「ピンク惑星」の塩分環境に関する新発見（Famous ‘Pink Planet’ Harbors a Salty Surprise）

2026-06-18 ノースウェスタン大学

Discovered in 2013, the Pink Planet orbits a sun-like star located 57 light-years from Earth. At roughly 25 times the mass of Jupiter, it sits near the fuzzy boundary between giant planets and brown dwarfs. So, astronomers refer to it as a “planetary-mass companion,” meaning that it’s a planet-sized object orbiting a star. Illustration courtesy of NASA/Goddard Space Flight Center

＜関連情報＞

• https://news.northwestern.edu/stories/2026/06/famous-pink-planet-harbors-a-salty-surprise
• https://iopscience.iop.org/article/10.3847/1538-3881/ae6919

JWST-TST高コントラスト：GJ 504 bの初の直接分光観測により、雲と金属濃縮の可能性が明らかに JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Clouds and Possible Metal Enrichment

Aneesh Baburaj, Jean-Baptiste Ruffio, Marshall Perrin, Jerry W. Xuan, William O. Balmer, Yayaati Chachan, Quinn M. Konopacky, Travis S. Barman, Mathilde Mâlin, Kielan K. W. Hoch,…
The Astronomical Journal  Published: 2026 June 18
DOI:10.3847/1538-3881/ae6919

Abstract

Characterizing the coldest directly imaged companions through direct spectroscopy has only recently become possible with the James Webb Space Telescope. We present moderate-resolution (R ∼ 2700) spectroscopic observations of the directly imaged planetary-mass companion (PMC), GJ 504 b, using the JWST/NIRSpec. The coldest imaged PMC of the pre-JWST era GJ 504 b is too faint for ground-based spectroscopy, with only photometric observations possible. Leveraging advanced postprocessing techniques with a forward-modeling framework, we detect the companion at high signal-to-noise (S/N > 300). We also present the first successful point-spread function (PSF) subtraction with angular differential imaging (ADI) in the NIRSpec point cloud, detecting GJ 504 b at S/N > 10 and reaching contrast limits <10⁻⁴. The extracted 2.9–5.3 μm spectra show strong signatures of several molecular species, including H₂O, ¹²C¹⁶O, CH₄, CO₂, NH₃, H₂S, ¹³C¹⁶O, and ¹²C¹⁸O. Atmospheric modeling of the spectra using petitRADTRANS yields an effective temperature = 564 ± 4 K, surface gravity logg= 4.87^+0.13_-0.12, metallicity [M/H] = 0.67^+0.13_-0.12, C/O ratio = 0.64^+0.02_-0.02, interstellar ¹²C/¹³C and ¹⁶O/¹⁸O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2^+8.4_-6.0M_Jup, which is in agreement with the mass range (19–27 M_Jup) obtained from ATMO evolutionary models, implying an age of 2.5–4.0 Gyr. Lastly, we compare the abundances of GJ 504 b to its primary, obtaining a stellar abundance of sulfur (S), superstellar carbon (C), and, possibly, oxygen (O). The observed metal enrichment tentatively supports a planetlike formation, but does not entirely exclude stellar abundances for GJ 504 b.