Planet beyond Pluto?

Recently, there has been news that astronomers have found another planet beyond Pluto. At the moment, astronomers think the new planet is about 10 Earth masses and has a period of 20,000 years.

The orbit is highly elliptical with a perihelion somewhere between 200 to 300 AU and an aphelion between 600 and 1200 AU. We can also assume that since the planet is out beyond the orbit of Pluto, it has a density similar to that of Pluto, about 2000 kg/m³.

What do these assumptions mean? It could explain why it took so long for astronomers to find this planet, if it is really there. Using Kepler's laws, specifically, P²=a³, where P is the period of the orbit in Earth years and a is the semi-major axis in AU, we can determine the semi-major axis of the planet. With P at 20,000 years, we find that a is about 740 AU.

Using the density of Pluto and the mass of the new planet (10 Earth masses = 6x1025 kg), we can estimate that the radius of Planet 9 is about 41,500 km. For a perihelion of 200 AU, the angular diameter is only 0.01" (arcseconds) which is typical of a star. It is also possible that the planet has a low albedo, so it is very dim.

So, while it is possible that there is a planet beyond Pluto, it took this long to find it because it is so far away.

Is Pluto a Planet?

In the past, I know I've been adamant to say that Pluto is not a planet. You can read them again here, here, and here. However, I've thought about things and may be coming around to categorizing Pluto as a planet.


Before, I had always said that there are two types of planets: terrestrial planets, like Earth, and Jovian planets, like Jupiter. But what if we redefined a planet to have a third type: dwarf planets. These are bodies in the solar system that are too large to be asteroids or comets but are not necessarily big enough to have swept all the smaller bodies from its orbit (meaning it could have co-orbital bodies), have densities somewhere between Jovian planets and terrestrial planets, and can be found anywhere in the solar system.


I did discuss dwarf planets in more detail in a previous post, when discussing Pluto (see first link above), so I may have unwittingly decided already that Pluto is a planet. In fact, if we make Pluto a planet again (which probably will happen sooner rather than later), we will also have add a few more planets to our solar system.

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

Eris and Makemake will definitely be added. Haumea, Sedna, and 2007 OR10 may as well. It all depends on where the lower limit of a dwarf planet's size will be if the IAU determines that.

This is what I love about science. It's a fluid subject that can easily change based on new information. I've changed my mind about Pluto (for now), and there may be other things I might change my mind about.

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.

The Discovery Of Pluto

Believe it or not, the discovery of Pluto was by accident. Astronomers had noted anomalies in the orbit of Neptune, and much like they did with Uranus' orbit, thought there was a planet lying outside Neptune's orbit which would give rise to the anomalies.  So astronomers began to look for it.

It wasn't until Kansas-born Clyde Tombaugh (it's only important that I mention he's from Kansas because so am I), using stereo imaging of photographic plates found what he thought was the new planet. However, the new object he found was Pluto.

Lowell Observatory Archives

Clyde Tombaugh

 

But after more measurements, the new planet was just not large enough to have the effect that they saw on Neptune. After some more calculations, Neptune's orbit was confirmed just using the masses of Neptune and Uranus.

So Pluto was there, but was not what was expected. If the orbit of Neptune had been calculated  correctly initially, it might have been a while longer before Pluto were discovered.