Update on Pluto

A while back, I posted an argument about why Pluto is not a planet. Recently, there has been a movement to reinstate Pluto as one of the planets in our solar system. Let's delve into this a bit further.


From the previous post, here are the IAU definitions of a planet and a dwarf planet.

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.

As to these current definitions, we can see that Pluto is not a planet. All the arguments I made are in the previously linked post. A nice thing about science is that new information can change our understanding of nature and the universe. Science is a fluid subject. Our perceptions can alter. So that is why it may be important to reinvestigate the idea of a planet.


If we were to redefine what makes a planet, we should be clear on what is and what is not a planet. Pluto is smaller than seven moons in our solar system, including our Moon. However, Mercury is also smaller than the two largest moons, Ganymede (orbiting Jupiter) and Titan (orbiting Saturn). We can all agree that Ganymede, Titan, the three other Galilean satellites, Triton (orbiting Neptune), and our Moon are NOT planet, they are moons. They orbit around planets which in turn orbit around the Sun. Pluto, only orbits the Sun (though it can be argued that it also orbits around the common center of mass of its system (Pluto, Charon, and its other orbital companions).


If the IAU does change the definition of a planet, it will have to get rid of the idea of co-orbiting bodies that are a significant fraction of the largest body's mass and radius. Remember, Charon is about 11.6% the mass of Pluto and has a radius about half that of Pluto. Our Moon is only 1.2% the mass of Earth and has a radius just over a quarter of the Earth. Looking at all the large satellites of the gas giant planets shows that all of them are significantly smaller in comparison to their parent planet than our Moon is to the Earth.


So if the definition is changed, what other objects in our solar system will have to be redesignated as a planet? Pluto is obviously the first. Eris will also have to redefined as it is a larger body than Pluto. After that, it depends on what the lower limit the IAU wants to use. Makemake may become a planet, Ceres may as well, though if Ceres does get redefined, than all the trans-Neptunian objects larger than Ceres will have to be classified as planets. Not only that, we will have to add a third type of planetary body along with terrestrial and Jovian. This will have to be something in between, though that type is not really a bridge between terrestrial and Jovian.


At the moment, I personally like the definition we have for planets. It's clear, concise, and makes a lot of sense. But as I said before, science can change and our understanding of what is going on can help us make more informed conclusions. Only if the IAU changes the definition of a planet, only then will Pluto, Eris, and some of the other dwarf planets/minor planets in our solar system become full-fledged planets.


We will learn more about Pluto once New Horizons reaches the Plutonian system in July of 2015. Then we will know more about Pluto and its sisters and may be able to make more informed conclusions about what they are.

1 Ceres

NASA's Solar System Explorations Page



Ceres was the first asteroid discovered in the Asteroid Belt on 1 January 1801 by Giuseppe Piazzi. When he first discovered it, he thought it was a planet, as it was a common belief at the time that there might be a planet between Mars and Jupiter because of the large gap between the two planets.
Because there was not enough information to determine the orbit of Ceres, it wasn't confirmed to exist until December of 1801. After close observation of Ceres, it was determined that it was not a planet, but something new. It was then that William Herschel coined the term "asteroid" as the object had a star-like appearance so was difficult to distinguish from the background stars. However, after observing Ceres and other asteroids over the course of days and weeks, it was apparent that asteroids move faster in the sky than the stars since they are much closer to Earth.

Ceres is small compared to the planets and many satellites, but it is the largest object in the Asteroid Belt by a wide margin. 

• It has a diameter of about 950 km or the size of Texas
• It's mass is 9.47x10²° kg, or about 0.00016% of the Earth. Despite its mass, Ceres contains 25% of all the mass in the Asteroid Belt
• It has a semi-major axis (orbital radius) of 2.76 AU

As shown in the opening picture, Ceres is relative spherical. Much like the planets, is an oblate spheroid with a wider diameter at the equator than the poles. This shows that it rotates on an axis uniformly. Other asteroids with irregular shapes kinda tumble and wobble in space as they rotate.

We've learned more about Ceres as telescopic observations became more sophisticated and advanced. Ceres does not fit into the three main types of asteroids mentioned in the previous post. In fact, like planets, it is differentiated (layered) with a rocky core, an icy mantel, and an outer crust.

NASA's Solar System Explorations Page

For the next two centuries, Ceres was still considered an asteroid as there was no other category of object it fit in. It's official designation was 1 Ceres as it was the first asteroid discovered and Ceres is its proper name. However, in 2006 when Pluto was demoted from planet status to minor planet, it was concluded that Ceres should be upgraded. Ceres is now officially considered a dwarf planet. It is too small to be a planet (and where the lower limit to be considered a planet is still murky) but too large and too regularly shaped to be an asteroid.

One final weird thing discovered on Ceres is that it is slightly active. An object as small as Ceres should not have any sort of eruptions going on, but in January of 2014, the Herschel Space Telescope discovered water vapor plumes emitted from the surface. We will learn more about Ceres in the coming years when the space probe Dawn arrives at Ceres and begins its exploration. Ceres is also the third most likely spot for crewed missions to visit after the Moon and Mars.