How many planets are in the Solar System?
Eight. The count dropped from nine in August 2006, when the International Astronomical Union agreed a definition of "planet" that Pluto does not meet.
Updated on October 7, 2026
The definition that set the number
IAU Resolution B5 asks three things of a planet: it orbits the Sun, it has enough gravity to have pulled itself into a nearly round shape, and it has cleared the neighbourhood around its orbit. Pluto passes the first two and fails the third: it shares its region with a great many other Kuiper Belt objects and is not gravitationally dominant there.
The resolution did not demote Pluto out of spite or by a narrow show of hands about its popularity. It was forced by discovery: Eris, found in 2005, is essentially Pluto-sized, and it was clear that dozens more would follow. Either the Solar System had dozens of planets or the definition needed a line. The IAU drew the line and created the category of dwarf planet for the bodies on the other side of it.
Source: União Astronômica Internacional (IAU) — Resolução B5 (2006) sobre a definição de planeta
The five recognised dwarf planets
These five are the ones the IAU formally recognises. Astronomers routinely treat several more trans-Neptunian objects as dwarf planets on the physical evidence, so you will see higher counts quoted; the official list is this one.
| Planet | Radius (km) | ua | Orbital period (a) | Recognised in |
|---|---|---|---|---|
| Ceres | 469.7 | 2.77 | 4.6 | 2,006 |
| Pluto | 1,188.3 | 39.48 | 247.94 | 2,006 |
| Haumea | 816 | 43.13 | 284.1 | 2,008 |
| Makemake | 715 | 45.43 | 306.2 | 2,008 |
| Eris | 1,163 | 67.78 | 558.1 | 2,006 |
Source: União Astronômica Internacional (IAU) — Resolução B5 (2006) sobre a definição de planeta
What about a ninth planet?
There is a live hypothesis that an undiscovered planet several times Earth’s mass orbits far beyond Neptune, proposed to explain the clustered orbits of some distant objects. It has not been found, the statistical case is contested, and surveys have steadily shrunk the places it could hide. Until something is imaged, the answer remains eight.
Source: NASA — Planetary Fact Sheet (NSSDCA) e NASA Science: Solar System
How to read this page
How to read the numbers here
Every value on "How many planets are in the Solar System?" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "How many planets are in the Solar System?" names its official source; the verification date sits at the top of the page.
Linked to the rest of the module
"How many planets are in the Solar System?" is one of this module’s reference pages; the glossary below defines the technical terms used here.
More about “How many planets are in the Solar System?”
How far out the dwarf planets live
The five recognised dwarf planets sit, on average, 39.7 astronomical units from the Sun — well past Neptune, which is just over 30. Their orbital periods range from 5 to 558 Earth years.
That distance, not just shape or size, is what makes finding a new dwarf planet so hard: the sunlight reaching them is faint, and the light they reflect back to us is fainter still — the inverse-square law counts twice.
Words on this page
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit
Exoplanet
A planet orbiting a star other than the Sun. Most were found indirectly, by the star’s faint dimming as the planet crosses in front of it (transit) or by the gravitational wobble it causes in the star (radial velocity), not by direct image.
Standard definition by NASA Exoplanet Archive · Exoplanet
Launch window
The span of time in which a rocket can lift off and still reach its planned orbit or destination. For a trip to another planet that window can last only a few weeks and only returns at the next favourable alignment, so missing it costs months or years of waiting.
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty