The Sims 4 Collectibles: Aliens
Aliens are rewarded to Sims who travel to space. Traveling to space requires you to purchase a Rocketship in buy mode, and have your Sims build it from scratch. Once you have a working rocketship, your Sims will need to explore space and complete text adventures. Collecting all aliens requires plenty of trips to space.
❗ Upgrading the Rocketship will help complete your Alien collection faster.