The planets in Planetarion are not just scattered throughout the universe. They are clustered into groups called Galaxies. Every galaxy has a similar number of planets in it, starting out at about 10 planets and increasing as the round progresses and more planets join.

Galaxy Politics

Minister of Development (yellow)

Minister of Communication (light blue)

Minister of War (red)

The Minister of war is responsible for arranging attacks and defences in a galaxy. He's generally the one, together with the Minister of Communication, that organises contact information on the forums and overview for defence. He may often also look for galaxies for his own galaxy to attack.

Note
Not all galaxies follow these ideas. Some planets play with friends and alliances as much or more than their own galaxy mates (especially for attacking). Therefore it's a good idea to also have an alliance.
The bottom line for defence and organisation is your galaxy, and generally "galaxy mates" know each other quite well by the end of the round. Communication is essential, which is why many galaxies use a form of communication such as IRC where they have their own private channel to discuss and organise galaxy issues. Mobile phones are often also used in emergencies, such as when the galaxy is being heavily attacked (although giving your mobile phone out is always optional).
It's generally up to the galaxy how much they play for fun and how active they are, as in any galaxy there's some people on 18 hours a day and some only 2-4 hours. As there's only a limited amount of people in a galaxy, some of the more serious players are determined to get active players, but even the smaller less active galaxies often have a certain amount of fun.

The galaxy is under attack !

When the galaxy is under attack, it is time to show what you are made of.
A lot of the planets will receive defense from their alliance. However, if a planet in an alliance is offline, then the alliance must be told before they can help (logical stuff huh?).
Alliances usually tell their members how to let the galaxy mates report incoming attacks. Some alliances let their members goto a channel on IRC and report it in PM (Private message). Others have more complicated ways. Either way, if a member of your galaxy is in an alliance, you should be told how to tell the alliance when someone is being attacked.
Sometimes in the round there will be uncovered planets, meaning they do not have (enough) ships defending them to make the attackers recall. This is where the galaxy really comes to life: you can send ships to each other, because your ETA is 5, which is much lower then any planet can get thats not in your galaxy. You can do a number things now: sit still and do nothing or prepare that defense fleet and help him or her. Sitting still doesn't do much good: you won't be very popular if you repeatedly refuse to defend and your galaxy will weaken because of the losses. You might get exiled or they will simply not defend you either. In the end they will not even bother to report your incoming hostile fleet(s) to YOUR alliance if you have one.
Reading the above must result into a simple conclusion: your galaxy will do better if you work as a team. If you are in an alliance, then defending your galaxy mates might be frowned upon by the alliance leaders: that defense fleet could have gone to a fellow alliance member, and maybe you are defending an alliance enemy. You might even be defending against your own alliance!