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That small dot moving left to right is 3I/ATLAS. It's a comet that was discovered on 1 July 2025. It's travelling at 58 km PER SECOND and isn't gravitationally bound to the Sun. Which means you're looking at a comet that formed around another star, passing through the Solar System.

3I/ATLAS was discovered with the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope in Chile. Its nucleus size is uncertain because of the comet's coma, but is likely substantially smaller than 24 km in diameter.

3I/ATLAS is the third confirmed extrasolar object in the Solar System, after 1I/ʻOumuamua and 2I/Borisov. There are likely very, very many more, but they're exceptionally tricky to detect. Similarly, we can expect comets born around the Sun to be passing through other planetary systems.

With RUBIN up and running, I wonder how many more of these we'll detect.