The planets around a star dubbed Kepler-36A occupy nearly the same orbital plane and sometimes approach within about 1.2 million miles of each other, just five times the Earth-moon distance.
"These are the closest two planets to one another that have ever been found," University of Washington astronomer Eric Agol said. "The bigger planet is pushing the smaller planet around more, so the smaller planet is harder to find."
The planets are designated Kepler-36b and Kepler-36c, with Planet b being a rocky planet like Earth, though 4.5 times more massive and with a radius 1.5 times greater.
Kepler-36c, which could be either gaseous like Jupiter or watery, is 8.1 times more massive than Earth and has a radius 3.7 times greater.
The larger planet was originally spotted in data from NASA's Kepler satellite, and researchers used a mathematical algorithm to analyze the data for a possible second planet.
"We found this one on a first quick look," Joshua Carter of the Harvard-Smithsonian Center for Astrophysics said.
"We're now combing through the Kepler data to try to locate more."