Catching up with arXiv.org‘s posts:
Earth and Planetary Astrophysics > arxiv.org/abs/2601.06759
Lisse, C. M. Bach, Y. P. Bryan, S. A. et al. SPHEREx Re-Observation of Interstellar Object 3I/ATLAS in December 2025: Detection of Increased Post-Perihelion Activity, Refractory Coma Dust, and New Coma Gas Species
11 Jan 2026
arxiv.org/abs/2601.08591
Choi, S. Ishigura, M. Takahashi, J. et al. Dust Properties of the Interstellar Object 3I/ATLAS Revealed by Optical and Near-Infrared Polarimetry
13 Jan 2026
Spoiler alert: the “New Gas Species” detected at the interstellar object are new to ATLAS (since detection 6-1/2 months ago), not new to comet-kind. Yes, the comet is a comet.
The beauty of SPHEREx is that it catches targets without even trying. Its wide field of view sweeps the sky, directed or not, and takes infrared data on whatever’s there. Based on the sweeping pattern, it happened to catch ATLAS soon after discovery; based on the continued sweep, ATLAS just re-entered the SPHEREx field of view. And what do we see now? Back in Summer 2025, the comet was far, cold, and dim. Now, the object has passed perihelion, warmed up, and grown much more active (well, for ATLAS- it was never a showy comet). The new gases detected by SPHEREx are CN (cyanide- not HCN) and the marker for organic chemicals, the C-H bond. Both such volatiles are common in comets, but apparently ATLAS’ outer mantle is depleted in them; we’re just seeing them now.
Polarimetry, now that’s a different take. Polarimeter instruments are sensitive to light scattered off dust particles; Choi et al. use this feature to infer the nature of grains in 3I’s coma… to the extent they can. Again, ATLAS is not that active a comet, isn’t shedding much, and makes a dim target.
Standard disclaimer: papers posted to arXiv are not at the same level as refereed science journals. Either way, I don’t think “here’s what we saw” is some bold claim that demands- requires- the same level of scrutiny as other things we’ve heard.