NASA has announced the discovery of 461 new planets, and among the 461 planets found, four could potentially hold liquid water. Now some NASA scientists say finding a planet much like Earth is just a matter of time.

NASA’s Kepler mission, whose goal is to find Earth-like planets orbiting other stars, has found 461 new potential planets, NASA announced Monday. The data were collected by Kepler between May 2009 and 2011, and more observations are needed to confirm the findings.

Christopher Burke, from the SETI Institute in Mountain View, Calif., was the lead scientist in charge of analyzing the data collected by Kepler’s telescope and said of the discovery, “There is no better way to kick off the start of the Kepler extended mission than to discover more possible outposts on the frontier of potentially life-bearing worlds.”

In order to discover potential planets, Kepler’s telescope analyzes the changes of brightness of a host star. Objects, such as planets or debris, will pass in front of the host star, changing its brightness as seen from Earth. This is called a transit, and three such transits have to be observed before the object is classified as a potential planet.

Once an object is classified as a potential planet, scientists conduct further observations to make sure. Including this recent discovery, NASA’s Kepler mission has discovered 2,740 potential planets that orbit 2,036 stars. Of all the potential planets, 1,290 are roughly the size of Neptune, 816 are “Super-Earth” size, roughly 1.25 to 2 times the diameter of Earth, 351 are roughly the size of Earth, 202 are similar in size to Jupiter and 81 are even larger than that. Smaller planets are harder to see, of course.

What is exciting NASA about the latest discovery is all the potential planets that are similar to or smaller than Earth as well as the number of host stars with multiple potential planets.

Jack Lissauer, a planetary scientist from NASA's Ames Research Center in Moffett Field, Calif., said, “The large number of multi-candidate systems being found by Kepler implies that a substantial fraction of exoplanets reside in flat multi-planet systems.” According to Lissauer, potential Earth-like planets are being discovered in systems much like our own, with several planets orbiting a host star.

Of the 461 planets found, four could potentially have liquid water as they orbit their host star in the “Goldilocks zone,” where conditions are "just right."

According to Steve Howell, a Kepler mission project scientist at Ames Research Center, finding a truly Earth-like planet is a real possibility. Howell says, “The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits, orbital periods similar to Earth's. It is no longer a question of will we find a true Earth analogue, but a question of when.”