Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune

We detect Ly alpha absorption from the escaping atmosphere of HD 63433c, a R = 2.67R (circle plus), P = 20.5 day mini-Neptune orbiting a young (440 Myr) solar analog in the Ursa Major Moving Group. Using Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph, we measure a transit depth of 11.1 +/- 1.5% in the blue wing and 8 +/- 3% in the red. This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second transit observation with HST. We do not detect Ly alpha absorption from the inner planet, a smaller R = 2.15R (circle plus) mini-Neptune on a 7.1 day orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium absorption for both planets. We measure the host star's X-ray spectrum and mid-ultraviolet flux with XMM-Newton, and model the outflow from both planets using a 3D hydrodynamic code. This model provides a reasonable match to the light curve in the blue wing of the Ly alpha line and the helium nondetection for planet c, although it does not explain the tentative red wing absorption or reproduce the excess absorption spectrum in detail. Its predictions of strong Ly alpha and helium absorption from b are ruled out by the observations. This model predicts a much shorter mass-loss timescale for planet b, suggesting that b and c are fundamentally different: while the latter still retains its hydrogen/helium envelope, the former has likely lost its primordial atmosphere.

Automated summary

Ly alpha absorption from the escaping atmosphere of HD 63433c, a mini-Neptune orbiting a young solar analog, was detected. Transit observations with the Hubble Space Telescope showed a transit depth of 11.1% in the blue wing and 8% in the red. The absorption signal is unlikely to be due to stellar variability and needs confirmation. No Ly alpha absorption was detected from the inner planet, and an upper limit of 0.5% on helium absorption for both planets was determined. A hydrodynamic code was used to model the outflow from both planets, which provided a reasonable match to the observed light curve but could not explain all the absorption features.