Why The Solar System Has Eight Planets And Not Nine

On 24 August 2006, at the close of the International Astronomical Union’s General Assembly in Prague, 424 astronomers raised yellow cards to decide what a planet is. When student volunteers had counted the votes, the solar system had eight planets. Pluto, a planet since its discovery in 1930, was not one of them.

Image Via: @SETIInstitute on X

One of the most common questions about space, how many planets are in the solar system, was settled by a show of hands. Two decades later the definition still stands in the union’s rules and in textbooks worldwide. Some scientists reject it, and a telescope in Chile is now searching for a world that could make the count nine again.

What Forced The Vote

The trouble began at Palomar Observatory in California. On 21 October 2003, a telescope there photographed a faint point of light moving slowly far beyond Neptune. The astronomers Mike Brown, Chad Trujillo and David Rabinowitz noticed it only in January 2005, on a second pass through the images, and announced the find on 29 July 2005.

The object, now called Eris, is about 2,326 km across, slightly smaller than Pluto, and about 27 per cent more massive. Early measurements suggested it might even be the larger of the two. If Pluto was a planet, there was no clear reason Eris was not. Other icy bodies in the same distant region, such as Quaoar and Sedna, suggested many more would follow. Astronomers faced a choice between a solar system with dozens of planets and a definition that drew a line.

The Three Tests Of A Planet

The union drew that line with three conditions. A planet must orbit the Sun. It must have enough mass for its own gravity to pull it into a nearly round shape. And it must have cleared the neighbourhood around its orbit, meaning it is the dominant body along its path.

Pluto passes the first two tests and fails the third. It shares its region of space with thousands of other objects in the Kuiper belt, the band of icy bodies beyond Neptune. A companion resolution made Pluto a dwarf planet: a body that orbits the Sun and is round but has not cleared its orbit. Eris joined it in that category.

The Planets In Order From The Sun

The four inner planets are made of rock and metal. Mercury, the smallest, circles closest to the Sun. Venus, wrapped in thick clouds over a carbon dioxide atmosphere, is the hottest planet, hotter even than Mercury. Earth is the only world known to host life. Mars, coloured red by iron oxide dust, holds Olympus Mons, the tallest volcano known in the solar system.

Beyond the asteroid belt lie the giants. Jupiter, the largest planet, is more massive than all the others combined. Saturn carries the most prominent ring system. Uranus spins on its side, its axis tilted by about 98 degrees. Neptune, the outermost planet, was found in 1846 through mathematics before anyone saw it, predicted from the pull it exerted on the orbit of Uranus.

The Scientists Who Disagree

Not everyone accepted the result. The 424 voters were a small share of the union’s roughly 9,000 members. Alan Stern, principal investigator of NASA’s New Horizons mission, argued that the third test was unworkable. Earth, Mars, Jupiter and Neptune also share their orbits with smaller bodies, so by a strict reading none has cleared its neighbourhood.

Stern’s spacecraft had left Earth on 19 January 2006, seven months before the vote, bound for what was then the ninth planet. New Horizons flew past Pluto on 14 July 2015. It found a world about 2,370 km across, with a heart-shaped plain of nitrogen ice that appears to be the largest known glacier in the solar system, and signs that its atmosphere has changed over time. To Stern and those who share his view, that geology is the mark of a planet. The union has not reopened its definition.

The Search For A Ninth Planet

The count could change again, this time through observation. In 2016, Konstantin Batygin and Mike Brown of the California Institute of Technology proposed that an undiscovered planet several times the mass of Earth orbits far beyond Neptune. Their evidence was indirect: a handful of distant icy bodies whose orbits cluster in the same direction, as if shepherded by an unseen mass.

Other astronomers argue that the clustering reflects where telescopes happened to look, and searches with several survey telescopes have not found the planet. The decisive test is now beginning in Chile. The Vera C. Rubin Observatory released its first images on 23 June 2025 and is starting a 10-year survey of the southern sky that will map distant solar system objects and look for the proposed world.

India’s exploration of the planets has followed the same eight. The Indian Space Research Organisation’s (ISRO) Mars Orbiter Mission entered orbit around Mars on 24 September 2014, on India’s first attempt. Its Venus Orbiter Mission, approved by the Union Cabinet on 18 September 2024 with a budget of ₹1,236 crore (₹12.36 billion), is planned for launch in March 2028.

If Rubin finds a ninth planet, the answer to the question will change once more. This time a telescope, not a vote, will settle it.

Key facts

• The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
• The International Astronomical Union adopted its definition of a planet on 24 August 2006 in Prague.
• Pluto, discovered in 1930, has been classed as a dwarf planet since that vote.
• Planet Nine, proposed in 2016, has not been observed.