Hydrogen and helium are the primary elements on those planets.
Solid or terrestrial planets are those whose primary mass is land. The terrestrial planets in the Earth's solar system include Earth, Mercury, Venus, and Mars.
planet A better answer might be "primary". Objects other than planets can have satellites.
The primary energy source that drives weather on terrestrial planets like Earth is the Sun. Solar energy drives processes such as the water cycle, which in turn influences atmospheric circulation, temperature patterns, and weather phenomena.
The planets in our solar system that start with the letter "N" are Neptune and, if considering dwarf planets, there is also the dwarf planet Haumea, which is sometimes associated with the name "N" because of its connection to the Hawaiian goddess. However, in terms of the eight primary planets, only Neptune fits the criteria.
The collection of planets and their star (primary) are called a solar system. The planets themselves are called "exoplanets." Once we can tell they fit our definition of planet (having cleared their orbits, etc.) we would probably just refer to them as planets.
If planets had no orbital momentum, the gravitational pull from the Sun would pull them into the Sun. Essentially, an orbit is a stable path where an object is constantly falling towards its primary and constantly missing.
There are a number of appropriate words. The most general is "primary", as in "primary object"; the satellite orbits around the primary. This says nothing about what the primary object IS; a sun, a planet, a moon, or an asteroid, any of which can (and often do) have satellites.
The Sun is the primary source of heat and light for all the planets in our solar system. It emits energy through nuclear fusion reactions in its core, which produces heat and light that reach all the planets, including Earth.
All of them. A few satellites are tidally locked to their primary (luna for example), but all the planets rotate.
Most of the outer planets, such as Jupiter and Saturn, contain hydrogen as the primary light element. Hydrogen is the most abundant element in the universe and makes up a significant portion of these gas giants' composition.
The sun is the primary source of external heat for all eight planets, although in the case of some planets it doesn't provide a lot of heat. The sun also is the largest single factor affecting the orbits of the eight planets.