Insights into Terrestrial Planet Compositions and Geophysics from Observations of Magma Worlds

There exists a remarkable population of short period transiting rocky exoplanets with temperatures >2,000 K, and masses ranging from about 8 Earth masses, such as the hot super-Earth 55 Cancri e, to that of Mercury or smaller, such as K2-22b. These planets are thought to have mineral atmospheres that are produced by the vaporisation of their magma surfaces, or large exospheres that are produced by sputtering of their atmospheres or exposed surfaces by intense stellar winds. Additionally, the smaller, low surface gravity hot rocky exoplanets have been found to be actively disintegrating and forming 'comet-like' dust tails.

Since their atmospheres and released gas and dust can be observationally constrained, these planets present the tantalising prospect of directly probing the composition of rocky planets. Sodium and calcium are promising species to detect given their low sublimation temperatures, large absorption cross-sections, likely presence in terrestrial planet compositions, and presence in Mercury's exosphere.

This poster presents the insights we gained from using high-resolution transmission spectroscopy to search for Na and Ca around 55 Cnc e and K2-22 b using several ground based telescopes. For 55 Cne e, we detected a tantilizing 5 σ signal of Ca+ on one night of observation, but a similar signal has not been detected since (despite our unprecedented limits). This may be related to variability of the star-planet system and the planet's magnetic field.

For K2-22 b, we did not detect absorption by Na or Ca+, but found lower-limits that are smaller than the expected magnitude of the signal based on the planet's estimated mass-loss rate and assuming a terrestrial composition. We attribute this non-detection to the probed gases being accelerated by the stellar wind and radiation pressure to high velocities, resulting in very broad Doppler shifted absorption signals that are hard to detect.

The implications of these results on probing rocky exoplanet compositions, constraining planetary magnetic fields, and understanding the environment around short-period rocky exoplanets are also outlined.