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Register a new userTESS Hunt for Young and Maturing Exoplanets (THYME) III: a two-planet system in the 400 Myr Ursa Major Group
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Exoplanets can evolve significantly between birth and maturity, as their atmospheres, orbits, and structures are shaped by their environment. Young planets ($<$1 Gyr) offer an opportunity to probe the critical early stages of this evolution, where planets evolve the fastest. However, most of the known young planets orbit prohibitively faint stars. We present the discovery of two planets transiting HD 63433 (TOI 1726, TIC 130181866), a young Sun-like ($M_*=0.99pm0.03$) star. Through kinematics, lithium abundance, and rotation, we confirm that HD 63433 is a member of the Ursa Major moving group ($tau=414pm23$ Myr). Based on the TESS light curve and updated stellar parameters, we estimate the planet radii are $2.15pm0.10R_oplus$ and $2.67pm0.12R_oplus$, the orbital periods are 7.11 and 20.55 days, and the orbital eccentricities are lower than about 0.2. Using HARPS-N velocities, we measure the Rossiter-McLaughlin signal of the inner planet, demonstrating that the orbit is prograde. Since the host star is bright (V=6.9), both planets are amenable to transmission spectroscopy, radial velocity measurements of their masses, and more precise determination of the stellar obliquity. This system is therefore poised to play an important role in our understanding of planetary system evolution in the first billion years after formation.
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Young exoplanets are snapshots of the planetary evolution process. Planets that orbit stars in young associations are particularly important because the age of the planetary system is well constrained. We present the discovery of a transiting planet larger than Neptune but smaller than Saturn in the 45 Myr Tucana-Horologium young moving group. The host star is a visual binary, and our follow-up observations demonstrate that the planet orbits the G6V primary component, DS Tuc A (HD 222259A, TIC 410214986). We first identified transits using photometry from the Transiting Exoplanet Survey Satellite (TESS; alerted as TOI 200.01). We validated the planet and improved the stellar parameters using a suite of new and archival data, including spectra from SOAR/Goodman, SALT/HRS and LCO/NRES; transit photometry from Spitzer; and deep adaptive optics imaging from Gemini/GPI. No additional stellar or planetary signals are seen in the data. We measured the planetary parameters by simultaneously modeling the photometry with a transit model and a Gaussian process to account for stellar variability. We determined that the planetary radius is $5.70pm0.17$ Earth radii and that the orbital period is 8.1 days. The inclination angles of the host stars spin axis, the planets orbital axis, and the visual binarys orbital axis are aligned within 15 degrees to within the uncertainties of the relevant data. DS Tuc Ab is bright enough (V=8.5) for detailed characterization using radial velocities and transmission spectroscopy.
We present the discovery of a transiting hot Jupiter orbiting HIP 67522 ($T_{eff}sim5650$ K; $M_* sim 1.2 M_{odot}$) in the 10-20 Myr old Sco-Cen OB association. We identified the transits in the TESS data using our custom notch-filter planet search pipeline, and characterize the system with additional photometry from Spitzer, spectroscopy from SOAR/Goodman, SALT/HRS, LCOGT/NRES, and SMARTS/CHIRON, and speckle imaging from SOAR/HRCam. We model the photometry as a periodic Gaussian process with transits to account for stellar variability, and find an orbital period of 6.9596$^{+0.000016}_{-0.000015}$ days and radius of 10.02$^{+0.54}_{-0.53}$ R$_oplus$. We also identify a single transit of an additional candidate planet with radius 8.01$^{+0.75}_{-0.71}$ R$_oplus$ that has an orbital period of $gtrsim23$ days. The validated planet HIP 67522 b is currently the youngest transiting hot Jupiter discovered and is an ideal candidate for transmission spectroscopy and radial velocity follow-up studies, while also demonstrating that some young giant planets either form in situ at small orbital radii, or else migrate promptly from formation sites farther out in the disk.
The detection and characterization of young planetary systems offers a direct path to study the processes that shape planet evolution. We report on the discovery of a sub-Neptune-size planet orbiting the young star HD 110082 (TOI-1098). Transit events we initially detected during TESS Cycle 1 are validated with time-series photometry from Spitzer. High-contrast imaging and high-resolution, optical spectra are also obtained to characterize the stellar host and confirm the planetary nature of the transits. The host star is a late F dwarf (M=1.2 Msun) with a low-mass, M dwarf binary companion (M=0.26 Msun) separated by nearly one arcminute (~6200 AU). Based on its rapid rotation and Lithium absorption, HD 110082 is young, but is not a member of any known group of young stars (despite proximity to the Octans association). To measure the age of the system, we search for coeval, phase-space neighbors and compile a sample of candidate siblings to compare with the empirical sequences of young clusters and to apply quantitative age-dating techniques. In doing so, we find that HD 110082 resides in a new young stellar association we designate MELANGE-1, with an age of 250(+50/-70) Myr. Jointly modeling the TESS and Spitzer light curves, we measure a planetary orbital period of 10.1827 days and radius of Rp = 3.2(+/-0.1) Earth radii. HD 110082 bs radius falls in the largest 12% of field-age systems with similar host star mass and orbital period. This finding supports previous studies indicating that young planets have larger radii than their field-age counterparts.
Young exoplanets can offer insight into the evolution of planetary atmospheres, compositions, and architectures. We present the discovery of the young planetary system TOI 451 (TIC 257605131, Gaia DR2 4844691297067063424). TOI 451 is a member of the 120-Myr-old Pisces--Eridanus stream (Psc--Eri). We confirm membership in the stream with its kinematics, its lithium abundance, and the rotation and UV excesses of both TOI 451 and its wide binary companion, TOI 451 B (itself likely an M dwarf binary). We identified three candidate planets transiting in the TESS data and followed up the signals with photometry from Spitzer and ground-based telescopes. The system comprises three validated planets at periods of 1.9, 9.2 and 16 days, with radii of 1.9, 3.1, and 4.1 Earth radii, respectively. The host star is near-solar mass with V=11.0 and H=9.3 and displays an infrared excess indicative of a debris disk. The planets offer excellent prospects for transmission spectroscopy with HST and JWST, providing the opportunity to study planetary atmospheres that may still be in the process of evolving.
Planets around young stars trace the early evolution of planetary systems. We report the discovery and validation of two planetary systems with ages $lesssim 300$ Myr from observations by the Transiting Exoplanet Survey Satellite. TOI-251 is a 40-320 Myr old G star hosting a 2.74 +0.18/-0.18 REarth mini-Neptune with a 4.94 day period. TOI-942 is a 20-160 Myr old K star hosting a system of inflated Neptune-sized planets, with TOI-942b orbiting with a period of 4.32 days, with a radius of 4.81 +0.20/-0.20 REarth, and TOI-942c orbiting in a period of 10.16 days with a radius of 5.79 +0.19/-0.18 REarth. Though we cannot place either host star into a known stellar association or cluster, we can estimate their ages via their photometric and spectroscopic properties. Both stars exhibit significant photometric variability due to spot modulation, with measured rotation periods of $sim 3.5$ days. These stars also exhibit significant chromospheric activity, with age estimates from the chromospheric calcium emission lines and X-ray fluxes matching that estimated from gyrochronology. Both stars also exhibit significant lithium absorption, similar in equivalent width to well-characterized young cluster members. TESS has the potential to deliver a population of young planet-bearing field stars, contributing significantly to tracing the properties of planets as a function of their age.
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