In the Legendary ending of Halo 3, the rear section of the UNSC Frigate Forward Unto Dawn containing Master Chief and Cortana, is seen drifting towards an unknown planet. Seconds later, a light shined across the surface of the planet. If a user freezes the cutscene, they can see that there is a large Marathon symbol on the surface of the planet.
Speculation
There are many speculations on what this unknown planet might be. Some people believe it is Onyx, although its destruction in Ghosts of Onyx makes this theory highly unlikely.
Another speculation of this planet is that it could be Tau Ceti IV and that the cyborg portrayed in the video game Marathon is actually the future Master Chief. This is a highly popular theory, as well as following up with all Halo canon to make this theory plausible in the Halo Universe. No one is entirely sure what that planet is.