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
Solar System unmapped. There are also ongoing studies of the region between Mercury and the Sun. Objects may yet be discovered in the Solar System's uncharted regions. Currently, the furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun, but as the Oort cloud becomes better known, this may change. The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 100 billion stars. The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. The Sun