A Cold and Super-Puffy Planet on a Prograde Orbit

TL;DR

TOI-4507 b addresses the challenge of characterizing a long-period, low-density exoplanet around a young star. The authors combine TESS and ASTEP photometry with RV data from HARPS, FEROS, and CORALIE, plus Rossiter-McLaughlin measurements and high-resolution imaging, to constrain the planet's radius, mass upper limit, density, and obliquity. They find a radius of about 8.2 Earth radii, an upper mass limit near 20 Earth masses, a density under 0.2 g/cm^3, and a prograde obliquity near -15 degrees, with hints of a possible outer companion. The results place TOI-4507 b among the youngest and longest-period super-puffs with obliquity measurements, informing theories of their formation, evolution, and atmospheric properties, and highlighting the potential for atmospheric characterization with JWST.

Abstract

We report the discovery of TOI-4507 b, a transiting sub-Saturn with a density $<$ 0.2 g/cm$^3$ on a 105-day prograde orbit around a 700 Myr old F star. The transits were detected using data from TESS as well as the Antarctic telescope ASTEP. A joint analysis of the light curves and radial velocities from HARPS, FEROS, and CORALIE confirmed the planetary nature of the signal by limiting the mass to be below 20 $M_\oplus$ at 95% confidence. The radial velocities also exhibit the Rossiter-McLaughlin effect and imply that the planet orbits the star in a prograde orbit with a sky-projected obliquity $λ=-15_{-44}^{+50}$ deg ($|λ|<80$ deg at $3σ$). With these characteristics, TOI-4507 is one of the longest-period systems for which the stellar obliquity has been measured, and the planet is among the longest-period and youngest ''super-puff'' planets yet discovered.

Figures (6)

• Figure 1: Different observations of TOI-4507 along with the best model. a) Phase-folded out-of-transit RVs taken with different instruments, along with the best model in red. b) RVs as a function of time showing the long-term trend detected at a $5\sigma$ confidence level. c-f) Different light curves along with the best model. Binned data are shown as solid blue points.
• Figure 2: High-resolution imaging observations of TOI-4507 taken with the 4.1 m SOAR telescope. We show the $5\sigma$ detection sensitivity and speckle autocorrelation functions. No companions to TOI-4507 within $3^{\prime\prime}$ are found in these observations.
• Figure 3: HARPS observations of the RM effect produced during the transit of TOI-4507 b, along with the best-fit model in red. Shaded areas show $1\sigma$, $2\sigma$, and $3\sigma$ models.
• Figure 4: Sky-projected obliquity $\lambda$ as a function of the orbital period (left) and age of the system (right). The population of Neptunes/sub-Saturns ($10<M_p/M_\oplus<90$) is shown in blue. Super-puff planets (Neptunes/sub-Saturns with $\rho_p<0.3$ g/cm$^3$) are shown in pink, with TOI-4507 highlighted as a star with a red edge. Other giant planet systems are shown as faint black points. Data from TEPCat with ages from the NASA Exoplanet Archive.
• Figure 5: Observed modulation in the light curves of TOI-4507 (upper panels) and the nearby source Gaia DR3 4657949658179626624 (bottom panels). a) Lomb-Scargle periodogram of the 2-min cadence TESS light curve of TOI-4507. A false alarm probability (FAP) level of 1% is marked with a red dashed line. The black dashed line marks the highest-power peak, indicating a period of 1.7 days. b) TESS light curve phased to the period of 1.7 days. All the work done here made use of the 2-minute cadence light curve, but for illustrative purposes only, the 30-minute cadence TESS light curve is shown here in blue, with binned data in red. c) Lomb-Scargle periodogram of the OGLE light curve of Gaia DR3 4657949658179626624, also showing a clear period of 1.7 days. d) OGLE light curve of Gaia DR3 4657949658179626624 phased to the period of 1.7 days.