Fictional planets of the Solar System 40 Eridani A. Counter-Earth was a hypothetical planet sharing an orbit with Earth, but on the opposite side of the Sun (hence Earth and Counter-Earth would always be invisible to each other). The idea of a counter-Earth has never been a serious scientific hypothesis in modern times. Phaëton is a name given to a supposed planet existing in the past between the orbits of Mars and Jupiter, which no longer exists, having become the Solar System's asteroid belt. Proposed not long after the discovery of multiple asteroids at the beginning of the 19th century, the idea that the asteroids were
Pioneer Terra of a probe and orbiter to Venus. Pioneer Terra Pioneer Terra is a region on the dwarf planet Pluto, north of Tombaugh Regio and east of Voyager Terra. It was discovered by the "New Horizons" spacecraft on 14 July 2015. It is named for the "Pioneer" program, which included "Pioneer 10" and "Pioneer 11", the first spacecraft to cross the asteroid belt and explore Jupiter and Saturn, as well as the first to explore the outer solar system; "Pioneer 6, 7, 8, and 9", which formed an "interplanetary weather network"; "Pioneer 1" through "5", which explored the Moon in various