In order to find a suitable planet for colonization you have to explore nearby systems. You can do this two ways: the first one is by pressing Explore from left menu and the second one is exploring directly from the system view.
Humans use satellites for explorations.
EXPLORE SCREEN
EXPLORING FROM SYSTEM SCREEN
Action result message. It will tell if exploring is possible or not.
Needed resources and information regarding the Explore action.
To explore a system you must provide its valid coordinates.
A list with exploring satellites already sent to star systems.
NOTE: Exploring from the System screen does not require system coordinates - they will be automatically filled.
To explore more than one system you may use group exploring. Specify desired number of systems you want to be explored and press the Send Satellites button.