Hubble observations of AU Mic b reveal stellar contamination limits atmospheric analysis
Transmission spectroscopy of the 20-million-year-old exoplanet AU Mic b with Hubble’s WFC3 was hampered by unstable scanning and strong stellar activity, leaving only weak constraints on the planet’s atmosphere.
Astronomers using the Hubble Space Telescope’s Wide Field Camera 3 obtained a 0.8-1.6 µm transmission spectrum of AU Mic b, a young sub-Neptune orbiting a nearby M-dwarf star. The planet’s age of about 20 million years and proximity (9.7 pc) make it a prime target for studying early atmospheric evolution.
During the observations the telescope experienced unstable scanning, producing a variable point-spread function that altered the orbit-to-orbit baseline. Although spectral binning reduced the impact, residual systematics limited the precision of the measurements.
The host star’s intense magnetic activity introduced additional complications. Rotational variability, spot crossings, and the transit-light-source (TLS) effect altered both the light curves and the derived spectrum. By decomposing the out-of-transit stellar spectral energy distribution, the team estimated a photospheric temperature of 3891 ± 37 K, a spot temperature of 3020 ± 69 K, and a spot filling factor of 0.33 ± 0.05.
"The spectrum is dominated by the TLS effect with weak atmospheric constraints, and the data prefer a relatively small scale height of less than 185 km at 3σ," the authors wrote.
Bayesian atmospheric retrievals indicated that any planetary atmospheric signal is likely compact and faint, with the TLS effect overwhelming potential spectral features across optical and infrared wavelengths. The study underscores the difficulty of isolating exoplanet atmospheres around active young stars and suggests that future observations with more stable instruments will be needed for clearer detections.
