Planets beyond Neptune

Lore & Background

Following the discovery of Neptune in 1846, astronomers noted residual discrepancies in the orbits of Uranus and Neptune. In 1848, Jacques Babinet proposed a planet named 'Hyperion' of roughly 12 Earth masses at 48 AU, but Urbain Le Verrier dismissed the idea. In 1879, Camille Flammarion and George Forbes used comet aphelia to suggest two trans-Neptunian planets, though skeptics questioned the data. Percival Lowell's Planet X hypothesis, pursued from the early 20th century, led to Clyde Tombaugh's discovery of Pluto in 1930, which initially seemed to validate the theory.

Reader's Guide

The search for planets beyond Neptune reflects the evolution of planetary science and the challenges of detecting distant objects. Pluto's discovery appeared to confirm Lowell's hypothesis, but by 1978 it was found too small to affect giant planets, prompting a brief search for a tenth planet. The search largely ended in the early 1990s when Voyager 2 data showed that Uranus's orbital irregularities were due to an overestimation of Neptune's mass. After 1992, discoveries of numerous icy trans-Neptunian objects led to the IAU's 2006 reclassification of Pluto as a dwarf planet. The concept revived in 2014 and 2016 with the hypothesis of Planet Nine, a super-Earth or ice giant on an inclined, eccentric orbit, based on clustered extreme trans-Neptunian objects. Observations with WISE have ruled out Saturn-sized objects out to 10,000 AU and Jupiter-sized objects out to 26,000 AU.

Did You Know?

• In 1850, James Ferguson 'lost' a star that was briefly thought to be a new planet, but it was later shown to be human error.
• In 1911, Indian astronomer Venkatesh Ketakar predicted a trans-Neptunian planet named Brahma with a mean distance close to Pluto's.
• The 2016 Planet Nine hypothesis predicts an orbit anti-aligned to clustered extreme trans-Neptunian objects.
• As of March 2014, WISE observations ruled out a Saturn-sized object out to 10,000 AU.