This is a question that many people have wondered about over the years as our knowledge of the solar system has evolved. In the past, schoolchildren were taught that there were 9 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. However, in recent years, astronomers have reconsidered whether Pluto should be classified as a planet. This has sparked an ongoing debate about how many planets there truly are.
Quick Answer
The quick answer is that by the most common current definition, there are only 8 planets in our solar system. The 8 planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto does not meet the criteria to be considered a planet under this definition and is now classified as a dwarf planet.
History of Changing Planet Definitions
To understand why Pluto is no longer considered a planet, we need to look at how the definition of what constitutes a planet has changed over time as new objects were discovered in our solar system. When Pluto was first discovered in 1930, it was declared the 9th planet. However, beginning in the 1990s, astronomers started discovering other large objects beyond Neptune that were similar to Pluto in size and orbit. This included objects like Haumea, Makemake, Ceres, and Eris in the region called the Kuiper Belt.
The discovery of these new Pluto-like objects led astronomers to reconsider the definition of a planet. With so many new large objects being found, it seemed necessary to come up with clearer criteria for what makes something a planet. This led to a 2006 International Astronomical Union (IAU) resolution that defined a planet as a celestial body that:
- Orbits the Sun
- Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
- Has “cleared the neighborhood” around its orbit
By this new definition, Pluto was deemed not to qualify as a planet because it did not meet the third criteria of having cleared its neighborhood. Its orbit overlaps with many Kuiper belt objects. Therefore, in 2006, Pluto was reclassified as a dwarf planet along with Ceres and others.
Number of Planets Under Different Definitions
| Definition | Number of Planets |
|---|---|
| Planets under original definition (before 1990s) | 9 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) |
| Planets under 2006 IAU definition | 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) |
| Possible planets under proposed definition including round objects in zone around star | 12+ (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, Eris, and more) |
As the table shows, the number of planets varies significantly depending on the specific criteria used in the definition. Some astronomers have proposed broader planet definitions that would include more objects orbiting our Sun.
Arguments for Pluto as a Planet
While Pluto is not considered a planet under the IAU definition, some scientists argue that it should retain its planet status. Reasons put forward for why Pluto could still be a planet are:
- Historic precedent – It was classified as a planet for over 70 years before 2006.
- Geophysical complexity – Pluto has an atmosphere, weather, seasons, and other complex structures that make it seem more planet-like than asteroid-like.
- Cultural importance – The public is used to thinking of Pluto as a planet and associate it with the other planets.
- Orbit differences – Pluto has a distinct orbital relationship with Neptune compared to other Kuiper Belt Objects.
Some astronomers say that the IAU’s planet definition is too narrow and that it should include any non-stellar round object orbiting a star. By some proposed definitions that focus more on round shape and orbit than clearing orbit, Pluto could potentially be classified once again as the 9th planet.
Reasons Pluto May Still Deserve to Be a Planet
| Reason | Explanation |
|---|---|
| Historic precedent | Considered a planet for over 70 years before reclassification |
| Geophysical complexity | Has atmosphere, weather, seasons – seems more planet-like than asteroid-like |
| Cultural importance | Public used to thinking of Pluto as major planet, associate it with other planets |
| Orbital dynamics | Pluto has a resonant orbit with Neptune unlike other Kuiper Belt Objects |
While the IAU maintains their 8 planet definition as of today, the debate continues even among astronomers about whether Pluto should remain in the planet club or not.
Should the Definition Be Changed?
Given these disagreements, some argue that the IAU definition should be revised or expanded in some way to be more inclusive. But others think the current definition works well and helps categorize different classes of objects. Arguments on either side include:
Arguments for Revising the Definition
- Make the definition based more on round shape and orbit rather than neighborhood clearing.
- Add a new classification like “major planet” that includes Pluto.
- Account for complex planetary evolution where clearing takes time.
- Emphasize cultural and historical importance over technical details.
Arguments Against Revising the Definition
- The current definition is useful in distinguishing between types of celestial objects.
- It’s objective and not based on cultural history or public opinion.
- Reopening the definition leads to more confusion rather than clarity.
- Other classifications like dwarf planet handle borderline objects like Pluto.
Ultimately, any definition will have pros and cons. But many astronomers think the IAU got it right in 2006 by drawing a distinction between planets and dwarf planets that emphasizes a dynamical difference – whether the objects are gravitationally dominant in their orbit. Still, both sides continue to bring up valid perspectives in this ongoing debate.
Conclusion
Based on the current official IAU definition used by most astronomers, there are only 8 planets in our solar system – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto and other dwarf planets do not meet all the criteria to qualify at this time. However, some scientists make reasonable arguments for why Pluto could still be considered a planet based on factors like its orbital dynamics and complex geology.
The debate over the definition of a planet will likely continue, as there are valid perspectives on both keeping the current definition or revising it to be more inclusive. But for now, the conventional accepted view in the scientific community remains that there are 8 major planets orbiting our Sun, with Pluto and similar dwarf planets constituting a separate category.
Regardless of these technical classifications, as one of the largest and most fascinating objects in our solar system, Pluto still maintains great cultural and scientific significance worthy of continued exploration and study. The flyby of the New Horizons spacecraft in 2015 revealed Pluto to be a stunning, complex world with mountains, glaciers, and an atmosphere, reminding us that whatever category it belongs to, Pluto is a destination that captures the imagination and continues to inspire planetary discovery.