code:

double colonyDesirablility = 1.0;

if (coloniesAtThisStar > 0)
{
   if (enemyColoniesAtThisStar > 0)
   {
      colonyDesirablility *= 1.5; //Oh, this is a contested system?  Contest it harder!
   }
   else
   {
      colonyDesirablility *= 0.8; //We've already claimed this system.
   }
}
else if (enemyColoniesAtThisStar > 0)
{
   colonyDesirablility *= 0.3; //This is someone else's system.  Colonizing here would be impolite.
}
else if (areAllConnectedStarsOurs)
{
   colonyDesirablility *= 0.9; //Let's try to expand to new horizons, not backfill old ones.
}
else
{
   colonyDesirablility *= 0.5; //Then this is unclaimed frontier, which could be vulnerable to attack.
   if (numberOfEnemyStarsThatWouldBeEnclosedByThisSystem > 0)
   {
      //It would be really, REALLY impolite to cut our enemy's colonies off from them.  And our maps wouldn't look so pretty.
      colonyDesirablility *= pow(0.3, numberOfEnemyStarsThatWouldBeEnclosedByThisSystem);
   }
   else
   {
      colonyDesirablility *= (1.0 +
         (numberOfUnclaimedStarsThatWouldBeEnclosedByThisSystem * 0.25) +  //Stars that would be ours without burning resources to colonize them?  I'm in!
         (double)numberOfOurStarsThatWouldBeEnclosedByThisSystem +  //More secure systems, you say?
         (isThisStarConnectedToOurHomeStar ? 1.0 : 0.0));  //Home system.  Kinda important to defend.
   }
}
colonyDesirablility *= colonyDesirablility;  //Square it to increase the gulf between desirable and undesirable planets.