Our Solar System (2) The outer planets

by Vladimir Martinusi

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.


Figure 1. The outer planets. The smallest circle is Earth’s orbit, and next to it is Mars.

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.


Figure 2. the motion of Mars as seen from Earth (daily rotation of Earth ignored).


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.


Figure 4. The Sun, Uranus and Neptune from an Earthbound perspective.


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”:

  1. It is in orbit around the Sun.
  2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
  3. 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.


Figure 5. Pluto is in the middle, Charon on the outer ellipse. The trajectories depict their orbits about their mutual mass center.

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.



Our Solar System (1) The inner planets

by Vladimir Martinusi

Everybody (well, almost) knows that our planet is located in a solar system where we revolve around our Sun together with seven other planets (once there were nine; a few years ago, Pluto was downgraded from planet to minor planet or dwarf planet). All planets move on almost circular orbits, and their orbits are located in almost the same plane. Moreover, all planets move in the same direction, let us say prograde (i.e. inverse to the sense of the clock hands spinning). This is true for an observer that looks at this plane from (let’s say) above it. An observer “below” this plane (i.e. on the other side) would see a retrograde motion of the planets. Basically, the perspective on everything in the Universe strongly depends on the location from which things are observed.

The inner planets are those who have the orbit with a radius smaller than Earth’s orbit radius. Starting from the closest to the Sun, they are Mercury and Venus. Mercury was known to the Sumerians around 3,000 B.C., while the oldest surviving astronomical manuscript mentioning Venus dates back to 1,600 B.C. and was written in Babylon.

Their motion is depicted in the animation below. This is what somebody which has the position fixed with respect to the Sun would see it.



Figure 1. Sun-bound view on the planetary motion.

Our ancestors did not even imagine this. Their perspective was earthbound, so they saw how this motion takes place with respect to the Earth. For them, the motion of the inner planets was intricate and counter-intuitive (see the animation below), nurturing  superstitions and generating serious scientific questions which remained unanswered for many centuries.


Figure 2. Earth-bound view of the planetary motion

At the beginning, the Earth was considered to be the centre of the Universe (and unfortunately, there are many scientific illiterates today still to believe it). The first heliocentric model was proposed by Aristarchus of Samos, that came to contradict the geocentric model of Heraclides Ponticus (although some claim the latter is the first to propose a heliocentric model).

The first breakthrough (and also a huge drawback) came with Claudius Ptolemy, who offered a quite accurate planetary model that could predict eclipses and explain the retrograde motion (we’ll come back to this one shortly). His model stated that planets move around Earth as a result of the composition of two motions: (1) the rotation on a small circle, called epicycle and (2) the motion of the centre of the epicycle on anotherr circle, called deferent.

If we take another look at Figure 2, we realise that Ptolemy was right: the man spoke what he saw… from Earth. The epicycle is the orbit of the planet as seen from the Sun, while all planets have the same deferent, the apparent orbit of the Sun around Earth. Too bad that Ptolemy put the later in the centre of the solar system. His error propagated for almost one and a half millennia, until Nicolaus Copernicus formulated (again) the first heliocentric model, sometime around 1510.


Figure 3. Copernican system (photo credit: Wikipedia)

Planetary retrograde motion

This phrasing obviously refers to the apparent motion of a planet with respect to Earth or another celestial body which is not the Sun. As depicted in the animation in Figure 2, at some point (let us take Mercury as example) the planet seems to move backwards with respect to Earth, namely around the point where its distance to the blue planet is minimal. Astronomically, it does not mean anything more than the two planets (Mercury and Earth) come at their closest relative distance (and no, folks, there is no energy emerging when this retrograde motion happens at the same time when our natural satellite, the Moon, is both at perigee and full).

Please stay tuned for the next articles. Meanwhile, I leave you contemplate both Sun-bound and Earth-bound apparent motions in one single synchronised animation.



The animations in this article were made by using Geogebra.