U.S. astronomers Tuesday said they have discovered two new planets beyond the solar system, creating a new category in extra-solar planets.

The National Science Foundation and National Aeronautics and Space Administration sponsored the team of scientists who made the discovery.

Geoffrey Marcy, of the University of California-Berkeley, said the new planets are about the size of Neptune -- which has 17 times the mass of Earth. Even still, compared to the other 120-plus extra-solar planets discovered so far, these two are rather tiny.

He said the new planets may just be the beginning -- that there are many more smaller discoveries to come even though these lower-mass planets are more difficult to find.

On Aug. 25, European astronomers said they had preliminary evidence of a third Neptune-sized extra-solar planet.

Instead of trying to get an actual image of the new planets -- even the biggest extra-solar planet is too far away for that -- the astronomers monitored candidate stars, looking for wobbles that might be caused by the gravitational pull of an orbiting planet.