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

Understand

We detect Lyman $\alpha$ absorption from the escaping atmosphere of HD 63433c, a $R=2.67 R_\oplus$, $P=20.5$ d mini Neptune orbiting a young (440 Myr) solar analogue in the Ursa Major Moving Group.

• Using HST/STIS, we measure a transit depth of $11.1 \pm 1.5$% in the blue wing and $8 \pm 3$% in the red.
• This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second visit with HST.
• We do not detect Lyman $\alpha$ absorption from the inner planet, a smaller $R=2.15 R_\oplus$ mini Neptune on a 7.1 d orbit.