppress CO2 absorption. The L 98-59 system is an excellent laboratory for comparative planetary studies of transiting multiplanet systems, and observations of the system via HST and JWST would present a unique opportunity to test the accuracy of the models presented in this study.
LHS 1140 is a nearby mid-M dwarf known to host a temperate rocky super-Earth (LHS 1140 b) on a 24.737-day orbit. Based on photometric observations by MEarth and Spitzer as well as Doppler spectroscopy from HARPS, we report the discovery of an additio
nal transiting rocky companion (LHS 1140 c) with a mass of $1.81pm0.39~{rm M_{Earth}}$ and a radius of $1.282pm0.024~{rm R_{Earth}}$ on a tighter, 3.77795-day orbit. We also obtain more precise estimates of the mass and radius of LHS 1140 b to be $6.98pm0.89~{rm M_{Earth}}$ and $1.727pm0.032~{rm R_{Earth}}$. The mean densities of planets b and c are $7.5pm1.0~rm{g/cm^3}$ and $4.7pm1.1~rm{g/cm^3}$, respectively, both consistent with the Earths ratio of iron to magnesium silicate. The orbital eccentricities of LHS 1140 b and c are consistent with circular orbits and constrained to be below 0.06 and 0.31, respectively, with 90% confidence. Because the orbits of the two planets are co-planar and because we know from previous analyses of Kepler data that compact systems of small planets orbiting M dwarfs are commonplace, a search for more transiting planets in the LHS 1140 system could be fruitful. LHS 1140 c is one of the few known nearby terrestrial planets whose atmosphere could be studied with the upcoming James Webb Space Telescope.
L 98-59 (TIC 307210830, TOI-175) is a nearby M3 dwarf around which TESS revealed three terrestrial-sized transiting planets (0.80, 1.35, 1.57 Earth radii) in a compact configuration with orbital periods shorter than 7.5 days. Here we aim to measure t
he masses of the known transiting planets in this system using precise radial velocity (RV) measurements taken with the HARPS spectrograph. We consider both trained and untrained Gaussian process regression models of stellar activity to simultaneously model the RV data with the planetary signals. Our RV analysis is then supplemented with dynamical simulations to provide strong constraints on the planets orbital eccentricities by requiring long-term stability. We measure the planet masses of the two outermost planets to be $2.46pm 0.31$ and $2.26pm 0.50$ Earth masses which confirms their bulk terrestrial compositions. We are able to place an upper limit on the mass of the smallest, innermost planet of $<0.98$ Earth masses with 95% confidence. Our RV plus dynamical stability analysis places strong constraints on the orbital eccentricities and reveals that each planets orbit likely has $e<0.1$ to ensure a dynamically stable system. The L 98-59 compact system of three likely rocky planets offers a unique laboratory for studies of planet formation, dynamical stability, and comparative atmospheric planetology. Continued RV monitoring will help refine the characterization of the innermost planet and potentially reveal additional planets in the system at wider separations.
We report the detection of a transiting super-Earth-sized planet (R=1.39+-0.09 Rearth) in a 1.4-day orbit around L 168-9 (TOI-134),a bright M1V dwarf (V=11, K=7.1) located at 25.15+-0.02 pc. The host star was observed in the first sector of the Trans
iting Exoplanet Survey Satellite (TESS) mission and, for confirmation and planet mass measurement, was followed up with ground-based photometry, seeing-limited and high-resolution imaging, and precise radial velocity (PRV) observations using the HARPS and PFS spectrographs. Combining the TESS data and PRV observations, we find the mass of L168-9 b to be 4.60+-0.56 Mearth, and thus the bulk density to be 1.74+0.44-0.33 times larger than that of the Earth. The orbital eccentricity is smaller than 0.21 (95% confidence). This planet is a Level One Candidate for the TESS Missions scientific objective - to measure the masses of 50 small planets - and is one of the most observationally accessible terrestrial planets for future atmospheric characterization.
We report on precise Doppler measurements of L231-32 (TOI-270), a nearby M dwarf ($d=22$ pc, $M_star = 0.39$ M$_odot$, $R_star = 0.38$ R$_odot$), which hosts three transiting planets that were recently discovered using data from the Transiting Exopla
net Survey Satellite (TESS). The three planets are 1.2, 2.4, and 2.1 times the size of Earth and have orbital periods of 3.4, 5.7, and 11.4 days. We obtained 29 high-resolution optical spectra with the newly commissioned Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) and 58 spectra using the High Accuracy Radial velocity Planet Searcher (HARPS). From these observations, we find the masses of the planets to be $1.58 pm 0.26$, $6.15 pm 0.37$, and $4.78 pm 0.43$ M$_oplus$, respectively. The combination of radius and mass measurements suggests that the innermost planet has a rocky composition similar to that of Earth, while the outer two planets have lower densities. Thus, the inner planet and the outer planets are on opposite sides of the `radius valley -- a region in the radius-period diagram with relatively few members, which has been interpreted as a consequence of atmospheric photo-evaporation. We place these findings into the context of other small close-in planets orbiting M dwarf stars, and use support vector machines to determine the location and slope of the M dwarf ($T_mathrm{eff} < 4000$ K) radius valley as a function of orbital period. We compare the location of the M dwarf radius valley to the radius valley observed for FGK stars, and find that its location is a good match to photo-evaporation and core-powered mass loss models. Finally, we show that planets below the M dwarf radius valley have compositions consistent with stripped rocky cores, whereas most planets above have a lower density consistent with the presence of a H-He atmosphere.
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