Exoplanets: rather than comparing two edges of a planet, measure the time lag in the transit to determine its atmosphere

A new method applied to WASP-94A b utilises the temporal signature of limb asymmetry, reducing the error in the measured molecular abundances by a factor of 30 to 50.

More and more transiting exoplanets are showing that their "morning" and "evening" edges differ, but interpreting these observations becomes the limiting step. The classical approach, which adjusts separate transit depths for each limb, produces strongly anticorrelated quantities and, when treated as independent, can bias the molecular abundances found to 3 sigma. Sagnick Mukherjee, Luis Welbanks and their colleagues propose instead to rely on the time signature of asymmetry: a shift in the mid-transit time according to the wavelength, measurable on existing light curves with a spherically symmetrical model, and not correlated to the mean depth.

By simultaneously adjusting the transmission spectrum and this "time lag spectrum", with an independent atmospheric column per limb, the method reduces the root mean square error over the abundances by a factor of 30 to 50 compared to the best of the current approaches. Applied to WASP-94A b over 0.8 to 5 microns (NIRISS/SOSS and NIRSpec/G395H data), it reveals a morning limb obscured by aerosols over the entire range, while the evening limb delivers the abundances of H2O, CO2, CO and H2S, with a temperature difference between limbs of 415 kelvin to 1 millibar.