More Like this

1. Planet(esimal)s around stars with TESS (PAST) III: A search for triplet He  i in the atmospheres of two 200 Myr-old planets

   https://doi.org/10.1093/mnras/stac3301  

   Gaidos, Eric ; Hirano, Teruyuki ; Lee, Rena A. ; Harakawa, Hiroki ; Hodapp, Klaus ; Jacobson, Shane ; Kotani, Takayuki ; Kudo, Tomoyuki ; Kurokawa, Takashi ; Kuzuhara, Masayuki ; et al ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   We report a search for excess absorption in the 1083.2 nm line of ortho (triplet) helium during transits of TOI-1807b and TOI-2076b, 1.25 and 2.5-R⊕ planets on 0.55- and 10.4-d orbits around nearby ∼200 Myr-old K dwarf stars. We limit the equivalent width of any transit-associated absorption to <4 and <8 mÅ, respectively. We limit the escape of solar-composition atmospheres from TOI-1807b and TOI-2076b to ≲1 and ≲0.1M⊕Gyr−1, respectively, depending on wind temperature. The absence of a H/He signature for TOI-1807b is consistent with a measurement of mass indicating a rocky body and the prediction by a hydrodynamic model that any H-dominated atmosphere would be unstable and already have been lost. Differential spectra obtained during the transit of TOI-2076b contain a He i-like feature, but this closely resembles the stellar line and extends beyond the transit interval. Until additional transits are observed, we suspect this to be the result of variation in the stellar He i line produced by rotation of active regions and/or flaring on the young, active host star. Non-detection of escape could mean that TOI-2076b is more massive than expected, the star is less EUV luminous, the models overestimate escape, or the planet has a H/He-poor atmosphere that is primarily moleculesmore »such as H2O. Photochemical models of planetary winds predict a semimajor axis at which triplet He i observations are most sensitive to mass-loss: TOI-2076b orbits near this optimum. Future surveys could use a distance criterion to increase the yield of detections.

   « less
2. A detailed analysis of the Gl 486 planetary system

   https://doi.org/10.1051/0004-6361/202243548  

   Caballero, J. A. ; González-Álvarez, E. ; Brady, M. ; Trifonov, T. ; Ellis, T. G. ; Dorn, C. ; Cifuentes, C. ; Molaverdikhani, K. ; Bean, J. L. ; Boyajian, T. ; et al ( September 2022 , Astronomy & Astrophysics)

   Context . The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R ⊕ and 3.0 M ⊕ . It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims . To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods . We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results . From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at θ LDD = 0.390more »± 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R * = 0.339 ± 0.015 R ⊕ . We also measure a stellar rotation period at P rot = 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble /STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R p = 1.343 −0.062 +0.063 R ⊕ and M p = 3.00 −0.12 +0.13 M ⊕ , with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope ( Webb ) observations.« less
3. TESS Hunt for Young and Maturing Exoplanets (THYME). VI. An 11 Myr Giant Planet Transiting a Very-low-mass Star in Lower Centaurus Crux

   https://doi.org/10.3847/1538-3881/ac511d  

   Mann, Andrew W. ; Wood, Mackenna L. ; Schmidt, Stephen P. ; Barber, Madyson G. ; Owen, James E. ; Tofflemire, Benjamin M. ; Newton, Elisabeth R. ; Mamajek, Eric E. ; Bush, Jonathan L. ; Mace, Gregory N. ; et al ( March 2022 , The Astronomical Journal)

   Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15R_⊕planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05R_J(9.5R_⊕) planet transiting a very-low-mass star (0.170 ± 0.015M_⊙) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5M_J. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5R_⊕planets.
4. The fundamentals of Lyman α exoplanet transits

   https://doi.org/10.1093/mnras/stac3414  

   Owen, James E. ; Murray-Clay, Ruth A. ; Schreyer, Ethan ; Schlichting, Hilke E. ; Ardila, David ; Gupta, Akash ; Loyd, R. O. Parke ; Shkolnik, Evgenya L. ; Sing, David K. ; Swain, Mark R. ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   Lyman α transits have been detected from several nearby exoplanets and are one of our best insights into the atmospheric escape process. However, due to ISM absorption, we typically only observe the transit signature in the blue-wing, making them challenging to interpret. This challenge has been recently highlighted by non-detections from planets thought to be undergoing vigorous escape. Pioneering 3D simulations have shown that escaping hydrogen is shaped into a cometary tail receding from the planet. Motivated by this work, we develop a simple model to interpret Lyman α transits. Using this framework, we show that the Lyman α transit depth is primarily controlled by the properties of the stellar tidal field rather than details of the escape process. Instead, the transit duration provides a direct measurement of the velocity of the planetary outflow. This result arises because the underlying physics is the distance a neutral hydrogen atom can travel before it is photoionized in the outflow. Thus, higher irradiation levels, expected to drive more powerful outflows, produce weaker, shorter Lyman α transits because the outflowing gas is ionized more quickly. Our framework suggests that the generation of energetic neutral atoms may dominate the transit signature early, but the acceleration of planetary materialmore »produces long tails. Thus, Lyman α transits do not primarily probe the mass-loss rates. Instead, they inform us about the velocity at which the escape mechanism is ejecting material from the planet, providing a clean test of predictions from atmospheric escape models.

   « less
5. TESS-Keck Survey. IX. Masses of Three Sub-Neptunes Orbiting HD 191939 and the Discovery of a Warm Jovian plus a Distant Substellar Companion

   https://doi.org/10.3847/1538-3881/ac3d38  

   Lubin, Jack ; Van Zandt, Judah ; Holcomb, Rae ; Weiss, Lauren M. ; Petigura, Erik A. ; Robertson, Paul ; Akana Murphy, Joseph M. ; Scarsdale, Nicholas ; Batygin, Konstantin ; Polanski, Alex S. ; et al ( January 2022 , The Astronomical Journal)

   Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI 1339), a bright (V= 9) and Sun-like (G9V) star, which TESS found to host three transiting planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with similar sizes from 3 to 3.4R_⊕. To further characterize the system, we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES and APF/Levy. We find thatM_b= 10.4 ± 0.9M_⊕andM_c= 7.2 ± 1.4M_⊕, which are low compared to most known planets of comparable radii. The RVs yield only an upper limit onM_d(<5.8M_⊕at 2σ). The RVs further reveal a fourth planet (e) with a minimum mass of 0.34 ± 0.01M_Jupand an orbital period of 101.4 ± 0.4 days. Despite its nontransiting geometry, secular interactions between planet e and the inner transiting planets indicate that planet e is coplanar with the transiting planets (Δi< 10°). We identify a second high-mass planet (f) with 95% confidence intervals on mass between 2 and 11M_Jupand period between 1700 and 7200 days, based on a joint analysis of RVs and astrometry from Gaia and Hipparcos. As a bright starmore »hosting multiple planets with well-measured masses, HD 191939 presents many options for comparative planetary astronomy, including characterization with JWST.

   « less