The Solar System

The Sun and 8 major planets in the Solar System

There are currently 8 planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In previous years the answer would have been 9, but Pluto has been demoted to dwarf planet status. In addition to this, another dwarf planet named Eris was discovered in 2005 beyond Pluto. Mercury, Venus, Earth, and Mars are the inner planets, while Jupiter, Saturn, Uranus, and Neptune are classified as gas giants. The 3 known dwarf planets, Pluto, Eris, and 2018VG18(Farout) are located in the far reaches of the Kuiper Belt which is a collection of icy bodies in the outer most region of the Solar System. The following is a brief overview of each planet.

Mercury – Closest To The Sun

Mercury and Sun

Mercury is the closest planet to the Sun and is also the smallest planet in the Solar System as well. The surface gravity is .38g(38 percent that of Earth where Earth is 1g). It orbits the sun in approximately 88 days which makes it the shortest orbital period of any of the planets. It is so close to the sun – the minimum distance is 28.5 million miles and the maximum distance is 43.5 million miles – that Mercury is only visible close to the horizon in the evening or morning, often appearing as a bright star. As the maximum and minimum distance figures imply, Mercury has an elliptical orbit. Also, Mercury has no known natural satellites.

Mercury’s surface is heavily cratered looking somewhat like the Moon’s surface. Since Mercury has no significant atmosphere to retain heat, there is a very wide temperature variance, which is more than any other planet in the Solar System. Temperatures on Mercury vary from -280 degrees fahrenheit at night to 800 degrees fahrenheit during the day along the equator, while the polar regions are constantly below – 130 degrees fahrenheit. Mercury completes two rotations for every three orbits around the sun, which means that Mercury has a VERY long day.

One interesting fact is that Mercury is NOT the hottest planet in the Solar System, that honor goes to Venus which can reach temperatures of 890 degrees fahrenheit. It is therefore the second hottest planet in the solar system. Something else very interesting is that organic water molecules have been found on Mercury at the polar regions, particularly in some craters. This means that ice can and probably does exist on Mercury, although in miniscule amounts. Two spacecraft have visited Mercury – Mariner 10 in 1974 and 1975 with a fly by, and Messenger which actually orbited the planet 4000 plus times before crashing into it’s surface in 2015.

Venus – Eternally Covered In Clouds

Forever cloaked with clouds

Venus is the second planet from the Sun with an orbital period of about 225 days and is very close in size to the Earth with a surface gravity of .904g, which is very close to Earths 1g. It orbits the Sun at a distance of 66.78 million miles at the closest to 67.69 million at the furthest. So much so that it is sometimes called Earths sister planet. Its rotation period is the longest of any planet in the Solar System at approximately 243 days which means the same side is closely locked in orientation to the Sun. The Venusian day is VERY long! Something very odd about Venus is that it rotates in the opposite direction to most other planets in our Solar System. Of course, the sun wouldn’t be visible beneath the extremely thick atmosphere of Venus, but if it were the sun would acutally rise in the west and set in the east!

Venus is the second brightest object in the night sky after the Moon, reaching an apparent magnitude of -4.6, bright enough to actually cast shadows. Very rarely Venus has even been visible in the daytime. Naturally, Venus makes a great object for amateur astronomers to observe with even a modest telescope.

As mentioned before, Venus has an extremely thick atmosphere. The Venusian atmosphere has a pressure 92 times that of the Earth on its surface, being 96% carbon dioxide. This accounts in large measure for the fact that Venus has by far the hottest temperature of any of the planets in the Solar System with a mean surface temperature of 863 degrees fahrenheit. The surface of Venus is very dry and desert-like with slab shaped rocks and possibly volcanos.

The USA spacecraft Mariner 2 flew by Venus in 1962 and the Russians landed Venera 7 on Venus in 1970. The Magellan Orbiter was launched from the Space Shuttle in 1989, arriving at Venus in 1991. This probe did extensive radar mapping of Venus as well as measuring its gravitational field, accounting for much of what we currently know about the Venusian surface topography. Venus has no known natural satellites. It also has the closest approach to the Earth of any of the planets – about 26 million miles.

Earth – Oasis Of The Solar System

Jewel of the Solar System

Now we come to Earth, the third planet from the Sun in our Solar System. The Earths orbit has a close approach to the Sun of 91.4 million miles and a long distance of 94.5 million miles. The rotational period is approximately 24 hours as it revolves around the Sun at about 66,600 miles per hour every 365 days. A tilt variance of 23.4 degrees on its axis accounts for the different seasons. The surface gravity is 1g; the gravity of the other planets will be compared by this standard. There is one natural moon which has a significant gravitational effect and has a mean distance of about 239,000 miles. The diameter of the Earth is roughly 7926 miles at the equator and 7900 miles at the poles; the rotational speed of about 1036 miles per hour at the equator accounts for the very slight flattening effect. Note that the rotational speed decreases with latitude until it becomes zero at the poles. This is easily calculated by dividing the Earths circumference at the equator, 24855 miles by 24 hours. Then it decreases by the cosine of the latitude until it reaches zero at the poles. The coldest place on Earth is Vostok Station in Antartica near the south pole at -128.6 degrees Fahrenheit and the hottest is 134.1 degrees Fahrenheit in Furnace Creek Ranch, California which is located in the Death Valley desert in the United States.

It is the only known object in the Universe to have intelligent life, or any kind of life for that matter. The Earth has an incredibly diverse spectrum of life which is beyond the scope of this article to go into. This ranges from the smallest creatures imaginable to absolute giants such as the Blue Whale. It is currently dominated by a somewhat intelligent form of life, the super-apes, also known as humans, who are gradually ascending the evolutionary scale. Although these creatures are making significant progress technologically, their long term survival is still in question due in large part to their still relatively primative state.

Mars – Is There Life?

The Red Planet

Mars is the fourth planet from the sun at a distance of 128.4 million miles at the closest and 154.8 million miles at the most distant and is the second smallest behind Mercury with a diameter at the equator of about 4200 miles, a little over half that of Earth. Temperatures range from -220 degrees Fahrenheit in the winter at the poles to over 70 degrees Fahrenheit in the summer near the equator – the average temperature being around -81 degrees Fahrenheit. Revolving around the Sun every 687 days Mars has an average orbital velocity of 53700 miles per hour. It has a distinctive red appearance and comes within 34 to 64 million miles of Earth at its closest approach. The reason for the range is because of Mars highly elliptical orbit around the Sun. Mars makes for a very interesting object in the night sky with even a fairly modest telescope.

The surface gravity is .3794g which is about 40 percent that of Earths. Mars actually has a thin atmosphere as well as seasons similar to Earth, although roughly twice as long. The surface of Mars seems to resemble the Moon in some respects because of the extensive cratering, as well as desert and polar regions of the Earth. Olympus Mons is the largest volcano and the second highest mountain yet discovered in our Solar System and Valles Marineras is the largest known canyon in the Solar System, with a length of over 1860 miles and a width of 370 miles across as well as 5 miles deep. By comparison the Earths Grand Canyon measures approximately 500 miles long and 18.6 miles across. Water actually exists on Mars, although only visible on the surface at the north polar region.

Face on Mars in Cydonia region

The Cydonia region of Mars is a very strange area with something that actually resembles a ‘face'(see above photo). Whether this is only a geographical anomaly or is something made by intelligent life forms eons ago has been the subject of much speculation and debate. Perhaps future space probes or manned missions to Mars will help resolve this enigmatic question. Mars has two very small moons, Deimos and Phobos which are 7.5 and 14 miles in diameter respectively. Various space probes have visited Mars from the 1960s to the present. These include the Mariner and Viking missions, the Mars Global Surveyor, and the Mars Reconnaissance Orbiter. It may well be that some primitive form of life actually exists on Mars; this is a question still awaiting some future time to be resolved.

Jupiter – Super Gas Giant

Jupiter with moon Io



Jupiter is the fifth planet from the Sun at a distance ranging from 740.5 million miles to 816.6 million miles. It is classified as a gas giant and in fact, it is so huge that it has two and one-half times the mass of all the other planets combined. Compared to the Earth Jupiter is unbelievably large; 1300 Earths could fit inside the volume of Jupiter! Jupiter has a surface gravity of2.528g, which is considerably greater than Earths – a 170 pound Earthman on Jupiter would weigh about 429.6 pounds!

Jupiter Compared to Earth

Jupiters distance from Earth ranges from 390682809 miles at the closest approach to 576682809 miles at the farthest. Temperatures can vary from -234 degrees Fahrenheit to a somewhat balmy(at least by Jupiter standards) 70 degrees Fahrenheit as you move deeper into its atmosphere. Despite this great distance, Jupiter is on average the third brightest object in the night sky behind the Moon and Venus. At its closest approach to Earth, it actually becomes the second brightest object in the night sky after the Moon. When this happens Jupiter reaches an apparent magnitude of -2.94, which is bright enough to cast shadows! Jupiters brightness and great size make it an ideal night sky object to observe with even the most modest of telescopes. The four largest moons of Jupiter are easily visible with only a pair of binoculars.

Jupiter is composed mostly of hydrogen with a fourth of its mass helium and makes a revolution around the Sun every 11.6 years at an orbital velocity of 29376 miles per hour. It has a diameter 11.6 times that of the Earth and a rotational speed of the outermost cloud tops at the equator of 28000 miles per hour; this compares to the Earths rotational speed of only 1000 miles per hour at the equator. This incredibly fast rotational speed creates violent storms in Jupiter’s atmosphere along with the various colors of latitudinal bands which are easily observed through a telescope. The Great Red Spot on Jupiter is the result of an incredibly violent storm and is also easily visible through a telescope. It is believed to have first been observed in 1665, over 350 years ago. More recent observations seem to show that the Great Red Spot is gradually fading which would indicate that the great storm is slowly abating.

Comet Shoemaker-Levy 9 broke apart and collided with Jupiter in July of 1994, which resulted in the first observation of this type of collision in the Solar System. Although it was believed for years that Jupiter had 12 moons, we now know that there are 79 known satellites, due in large part from the space probes sent to Jupiter which include Galileo and Juno. Future planned space probes to Jupiter include JUICE – Jupiter Icy Moon Explorer from the European Space Agency due to launch in 2022 – and NASA’s Europa Clipper in 2025. There can be little doubt that these future space probes will reveal much more exciting discoveries.

Saturn – The Ringed Planet

Beautiful Rings Of Saturn

Saturn is the sixth planet from the Sun, with an average distance of 886 million miles, and is the second largest planet in our Solar System behind Jupiter. Surface gravity is 1.065g, very similar to Earth. The orbit of Saturn varies from a close distance to the Sun of 839 million miles to a far distance of 934 million miles, and it travels around the Sun with a velocity of about 21000 miles per hour. Temperatures range from -188 degrees Fahrenheit to -300 degrees Fahrenheit. A Saturn year is about 29.5 Earth years. It also has the distinction of being the only ringed planet in Our Solar System, at least as far as any visible rings are concerned – Jupiter, Uranus, and Neptune all have rings which are invisible to amateur astronomers on Earth. Saturn is also classified as a gas giant and is about 95 times as massive as Earth with a diameter roughly 9 times that of Earth. Based on space probe data it is believed to have a solid core of iron and nickel surrounded by metallic hydrogen and helium and then a gaseous layer beyond that composed of methane, ammonia, nitrogen, and even some oxygen. An odd fact about Saturn is that it is the only planet in the Solar System with an average density less than water – about 30 percent less. This means that Saturn could float on water if it were possible to have that much water! The bands of Saturn appear somewhat less distinct and murkier than those of Jupiter. The orbital velocity at the equator is approximately21600 miles per hour and wind speeds on Saturn can reach 1100 miles per hour – this is higher than those on Jupiter(only about 224 miles per hour) but actually less than those on Neptune(1300 miles per hour).

Of course, the most prominent feature of Saturn is its magnificent ring structure which is easily visible through even a modest telescope from Earth, despite Saturn being 746 million miles at its closest approach. At its farthest distance from Earth, Saturn is on the opposite side of the Sun and over 1 billion miles away. The Rings of Saturn are composed mostly of ice crystals with some rock debris as well and are the most extensive ring system in the Solar System. The objects in it range from micrometers in size to a meter and beyond which all orbit Saturn.

Rings of Saturn as they might appear from one of its moons

A number of space probes from the European Space Agency and the United States have visited to investigate its physical characteristics including the rings themselves. These include the Pioneer 11 flyby in September of 1979 at a distance of 12400 miles, Voyager 1 flyby in November of 1980 with a close approach of 3300000 miles, Voyager 2 flyby in August of 1981 approaching within 13 million miles, and the Cassini-Huygens space probe which went into orbit around Saturn in July of 2004 after a flyby of Phoebe(one of Saturns moons). The Cassini probe lasted until September 2017 when it flew through the gaps in Saturns rings and then entered the atmosphere and was destroyed. The information obtained by Cassini was invaluable and will probably continue to be analyzed for years to come.


Cassini space probe orbiting Saturn

Saturn has 62 moons, only 13 of which have diameters larger than 31 miles. Saturns five brightest moons can easily be seen through a telescope, but only its largest moon Titan can be seen through a pair of binoculars. There can be little doubt that future space probes from NASA(and possibly the European Space Agency) will visit Saturn in the future, uncovering even more exciting revelations about the beautiful ringed planet.

Uranus – The Blue Icy Giant

Uranus the blue icy giant

Uranus, the seventh planet from the Sun, is the blue icy giant. It is about 1.7 billion miles away at the closest point and 1.89 billion miles away at its farthest. Uranus is traveling around the Sun with an orbital speed of approximately 15211 miles per hour and it takes Uranus about 84 Earth years to complete one revolution. It has the 3rd largest diameter and 4th greatest mass of all the planets in the Solar System. The surface gravity is .886g. Uranus(and Neptune as well) is classified as an ice giant because it is composed of different substances than Jupiter and Saturn. The main composition of Uranus is oxygen, nitrogen, carbon, and sulfur as opposed to mostly hydrogen and helium for Jupiter and Saturn. Uranus is also lacking a metallic hydrogen mantle deep below the surface like Jupiter and Saturn. Uranus does have some hydrogen and helium in its upper atmosphere, but due to its great distance from the Sun and its heavier element composition, it remains in a mostly icy state.

Uranus has the greatest eccentricity of any planet in the Solar System except for the dwarf planets Pluto, Eris, and Farout. This simply means that Uranus has the greatest variance between closest and farthest distances from the Sun. This blue giant has a diameter at its equator of 31763 miles and has a rotational velocity of 5791 miles per hour and winds can reach 560 miles per hour in its upper atmosphere. To give the size difference another way, if the Earth were a large apple, Uranus would be a basketball! Uranus is the coldest true planet in the Solar System with a minimum temperature of -371 degrees Fahrenheit. No wonder it is classified as an ice giant!

Uranus as a total of 27 known moons, 5 of which are considered major or large moons. These five moons are Miranda, Ariel, Umbriel, Titania, and Oberon. The largest of these are named Titania which is the eighth largest moon in the Solar System and about one-twentieth the size of Earths Moon. All five of these moons are relatively dark objects with a variance of Bond Albedo between 10 and 23 percent – Bond Albedo is defined as the fraction of total electromagnetic power(including and proportional to visible light) falling on an astronomical body and being radiated back out into space. It seems unlikely therefore that any of these moons would be easily visible from Earthbound amateur telescopes. There is a ring system as discovered by space probes but it is probably not visible from Earth, certainly not with amateur telescopes. Uranus itself, however, is easily visible with even a modest telescope as a glowing blue dot and also is visible with binoculars and even occasionally without any optics at all.

The Voyager 2 probe, launched in August 1977 had a flyby of Uranus in January 1986 discovering more about its ring structure and 10 new moons. A Uranus Orbiter and Probe is currently being planned as a mission concept by NASA.

Neptune – Strange Icy Blue Giant

Neptune the strange Icy Giant

Neptune is really a strange icy giant because of some of its anomalies, but we’ll get into that later. Right now let’s get some basic facts. Neptune is the eighth planet from the Sun and so is the farthest true planet in the Solar System. It orbits the Sun at a range of 2.77 billion miles at the closest point to 2.82 billion miles at the farthest with an average orbital speed of 12146 miles per hour. Thus Neptune makes a complete revolution around the Sun in about 164.8 years. It has the fourth largest diameter and the third most mass of the planets in our Solar System. The surface gravity is 1.14g. It is almost a twin to Uranus in size – Neptune has 17 times the mass of Earth whereas Uranus has 15 times the mass. Fifty-seven Earths would fit inside Neptune compared to 63 Earths inside Uranus. The atmosphere of Neptune is composed mainly of hydrogen and helium which is similar to Jupiter and Saturn but differs in the amount of icy methane and water as well as small amounts of nitrogen. Increased amounts of methane in Neptunes outer atmosphere can at least partially account for its deep blue appearance.

Neptune has more active weather patterns than Uranus

Neptune has much more active weather patterns in its atmosphere than Uranus, which has a somewhat hazy blue appearance – see above photo of Neptune compared to the previous photo of Uranus. With a rotational speed of 5996 miles per hour at the equator, Neptune has the strongest sustained winds of any planet in the Solar System, about 1300 miles per hour. The Great Dark Spot of Neptune is indicative of this incredibly violent weather system – again, see above photo from Voyager 2 flyby in 1989. Neptune’s outer atmosphere is the second coldest in the Solar System at -361 degrees fahrenheit(Uranus is slightly colder at -371 degrees fahrenheit). Although Neptune is at a much greater distance from the Sun than Uranus(about 50 percent) and receives about 40 percent less sunlight, it still has much more active weather patterns than Uranus. Also, the two planets have almost the same surface temperature. This can be accounted for in large part to the relatively high level of internal heat given off by Neptune’s core. Neptune radiates into space about 2.6 times the energy it receives from the Sun compared to only 1.1 times the energy for Uranus.

A very slight ring structure was discovered around Neptune in 1984 and was confirmed by the Voyager 2 flyby in 1989. Neptune has 14 known satellites which are probably not easily visible through an Earthbound amateur telescope. The names of Neptunes largest moons are Triton(largest), Nereid, Proteus, Naiad, Thalassa, Despina, Galatea, and Larissa. A very small moon named Hippocamp was discovered in 2013 by the Hubble Telescope. Neptune itself is visible through a good amateur telescope if one knows where to look to find it since it is not visible without optical aid. With such a telescope Neptune might appear as a dark blue dot about half the size of a pea held at arms length.

Pluto – Dwarf Planet

Pluto the dwarf planet as seen by New Horizons space probe

Instead of being a full-fledged member of the regal order of planets in our Solar System, Pluto has been demoted to dwarf planet status in recent years. Nevertheless many still consider Pluto a ‘full’ planet so I would be remiss if I didn’t discuss it here.

Pluto is a dwarf planet that resides in the Kuiper belt, which is a group of icy bodies orbiting the Sun beyond Neptune. It was discovered by Clyde Tombaugh in 1930 and for many years was considered to be the ninth planet in the Solar System. It revolves around the Sun in a highly eccentric orbit at a distance of 2.757 billion miles at the closest to 4.583 billion miles at the farthest. The average orbital speed is 10446 miles per hour and Pluto takes 248 years to make a complete revolution around the Sun. With the great distance from the Sun, the surface temperature on Pluto ranges from -369 degrees Fahrenheit to -387 degrees Fahrenheit  – the latter being one of the coldest in the Solar System and even colder than Uranus and Neptune. The surface gravity is .063g because of its small mass – a 170-pound man on Earth would weigh only 10.71 pounds, much less than even on the moon with its .1654g(28.12 pounds). The diameter of Pluto is 1474 miles compared to about 2159 miles for the Moon. Also, Pluto has about one-sixth the mass of the Moon and roughly one-third of its volume. So its really very easy to see why Pluto has been demoted to dwarf status among the planets.

Pluto rotates around its axis at only 29.2 miles per hour at the equator; the length of its day is 6 days 9 hours 17 minutes and 36 seconds. The composition of Pluto is mainly icy and rocky material – this includes nitrogen, methane, carbon monoxide, and various rocky substances.

Pluto has five known moons named Charon, Nix, Hydra, Styx, and Kerberos. Charon is the largest of these and also the closest to Pluto; it is tidally locked with Pluto with each face pointing to the other constantly. Charon is one-half the diameter of Pluto and one-eighth of its mass. Until discoveries by the New Horizons space probe, it was thought that the two were actually a single planet. With an apparent magnitude of 13.65 to 16.3, with the average being 15.1, Pluto is probably too faint for even the best amateur telescopes.

Eris & Farout – Dwarf Planets Beyond Pluto Deep In The Kuiper Belt

Eris is a dwarf planet that resides even beyond Pluto in the Kuiper belt – it is about 27 percent more massive than Pluto although it is slightly smaller by volume. Eris was not discovered by a space probe as one might think, but actually by a team from Palomar Observatory in January of 2005. It has not even been visited by a space probe yet so, unfortunately, there is no photo of Eris to display.

The diameter of Eris is 1445 miles(compared to 1430 for Pluto) and it revolves around the Sun about every 558 years at an orbital velocity of 7681.2 miles per hour. Eris makes its closest approach to the Sun at 5.723 billion miles and its farthest distance at 14.602 billion miles. This gives Eris an eccentricity in its orbit of .44068 which is much greater than Plutos .2488. The only known moon of Eris is named Dysnomia with an estimated diameter of 435 miles – this figure was achieved by radiometric observation. Observations from the Hubble Space Telescope found Eris to be about 500 times fainter than Pluto in the visible portion of the electromagnetic spectrum. The surface gravity of Eris is about .083g(compared to .063g for Pluto) and it has a surface temperature range of -405.67 degrees Fahrenheit to -360.4 degrees Fahrenheit.

Until recently it was thought that Eris was the farthest known object in the Solar System. That was until an object called 2018 VG18, nicknamed Farout, was discovered on November 10th, 2018. This object orbits the Sun at a distance of about 11.8 billion miles, over three times the distance of Pluto from the Sun and twice that of Eris. It is believed to have a diameter of around 310.7 miles while making a revolution around the Sun roughly every 929 years. With an apparent magnitude of 24.6, it is incredibly faint and of course far from the ability of any amateur astronomer to observe it.

How Many Planets Are In Our Solar System?

The technically correct answer is 8 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. I included the dwarf planets Pluto, Eris, and Farout as well since they certainly deserve to be mentioned and they add even more intrigue and mystery to the story of our Solar System. When Pluto was discovered in February of 1930 it was then considered to be the ninth planet in our Solar System, and so it remained for many decades thereafter until Pluto was demoted to a dwarf planet in 2006. So some may argue that there are 9 planets in the Solar System, while others may insist there are 10 or even 11 planets. Regardless, more exciting discoveries surely will come in the future; even now there are some scientists who suspect another massive celestial body, or planet, even further out in the icy netherworld of the Kuiper belt. So stay tuned, the final answer may not be in yet!

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