Beyond the orbit of our planet there is the vast remainder of the solar system. The Earth-Sun average distance is about 149.6 million kilometers, and with an exercise of imagination we can scale it to 1 (astronomical unit or A.U.). The most distant planet, Neptune, is situated in this case at about 30 A.U. (on average) from the Sun, while the closest outer planet, Mars, has its orbital radius of approximately 1.52 A.U. On a true-scale representation (orbital periods adjusted accordingly), the dance of the outer planets is depicted in Figure 1.
The closest to Earth of the outer planets is Mars, the red planet. It was known to the ancient Sumerians as Nergal, a god of war and plague. Around 1500 BC, the ancient Egyptians were familiar not only with the planet itself, but also with its retrograde motion (see Figure 2). In East Asian cultures, Mars is referred to as the fire star. In Greek and Roman mythologies, it is associated to the god of war, from which it took the name we use today.
The motion of Mars from an Earthbound perspective is depicted in the animation in Figure 2.
Beyond Mars lie the biggest planets of our Solar System. Their combined masses sum up to about 99.8% of the mass of all planets (that leaves only 0.2% mass for Earth and the inner planets!). However, this total mass of the outer planets represents only 1.40 Jupiter masses, so we have now a scale of the dimensions of the gas giant that is beyond Mars.
Jupiter is the most massive planet of our solar system. Its mass is 317.8 times bigger than the mass of Earth and 2.5 times the mass of all other planets combined. However, it is a tiny fraction of the mass of the Sun, which is 1047.348 times bigger than Jupiter’s.
Jupiter was known as Marduk by the ancient Babylonians and Zeus by the ancient Greeks. It is the third brightest celestial body in Earth’s night sky, after the Moon and Venus. Due to its huge mass, Jupiter is one of the main factors that cause the existence of life on our planet, since it practically acts as an asteroid shield (together with Saturn) through its gigantic gravity influence. More on Jupiter and the celebrated Juno mission here.
The next giant is Saturn, also known from prehistoric times because it is visible with the naked eye from Earth. The first to observe Saturn with a telescope was Galileo Galilei, who was the first to notice, in 1610, its famous rings. The planet was known to the ancient Babylonians, to the ancient Chinese (as “the Earth Star”), to the ancient Hindu (“Shani”) and to the ancient Israelites (Shabbathai).
The dance of the Sun, Jupiter and Saturn as seen from Earth is depicted in Figure 3.
Beyond the orbit of Saturn there are two more planets, Uranus and Neptune. Although visible with the naked eye, Uranus was first recognized as a planet by William Herschel in 1781, and it is the first planet discovered with the help of the telescope. The year 1781 marks the first major expansion of the known solar system.
Neptune is the farthest planet of the solar system. Its discovery was intensely disputed in the nineteenth century between John Couch Adams and Urbain Le Verrier. Eventually, Le Verrier measurements and estimations were far more accurate than Adams’, and a short time after its discovery, in 1845-46, Neptune bared the name “Le Verrier’s planet”. However, its current name was proposed by Le Verrier himself, and it was internationally accepted as so shortly after its discovery.
The motions of Uranus and Neptune as seen from Earth are depicted in Figure 4.
Well, these are all the celestial bodies that we, humans, use to call “major planets” of our solar system. Obviously, they are not the only “planets”.
Pluto: not a planet anymore
Recently, Pluto was downgraded from the status of “planet” to that of “dwarf planet”. This move was made by the International Astronomical Union (IAU) in August 2006, when it established the criteria that need to be fulfilled by a celestial body in order to have the title of “full scaled planet”:
- It is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
“Clearing the neighborhood” means that the planet should be the gravitationally dominant body of its orbit, which is not the case. The cause of Pluto’s downgrade is its orbit companion Charon, which is almost half of its size and its mass is 12.2% with respect to Pluto’s mass. Pluto and Charon are tidally locked (meaning they show to each other the same face, while orbiting together about the center of mass of their ensemble). For comparison, the Moon is tidally locked to Earth, meaning that we see from here the same face. By contrary, the Earth is not tidally locked with the Moon, as Pluto is with its companion.
Figure 5 shows the orbits of Pluto and Charon about their common center of mass. This center of mass is in orbit about the Sun, which makes the “orbital dance” of Pluto and Charon quite intricate.
Well, these are the outer planets of our solar system. The inner planets were briefly discussed here.
Besides these large bodies, bilions of bilions of other celestial objects populate the inter and outer planetary space and orbit about the Sun. About the most relevant ones (dwarf planets, moons, notable asteroids and comets) — in future posts. Stay tuned.
The animations in this article were made by using GeoGebra.