Nine Planets

NOTE: When this song was written and recorded, there were nine planets. We never thought this song would be the one that needed the most major revision. A new version, called "How Many Planets" will be made available here in late 2020.

The Solar System consists of the Sun - an average, yellow, middle-aged star - and all the objects that orbit it: some rotating, spherical bodies called planets or dwarf planets; tens of thousands of rocky asteroids; and, reaching out almost halfway to the nearest star in an icy, spherical halo, billions of comets. Locked within their orbital motions are records of the laws of physics. Scientists combine our understanding of these laws and careful observations of the orbits and compositions of these objects to paint a picture of the birth and evolution of the Solar System. It is a fascinating story, and we are finding new and surprising chapters all the time.

Family Portrait

Ancient astronomers watched for patterns in the sky. They noticed that the stars maintained their relative positions with respect to each other. They saw that the bright stars often looked like something familiar in their world and named these groupings accordingly - the bright stars that trace out the Big Dipper in the northern hemisphere skies were referred to in some cultures as the Great Bear, or the Drinking Gourd. The names of groups of stars are referred to as 'constellations'. Modern astronomers still use constellations as a way of referring to positions on the sky in much the same way as we all use the names of oceans and countries to refer to positions on the Earth.

The ancients also carefully watched the Sun and the Moon, and realized that their motions were regular and periodic. The Moon stayed in a narrow band on the sky, passing through only some of the constellations through its 29.53-day cycle from one full Moon to the next. These constellations were given special status - the constellations of the zodiac. These ancient astronomers also figured out that the Sun follows an almost identical path through the sky over the course of one year. On any given day the Sun and the Moon are at different positions along the zodiac, but they are each always someplace along the zodiac.

Modern astronomers call the Sun's path through the sky the 'ecliptic'. We now know that this path is determined by two things: the Earth's orbital period about the Sun (which causes the Sun to travel once around the ecliptic in one year), and the inclination of the Earth's spin axis (which is tilted 23.5 degrees away from straight up and down, with respect to the plane that the Earth orbits in).

With the stars organized into constellations and the paths of the Sun and the Moon well mapped, the ancient astronomers had one last feature of the sky to figure out: the Wanderers. The Wanderers were objects that looked like stars but did not stay still with respect to the other stars, nor follow a simple path like the Sun and Moon. One thing was certain: like the Sun and the Moon, the Wanderers restricted themselves to the constellations of the zodiac. (By now it should be obvious why ancient astronomers assigned mystic significance to these particular constellations!) Some Wanderers were never seen overhead; instead they were features of the early evening or early morning sky. These were named Venus and Mercury. Other Wanderers could be seen overhead, but they traced out a fascinating motion: they performed a loop-the-loop during their trek across the ecliptic. These were called Mars, Jupiter, and Saturn. Ancient Egyptian astronomers debated the nature of the heavens: was the Earth at the center of the Universe, or was the Sun instead at the center, and the Earth and Wanderers in orbit around the Sun?

This debate was reopened during the Renaissance, in large part due to the curiosity of Galileo, who decided to look at the sky through a telescope. What he saw ultimately resolved the debate, and opened the doors to our modern understanding of the Universe. Venus goes through phases, like the Moon. At the same time it regularly grows larger and smaller--a full Venus is smaller in apparent size than a crescent or new Venus. Galileo knew this could only happen if Venus shines by light reflected from the Sun, and if Venus orbits the Sun, not the Earth. Venus is smallest when it is farthest from us on its orbit around the Sun. Jupiter was seen to be a disk with four tiny objects circling around it with periods of a few days to a couple of months. Galileo saw that these objects orbited Jupiter, and not the Earth, which independently proved that all heavenly bodies did not revolve around Earth. Galileo was also the first person to see the rings of Saturn, now regarded as one of the most majestic sights in the Solar System. Galileo's observations ultimately forced the scientific community of his day to reject the Earth-centered Universe in favor of a model put forth by Copernicus with the Sun at the center. This process is known as the 'Copernican Revolution'. It didn't come without a personal cost to Galileo. Since his observations were seen by some in the church as being in direct conflict with Biblical teachings, Galileo spent the last years of his life under house arrest.

The Solar System as a Whole

What we now know about the planets starts with Galileo, but it certainly doesn't end there. For each planet, curious scientists who yearned to know more made careful observations, designed and developed more powerful telescopes, and collected ever more informative data. Today's planetary scientists get much of their information from space probes launched from Earth into orbits that give them a close-up look of the planets.

We now understand that the rotation of the Sun and the orbits of the planets about the Sun are the result of the initial spin that must have been present in the large cloud of gas and dust from which the Solar System formed 4.6 billion years ago. Our current picture of the birth of the Solar System starts when this cloud begins to collapse under the influence of gravity. The molecules begin to clump together as the cloud as a whole reels inward. As the cloud condenses and a central stellar core starts to form, the heavier material - rocks and metals - sinks toward the center of gravity. The lighter material - mostly hydrogen, and some dusty ice - remains in the outskirts of the cloud. The rocky bits begin to collect together into the planets nearest the Sun (the terrestrial planets: Mercury, Venus, Earth and Mars).

Some rocky bits never made it to planet status. They are the leftovers of terrestrial planet formation and they now reside in the asteroid belt, just beyond the orbit of Mars. The gassy and icy parts of the initial cloud also condensed into planets: Jupiter, Saturn, Uranus and Neptune - the Gas Giants. The icy chunks that didn't merge into the Gas Giants were flung out to the edge of our Solar System by gravitational interactions with these huge bodies. The Kuiper Belt beyond Neptune is home base for the short period (less than 200 years) comets, with the vast majority of comets occupying the Oort Cloud. The Oort Cloud is the home to billions of comets, whose total mass is estimated at about 40 Earths - pieces of a gas giant that wasn't meant to be.

A Tour of the Solar System

Keeping the overall picture outlined above in mind, let's take a short trip to each of the planets (and one very special dwarf planet) in our Solar System. Like a family, each member is unique, and has an important place within the group. And each family member shares certain traits of the family too, unless something catastrophic has occurred to change that. In the case of the Solar System, comets and asteroids that get knocked out of their orbits can collide with a planet, virtually erasing its primeval spin, and maybe even altering its original composition. We'll talk about distances from the Sun in terms of the Earth's distance. The Earth is defined as 1.0 Astronomical Unit (AU) away from the Sun.

Photo of Mercury


The closest planet to the Sun, it goes around the Sun once in 88 days. It orbits at a distance of 0.38 AU. Being so close to the Sun, Mercury is difficult to observe from Earth - it is never visible except just after sunset or just before sunrise. Mercury spins around 3 times on its axis for every two orbits about the Sun, and has practically zero orbital inclination. Its surface is covered with craters, much like the Moon, although it also has smooth regions that are probably evidence of ancient volcanoes. It does not show any current plate tectonics, but some of its surface features are probably due to compression of the outer crust. It has a large iron core that occupies most of the interior of the planet. Mercury is the smallest of the terrestrial planets (about 40% the diameter of Earth), and doesn't hold on to any significant atmosphere. Parts of its surface get very hot from the Sun beating directly down on it. It has no natural satellite (moon). The MESSENGER mission is our most recent mission to Mercury. It had been 30 years since the previous mission, Mariner 10, which had mapped less than half its surface!


Venus radar map The second planet from the Sun is our nearest neighbor, Venus. It has an orbital period of 225 days and is at a distance of 0.72 AU from the Sun. In many ways, Venus is practically the Earth's twin. It's about 95% the diameter of Earth and 80% of Earth's mass and so has virtually the same density. Its surface is not covered with ancient craters like that of the Moon and Mercury. But there are very important differences between Venus and the Earth that make life on the planet impossible for humans. Venus has a very dense, thick atmosphere. The atmospheric pressure on the surface is 90 times that on Earth. You'd have to go down one kilometer in the ocean to experience what that pressure feels like! Trapped in the clouds are droplets of sulfuric acid (known here on Earth as battery acid). These droplets never reach the surface of the planet. The dense atmosphere is composed mainly of carbon dioxide, giving rise to a very large greenhouse effect: light from the Sun reaches the surface, is re-radiated as heat, but gets trapped by the dense atmosphere. This causes the surface temperature to increase to over 700 Kelvin (truly hot enough to melt lead). Venus probably had significant oceans at one point, but this incredible temperature caused them to boil off. Another major difference is the rotational period of the planet. It is actually rotating slower than it orbits the Sun. One turn on its axis takes 243 days! Also, it rotates in a direction different from all the other planets and the Sun. Its rotation period is related to Earth's orbit in such a way that each time Earth and Venus have their closest approach, the same side of Venus is pointed toward Earth. The lack of craters on the surface is due to volcanoes, some of which may still be active. Another factor is that small impacting bodies are completely burned up in their path through the dense Venus atmosphere. Venus is the first planet to be visited by an unmanned landing craft from Earth. It has the most circular orbit of any planet. Like Mercury, Venus has no natural satellite.

Photo of Earth from space


Your home planet, my home planet, everyone we've ever heard of's home planet! At a distance of 1 AU from the Sun it takes Earth 1 year to orbit the Sun. Our 23.5 degree inclination (axial tilt) and 24 hour day give rise to day and night lengths and seasons whose variations are not too taxing on the life forms that have developed here. Earth is the largest of the terrestrial planets. While humans have been exploring Earth for millennia, we've only just gotten our first glimpses of the planet as a whole in the 20th Century. The Earth is composed of several layers: a crust, upper mantle, lower mantle, outer core and inner core. The crust is quite a bit thinner on the ocean floors than at the continents. The crust is solid, as is the inner core. Separate regions of the crust reside on distinct plates, which are flowing atop the more fluid mantle, a process known as plate tectonics. Of the terrestrial planets, plate tectonics only occur on Earth. The other layers deform easily. Most of the Earth's mass is in the mantle and core; the crust makes up only a small fraction. The core is made up mostly of nickel and iron and reaches very high temperatures: hotter than the surface of the Sun! What we know of the interior of our planet is largely due to studying the vibrations that result from Earthquakes (seismic studies). Except for volcanoes, which spew out some of the upper mantle, we directly know only of the tiny crust of Earth. Earth is composed mostly of iron (34%) followed by oxygen (30%) and other metals. Over 70% of the Earth's surface is covered by oceans of liquid water. Liquid water is essential to life as we know it, and the Earth is the only body in the Solar System that allows liquid water to exist at its surface. The large surface area of the oceans acts to moderate the temperature variations of the planet. The Earth is the densest planet in the Solar System. In orbit about the Earth is a rather large natural satellite - the Moon. (See "Lunar Love" for more details on the Moon!) Most theories of the formation of the Moon say that a major impact between Earth and a planet possibly half the size of Mars kicked up enough of Earth's early crust to form the Moon. Perhaps this event was also the catalyst for the variation of the thickness of the Earth's crust, allowing the liquid water to collect in oceans and beginning the process of plate tectonics. (For more information on Earth, see "Habitable Zone".)


Orbiting at a distance 1.6 AU from the Sun and circling the Sun in 1.9 Earth years is the last of the terrestrial planets. Mars is only about half the size of Earth. There is convincing evidence of surface erosion, leading scientists to believe that at one time a large amount of water existed on the surface. Recent images from the Mars Global Surveyor satellite suggest that some water may still be liquid under the surface of Mars, and that it has flowed recently and given rise to some suggestive geological features.

comparison of channels on Mars and the Earth

Mars' orbital eccentricity combined with its axial tilt give rise to seasonal variations in the Southern Hemisphere which are markedly more intense than in the North. Once per Martian year, huge dust storms sweep across the Southern portions of the planet. Mars also has ice caps of frozen water and carbon dioxide. The lower gravity of Mars has allowed much of its atmosphere to escape into space; what's left is mostly carbon dioxide. Mars has two moons, Phobos and Deimos, which are probably captured bodies from the neighboring asteroid belt. They are much smaller than Earth's Moon.

Image of Jupiter from Hubble Space Telescope


The king of the Solar System, Jupiter contains two thirds of the mass of the Sun's planets. At a distance of 5.2 AU, it takes nearly 12 years to orbit the Sun. Its composition is more like that of the Sun than the terrestrial planets, mainly hydrogen and helium. However, unlike the Sun, it does not fuse hydrogen into helium - the internal pressure and temperature never get high enough. Still, the interior pressure of Jupiter is estimated to be about 100 million times that at the surface of Earth. As the pressure increases in Jupiter's interior, the hydrogen atoms get pushed together to the density of a liquid. Even further in, the enormous pressure strips electrons from the hydrogen atoms. A material with such freely flowing electrons is called a metal. In fact, since there is so much liquid metallic hydrogen at the cores of Jupiter and Saturn, it is probably the most abundant metal in our Solar System! Jupiter's atmosphere is composed of three distinct cloud layers, which give the planet its distinctive appearance. At high altitudes, hydrogen behaves like a typical gas. The cause of the reddish/orange coloring of some of the bands is not yet well established. It is probably the result of warmer gases rising up from a lower layer in the atmosphere that contains carbon monoxide, methane, and other compounds. Brewing in the Jovian atmosphere for hundreds of years is the large reddish feature referred to as the Great Red Spot. While the feature has been seen on the surface of Jupiter for at least the last 300 years, details of its size and shape change with time. It appears it is a stable, though dynamic feature of the atmosphere. The planet has a strong magnetic field that would look as large as the Moon in our sky if we could see it directly.

Photo of the 4 largest moons of Jupiter Jupiter has at least sixteen moons in orbit about it. The four largest of these, Io, Europa, Ganymede and Callisto (left to right in the photo) , are easily seen in a telescope, and were first spied by Galileo. Each is similar in composition and size to the terrestrial planets. Io is about the size of Earth's Moon. Its surface is scarred by intense, ongoing volcanic eruptions. The constant volcanic activity is driven by the very strong tides raised on Io by massive and nearby Jupiter. Europa is slightly smaller than Earth's Moon. It is covered by ice and has no impact craters on its surface. It is the smoothest object in the Solar System, with no feature above 1 km in height. Some scientists believe there is a liquid ocean under the ice layer, upon which the ice layers ride like rafts. Europa is one of the most logical places to look for signs of bacterial life outside of Earth, because of the implied presence of liquid water. Ganymede is the largest moon in the Solar System, and is bigger than Mercury or Pluto and about 3/4 the size of Mars. Ganymede's surface is complex, with dark, heavily cratered regions coexisting with bright, smoother regions. The crust is believed to be a thick layer of ice. Callisto is about the size of Mercury. It is heavily cratered; in fact it is the only Solar System body whose surface shows virtually no signs of having been resurfaced since the cratering era, roughly 4 billion years ago.

Photo of Saturn


Another gas giant, Saturn is about one-third the mass of Jupiter. It is 9.6 AU from the Sun and orbits once every 29.5 years! Around it is a belt of bright rings. The Voyager 1 and 2 spacecraft flew close to these rings, and determined they were composed mostly of ice chunks: from mostly pebble sized to as big as a house. The rings are confined to Saturn's orbital plane (around its equator). Some regions of the rings are relatively empty. These gaps are caused by the gravity of satellites outside of the rings forcing particles into slightly different orbits. Saturn also has numerous moons, at least 18. One of them is Titan, a moon slightly bigger than Mercury. Titan's large mass has allowed it to maintain a significant atmosphere, although it contains gases poisonous to humans. Cloud layers within the atmosphere kept the Voyager craft from directly viewing the surface of this moon. The Cassini mission visited Saturn and Titan, and a probe was launched to pass through Titan's atmosphere and land on its surface, beaming up-close images back to Earth. Saturn also has a small satellite named Hyperion. Shaped like a potato, Hyperion is believed to be tumbling chaotically about its long axis.

Photo of Uranus


Orbiting at 19.2 AU and taking 84 years to orbit the Sun, Uranus is the next gas giant. A significant amount of methane gas gives this planet a distinctive bluish hue. It has a ring system, discovered in 1977, much smaller than Saturn's, composed of nine distinct rings. An interesting feature of Uranus is its very high inclination of about 97 degrees. In other words, it is practically lying on its side as it orbits the Sun. This means the North Pole is pointed toward the Sun for roughly 42 years, followed by 42 years of darkness as the South Pole gets its turn. Uranus has at least 21 satellites.


Photo of Neptune The final gas giant in the Solar System is Neptune. It is similar in size and composition to Uranus. Its blue color is also due to the presence of methane gas in its large atmosphere. It is very windy, probably the windiest place in the Solar System. It has cloud features in its upper atmosphere, the most notable being a great dark spot, a large storm system the size of the Earth. This spot was observed by Voyager when it encountered the planet. Recent HST observations do not show the storm! Neptune is 30 AU from the Sun and orbits with a period of 165 years. It has eight known satellites, the largest of which is Triton. Triton is slightly smaller than Earth's Moon, and has a hazy nitrogen/methane atmosphere. It orbits Neptune in a direction opposite the direction nearly everything else in the Solar System rotates. Triton is slowly spinning into Neptune and will one day be ripped apart by tidal forces, most likely forming a ring around Neptune.


image of Pluto from New Horizons flyby Since its discovery in the 1930's astronomers puzzled over Pluto's peculiarities. What was a small body of ice and rock doing out among the gas giants? Why was its orbit inclined with respect to the rest of the Solar System? When its moon Charon was discovered, scientists realized that the planet-to-satellite mass ratio was the highest of any system. Why was its eccentricity so high that at some times it actually orbits closer to the Sun than Neptune does? It didn't seem to fit with the picture of Solar System formation in which large gaseous planets were formed further out from the smaller rocky bodies. If that weren't weird enough, several satellites of the gas giants (and even Earth's own Moon) are larger than the planet Pluto. Adding to the confusion: due to its very small size and incredible distance from the Sun (nearly 40 AU), Pluto is extremely challenging to observe. Beginning in the 1990s, some astronomers discovered fairly large bodies beyond the orbit of Neptune that are now considered an important piece of the solution to the Pluto puzzle. Known as Trans-Neptunian Objects (TNOs), these bodies of mostly ice and rock inhabit the region beyond Neptune, with some of them in a 3:2 orbital resonance with Neptune. As more and more TNOs are discovered, some with masses approaching the mass of Pluto's moon, more astronomers decided that Pluto is probably the largest member of this class of objects and have removed it from the "planet" category and it's now referred to as a "dwarf planet." Moreover, this band of icy chunks marks the beginning of a large region of comet nuclei known as the Kuiper Belt (see "A Little Bit of Rock"). Is Pluto a planet? Or the giant of the Kuiper Belt? What do you think? In 2015, the New Horizons spacecraft flew by Pluto offering us amazing images and new questions.

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