How big does a planet have to be to be a planet?

Question

Pluto was a planet but now it's a dwarf planet .

Answer 1

The resolution adopted by the IAU states that, within the Solar System, a planet is a celestial body that:

(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;
(c) has cleared the neighbourhood around its orbit.

As a result of this definition, the Solar System is now considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Those objects which fulfil criteria (a) and (b) but not (c) – Ceres, Pluto and Eris – are categorized as dwarf planets.

Beyond the Solar System, there have been more than two hundred objects discovered orbiting other stars. The IAU defines these objects as "extrasolar planets" if they:

(i) orbit a star or stellar remnants;
(ii) each have a mass below that required for the thermonuclear fusion of deuterium;
(iii) each fulfil the minimum mass/size requirement for planetary status in the Solar System.
This is a provisional definition which has not been formally adopted. The IAU also has not decided on whether the term "planet" should be extended to free-floating objects of planetary mass outside star systems, except to exclude those in young star clusters.



The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, currently defines "dwarf planet" as a celestial body that, within the Solar System,

(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;
(c) has not cleared the neighbourhood around its orbit; and
(d) is not a satellite
At current, the term only applies to our solar system.

The IAU's stance on dwarf planets within our solar system has been formally adopted by the IAU general assembly (in 2006). It differs from the definition of "planet" in that the dwarf planet has not cleared its orbital neighbourhood. Before the adoption of the 2006 resolution, there was no formally specified scientific definition of "dwarf planet". The category is distinguished from "planets" and "small solar system bodies".

Under the IAU's definition, our solar system is considered to have three dwarf planets: Ceres (the smallest), Pluto and Eris (the largest). However, many more objects could be added to the list once it has been determined whether they fulfill criteria (b).

Answer 2

The IAU has come up with an inconsistent, non-rigorous definition of what a planet is. And unlike common sense and every other definition in the IAU that uses the term "dwarf", a "dwarf planet" is not a small planet, it is something entirely different.

If you take a look at "Sky and Telescope", "Astronomy", or any of the other publications dedicated to space, they are unanimous in their condemnation of the IAU's definition. Maybe Pluto isn't a planet, but by their definition neither is Uranus.

So stay tuned. The IAU will meet again in a couple of years, and almost assuredly they will try, again, to come up with a scientific definition of what a planet is.

Answer 3

The IAU's definition is pretty flakey. One standard that makes sense is that it has to be big enough for gravity to make it spherical. Ceres at 590 or so miles across is about as small as a ball of rock can get and still be a sphere.

Answer 4

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Answer 5

it has more to do with its qualities than size.

for example, if it orbits a star, it is a planet.

hope this helps.