The First Near-Infrared Transmission Spectrum of HIP 41378 f, a Low-Mass Temperate Jovian World in a Multi-Planet System

Kavli Affiliate: Andrew M. Vanderburg

| First 5 Authors: Munazza K. Alam, James Kirk, Courtney D. Dressing, Mercedes Lopez-Morales, Kazumasa Ohno

| Summary:

We present a near-infrared transmission spectrum of the long period (P=542
days), temperate ($T_{eq}$=294 K) giant planet HIP 41378 f obtained with the
Wide-Field Camera 3 (WFC3) instrument aboard the Hubble Space Telescope (HST).
With a measured mass of 12 $pm$ 3 $M_{oplus}$ and a radius of 9.2 $pm$ 0.1
$R_{oplus}$, HIP 41378 f has an extremely low bulk density (0.09 $pm$ 0.02
g/cm$^{3}$). We measure the transit depth with a median precision of 84 ppm in
30 spectrophotometric channels with uniformly-sized widths of 0.018 microns.
Within this level of precision, the spectrum shows no evidence of absorption
from gaseous molecular features between 1.1-1.7 microns. Comparing the observed
transmission spectrum to a suite of 1D
radiative-convective-thermochemical-equilibrium forward models, we rule out
clear, low-metallicity atmospheres and find that the data prefer
high-metallicity atmospheres or models with an additional opacity source such
as high-altitude hazes and/or circumplanetary rings. We explore the ringed
scenario for this planet further by jointly fitting the K2 and HST light curves
to constrain the properties of putative rings. We also assess the possibility
of distinguishing between hazy, ringed, and high-metallicity scenarios at
longer wavelengths with JWST. HIP 41378 f provides a rare opportunity to probe
the atmospheric composition of a cool giant planet spanning the gap between the
Solar System giants, directly imaged planets, and the highly-irradiated hot
Jupiters traditionally studied via transit spectroscopy.

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