The Cheops mission -- for CHaracterising ExOPlanets Satellite -- will launch in 2017, a release from ESA headquarters in Paris said Friday.
The satellite will monitor stars' brightness for telltale signs of a "transit" as a planet passes briefly across a star's face.
This will allow an accurate measurement of the radius of the planet and help scientists understand more about the formation of planets from a few times the mass of Earth -- "super-Earths" -- up to Neptune-sized worlds, the ESA said.
It will also identify planets with significant atmospheres, it said.
"By concentrating on specific known exoplanet host stars, Cheops will enable scientists to conduct comparative studies of planets down to the mass of Earth with a precision that simply cannot be achieved from the ground," said Professor Alvaro Gimenez-Canete, ESA Director of Science and Robotic Exploration.
Cheops will operate in low-Earth orbit at an altitude of 500 miles, with a planned mission lifetime of 3.5 years, the ESA said.