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Abstract We report initial observations aimed at the characterization of a third interstellar object. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity ofe ∼ 6.1, perihelion ofq ∼ 1.36 au, inclination of ∼175°, and hyperbolic velocity ofV∞ ∼ 58 km s−1. We report deep stacked images obtained using the Canada–France–Hawaii Telescope and the Very Large Telescope that resolve a compact coma. Using images obtained from several smaller ground-based telescopes, we find minimal light-curve variation for the object over a ∼4 day time span. The visible/near-infrared spectral slope of the object is 17.1% ± 0.2%/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids). Moreover, 3I/ATLAS will be observable through early 2025 September, then unobservable by Earth-based observatories near perihelion due to low solar elongation. It will be observable again from the ground in late 2025 November. Although this limitation unfortunately prohibits detailed Earth-based observations at perihelion when the activity of 3I/ATLAS is likely to peak, spacecraft at Mars could be used to make valuable observations at this time.
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This content will become publicly available on August 28, 2026
The Kinematic Age of 3I/ATLAS and Its Implications for Early Planet Formation
Abstract The recent discovery of the third interstellar object (3I/ATLAS) expands the known census from two to three and significantly improves statistical inferences regarding the underlying Galactic population. In this Letter, we argue that cometary activity likely significantly contributes to 3I/ATLAS’s brightness since the nuclear size inferred when assuming an asteroidal reflectance implies an untenable interstellar object mass per star. Relative to the Sun, 3I/ATLAS exhibits a high excess velocity ofv∞= 58 km s−1, which implies that 3I/ATLAS is relatively old in comparison to previous interstellar objects. Here, we calculate the posterior distribution of ages implied by the kinematics of the interstellar objects and find that 3I/ATLAS is likely ∼3–11 Gyr old, assuming that the interstellar object and stellar age–velocity dispersion relations are equivalent. We also calculate the distribution of host star metallicities and find that 3I/ATLAS has a 12% chance of originating from a star with [Fe/H] ≤ −0.4. These results show that interstellar object formation is likely efficient at low metallicities and early in the history of the Galaxy. Finally, we estimate the interstellar object formation rate throughout Galactic history implied by these three objects. As future interstellar objects are discovered, the framework presented here can be applied to further refine this calculation. Comparison between the interstellar object and stellar formation histories will provide unique insights into the history of stellar system formation in the Galaxy.
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- Award ID(s):
- 2303553
- PAR ID:
- 10646049
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 990
- Issue:
- 1
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L14
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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