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.
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