“I think the sense of excitement stems in part from the timing of these discoveries,” Dr. Laughlin said. Oumuamua and Borisov, he added, augur well for a new telescope the National Science Foundation is building in Chile called the Large Synoptic Survey Telescope, which will sweep the entire sky every few days, producing in effect a movie of the universe.

That telescope will be superbly positioned to find more interloper comets, perhaps even in time to send probes to greet them with Deep Impact-style missions. “The situation is reminiscent of when the first exoplanets were detected,” Dr. Laughlin said.

That discovery occurred in 1995, shortly before the Spitzer Space Telescope, which was built without exoplanets in mind, was launched.

Astronomers have long suspected that if anything came calling from another star system, it would be comets. New stars and planetary systems are surrounded by vast clouds of icy leftover fragments, so the story goes. These snowballs are easily dislodged by passing stars and knocked hither and fro — many inward toward their mother star and its planets, but others outward across the galaxy.

Until now, astronomers have lacked telescopes big and sensitive enough to detect them. Now, with telescopes like the Large Synoptic Survey Telescope and the Pan-STARRS in Hawaii, which discovered Oumuamua, they do.