Net.mesis, the Invisible Sun, orbits the sun of the Loonivearth, far beyond the Oort Cloud, high above the plane of the ecliptic. It has a highly elliptical polar orbit 26 million years long. When Net.mesis approaches close, it disrupts the Oort Cloud, sending comets screaming into our solar system and causing periodic cataclysms, one of which killed the dinosaurs. (In one version of Hypertext Time, anyway.)
Net.mesis is a pulsar, sending out a blaze of light and energy every 1.23 seconds. Said blaze runs parallel to the solar system's orbital plane, making it undetectable from Earth.
The planet Nox orbits Net.mesis.