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Register a new userA super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270
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Added by
Maximilian G\\\"unther
Publication date
2019
fields
Physics
and research's language is
English
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We report the Transiting Exoplanet Survey Satellite discovery of three small planets transiting one of the nearest and brightest M dwarf hosts to date, TOI-270 (TIC 259377017; K-mag 8.3; 22.5 parsec). The M3V-type star is transited by the super-Earth-sized TOI-270 b (1.247+0.089-0.083 R_earth) and the sub-Neptune-sized exoplanets TOI-270 c (2.42+-0.13 R_earth) and TOI-270 d (2.13+-0.12 R_earth). The planets orbit close to a mean-motion resonant chain, with periods (3.36, 5.66, and 11.38 days) near ratios of small integers (5:3 and 2:1). TOI-270 is a prime target for future studies since: 1) its near-resonance allows detecting transit timing variations for precise mass measurements and dynamical studies; 2) its brightness enables independent radial velocity mass measurements; 3) the outer planets are ideal for atmospheric characterisation via transmission spectroscopy; and 4) the quiet star enables future searches for habitable zone planets. Altogether, very few systems with small, temperate exoplanets are as suitable for such complementary and detailed characterisation as TOI-270.
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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 Martir (Mexico). 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$_{oplus}$ and an orbital period of 10.9 days. The outer, smaller planet has a radius of $R=1.56_{-0.13}^{+0.15}$ R$_{oplus}$ 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 $M_mathrm{p}$ = $13.5_{-9.0}^{+11.0}$ $mathrm{M_{oplus}}$ ($<36.8$ $mathrm{M_{oplus}}$ at 2-$sigma$) for TOI-1266 b and $2.2_{-1.5}^{+2.0}$ $mathrm{M_{oplus}}$ ($<5.7$ $mathrm{M_{oplus}}$ at 2-$sigma$) for TOI-1266 c. We find small but non-zero orbital eccentricities of $0.09_{-0.05}^{+0.06}$ ($<0.21$ at 2-$sigma$) for TOI-1266 b and $0.04pm0.03$ ($<0.10$ at 2-$sigma$) for TOI-1266 c. The equilibrium temperatures of both planets are of $413pm20$ K and $344pm16$ 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.
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Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R_Sun, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1 R_Earth planet on a 0.95 day orbit and a 2.3 R_Earth planet on a 5 day orbit. Because the host star is small the loss of light during these planets transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.
We report the discovery of a new planetary system with three transiting planets, one super-Earth and two sub-Neptunes, that orbit EPIC,249893012, a G8,IV-V evolved star ($M_star$,=,1.05,$pm$,0.05,$M_odot$, $R_star$,=,1.71,$pm$,0.04,$R_odot$, $T_mathrm{eff}$,=5430,$pm$,85,K). The star is just leaving the main sequence. We combined ktwo photometry with IRCS adaptive-optics imaging and HARPS, HARPS-N, and CARMENES high-precision radial velocity measurements to confirm the planetary system, determine the stellar parameters, and measure radii, masses, and densities of the three planets. With an orbital period of $3.5949^{+0.0007}_{-0.0007}$ days, a mass of $8.75^{+1.09}_{-1.08} M_{oplus}$ , and a radius of $1.95^{+0.09}_{-0.08} R_{oplus}$, the inner planet b is compatible with nickel-iron core and a silicate mantle ($rho_b= 6.39^{+1.19}_{-1.04}$ g cm$^{-3}$). Planets c and d with orbital periods of $15.624^{+0.001}_{-0.001}$ and $35.747^{+0.005}_{-0.005}$ days, respectively, have masses and radii of $14.67^{+1,84}_{-1.89} M_{oplus}$ and $3.67^{+0.17}_{-0.14} R_{oplus}$ and $10.18^{+2.46}_{-2.42} M_{oplus}$ and $3.94^{+0.13}_{-0.12} R_{oplus}$, respectively, yielding a mean density of $1.62^{+0.30}_{-0.29}$ and $0.91^{+0.25}_{-0.23}$ g cm$^{-3}$, respectively. The radius of planet b lies in the transition region between rocky and gaseous planets, but its density is consistent with a rocky composition. Its semimajor axis and the corresponding photoevaporation levels to which the planet has been exposed might explain its measured density today. In contrast, the densities and semimajor axes of planets c and d suggest a very thick atmosphere. The singularity of this system, which orbits a slightly evolved star that is just leaving the main sequence, makes it a good candidate for a deeper study from a dynamical point of view.
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 Exoplanet 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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