Water Ice on a 'Cold Jupiter'

A giant planet 12 light-years from Earth appears to have water-ice clouds in its atmosphere, based on new James Webb Space Telescope observations of Epsilon Indi Ab. (arxiv.org) (eurekalert.org) Astronomers read exoplanet atmospheres by splitting a planet’s light into a spectrum, a kind of barcode that shows which gases and particles are present. In this case, the team used the James Webb Space Telescope to directly image Epsilon Indi Ab, instead of waiting for the planet to cross in front of its star. (eurekalert.org) (arxiv.org) The planet is a cold gas giant, with an estimated temperature of about 200 to 300 kelvin, and the paper describes it as the closest known super-Jupiter. The researchers updated its mass to 7.6 ± 0.7 times Jupiter’s mass and reported an orbital eccentricity of 0.24 with asymmetric uncertainties. (arxiv.org) The new data came from the telescope’s Mid-Infrared Instrument, or MIRI, at 11.3 micrometres, combined with earlier measurements. Epsilon Indi Ab was 0.88 ± 0.08 magnitudes brighter at 11.3 micrometres than at 10.6 micrometres, a pattern the team says points to ammonia in the atmosphere. (arxiv.org) But the ammonia feature was weaker than cloud-free models predicted. The authors said their preferred explanation is thick water-ice clouds, which would mute the ammonia signal and also dim the planet at near-infrared wavelengths. (arxiv.org) That result cuts against many widely used exoplanet atmosphere models, which the Max Planck Institute for Astronomy team said often simplify or omit clouds. The same paper says a small but growing sample of cold giant exoplanets also looks fainter than expected between 3 and 5 micrometres, which fits the cloud idea. (eurekalert.org) (arxiv.org) Most exoplanet atmospheres studied so far have belonged to hot gas giants close to their stars, because those worlds are easier to detect when they transit. Epsilon Indi Ab gives astronomers a rarer look at a colder Jupiter analogue, the kind of planet that more closely resembles the outer giants in our own solar system. (eurekalert.org) The paper is titled “A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter,” and arXiv lists it as accepted by *The Astrophysical Journal Letters*. The team said more James Webb observations from 3 to 20 micrometres should test whether the water-ice cloud signal holds up. (arxiv.org) (scienceblog.com)