Planet Alpha

In chapter 3 shortly after discovering

you will be in an open area standing on stone and having a stone pillar in the background which has a round opening. The pillar is lighting up as soon as you are standing on the glowing plattform where you can use your power of changing day/night. Press or to change the stars to match it to the symbols on the pillar.
After that run to the right. You have to fall down and then turn left until you see an glowing orb. This orb will take to a parallel universe which is dark and without life and plants. It also has less gravity than the "Planet Alpha" world. Get all the way to the end of that universe until you stand within 4 glowing pyramids. These will then form a new pyramide and an object above your head. Jump and pick it up. It will be your first artefact.
Bingo, achievement "Exploring the Unknown" unlocked.

For those who want a video solution, I found a full walkthrough on YouTube. So credit for the uploader "Virtual Conflux" for that video. The timestamp for this achievement is at 26:00 min.