What does this circular icon mean?

Sometimes I get this reward, but I have no idea what it means.

Hovering over the icon doesn't reveal a tool-tip.

That icon indicates ship or planet sensor range. It denotes how far away you can see enemy ships and colonies. The higher, the better. There is no downside to having larger sensor ranges (other than improvement maintenance costs).

Ships can get sensor package upgrades, which will allow them to see further, and make ideal upgrades for scout ships. These upgrades increase sensor range by:

• Long Range Sensors: +4
• Brainwave Sensors: +6
• Dusty Sensors: +8

Star Systems also have sensor upgrade packages, which the primary use of are for early detection and warning systems. The further away you can detect incoming enemy ships, the more time you have to move ships to defend the planet. These upgrades each provide 10% to sensor range:

• Collaborator Networks
• Signal Laser Array

Sophons also get a racial unique upgrade that provides +20% to sensor range: the Predictive Sensor Array.

Your heroes can also help increase the sensor range of planets with Abilities:

• Security Specialist: +4 to Star Systems
• Foresight: +2 to both Fleets and Star Systems (Only the applicable bonus is applied; can't use both at the same time with the same hero)

I couldn't find any data about star system's initial range, unfortunately. Each planet should have the same range, sans improvements/hero abilities, once they become full colonies. Note that planet influence increases with population; that's the colored circle around your planet; sensor range is the colorless circle around ships and planets.