Must Know Astronomy – Order, Composition And Size Of Planets In Our Solar System

Must Know Astronomy - Order, Composition And Size Of Planets In Our Solar System

Astronomy, when considered in total, is a bewildering array of facts, numbers and datapoints. In fact, if you search wikipedia for the word “planet” you will get 31 pages of material. When studying any science, the urge to become overwhelmed is everpresent and with so much information, it is often difficult to know where to start – but – If you resist the urge to become scattered and restrict your attention to some basic “must knows” your study will overcome inertia and you will have your starting point.

Three Astronomy Must Knows are: 1) Planetary Order, 2) Composition and 3) Relative Planetary Size.


There are 8 planets that orbit around our Sun, so we can say that there are 8 planets in our solar system. These planets are named according to their proximity to the sun (closest to farthest away). The memory device MVEM (emvem) and JSUN(jaysun) helps to remember the correct order of the planets.

SUN — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

the memory device MVEM and JSUN is also useful because it separates the planets by composition. The first 4 planets, MVEM, are terrestrial planets (composed of rock and solid ground like Earth). The last 4 planets, JSUN, are sometimes referred to as “Gas Giants” because rather than being composed mainly of rock and ground they are composed of mainly gas and ice.


There are two basic ways to consider planetary size: 1) Planetary Mass and 2) Equatorial Diameter.

Planetary Mass:

Planetary mass is simply a measurement of the amount of matter composing a planet. For most matter, the amount of mass is measured by putting the matter on a scale and weighing it. If you were to weigh the planets you would find that their masses rank as follows:

Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury

From this list you can see that the the outer “Gas Giants” are more massive than the terrestrial planets. In fact Jupiter is nearly 318 times more massive than Earth while, by contrast, it would take 16 Mercuries to equal the mass of 1 Earth.

Eqatorial Diameter:

When we think of the “bigness” of a planet, we usually think in terms of equatorial diameter or “how big around is it?” Any sphere has an eqatorial diameter. The equator of a sphere is a ring around the center of the sphere that cuts the sphere into two equivalent halves. Ranking the planets by equatorial diameter generates a list that is very similar to ranking them by mass.

Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury

The only difference between this ranking and the ranking by mass is that Uranus and Neptune are switched in position. Neptune is more dense than Uranus. So while it has slightly more mass, the actual sphere of Neptune is smaller than the sphere of Uranus. Neptune, therefore, ranks as slightly smaller planet than Uranus.

Astronomy is a vast topic. Committing to memory the astronomy basics of planetary order, composition, and size is a great start. Even though this information is basic, only a very few people have this information memorized. In science, knowledge of the basics is king , a journey of a million miles begins with the first step.