Its planet status is a matter of ongoing debate. Scientists who hold to the geophysical planet definition, including New Horizons Principal Investigator Alan Stern, never stopped viewing Pluto as a planet. They hold to the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal…

Yes, according to the geophysical planet definition, Ceres is a planet. 19th century astronomers could not resolve it into a disk and therefore did not know it is in hydrostatic equilibrium. Today, we know it is spherical, meaning its demotion was in error. Ceres is much more like the terrestrial planets than it is…

Actually, Pluto has very much the same characteristics and properties as the terrestrial planets. It is spherical, has geology and weather, and is differentiated into core, mantle, and crust. Like Earth, it has nitrogen in its atmosphere and has a large moon formed via a giant impact. Earth has far more in common with…

Maybe, but so far, the number is more like 50. There is absolutely no scientific merit to artificially keeping the number of solar system planets small. The solar system has whatever number of planets it has. Should we say there cannot be billions of stars or billions of galaxies, or that Jupiter can have only four…

There are no objects the size of Pluto that aren't planets—at least not for those who hold to the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. In other words, if the object is large enough…

No, there is no consensus among astronomers as to Pluto's planet status, and this is not a determination of the IAU, even if its leadership wants you to think so. Science is not decided by decree of an "authoritative body." That is dogma. In the case of Pluto, we are talking not about the facts themselves but about…

There are just as many scientists who do view Pluto as a planet as there are those who do not.

There are several factual errors in this article. First, Eris is not bigger than Pluto. It was initially thought to be bigger when it was discovered in 2005, but in November 2010, a team of astronomers led by Dr. Bruno Sicardy obtained a more accurate measurement of it when it occulted a star, and determined Eris is…

Those measurements are incorrect. Eris is not larger than Pluto. Eris is approximately 2,326 km while Pluto is approximately 2,338 km.

Pluto IS a planet. It is well beyond the threshold for being rounded by its own gravity; that is what makes it a planet.

This is not complete: Add diameter of Pluto (2,338 km) + diameter of Ceres (974.6 km) +diameter of Makemake (1,300-1,900 km) +diameter of Haumea (1,960 X 1,518 X 996 km) +diameter of Eris (2,326 km).

There aren't only nine planets. There are at least 13 and likely many more. There is no need to memorize a list of names; the important thing is to understand the types of planets and their characteristics.

Mike Brown didn't start the argument, which has been going on since Pluto's discovery in 1930. Tyson started the latest round in 2000 with the renovation of the Hayden Planetarium.

Supporters of Pluto being a planet have New Horizons Principal Investigator Dr. Alan Stern on our side. He just happens to be the leading scholar of Pluto in the world.

Pluto is NOT smaller than any Kuiper Belt Objects. It is the largest known KBO. Its composition is nothing like that of a comet. The largest comet known is tiny compared to Pluto and does not have any of the atmospheric or geological processes Pluto has. Yes, there are dozens of planets in our solar system. That does…

No, it doesn't make sense. It doesn't make sense to say dwarf planets are not planets or to say an object is not a planet simply because you don't want to "open the floodgates." If our solar system has hundreds of planets, then that is what it has. If the existence of hundreds of planets "destroys the notion of…

There are dozens of planets orbiting our sun. Any artificial limiting of the number simply for convenience is not a sound scientific decision.

A planet isn't any object that orbits the sun; it's any object that orbits the sun and is large enough to be rounded by its own gravity. The latter makes it a complex world very different than a comet. Chances are there are not millions of objects large enough to be spherical orbiting the sun. But those that are are…

Composition is more important than their orbits. Composition tells us what an object is made of. It reveals the object's intrinsic properties. These, not location, are what should determine an object's classification. If you look only at orbits, you could end up with the same object being considered a planet in one…

The ecliptic is merely the path of the Earth around the sun, not the path of the sun. Mercury does not orbit on the ecliptic either. Giant exoplanets have orbits far more elliptical than Pluto. And Pluto is not that different from the other planets. As several people here pointed out, Earth has more in common with…