UFOs are bulky and round-shaped space ships. The most common ones are purple and blue with flashing yellow and white lines. There is also a second type of UFOs that is blue with flashing red lines. It is unknown who created the UFOs, but it is known that they contain items.
Sonic the Hedgehog CD
In the Special Stages of Sonic the Hedgehog CD, the player's goal is to destroy all six UFOs in each arena within a hundred seconds or they will get a Time Over. Here, UFOs hover around constantly and Sonic has to Spin Jump into them to destroy them. He then either gets a Super Ring (ten bonus Rings) or Sneakers. However, the UFOs can be tricky to hit as they can perform sharp turns that makes it hard for the player to target them.
Every time the timer hits the twenty second point, a blue UFO appears in the Special Stage that grants a Time Bonus (increases the timer thirty extra seconds) after destroying it. After destroying all UFOs, a Time Stone falls into Sonic's possession.
Like in Sonic the Hedgehog CD, UFOs in this game appear exclusively in the Special Stages. This time around, only one UFO appear in each Special Stage, and each of them is holding one of the Chaos Emeralds in their claws. These UFOs are racing down the paths in the Special Stages, and it is the player's objective to chase them down to obtain their Chaos Emerald. However, the UFOs can be tricky to chase down as they can perform sharp turns that are hard for the player to mimic.
In very early screenshots for the original Sonic the Hedgehog, numerous UFOs fly around in the sky of Marble Zone. This is never seen in the final version of the game. However, the 2005 the mobile phone port has one frame of the UFO inside the game's code. In the 2013 re-release version of the original Sonic the Hedgehog, UFOs will fly around in the sky by activating the object's placement with Debug Mode in Marble Zone.
When the player has completed Sonic the Hedgehog CD by getting the good ending, the game unlocks the DA Garden feature. In the background of DA Garden, there are numerous objects and characters flying across the screen. One of them is a green and yellow-flashing space shuttle that flies in a zigzag format.
In the 2011 version of Sonic the Hedgehog CD, the UFOs that carry the Speed Shoes power-up are colored blue, most likely to better differentiate between the purple ones, which carry the Ring power-up. In the original, this difference was only visible from a short distance, with gold stripes indicating rings and silver indicating the Speed Shoes.