From what we heard last year, Google was speculated to be working on a third OS called Fuchsia. There was only slight information at that time and we knew the fact that it exists, it would be open source and could run on everything from embedded devices to smartphones and PCs.
Android Police had done some investigation and found that the OS is based on Magenta Kernel, which is in-turn based on the LittleKernel project. Both these kernels were designed to run on embedded devices. The UI is based on Flutter, which is “a heavily optimized, mobile-first 2D rendering engine (with excellent support for text)” and a little more than that. Apparently, Google decided to go with Dart as the programming language. Android Police also pointed out that the OS uses Escher, a rendering engine with full OpenGL and Vulkan support.