There are eight planets in the Solar System. This statement makes a lot of people angry for several different reasons. The obvious group to respond with anger is the 'people for Pluto,' who have an unwavering dedication to the little planet that could. It's scientifically recognized as a dwarf Planet, and is still one step up from a Kuiper Belt Object (KBO), so it's doing well. Far beyond Pluto, in the outer recesses of our Solar system, you may have heard of a potential Super-Earth-sized Planet recently theorized by Konstantin Batygin and Mike Brown. This is the other reason people would be upset about my initial statement. There is a strong desire to find this planet, and whoever does will undoubtedly become part of scientific history.
Yes, even though there are currently eight planets, the planet nine theory is strong, and I would be very happy to see astronomers find this planet. In the flurry of scientists working on the discovery of the planet, there is a lot of theorizing that can be done. Using what we know of planetary formation and the evolution of our Solar System, scientists from Germany have come up with a theory of what this planet might be like and what it is made of.
The assumption they started with is that planet 9 is an 'ice giant,' similar to planets like Uranus and Neptune, that formed with the rest of the Solar System 4.6 Billion years ago. It should have an envelope of Hydrogen and Helium, and it should be very cold due to the lack of sunlight, but not as cold as you might think. Planets of this size radiate heat outward as their core cools. This puts the planet as a temperature of 47 Kelvin, about -226 degrees Celsius.
Because of the lack of sunlight the planet would receive, it radiates 1000x as much energy as it reflects from the Sun, meaning it is much brighter in infrared than it is in the visible spectrum. The researchers also gave some insights on the size and mass of the planet. A 10 Earth mass world with these properties would be about 3.7 Earth radii, putting it between Earth and Neptune in size.
It's certain that we will hear about this theorized planet again, and again, until it is either discovered or its existence is disproven. Either way we are experiencing in real time, the incremental steps of science, the marriage of theory and experiment, and the tireless effort to solve the mysteries of nature.