The Titius–Bode law (sometimes termed just Bode’s law) is a hypothesis that the bodies in some orbital systems, including the Sun’s, orbit at semi-major axes in a function of planetary sequence. The hypothesis correctly predicted the orbits of Ceres and Uranus, but failed as a predictor of Neptune’s orbit.

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When originally published, the law was approximately satisfied by all the known planets — Mercury through Saturn — with a gap between the fourth and fifth planets. It was regarded as interesting, but of no great importance until the discovery of Uranus in 1781 which happens to fit neatly into the series. Based on this discovery, Bode urged a search for a fifth planet. Ceres, the largest object in the asteroid belt, was found at Bode’s predicted position in 1801. Bode’s law was then widely accepted until Neptune was discovered in 1846 and found not to satisfy Bode’s law. Simultaneously, the large number of known asteroids in the belt resulted in Ceres no longer being considered a planet at that time. Bode’s law was discussed as an example of fallacious reasoning by the astronomer and logician Charles Sanders Peirce in 1898.^[3]

The discovery of Pluto in 1930 confounded the issue still further. While nowhere near its position as predicted by Bode’s law, it was roughly at the position the law had predicted for Neptune. However, the subsequent discovery of the Kuiper belt, and in particular of the object Eris, which is larger than Pluto yet does not fit Bode’s law, have further discredited the formula.