Possible Hycean conditions in the sub-Neptune TOI-270 d
Astronomy & Astrophysics(2024)
摘要
The JWST has ushered in a new era in atmospheric characterisations of
temperate low-mass exoplanets with recent detections of carbon-bearing
molecules in the candidate Hycean world K2-18 b. We investigated JWST
observations of the TOI-270 system, with two sub-Neptunes simultaneously
transiting the nearby M dwarf during the visit. We report our atmospheric
characterisation of the outer planet TOI-270 d, a candidate Hycean world, with
JWST transmission spectroscopy using the NIRSpec G395H instrument in the
2.7-5.2 μm range, combined with previous observations obtained with the HST
WFC3 spectrograph (1.1-1.6 μm). The spectrum reveals strong signatures of
CH4 and CO2 at 3.8-4.9σ and 2.9-3.9σ confidence, respectively,
and no evidence of NH3. The abundant CH4 and CO2, at 0.1-1
the non-detection of NH3 are similar to the findings reported for K2-18 b and
consistent with predictions for a Hycean world with a planet-wide ocean under a
H2-rich atmosphere. We also report evidence of CS2 at a 2.3-3.0σ
confidence and a potential inference of H2O at 1.6-4.4σ, depending on
the data analysis approach, and discuss possible interpretations of these
results. The spectrum does not provide strong constraints on the presence of
clouds or hazes in the observable atmosphere, nor any evidence for the effects
of stellar heterogeneities, which is consistent with previous studies. For the
smaller inner planet TOI-270 b, we find that the spectrum is inconsistent with
a featureless spectrum at 3σ, showing some preference for an H2-rich
atmosphere in a super-Earth. We discuss the implications of our findings and
future prospects.
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