You need first to build a Comms Satellite.
Your planet's Communication Menu gives you the ability to scan planets:
The Resource Scan lists detailed planet resource and space information. It includes the planets abundance percentages of resources and the planets available land and orbital space.
( 500 per scan.)
The Surface Scan lists all the information provided in the Resource Scan in addition to all buildings, Colonists and Soldiers currently stationed on the planet. These informations are useful if you want to invade a planet.
( 1000 per scan.)
The Fleet Scan lists all ships currently orbiting the planet or going to the planet. These informations are useful for space combat.
( 2000 per scan.)