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Bergsten, G. J., Pascucci, I., Hardegree-Ullman, K. K., Fernandes, R. B., Christiansen, J. L., & Mulders, G. D. (2023). No Evidence for More Earth-sized Planets in the Habitable Zone of Kepler's M versus FGK Stars. Astron. J., 166(6), 234.
Abstract: Reliable detections of Earth-sized planets in the habitable zone remain elusive in the Kepler sample, even for M dwarfs. The Kepler sample was once thought to contain a considerable number of M-dwarf stars ( T-eff < 4000 K), which hosted enough Earth-sized ([0.5, 1.5] R-circle plus) planets to estimate their occurrence rate (eta(circle plus)) in the habitable zone. However, updated stellar properties from Gaia have shifted many Kepler stars to earlier spectral type classifications, with most stars (and their planets) now measured to be larger and hotter than previously believed. Today, only one partially reliable Earth-sized candidate remains in the optimistic habitable zone, and zero in the conservative zone. Here we performed a new investigation of Kepler's Earth-sized planets orbiting M-dwarf stars, using occurrence rate models with considerations of updated parameters and candidate reliability. Extrapolating our models to low instellations, we found an occurrence rate of eta(circle plus) = 8.58( – 8.22 )(+ 17.94) % for the conservative habitable zone (and 14.22 (- 12.71) (+ 24.96 )% for the optimistic one), consistent with previous works when considering the large uncertainties. Comparing these estimates to those from similarly comprehensive studies of Sun-like stars, we found that the current Kepler sample does not offer evidence to support an increase in eta(circle plus) from FGK to M stars. While the Kepler sample is too sparse to resolve an occurrence trend between early and mid-to-late M dwarfs for Earth-sized planets, studies including larger planets and/or data from the K2 and TESS missions are well suited to this task.
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Bergsten, G. J., Pascucci, I., Mulders, G. D., Fernandes, R. B., & Koskinen, T. T. (2022). The Demographics of Kepler's Earths and Super-Earths into the Habitable Zone. Astron. J., 164(5), 190.
Abstract: Understanding the occurrence of Earth-sized planets in the habitable zone of Sun-like stars is essential to the search for Earth analogs. Yet a lack of reliable Kepler detections for such planets has forced many estimates to be derived from the close-in (2 < P-orb < 100 days) population, whose radii may have evolved differently under the effect of atmospheric mass-loss mechanisms. In this work, we compute the intrinsic occurrence rates of close-in super-Earths (similar to 1-2 R-circle plus and sub-Neptunes (similar to 2-3.5 R-circle plus) for FGK stars (0.56-1.63 M-circle dot) as a function of orbital period and find evidence of two regimes: where super-Earths are more abundant at short orbital periods, and where sub-Neptunes are more abundant at longer orbital periods. We fit a parametric model in five equally populated stellar mass bins and find that the orbital period of transition between these two regimes scales with stellar mass, like P-trans proportional to M-*(1.7 +/- 0.2). Ptrans These results suggest a population of former sub-Neptunes contaminating the population of gigayear-old close-in super-Earths, indicative of a population shaped by atmospheric loss. Using our model to constrain the long-period population of intrinsically rocky planets, we estimate an occurrence rate of Gamma(circle plus) = 15(-4)(+6)% for Earth-sized habitable zone planets, and predict that sub-Neptunes may be similar to twice as common as super-Earths in the habitable zone (when normalized over the natural log-orbital period and radius range used). Finally, we discuss our results in the context of future missions searching for habitable zone planets.
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Clark, J. T., Addison, B. C., Okumura, J., Vach, S., Errico, A., Heitzmann, A., et al. (2023). Spinning up a Daze: TESS Uncovers a Hot Jupiter Orbiting the Rapid Rotator TOI-778. Astron. J., 165(5), 207.
Abstract: NASA's Transiting Exoplanet Survey Satellite (TESS) mission has been uncovering a growing number of exoplanets orbiting nearby, bright stars. Most exoplanets that have been discovered by TESS orbit narrow-line, slow-rotating stars, facilitating the confirmation and mass determination of these worlds. We present the discovery of a hot Jupiter orbiting a rapidly rotating (v sin (i) = 35.1 +/- 1.0 km s(-1) early F3V-dwarf, HD 115447 (TOI-778). The transit signal taken from Sectors 10 and 37 of TESS's initial detection of the exoplanet is combined with follow-up ground-based photometry and velocity measurements taken from MINERVA-Australis, TRES, CORALIE, and CHIRON to confirm and characterize TOI-778 b. A joint analysis of the light curves and the radial velocity measurements yields a mass, a radius, and an orbital period for TOI-778 b of 2.76(-0.23)(+0.24) M-J, 1.370 +/- 0.043 R-J, and similar to 4.63 days, respectively. The planet orbits a bright (V = 9.1 mag) F3-dwarf with M = 1.40 +/- 0.05 M-circle dot, R = 1.70 +/- 0.05 R-circle dot, and log g = 4.05 +/- 0.17. We observed a spectroscopic transit of TOI-778 b, which allowed us to derive a sky-projected spin-orbit angle of 18 degrees +/- 11 degrees, consistent with an aligned planetary system. This discovery demonstrates the capability of smaller-aperture telescopes such as MINERVA-Australis to detect the radial velocity signals produced by planets orbiting broad-line, rapidly rotating stars.
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