Updated JWST measurements refine the view of giant planet HIP 65426 b

Updated James Webb Space Telescope images provide revised measurements of the position and brightness of the young giant planet HIP 65426 b. Its overall properties still fit a cloudy atmosphere and a temperature estimate of about 1,436 kelvins. A search around its star also identified 21 candidate sources, but none of the candidates was classified as a planet.

Star system: HIP 65426 | Distance from the Sun: 351 light years

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Updated JWST measurements refine the view of giant planet HIP 65426 b

Star system: HIP 65426 | Distance from the Sun: 351 light years

Updated measurements of HIP 65426 b revise its apparent position and brightness in some James Webb Space Telescope images. The planet’s measured separation from its star, position angle, and flux differ significantly from earlier results, although the measurements remain consistent within 2.2, 1.5, and 3.7 standard deviations, respectively. Flux means the amount of light received from an object; position angle describes its direction from the star.

The measurements come from JWST coronagraphic images taken with its NIRCam and MIRI instruments. A coronagraph blocks much of a star’s light so that nearby, fainter objects are easier to see. Updated processing methods changed the measured brightness and position in some filters, which capture different ranges of infrared light. The largest separation changes appeared in filters F444W and F1550C, the largest position-angle change in F250M, and the largest flux changes in F250M, F300M, F410M, F1140C, and F1550C.

HIP 65426 b’s overall spectrum, or pattern of brightness across wavelengths, remains consistent with a young giant planet whose atmosphere contains clouds. The best-fitting models give it an estimated effective temperature of about 1,436 kelvins. The images also revealed 21 candidate sources near the star, in addition to the known planet. Thirteen were classified as astrophysical sources: eight background stars, four galaxies, and one source of unknown type. None was classified as a planet. One galaxy looked like a point of light in MIRI images but appeared extended in NIRCam images, showing why a single MIRI image may not be enough to identify a faint companion.

Key findings
Revised measurements of HIP 65426 b differ from earlier values but remain consistent within 2.2 standard deviations in separation, 1.5 in position angle, and 3.7 in flux.
Changes from updated image processing were strongest in particular infrared filters, with the affected filters varying for position, direction, and brightness.
The planet’s spectrum remains consistent with a cloudy atmosphere and an estimated effective temperature of about 1,436 kelvins.
Of 21 additional candidate sources, 13 were classified as eight background stars, four galaxies, and one unknown source; none was classified as a planet.
A galaxy that looked point-like in MIRI images was extended in NIRCam images, highlighting a limitation of relying on single-epoch MIRI imaging alone to search for companions.

Scientific study

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems IX: A Multi-Wavelength Framework for Source Characterization in JWST Coronagraphic Imaging of the HIP 65426 System