The planets, called Kepler-20e and Kepler-20f, are the smallest planets outside the solar system confirmed around a star like the Sun, NASA said in a statement.
The planets are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface.
"This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them," Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said in the statement.
The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth.
Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets are in a five-planet system called Kepler-20, about 1,000 light-years away in the constellation Lyra.
Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days.
Kepler-20f, at 800 degrees Fahrenheit, is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit, would melt glass.
The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars as planets cross in front their stars.
NASA is an acronym for National Aeronautics and Space Administration.