Here's a brain dump of what I learned after talking to a couple of sources of mine who spoke on the condition of anonymity, but whom I believe are in the know about the project.
Jupiter is going to be a new user interface (UI) library for Windows, built alongside Windows 8. It will be a thin XAML/UI layer on top of Windows application programming interfaces and frameworks for subsystems like graphics, text and input. The idea is Jupiter will bring support for smoother and more fluid animation, rich typography, and new media capabilities to Windows 8 devices. (Not surprisingly, the more fluid UI capabilities also are on the feature set list for Silverlight 5.)
The high-level goal for Jupiter is to help Microsoft revitalize a world where developers write applications tailored for a specific platform. The days of "killer apps" optimized for Windows driving demand for Windows PCs are waning (if not already long gone). Microsoft's hope with Jupiter is to provide Microsoft and third-party developers with a new framework, plus the next versions of Microsoft's various development tools, to build what Microsoft is calling "immersive" applications.
Immersive apps are not meant to be Windows desktop apps. Nor are they necessarily pure Web apps. They are applications that will be built using C#, Visual Basic (and maybe C++). These apps will be developed using the new Windows 8 app model and take advantage of its inherent servicing and packaging technologies and that will be available via the anticipated Windows 8 app store.
Because Jupiter will be built off the same core XAML technology used in Windows Phone and Silverlight, there's a good chance some of the Silverlight code developers already have written will be able to be reused to develop this new class of apps. Does this mean Windows Phone apps will automatically work on Windows 8 and be available from the Windows 8 app store? I don't know but I am doubtful.
One of my contacts described Jupiter this way: "It has to do with XAML + Native Code on slate/iPad-like devices. I think this is Microsoft's approach for putting Windows on the smaller device without the bloat."
For now, Jupiter is supposedly a Windows 8 thing only, but could potentially be adapted to work with older versions of Windows and maybe Windows Embedded operating systems, as well. Jupiter will actually ship as part of Windows 8, I am hearing from my contacts. A subset of Jupiter also will ship as part of a future version of the .Net Framework, according to what my sources said of Microsoft's plans.
Microsoft officials are not commenting on Jupiter. That's not too surprising, as we heard from Microsoft execs at this week's Consumer Electronics Show, they aren't even willing to acknowledge that Windows 8 is what they're calling the next version of Windows....
Any Windows, Windows Phone and/or Silverlight developers out there have any thoughts to add (or questions to ask) about Jupiter? I, for one, am curious whether Jupiter will be part of Windows 8 on both the newly announced SoC ARM/AMD/Intel systems and existing generation of 32/64-bit PCs or not...
Update (January 7): Soma Somasegar, Senior Vice President of Microsoft's Developer Division, responded directly to me on Friday with a comment on this post. He reiterated that Microsoft is not yet ready to talk about the next version of Windows, but did say that "some of the information in this post is not right and out of date, not reflecting Microsoft's current thinking." When I asked for more information about which parts of this information were incorrect, Somasegar declined to comment.