"The discovery gives us a hint that finding a second Earth is not just a matter of if, but when," said Thomas Zurbuchen, associate administrator of the Science Mission Directorate at NASA Headquarters. "You can just imagine how many worlds are out there that have a shot at becoming a habitable ecosystem that we can explore."
Three of the newly discovered planets are in what's known as the habitable zone, the area around a star where a rocky planet is most likely to have liquid water, according to NASA. The Hubble Space Telescope has started to scan four of the planets, including the three found in the habitable zone.
The planets were found by astronomers using ground and space telescopes around an ultracool, red dwarf star named TRAPPIST-1, according to the European Organisation for Astronomical Research in the Southern Hemisphere (ESO).
The findings were published Wednesday in the scientific journal Nature. The paper's lead author, astronomer Michael Gillon, described the planets as "the seven wonders of TRAPPIST-1."
"The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our sun," said Amaury Triaud, one of the co-authors of the Nature paper. "Planets would need to be in far closer orbits than we see in the solar system if there is to be surface water. Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1."
Scientists characterized the discovery as a significant leap in the search for alien life during a news conference on Wednesday afternoon.
"Answering the question, 'Are we alone?' is a top science priority, and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal," Zurbuchen said.
The planets were spotted when scientists noticed dips in TRAPPIST-1's light output, according to ESO. The discovery was made using Spitzer, an infrared telescope that trails Earth as it orbits the sun, NASA officials said.
Astronomers determined that the dips were caused by the planets as they passed between the star and Spitzer.
The planets have been temporarily named with letters, TRAPPIST-1b through TRAPPIST-1h, in order of increasing distance from their parent star.