Caltech/IPAC Lunch Seminar

Despite its name, the Neptune desert is not totally devoid of planets. How these planets arrived and persist in the desert is an open question, potentially a result of a combination of high atmospheric metallicity and low stellar activity. Complicating this, migration may also be a major factor shaping the desert, as stars hosting desert planets have high metallicity much like Hot Jupiter hosts. A natural explanation may be that some desert planets are stripped Hot Jupiter cores, and we test this using the hottest known exo-Neptune and desert denizen LTT 9779 b. We observe strong CO2 absorption and find evidence for H2O in LTT 9779 b's atmosphere from JWST /NIRSpec G395H secondary eclipse spectroscopy, taken from phase curve observations. Previous Spitzer observations hinted at (but couldn't confirm) molecular absorption at 3.5 and 4.5μm. JWST /NIRISS transmission spectroscopy found no strong molecular absorption due to aerosols or high metallicity, while a NIRISS phase curve found water absorption and high albedo due to silicate clouds. Analysis of the full NIRSpec phase curve shows CO and CO2, as well as evidence for H2O, presented separately by R. Ashtari et al. (2025). Our atmospheric modeling indicates that LTT 9779 b has high metallicity ([Fe/H]=∼120×Solar) and stellar C/O=0.62. With its high atmospheric metallicity and large core mass fraction, LTT 9779 b may be a stripped Hot Jupiter that underwent Roche lobe overflow as it migrated to its current ultrahot orbit.