The sun lies at the heart of the solar system, where it is by far the largest object. It holds 99.8 percent of the solar system's mass and is roughly 109 times the diameter of the Earth — about one million Earths could fit inside the sun. The visible part of the sun is about 10,000 degrees Fahrenheit (5,500 degrees Celsius), while temperatures in the core reach more than 27 million F (15 million C), driven by nuclear reactions. The sun is one of more than 100 billion stars in the Milky Way. It orbits some 25,000 light-years from the galactic core, completing a revolution once every 250 million years or so.

Formation & Evolution

The sun is relatively young, part of a generation of stars known as Population I, which are relatively rich in elements heavier than helium. The sun was born about 4.6 billion years ago. Many scientists think the sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun.

Chemical composition

Just like most other stars, the sun is made up mostly of hydrogen, followed by helium. Nearly all the remaining matter consists of seven other elements — oxygen, carbon, neon, nitrogen, magnesium, iron and silicon. For every 1 million atoms of hydrogen in the sun, there are 98,000 of helium, 850 of oxygen, 360 of carbon, 120 of neon, 110 of nitrogen, 40 of magnesium, 35 of iron and 35 of silicon. Still, hydrogen is the lightest of all elements, so it only accounts for roughly 72 percent of the sun's mass, while helium makes up about 26 percent. - See more at:

Internal structure and atmosphere

The sun and its atmosphere are divided into several zones and layers. The solar interior, from the inside out, is made up of the core, radiative zone and the convective zone. The solar atmosphere above that consists of the photosphere, chromosphere, a transition region and the corona. Beyond that is the solar wind, an outflow of gas from the corona.

The core extends from the sun's center to about a quarter of the way to its surface. Although it only makes up roughly 2 percent of the sun's volume, it is almost 15 times the density of lead and holds nearly half of the sun's mass.

Next is the radiative zone, which extends from the core to 70 percent of the way to the sun's surface, making up 32 percent of the sun's volume and 48 percent of its mass. Light from the core gets scattered in this zone, so that a single photon often may take a million years to pass through.

The convection zone reaches up to the sun's surface, and makes up 66 percent of the sun's volume but only a little more than 2 percent of its mass. Roiling "convection cells" of gas dominate this zone. Two main kinds of solar convection cells exist — granulation cells about 600 miles (1,000 kilometers) wide and supergranulation cells about 20,000 miles (30,000 kilometers) in diameter.

The photosphere is the lowest layer of the sun's atmosphere, and emits the light we see. It is about 300 miles (500 km) thick, although most of the light comes from its lowest third. Temperatures in the photosphere range from 11,000 F (6,125 C) at bottom to 7,460 F (4,125 C) at top. Next up is the chromosphere, which is hotter, up to 35,500 F (19,725 C), and is apparently made up entirely of spiky structures known as spicules typically some 600 miles (1,000 km) across and up to 6,000 miles (10,000 km) high.

After that is the transition region a few hundred to a few thousand miles or kilometers thick, which is heated by the corona above it and sheds most of its light as ultraviolet rays. At the top is the super-hot corona, which is made of structures such as loops and streams of ionized gas. The corona generally ranges from 900,000 F (500,000 C) to 10.8 million F (6 million C) and can even reach tens of millions of degrees when a solar flare occurs. Matter from the corona is blown off as the solar wind.

Inner Planets

The inner four planets are called terrestrial planets. That means that they are like the earth in some ways. The terrestrial planets Mercury, Venus, Earth, Mars, and the earth's moon have similar compositions and densities. These planets are close to the sun, rocky, and dense. They are fairly small. They have few moons and no rings. Their environments are oxidized; that is, oxygen dominates the chemistry. Meteors, tectonic activity, and erosion have modified the surfaces of the terrestrial planets.

The atmospheres of the terrestrial planets have changed as a result of sunlight (which produces chemical reactions, such as those that break apart water molecules), escape of light gases, volcanic activity, and in the case of our planet, biological activity.
The main stages of evolution of the terrestrial planets are:

  • formation
  • impacts (during the period of early bombardment)
  • flooding of basins with lava
  • lessening of impacts, and, in some cases, the formation of an atmosphere
  • plate tectonics
  • lower activity
  • cooling, inactivity

Mercuryis the planet closest to the Sun, but surprisingly not the hottest. Because it has almost no atmosphere, there is no greenhouse effect to trap the heat, so temperatures vary from extreme hot to extreme cold. In fact it has the widest day/ night temperature change in the solar system. Its surface of craters resembles the Moon because it has been bombarded by comets etc. The planet was named for the Roman god Mercury, a winged messenger, and it travels around the Sun faster than any other planet. Mercury is difficult to see from Earth.

  • Size: 0.382 the diameter of Earth;
  • Diameter: 3,032.4 miles (4,880 km)
  • Atmosphere: A thin mixture of helium (95%) and hydrogen
  • Temperature: The sunlit side can reach up to 950°F (510°C) and the dark side can drop as low as -346°F (-210° C),avg day temp = 662°F,avg night temp= -274°F
  • Rotation (1 Day) = 59 Earth days
  • Revolution (1 Year) = 88 Earth days
  • Your weight: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mercury.
  • Distance from Earth: 57 million miles, at the closest point in its orbit
  • Mean Distance from Sun: 36 million miles (57.9 million km)
  • Satellites: 0
  • Rings: 0

Venusis the brightest planet in the sky because it is very reflective - it is covered in clouds. It has a tremendous greenhouse effect that does not allow the heat to escape leaving the temperature to hover around 880°F which makes it the hottest planet in the solar system. Venus is often called Earth's twin because the two planets are close in size, but that's about the only similarity. Venus' rotation is not only extremely slow but it is also backward or retrograde. Venus is named after the Roman goddess of love and beauty and appears as a bright, white disk from Earth.

  • Size: About 650 miles smaller in diameter than Earth
  • Diameter: 7,519 miles (12,100 km)
  • Surface: A rocky, dusty, waterless expanse of mountains, canyons, and plains, with a 200-mile river of hardened lava
  • Atmosphere: Carbon dioxide (95%), nitrogen, sulfuric acid, and traces of other elements
  • Rotation (1 day) = 243 Earth days (retrograde rotation or "backward")
  • Revolution (1 year) = 225 Earth days
  • Your weight: If you weigh 100 pounds on Earth, you would weigh 88 pounds on Venus.
  • Mean Distance from Sun: 64.24 million miles (108 million km)
  • Satellites: 0
  • Rings: 0

Earthis the third planet from the sun and is 4.6 billion years old as determined by radioactive dating. It is 5.5 times more dense than water on average. Its inner core is made up of solid iron with a molten outer core that produces it's magnetic field. Earth's atmosphere is made up of 78% Nitrogen, 21% Oxygen, and traces of other elements. Our greenhouse effect regulates the temperature on Earth and is essential for our survival. Also, Earth is not perfectly round; it bulges at the equator and is flatter at the poles.

  • Size: Four planets in our solar system are larger and four are smaller than Earth
  • Diameter: 7,926.2 miles (12,756 km)
  • Surface: Earth is made up of water (70%), air, and solid ground. It appears to be the only planet with water
  • Atmosphere: Nitrogen (78%), oxygen (21%), other gases
  • Rotation of its axis: 23 hours, 56 minutes, 4 seconds
  • Rotation around the Sun: 365.2 days
  • Mean Distance from Sun: 92.9 million miles (149.6 million km)
  • Satellites: 1
  • Rings: 0

Marsis the fourth planet from the Sun, situated between Earth and Jupiter. The surface of Mars contains silicon dioxide and iron oxide. The iron oxide is rust, which gives it a reddish color. Mars has two moons Phobos and Deimos. Like our moon, the two moons keep the same side toward Mars as they revolve around the planet. They are small and appear to be captured asteroids. Mars has the largest volcanic mountain in the solar system, Mt. Olympus, which is 2.5 times the size of Mt. Everst. The poles of Mars are covered with frozen water and frozen carbon dioxide producing white polar ice caps. There can be no liquid water due to the thin, low pressure atmosphere which allows water to vaporize.

  • Size: About one-half the size of Earth in diameter
  • Diameter: 4,194 miles (6,794 km)
  • Surface: Canyons, dunes, volcanoes, and polar caps of water ice
  • Atmosphere: carbon dioxide (95%)
  • Temperature: as low as -305°F (-187°C)
  • Rotation of its axis: 24 Earth hours, 37 minutes
  • Rotation around the Sun: 687 Earth days (almost 2 Earth years)
  • Your weight: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mars.
  • Distance from Earth: 35 million miles (56 million km) at the closest point in its orbit
  • Mean Distance from Sun:141.71 million miles (227.9 million km)
  • Satellites: 2
  • Rings: 0

Outer Planets

The next four planets out from the sun are Jupiter, Saturn, Uranus, and Neptune. They are collectively referred to as the Jovian planets (means Jupiter like). They are significantly different from the terrestrial planets. Besides being farther from the sun, the Jovian planets are much larger, are more gaseous, have lower densities, and all have some sort of rings. Their large sizes are responsible for increased gravity which can hold onto lighter elements like gases.

A belt of asteroids (fragments of rock and iron) between Mars and Jupiter separate the four inner planets from the five outer planets. Pluto, which fits in with neither the terrestrial or the Jovian planets, is actually considered a Kuiper Belt object which we will see in more depth on theKuiper Beltpage.

Jupiteris the largest planet in the solar system. It's diameter is about 11 times that of Earth. You could fit over 1,000 Earths inside! It is a giant ball of gas with no solid surface. It does however have a liquid center due to the high pressures forcing it into a liquid and a small solid core. Jupiter has 63 natural satellites (or moons). The largest four are Io, Europa, Ganymede, and Callisto. Viewed through a large telescope, Jupiter is stunningly colorful and looks like a disk covered with bands of blue, brown, pink, red, orange, and yellow. Its most distinguishing feature is the Great Red Spot, an intense, giant windstorm larger in size than Earth, which has continued for centuries without any signs of dying down.

  • Size: 11 times the diameter of Earth
  • Diameter: 88,736 miles (142,800 km)
  • Surface: A hot ball of gas and liquid
  • Atmosphere: Whirling clouds of colored dust, hydrogen, helium, methane, water, and ammonia.
  • Temperature: -234°F (-148°C) average
  • Rotation of its axis: 9 hours and 55 minutes
  • Rotation around the Sun: 12 Earth years
  • Weight: If you weigh 100 pounds on Earth, you would weigh 265 pounds on Jupiter.
  • Distance from Earth: At its closest, 370 million miles (591 million km)
  • Mean Distance from Sun: 483.88 million miles (778.3 million km)
  • Satellites: 63
  • Rings: 4

Saturnis the second-largest planet and has majestic rings surrounding it. Saturn's seven rings are flat and lie inside one another. They are made of billions of ice covered particles so they are not solid bodies they are orbiting debris. Ice reflects light, that is why they are so bright and we can see them through a regular telescope from Earth. Saturn's density is lighter than water (0.7) which means if you had a body of water big enough to hold it, it would float! It's composition is 73% Hydrogen and 26% Helium and 1% other which is similar to both Jupiter and the sun.

  • Size: About 10 times larger than Earth in diameter
  • Diameter: 74,978 miles (120,660 km)
  • Surface: Liquid and gas
  • Atmosphere: Hydrogen and helium
  • Temperature: -288°F (-178°C)
  • Rotation of its axis: 10 hours, 40 min
  • Rotation around the Sun: 29 Earth years!
  • Your weight: If you weigh 100 pounds on Earth, you would weigh 107 pounds on Saturn.
  • Distance from Earth: 744 million miles at the closest point
  • Average distance from Sun: 887.14 million miles (1,427 million km)
  • Satellites: 40+
  • Rings: 1,000?

Uranusis a greenish-blue planet, twice as far from the Sun as its neighbor Saturn. It's axis of rotation is sideways and it takes 84 years to revolve around the sun! It has approximately 20 years of daytime and 20 years of nighttime because one pole faces the sun for a couple of decades and then the other for a couple. Uranus wasn't discovered until 1781 by William Herschel who named it GeorgiumSidus (the Georgian star) after the English king, George III. Later its name was changed to Uranus, after an ancient Greek sky god, since all the other planets had been named after Roman and Greek gods.

  • Size: 4 times larger than Earth in diameter
  • Diameter: 32,193 miles (51,810 km)
  • Surface: Little is known
  • Atmosphere: Hydrogen, helium, and methane
  • Temperature: uniform temperature of -353°F (-214°C)
  • Rotation of its axis: 17 hours
  • Rotation around the Sun: 84 Earth years
  • Your weight: Not known
  • Distance from Earth: At the closest point, 1,607,000,000 miles
  • Mean Distance from Sun: 1,784 million miles (2,870 million km)
  • Satellites: 27
  • Rings: 11

Neptune, the 8th planet from the sun, is named for an ancient Roman sea god, and is a stormy blue planet about 30 times farther from the Sun than Earth. Close-up photographs from Voyager revealed the "Great Dark Spot", a large (19,000 mile wide) high-pressure cyclone rotating counter-clockwise. It has recently disappeared however. Neptune was discovered when astronomers realized that something was exerting a gravitational pull on Uranus, and that it was possible that an unknown planet might be responsible. Through mathematical calculations, astronomers determined there was indeed an undiscovered planet out in space a year before it was actually seen for the first time through a telescope (in 1846).

The Voyager 2 spacecraft flew by Neptune in 1989. It showed that the planet is, basically, a twin of Uranus based on its size, density, and chemical composition. Like Uranus, Neptune is distinctly colored light blue, from the CH4 in its atmosphere. Neptune's period of revolution is 165 years.

  • Size: Almost 4 times the size of Earth in diameter
  • Diameter: 30,775 miles (49,528 km)
  • Surface: A liquid layer covered with thick clouds and with constant, raging storms
  • Atmosphere: Hydrogen, helium, methane, and ammonia
  • Temperature: -353°F (-214°C)
  • Rotation of its axis: 16 hours
  • Rotation around the Sun: 165 Earth years
  • Your weight: Not known
  • Distance from Earth: 2,680,000,000 miles at closest point
  • Mean Distance from Sun: 2,796.46 million miles (4,497 million km)
  • Satellites: 13
  • Rings: 4