JWST images 30% larger exoplanet

Natasha Batalha and colleagues posted two new studies on May 14 that sharpen the picture of LTT 1445A b, one of the nearest small rocky exoplanets known to transit an M dwarf star. One paper presented the first James Webb Space Telescope transmission spectrum of the planet from 3 to 5 microns, while a second study used existing JWST eclipse data to test what kind of atmosphere, if any, could still fit the measurements. (arxiv.org 1) (arxiv.org 2) NASA’s Exoplanet Archive lists LTT 1445A b at 1.34 times Earth’s radius on a 5.36-day orbit in the LTT 1445 system, about 6.9 parsecs from Earth. (arxiv.org) Earlier Hubble work had already suggested the planet was a strong target for atmosphere searches because it is the closest known rocky planet transiting an M dwarf. ### Which planet are astronomers looking at? LTT 1445A b is a confirmed rocky planet orbiting the star LTT 1445A, according to NASA’s Exoplanet Archive. The archive gives the planet a radius of 1.34 Earth radii, which is roughly 30% larger than Earth, and an orbital period of 5.36 days. (arxiv.org) The LTT 1445 system sits 6.9 parsecs away, the Hubble study said, making it the closest transiting M-dwarf system and one of the best nearby rocky targets for atmospheric work. That earlier paper put the planet near 424 Kelvin and said prior observations had ruled out a light hydrogen-helium envelope but had not settled whether the world was bare rock, cloudy, or wrapped in a heavier atmosphere. (exoplanetarchive.ipac.caltech.edu) ### What did JWST newly measure? The May 14 JWST COMPASS paper by Batalha and co-authors reported the first JWST transmission spectrum of LTT 1445A b using NIRSpec/G395H over 3 to 5 microns. (exoplanetarchive.ipac.caltech.edu) The team said it used a single transit visit and two independent data-reduction pipelines. The same paper said the data reached a median precision of about 23 parts per million in one detector channel set and about 36 parts per million in the other. At that precision, the authors said, they found no statistically significant spectral features and could only place limits on atmospheric metallicity rather than claim a detection. (arxiv.org) ### Did the new work show a temperate world or a lava planet? Nicholas Wogan and co-authors did not say JWST had shown a temperate planet. Their May 14 paper said a bare-rock model is adequate to explain the existing MIRI Low Resolution Spectroscopy eclipse data, meaning “an atmosphere does not need to be invoked.” (arxiv.org) The same study described LTT 1445A b as a case study in how to extract pressure and composition limits from rocky-planet eclipse spectra. It set 2-sigma upper limits on several gases if an atmosphere exists, including about 1 bar for oxygen, nitrogen or carbon monoxide, about 0.1 bar for carbon dioxide, about 10^-3 bar for water vapor and about 10^-4 bar for sulfur dioxide. (arxiv.org) No primary source reviewed here says JWST has already established LTT 1445A b as a lava planet. The published and preprint material instead says the present data are consistent with a featureless or bare-rock interpretation and that thicker atmospheres can be constrained further with additional observations. (arxiv.org) ### Why are M-dwarf rocky planets central to this work? The Wogan paper said determining whether temperate rocky planets around M stars retain atmospheres is “currently a central goal” of exoplanet astronomy. The authors framed LTT 1445A b as a test case for climate-constrained Bayesian retrievals that could be applied across a broader population of rocky planets. (arxiv.org) The Batalha paper made a narrower point about the present observations. Its authors said JWST-only data can rule out some lower-metallicity atmospheres, and that combining JWST with Hubble extends those limits further, while “future analyses both in transit and emission” will determine whether detectable atmospheric features emerge. (arxiv.org) ### What comes next for this target? Scheduled JWST MIRI F1500W observations are the next named step in the Wogan paper. The authors wrote that those measurements could detect one of the thicker atmospheres still allowed by current data — specifically 1 bar of oxygen or 0.01 bar of carbon dioxide — if the observations achieve precision of 20 parts per million or better. (arxiv.org) May 14 is also the posting date for the two new papers that now define the state of play for LTT 1445A b. One is accepted at The Astronomical Journal, and the other was submitted to the AAS journals, according to the arXiv records. (arxiv.org) (arxiv.org)