This is the greatest number of exoplanets astronomers have found in the habitable zone of a single star.

Three of the planets exist in the "habitable zone," the orbital region around the star where conditions most likely could support life.

NASA's Spitzer Space Telescope has identified seven Earth-sized planets orbiting a star 40 light-years away, and all of them could host water or life.

"The discovery gives us a hint that finding the next Earth is not a question of if, but of when," said NASA Science Directorate head Thomas Zurbuchen, in a press conference Wednesday.

The findings were also published in the journal Nature. The study's lead author was Michael Gillon, a researcher from the Université de Liège in Belgium

Over the last 20 years or so, astronomers have discovered thousands of exoplanets using higher powered telescopes, but only a few among them appear to be habitable planets.

The planets orbit a star called TRAPPIST-1, an ultra-cool red dwarf star far smaller than Earth's sun. The planets in the system are far closer to the star than planets in our own solar system.

The location of these planets makes them especially good candidates for further study. The Hubble Space Telescope has begun looking at four of the planets, including the three inside the habitable zone.

"This is the most exciting result I have seen in the 14 years of Spitzer operations," said Sean Carey, manager of NASA's Spitzer Science Center at Caltech/IPAC in California, in a news release accompanying the announcement. "Spitzer will follow up in the fall to further refine our understanding of these planets so that the James Webb Space Telescope can follow up. More observations of the system are sure to reveal more secrets."