Stardust and eternity – 3.1.6

Planets of the Solar System

The Solar System is composed of objects that are gravitationally bound to our star. Apart from the Sun and the eight major planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune – this definition also includes minor bodies, such as their moons, asteroids, comets, and dwarf planets such as Pluto.

As for the major bodies, planets revolve in the same direction around the Sun – a Main Sequence star of spectral type G, which is the largest object in our solar system and contains about 99.9% of its total mass. All the major planets revolve in the same counterclockwise direction around the Sun, as seen from above the North Pole; the different orbits are close to the same “ecliptic plane” because they were originally condensed from the same disk of gas and dust that shaped our Solar System. The orbits are almost circular but slightly elliptical around the Sun – located at one focal point –as they are affected by the gravitational interactions of other planets and stars. Major planets vary in size, atmosphere, and internal structure, but can be ideally divided into two major classes, namely the “terrestrial” (Earth-like) and “Jovian” (Jupiter-like) planets.

The four inner planets are Mercury, Venus, Earth and Mars – ordered from closest to farthest from the Sun. These “terrestrial” planets are characterised by rocky, solid surfaces and have many other common properties that distinguish them from the “jovian” planets. Specifically, they are relatively small and have higher average densities. Earth – the “third rock around the Sun” – is the largest of them. Venus, defined as “Earth’s twin”, is just a little bit smaller than the Earth; while Mercury – a moon-like planet without atmosphere – is the smallest of the major objects, being just a little over 1/3 the size of our planet. Venus is the brightest planet at night sky because is our closest neighbour and has the highest albedo, reflecting as much as 3/4 of the sunlight compared to only 7% reflected by the Moon. The pressure of its atmosphere, composed mostly of CO₂ – or carbon dioxide – is almost 100 times that of the Earth at sea level and the “greenhouse effect” keeps its surface, day or night, extremely hot (460° Celsius!). Mars, the outermost of the terrestrial planets, is about half the size of the Earth. Although its atmosphere has a similar composition to that of Venus, its pressure is 10,000 times lower; as a result, Mars does not suffer from a strong greenhouse effect, although its temperature excursions – due to its very tenuous atmosphere – can exceed even 100° Celsius.

The four outer planets are Jupiter, Saturn, Uranus, and Neptune – ordered from closest to farthest from the Sun. These “jovian” planets are much larger than the terrestrial planets; furthermore, they lack solid surfaces and contain Earth-sized solid cores. The cores are enveloped into large gaseous atmospheres, mostly composed of light elements, such as hydrogen and helium. Jupiter is the largest, about 11 times larger than Earth but with a density less than 1/4 that of our planet. Its distinguishing feature is the gigantic red spot, a storm large enough to engulf our Earth, which has been going on for hundreds of years. Saturn, the second-largest planet in the Solar System – about 9 times larger than the Earth – has an even lower average density than water. Its main feature is its extended ring system, composed of small pieces of rock and ice, visible even with a small telescope. Both Jupiter and Saturn have about 80 satellite moons, which belong to the category of minor objects in the solar system. Finally, the two most distant planets, Uranus and Neptune, are slightly denser and about 4 times the size of the Earth.

Until 2006, Pluto – an icy terrestrial planet orbiting beyond Neptune and smaller than the Moon – was also listed among the major planets of our solar system. However, this “dwarf planet” (as it is now called) has an irregular, highly elliptical orbit inclinated with respect to the ecliptic plane, thus implying that it actually is a member of the Kuiper Belt – the doughnut-shaped ring of icy cometary objects orbiting around the Sun just beyond the orbit of Neptune.

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Planets of the Solar System – Picture by 0fjd125gk87 – Pixabay

Further resources

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Images

Planets (IAU, M. Kornmesser)

Copernicus Manuscript (ESO)

Map of the Solar System (NASA, JPL-Caltech, R. Hurt)

Rocky worlds (NASA)

Radius, mass density and interior structure of terrestrial bodies (K. R. Lang, Tufts University)

Mercury from Passing BepiColombo (ESA, JAXA, BepiColombo, MTM)

A Venus Flyby (NASA, JHUAPL, Naval Research Lab, Guillermo Stenborg and Brendan Gallagher)

Venus (K. R. Lang, Tufts University)

Mars Opposition (NASA, ESA, and STScI)

Mars (K. R. Lang, Tufts University)

The gas giants (NASA)

Jupiter and Europa (NASA, ESA, A. Simon, and M. H. Wong and the OPAL team.)

Inside Jupiter (K. R. Lang, Tufts University)

Saturn and its rings (NASA, ESA, A. Simon and the OPAL Team, J. DePasquale (STScI))

Inside Saturn (Kenneth R. Lang, Tufts University)

Uranus and Neptune (Hubble) (NASA, ESA, Mark Showalter (SETI Institute), Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley), Andrew I. Hsu (UC Berkeley)

Inside Uranus and Neptune (Kenneth R. Lang, Tufts University)

Artist’s impression of Mars four billion years ago (ESO/M. Kornmesser/N. Risinger)

Spectroscopic Analysis of Three Planets (NASA and C. Godfrey (STScI)

Videos

Solar System 101 | National Geographic

Introduction to the Solar System Crash Course Astronomy

Mercury 101 | National Geographic

Mercury: Crash Course Astronomy

Venus 101 | National Geographic

Venus: Crash Course Astronomy

Mars 101 | National Geographic

Mars: Crash Course Astronomy