Search

Neutral sodium (Na i ) is an alkali metal with a favorable absorption cross section such that tenuous gases are easily illuminated at select transiting exoplanet systems. We examine both the time-averaged and time-series alkali spectral flux individually, over 4 nights at a hot Saturn system on a ∼2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO observations are analyzed, providing new constraints. We recover the previously confirmed residual sodium flux uniquely when averaged, whereas night-to-night Na i varies by more than an order of magnitude. On HARPS/3.6 m Epoch II, we report a Doppler redshift at v _Γ,NaD = + 9.7 ± 1.6 km s ^−1 with respect to the planet’s rest frame. Upon examining the lightcurves, we confirm night-to-night variability, on the order of ∼1%–4% in NaD, rarely coinciding with exoplanet transit, not readily explained by stellar activity, starspots, tellurics, or the interstellar medium. Coincident with the ∼+10 km s ^−1 Doppler redshift, we detect a transient sodium absorption event dF _NaD / F _⋆ = 3.6% ± 1% at a relative difference of Δ F _NaD ( t ) ∼ 4.4% ± 1%, lasting Δ t _NaD ≳ 40 minutes. Since exoplanetary alkali signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly ∼−5.7 km s ^−1 , the radial velocity is rather at +15.4 km s ^−1 , far larger than any known exoplanet system. Given that the redshift magnitude v _Γ is in between the Roche limit and dynamically stable satellite orbits, the transient sodium may be a putative indication of a natural satellite orbiting WASP-49 A b.

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na i) and potassium (K i) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal . This tidal energy is also focused into the satellite driving an ∼105±2 higher mass-loss rate than Io’s supply to Jupiter’s Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼1010±1 Na cm−2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter’s extended (∼1000 RJ) Na exosphere and Jupiter’s atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the “high-altitude” Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles. [ABSTRACT FROM AUTHOR]