The UFO
An UFO is a clickable that comes to visit every 10 hours, real-time. It appears on the space between the Earth and the Moon and stays only for 15 minutes. There is an associated quest for finding it.
You can check for the time of its next arrival with this calculator
(orginal this is the original link but because of it having no file prefix it doesn't work properly so the link above is it with the file prefix)
Important Note[]
For the moment there is a difficulty clicking on the UFO because out of its 4 frames only 1 is clickable. If you can't get the achievement after trying to click it try rebooting your game or scrolling a little where the UFO is and try clicking on it as soon as you stop scrolling.
Bug[]
There is a known bug on the web version of the game where the UFO won't load. You can see a report on the discord
The bug can be bypassed to trigger the quest completion by randomly clicking around the section where the UFO should be.
If clicking on the UFO itself doesn't trigger the quest completion, try clicking around, it has a weird hitbox.