Phalanx is a building that you can construct only on Moons.
You can use Phalanx to detect fleet movement of a planet.
Sample screenshot:

1.Go to a moon where you have a phalanx through Overview (click moon icon) or Buildings list at top navigaton bar.
2.Once you are to the moon, go to Galaxy.
3.You can now scan (Phalanx Level * Phalanx Level - 1) systems away (You cannot phalanx to other galaxies)
All fleets from or to this planet will appear in your phalanx.
It costs 5000 deuterium per usage.
Limitations:
-Only planets can be scanned. You cannot scan a moon or outer space (expedition slot).
-Missions towards or from a MOON are completely hidden at all times.
Notes:
-You do not get a warning if someone has phalanxed you.
-Keep an eye for enemy phalanxes by spying their moons to see the level of their phalanx.
-If someone fleet saves from a moon to a df, and the moon is destroyed, the fleet will then show on planet phalanx.
(If you still have questions, please post below)