On Archive.org someone has uploaded an old build of PolarisOS. This is a stripped-down version of Windows 10 intended to run on low-cost computers with ARM architecture. The early version does not contain a user interface.
User scamdisk has uploaded PolarisOS Build 16299 to Archive.org. In the description, he mentions that PolarisOS is a cancelled edition of Windows Core OS, intended for laptops and desktops. There is no shell, and the system is unable to load one.
In order to boot PolarisOS, drivers must first be sideloaded. The uploader gives some basic instructions to get the system running on a tablet or phone with an ARM processor.
PolarisOS is an early version of Windows Core OS. It is a Windows version where a lot of legacy code has been removed to make the system run faster on slower hardware. It should also improve the battery life of mobile devices.
Windows Core OS has now been scrapped, but Microsoft has worked on many similar projects over the years. In 2012 it released Windows RT, a stripped-down version of Windows 8 for ARM tablets, and, according to persistent rumours, Microsoft will release Windows 10X very soon. This is also a lighter version of Windows, with the intention of competing with ChromeOS.