Why Pluto Lost its Planet Title

A Hubble Space Telescope image of Pluto and its moons. Charon is the largest moon close to Pluto. The other four bright dots are smaller moons discovered in 2005, 2011 and 2012.

A Hubble Space Telescope image of Pluto and its moons. Charon is the largest moon close to Pluto. The other four bright dots are smaller moons discovered in 2005, 2011 and 2012. ( Source: NASA)


Have you ever wondered what Pluto could have possibly done to lose it’s position as the ninth planet from the Sun? Well, it’s all about gaining a better understanding of our solar system.

Pluto, once dubbed Planet X, was originally discovered in 1930 by Clyde Tombaugh and was then later officially classified as the ninth planet from the Sun. As time went on Pluto continued to be considered to be the largest object past Neptune, which justified it’s planetary title, but as technology improved we began to find more and more objects in the outer solar system.

These objects in the outer solar system all orbit the Sun and are collectively known as the Kuiper Belt, which was officially discovered in 1992. The objects in the Kuiper Belt are numerous and made up of mostly of rock, metal, and ice. With more and more objects being discovered, it was likely that we would soon find a Kuiper Belt object that was larger than Pluto, which was eventually found in 2005. The object found was discovered by Mike Brown and his team and was later named Eris, which is 2,600 km across compared to Pluto’s 2,400 km. However, this information may be inaccurate as recent measurements have shown Eris to be much smaller than initially stated, in some cases it may actually be smaller than Pluto.

With all of these new discoveries it was becoming harder and harder to justify Pluto being our ninth planet. What really helped put an end to Pluto’s planet-hood was when we compared Pluto to the IAU’s (International Astronomical Union) definition of a planet which states that in order for an object to be considered a planet, it must meet the following requirements:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has “cleared the neighborhood” around its orbit. ( This means it must be the largest and most gravitationally dominant object in it’s orbital path)

Pluto runs into a big problem with the third requirement as the would-be planet is less than 1% of the mass of any other object in it’s orbit. This officially makes Pluto a Dwarf Planet and thus no longer the ninth planet in our solar system. Regardless of Pluto’s new title, we will always have a place in our hearts for the tiny cold rock. In fact, we will be visiting Pluto in 2015 as NASA’s Horizons spacecraft will be reaching the Dwarf Planet after a 3 billion mile, 10 year journey to explore Pluto and the Kuiper Belt.

Also, it’s important to note that many disagree with these recent changes to Pluto’s classification and are advocating the overturning of the IAU definition of a planet that caused this debate and downgrade of Pluto. You can read more information about this effort on LaurelsPlutoBlog.




  1. Laurel Kornfeld

    As a fellow astronomer, I find this article extremely disappointing. It presents one side of an ongoing debate as fact when this is hardly the case. Many, many astronomers still do consider Pluto a planet. Only four percent of the IAU voted on the controversial demotion in 2006, and most are not planetary scientists. Their decision was immediately opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. Stern is the person who first coined the term “dwarf planet,” but he intended it to designate a third class of planets in addition to terrestrials and jovians, not to designate non-planets. The 424 IAU members who voted in 2006 misappropriated his term. Second, Eris is NOT larger than Pluto. It was initially thought to be, but in November 2010, Eris occulted a star, enabling astronomers to obtain a much more accurate measure of its size. They found it is smaller than they thought, less than 1454 miles in diameter (2340 km). Pluto is about 1455 miles in diameter (2342 km), meaning it is marginally larger. Third, the so-called “third requirement” for planethood was contrived by astronomers with a deliberate agenda of excluding Pluto. It makes little sense in terms of defining planets, as no planet fully clears its orbit of asteroids, and Neptune does not clear its orbit of Pluto. Applied literally, this “requirement” would leave the solar system with no planets at all! It also is biased in favor of planets closer to their parent stars. The further a planet is from its star, the larger an orbit it has to “clear.” If Earth were in Pluto’s orbit, it would not clear that orbit either. A definition that could result in the same object being a planet in one location and not a planet in another makes absolutely no sense. That’s not to mention that many exoplanets have highly inclined orbits that take them through asteroid fields as well.

    So far, no planet larger than Pluto has been found in the Kuiper Belt. Even if one is, that doesn’t make Pluto and that planet non-planets. Pluto, Eris, and the other large Kuiper Belt Objects are complex worlds with all the features of planets, just smaller. They have geology and weather, and many are layered into core, mantle, and crust, just like Earth is. Pluto may even harbor a subsurface ocean. To blur the distinction between these little planets and the majority of tiny, shapeless Kuiper Belt Objects is simply bad science.

    To many astronomers, Pluto’s planethood has not ended because dwarf planets are simply another class of planets, just like dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies. Pluto is a lot more than “a tiny, cold rock.” If we count dwarf planets as a subclass of planets, Pluto is the solar system’s tenth of 14 planets and counting: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon (Pluto-Charon is a binary planet system since the two objects orbit a common center of gravity), Haumea, Makemake, and Eris.

  2. Mike Wrathell

    Riddle me this: our Sun is a yellow dwarf star, yet a star in good standing. Dwarf galaxies are considered galaxies in good standing. Why then are dwarf planets not considered planets? Why the glaring inconsistency? Could it be that the IAU Executive Committee ramrodded that rancid resolution down the throats of its membership without proper vetting or notice, so much so that one member even confessed to being threatened with the destruction of his career?


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