Can I run Ubuntu on something other than Linux Kernel?

Solution 1:

TL;DR

You cannot remove the Linux kernel from Ubuntu, without irrevocably crashing your entire system but in the future you might be able to replace it.

The long answer:

This is due to the history of GNU/Linux:

Once upon a time, there was a guy called RMS that wanted to keep software to be free¹. He ended up publishing the source of his software (what we call "Open-Sourcing" today) and eventually this all became the GNU Core Utilities after a few other people helped him out.

However, these utilities missed just one little thing to be 100% free and that was a Kernel to run on as the GNU Utilities were running on UNIX System V and its clones and this is where we bring Linus Torvalds in: he developed a Monolithic Kernel² and published his source too with the understatement of the XXth century:

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.³

The kernel is the bit of software that talks to the hardware, so manages the Memory, Disks, Keyboard, Mouse, Screen, Display Adapter, Speakers, Webcam, ... so you cannot run any OS without some kind of kernel.

Fast forward a quarter of a century and the GNU Utilities still run on that same kernel (in the meantime named "Linux")

Ubuntu = Linux Kernel + GNU Core Utilities + a ton of other software.⁴

So: No, today you cannot delete the kernel, but in a few years when the GNU HURD from the Free Software Foundation's⁵ multiserver microkernel comes out of development⁶ you will be able to replace the Linux Kernel with the Hurd one but you can never run any OS without some kind of kernel.

Alternatively,

You can run the GNU Core Utilities on another OS entirely:

BSD or one of its variants: MacOS
Windows (32-bit only)

But these also have kernels, though none of the above use Linux kernels.

_{Note 1: "Free" as in Freedom, not Free beer ;-).}
_{Note 2: GNU Hurd took the decision to go for a multiserver microkernel because they wanted the kernel to be distributed amongst multiple machines because RAM was so expensive at the time.}
_{Note 3: OK, maybe Houston, we have a problem is the biggest understatement of the century...}
_{Note 4: I.E. All the software you install yourself. :-).}
_{Note 5: the FSF is the copyright holder of the GNU Core Utilities.}
_{Note 6: The GNU Hurd multiserver microkernel has been in development for 20+ years, so don't expect anything in the next couple of years (maybe decades)}
_{Note 7: Please be aware that changing Ubuntu's Linux Kernel would make the concoction you just created off-topic on this site! (As it would not be Ubuntu any more)}

Solution 2:

You have to have an Operating System. Because this site is called Ask Ubuntu your question must be reinterpreted to fit into the rules:

What operating systems (kernels) other than Linux Kernel will Ubuntu run on?

One answer would be Windows 10 with the WSL (Windows Subsystem for Linux) option turned on. You can also run most, if not all, the GNU utilities there. Extra steps are needed though to run the Ubuntu GUI desktop in Windows 10:

What's the easiest way to run GUI apps on Windows Subsystem for Linux as of 2018?

Solution 3:

Yes, such project already exists

UbuntuBSD brings together FreeBSD kernel and Ubuntu userspace. This project has been covered in several Linux online journals, including itsfoss. While the project has been discontinued, you can still use the last available version.

Please note that this distro would be off-topic here, as discussed multiple times on Meta site, since this is NOT an official Ubuntu flavor