Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets

• Published in: The Astrophysical journal Vol. 885; no. 2; pp. 168 - 186
• Main Authors: Oza, Apurva V., Johnson, Robert E., Lellouch, Emmanuel, Schmidt, Carl, Schneider, Nick, Huang, Chenliang, Gamborino, Diana, Gebek, Andrea, Wyttenbach, Aurelien, Demory, Brice-Olivier, Mordasini, Christoph, Saxena, Prabal, Dubois, David, Moullet, Arielle, Thomas, Nicolas
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10-11-2019; IOP Publishing; American Astronomical Society
• Subjects: Astrophysics; Egress; Exosphere; Extrasolar planets; Gas giant planets; High altitude; Ionization; Jupiter; Jupiter atmosphere; Orbit decay; Physics; planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: detection; planets and satellites: dynamical evolution and stability; planets and satellites: magnetic fields; planets and satellites: physical evolution; Potassium; Satellites; Sciences of the Universe; Searching; Sodium; Solar system; Spectroscopy; Spectrum analysis; Tidal energy; Tidal power; Time dependence; Volatile compounds; Volcanic gases
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab40cc

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.