Exoplanetology: Exoplanets & Exomoons

JWST-TST High Contrast: Living on the Wedge, or, NIRCam Bar Coronagraphy Reveals CO2 in the HR 8799 and 51 Eri Exoplanets’ Atmospheres

By Keith Cowing

Status Report

astro-ph.EP

March 19, 2025

Filed under 51 Eridani, exoplanet, HR 8799, HR 8799 e, JWST, Spectroscopy

JWST-TST High Contrast: Living on the Wedge, or, NIRCam Bar Coronagraphy Reveals CO2 in the HR 8799 and 51 Eri Exoplanets’ Atmospheres — SED of 51 Eri b and best-fitting Exo-REM models. The top panel illustrates the transmission functions for each photometric filter. The middle panel shows the GPI (black circles, Macintosh et al. 2015; Rajan et al. 2017) and SPHERE (black squares, Samland et al. 2017; Brown-Sevilla et al. 2023) low-resolution spectra of the planet, as well as ground-based L’ and M band photometry (blue squares, Rajan et al. 2017), and the measurements from this work (gold hexagon, for our F410M detection, and gold downward facing triangles for our F335M, F430M, and F460M nondetections). Over plotted is the best-fitting Exo-REM model in magenta, and then 1, 2 σ envelopes showing the range of accepted models. The bottom panel illustrates the residuals to the best-fitting model; there are noticeable residuals in the J band GPI spectrum and the H band SPHERE spectrum that could be related to speckle noise or instrument throughput. The fit shows good agreement to the L’ band and F410M photometry, which constrain the CH4 and CO2 absorption slopes. The M band photometry from Rajan et al. (2017) lies slightly above the best-fitting model, but in general is consistent with strong CO absorption due to vertical mixing. — astro-ph.EP

High-contrast observations with JWST can reveal key composition and vertical mixing dependent absorption features in the spectra of directly imaged planets across the 3-5 μm wavelength range.

We present novel coronagraphic images of the HR 8799 and 51 Eri planetary systems using the NIRCam Long Wavelength Bar in an offset “narrow” position. These observations have revealed the four known gas giant planets encircling HR 8799, even at spatial separations challenging for a 6.5 m telescope in the mid-infrared, including the first ever detection of HR 8799 e at 4.6 μm. The chosen filters constrain the strength of CO, CH₄, and CO₂ absorption in each planet’s photosphere.

The planets display a diversity of 3-5 μm colors that could be due to differences in composition and ultimately be used to trace their formation history. They also show stronger CO₂ absorption than expected from solar metallicity models, indicating that they are metal enriched.

We detected 51 Eri b at 4.1 μm and not at longer wavelengths, which, given the planet’s temperature, is indicative of out-of-equilibrium carbon chemistry and an enhanced metallicity. Updated orbits fit to the new measurement of 51 Eri b validate previous studies that find a preference for high eccentricities (e=0.57+0.03−0.09), which likely indicates some dynamical processing in the system’s past.

These results present an exciting opportunity to model the atmospheres and formation histories of these planets in more detail in the near future, and are complementary to future higher-resolution, continuum-subtracted JWST spectroscopy.

William O. Balmer, Jens Kammerer, Laurent Pueyo, Marshall D. Perrin, Julien H. Girard, Jarron M. Leisenring, Kellen Lawson, Henry Dennen, Roeland P. van der Marel, Charles A. Beichman, Geoffrey Bryden, Jorge Llop-Sayson, Jeff A. Valenti, Joshua D. Lothringer, Nikole K. Lewis, Mathilde Mâlin, Isabel Rebollido, Emily Rickman, Kielan K. W. Hoch, Rémi Soummer, Mark Clampin, C. Matt Mountain

Comments: Published in AJ, associated with press release this https URL. 32 pages, 16 figures, 4 appendices, 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2503.13608 [astro-ph.EP] (or arXiv:2503.13608v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.13608
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Related DOI:
https://doi.org/10.3847/1538-3881/adb1c6
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Submission history
From: William Balmer
[v1] Mon, 17 Mar 2025 18:00:48 UTC (11,321 KB)
https://arxiv.org/abs/2503 .13608
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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