How many planets are there in the Solar System? Well, it seems not so easy to say. Our current classifications say there are 8 planets.
The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal.
Planet Nine, sometimes incorrectly referred to as Planet X, is a hypothetical planet in the outer region of the Solar System. Does it exist? Well, we don't know yet.
As we said, its gravitational effects could explain the unusual clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These eTNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects.
But how Planet Nine would be, if it existed?
Based on earlier considerations, it would be a super-Earth sized planet, with a mass of five to ten times that of the Earth, and a huge elongated orbit, 400 to 800 times as far from the Sun as the Earth. If the Sun was in New York and the Earth was in Washington, one would have to cover about ten times the distance from New York to Beijing to reach planet Nine!
Konstantin Batygin and Michael E. Brown suggested that Planet Nine could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others proposed that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.
Anyway, if it existed, we would not have any doubt about its planet nature.
In fact, according to IAU (international astronomical union), a planet is a celestial body which:
1 - is in orbit around the Sun;
2 - has sufficient mass to assume hydrostatic equilibrium (a nearly round shape);
3 - has "cleared the neighbourhood" around its orbit.
(Among other things, this definition caused Pluto to no longer be classified as a planet, a change from how it had been widely considered until that point. We want to spend some words here reporting a fun fact: when Pluto was finally classified as a dwarf planet, some people joined the “Pluto is a planet” protest. Here are some photos taken during those protests.)
If Planet Nine exists, its mass is sufficient to clear its orbit of large bodies in 4.6 billion years, the age of the Solar System, and its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by the above definition.
As of February 2021, no observation of Planet Nine had been announced.
But if planet nine exists, why can't we find it?
The answer is that while we can see the effects of this hypothetical planet, finding it is a different matter.
In fact, in order to find planets, astronomers make use of various methods.
The most famous, and perhaps the one you’ve heard of, is the transit method. From our position, if a planet crosses across its star, we can see the dip in light that causes.
Measuring three of these dips, we can work out the mass and orbit of the planet. Not all planets transit their star with respect to us, however, so we can’t use this for every star.
Another method is called the radial velocity. This involves noting the tiny, tiny gravitational tug a planet exerts on its star. For smaller planets in wide orbits, this is incredibly difficult, but for larger planets in tighter orbits, such as hot Jupiters, this can be quite useful.
Then there’s gravitational microlensing...