We just spotted only the third object ever to wander into our solar system from another star. It’s a total cosmic tourist.
March 27, 2026
Original Paper
A post-perihelion constraint on the CO$_{2}$/H$_{2}$O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines
arXiv · 2603.25002
The Takeaway
Following the famous 'Oumuamua and Comet Borisov, this new object (3I/ATLAS) was born in a different star system. Its unusual chemical makeup, which is extremely rich in carbon dioxide, gives scientists a rare direct look at the 'blueprints' used to build planets in distant parts of the galaxy.
From the abstract
We present high-resolution optical spectroscopy of interstellar comet 3I/ATLAS (C/2025 N1) obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope on UT 2026 January 7, when the comet was on its outbound trajectory at a heliocentric distance of $r_{\mathrm{h}} = 2.87$ au. The spectra cover the forbidden atomic oxygen lines, [O~I], at 557.7, 630.0, and 636.4 nm. The [O~I] red-doublet intensity ratio $I_{630.0}/I_{636.4} = 2.91 \pm 0.21$ matches the optically thin branching