Zodiacal Exoplanets in Time (ZEIT). IX. A Flat Transmission Spectrum and a Highly Eccentric Orbit for the Young Neptune K2-25b as Revealed by Spitzer

Abstract

Transiting planets in nearby young clusters offer the opportunity to study the atmospheres and dynamics of planets during their formative years. To this end, we focused on K2-25b -a close-in (P =3.48 days), Neptune-sized exoplanet orbiting a M4.5 dwarf in the 650 Myr Hyades cluster. We combined photometric observations of K2-25 covering a total of 44 transits and spanning > 2 yr, drawn from a mix of space-based telescopes (Spitzer Space Telescope and K2 ) and ground-based facilities (Las Cumbres Observatory Global Telescope network and MEarth). The transit photometry spanned 0.6-4.5µm, which enabled our study of K2-25b's transmission spectrum. We combined and fit each dataset at a common wavelength within a Markov Chain Monte Carlo framework, yielding consistent planet parameters. The resulting transit depths ruled out a solar-composition atmosphere for K2-25b for the range of expected planetary masses and equilibrium temperature at a > 4σ confidence level, and are consistent with a flat transmission spectrum. Mass constraints and transit observations at a finer grid of wavelengths (e.g., from the Hubble Space Telescope) are needed to make more definitive statements about the presence of clouds or an atmosphere of high mean molecular weight. Our precise measurements of K2-25b's transit duration also enabled new constraints on the eccentricity of K2-25's orbit. We find K2-25b's orbit to be eccentric (e > 0.20) for all reasonable stellar densities and independent of the observation wavelength or instrument. The high eccentricity is suggestive of a complex dynamical history and motivates future searches for additional planets or stellar companions.

The Habitable Zone Planet Finder Reveals a High Mass and Low Obliquity for the Young Neptune K2-25b

“…To explain the flat broadband transmission spectrum, they favored a scenario where K2-25b has a cloudy atmosphere using a predicted mass from exoplanet mass-radius relations. Although K2-25 could have a cloudy atmosphere, we show in this paper that the apparent flat broadband transmission spectrum of K2-25b could also be partially explained by the larger observed mass of K2-25b than the mass assumed in Thao et al (2020). Kain et al (2020) performed long-term photometric monitoring of K2-25 using the MEarth Observatories (Irwin et al 2015) to monitor the activity of the star and look for evidence of transit timing variations (TTVs) in the transits of K2-25b, which could be suggestive of an additional planet orbiting in the system.…”

“…Recently, two groups discussed additional transit follow-up observations of K2-25b. Thao et al (2020) studied the transit depth of K2-25b as a function of wavelength using photometric observations at different wavelengths from K2, the MEarth Observatories (Irwin et al 2015), and the Las Cumbres Global Telescope Network (LCOGT) in the optical and the Spitzer Telescope in the NIR and found no significant evidence of changes in transit depth as a function of wavelength. To explain the flat broadband transmission spectrum, they favored a scenario where K2-25b has a cloudy atmosphere using a predicted mass from exoplanet mass-radius relations.…”

“…The stellar parameters used in this work are summarized in Table 2 and are broadly adopted from the values presented in Mann et al (2016a) and Thao et al (2020). Thao et al (2020) estimated their stellar parameters (including the stellar radius and mass) using the empirically calibrated relations of Mann et al (2015Mann et al ( , 2019.…”

“…The stellar parameters used in this work are summarized in Table 2 and are broadly adopted from the values presented in Mann et al (2016a) and Thao et al (2020). Thao et al (2020) estimated their stellar parameters (including the stellar radius and mass) using the empirically calibrated relations of Mann et al (2015Mann et al ( , 2019. As an additional test of these parameters, we performed an independent spectral energy distribution (SED) and isochrone fit using the EXOFASTv2 package (Eastman 2017;Eastman et al 2019), placing informative Gaussian priors on the known metallicity ([Fe/ H]=0.15±0.03) and age ( =  age 730 50 Myr) of the star derived from its Hyades cluster membership (Mann et al 2016a), along with its known distance from Gaia.…”

Updated Parameters and a New Transmission Spectrum of HD 97658b

“…dwarf in the 650 Myr old Hyades cluster (Mann et al 2016;Thao et al 2020). The unusually large size (3.43 R ⊕ ) of K2-25b in comparison to other planets with similar orbital periods (3.485 days) and the fact that it is orbiting a young star suggest that K2-25b could represent an early or intermittent phase of planetary evolution, where young Neptunes are in the process of losing their atmospheres (Mann et al 2016).…”