Harper College


Sun: In Depth (NASA)

The sun is the closest star to Earth, at a mean distance from our planet of 149.60 million kilometers (92.96 million miles). This distance is known as an astronomical unit (abbreviated AU), and sets the scale for measuring distances all across the solar system. The sun, a huge sphere of mostly ionized gas, supports life on Earth. The connection and interactions between the sun and Earth drive the seasons, ocean currents, weather and climate. It is the center of our solar system.

About one million Earths could fit inside the sun. It is held together by gravitational attraction, producing immense pressure and temperature at its core. The sun has six regions - the core, the radiative zone, and the convective zone in the interior; the visible surface (the photosphere); the chromosphere; and the outermost region, the corona. The sun has no solid surface.

At the core, the temperature is about 15 million degrees Celsius (about 27 million degrees Fahrenheit), which is sufficient to sustain thermonuclear fusion. The energy produced in the core powers the sun and produces essentially all the heat and light we receive on Earth. Energy from the core is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the convective zone. The temperature drops below 2 million degrees Celsius (3.5 million degrees Fahrenheit) in the convective zone, where large bubbles of hot plasma (a soup of ionized atoms) move upwards.

The sun's surface - the photosphere - is a 500-kilometer-thick (300-mile-thick) region, from which most of the sun's radiation escapes outward and is detected as the sunlight we observe here on Earth about eight minutes after it leaves the Sun. Sunspots in the photosphere are areas with strong magnetic fields that are cooler, and thus darker, than the surrounding region. Sunspot numbers fluctuate every 11 years as part of the sun's magnetic activity cycle. Also connected to this cycle are bright solar flares and huge coronal mass ejections that blast off the sun.

The temperature of the photosphere is about 5,500 degrees Celsius (10,000 degrees Fahrenheit). Above the photosphere lie the tenuous chromosphere and the corona (crown). Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere can be seen as a red rim around the sun while the corona forms a beautiful white crown with plasma streaming outward, forming the points of the crown.

Above the photosphere, temperature increases with altitude, reaching as high as 2 million degrees Celsius (3.5 million degrees Fahrenheit). The source of coronal heating has been a scientific mystery for more than 50 years. Likely solutions emerged from observations by the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE) missions, but the complete answer still evades scientists. Recent missions - Solar Terrestrial Relations Observatory (STEREO), and the Solar Dynamics Observatory (SDO) - greatly improved our knowledge of the corona, getting us still closer to the answer. They also give us an unprecedented understanding of the physics of space weather phenomena such as solar flares, coronal mass ejections, and solar energetic particles. Space weather can adversely affect our technology in space and on Earth; these missions help us to develop space weather reports.




  • Sun
  • Sun Spot
  • Solar Flare
  • Sun Diagram
  • Sun Symbol
Physical Characteristics Specifications
Mean radius 696,342 km
110 Earths
Mass 1.989 x 1030 kg
3.33 x 105 Earths
Mean density 1.408 g/cm3
Surface gravity 274.0 m/s2
28 g
Rotation 25.38 d
Stellar Properties Specifications
Classification G2V
Absolute Magnitude 4.83
Temperature 5,778 K
Luminosity 3.846 x 102 W
Galactic Orbit Specifications
Distance 27,200 light years
Orbital Period 2.25 x 108 years


Mercury's eccentric orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear more than three times as large as it does when viewed from Earth. Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (430 degrees Celsius). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit (-180 degrees Celsius).

Because Mercury is so close to the sun, it is hard to directly observe from Earth except during dawn or twilight. Mercury makes an appearance indirectly -- 13 times each century, observers on Earth can watch Mercury pass across the face of the sun, an event called a transit. These rare transits fall within several days of 8 May and 10 November. The first two transits of Mercury in the 21st century occurred 7 May 2003, and 8 November 2006. The next are 9 May 2016, and 11 November 2019.

Mercury speeds around the sun every 88 days, traveling through space at nearly 50 km (31 miles) per second, faster than any other planet. One Mercury solar day (one day-night cycle) equals 175.97 Earth days.

Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking micrometeoroids. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury's magnetic field at the surface has just one percent the strength of Earth's, it interacts with the magnetic field of the solar wind to episodically create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When the ions strike the surface, they knock off neutrally charged atoms and send them on a loop high into the sky.

Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. Very large impact basins, including Caloris (1,550 km, or 960 miles, in diameter) and Rachmaninoff (306 km, or 190 miles), were created by asteroid impacts on the planet's surface early in the solar system's history. While there are large areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed as the planet's interior cooled and contracted over the billions of years since Mercury formed.

Mercury is the second densest planet after Earth, with a large metallic core having a radius of about 2,000 km (1,240 miles), about 80 percent of the planet's radius. In 2007, researchers used ground-based radars to study the core, and found evidence that it is partly molten (liquid). Mercury's outer shell, comparable to Earth's outer shell (called the mantle and crust), is only about 400 km (250 miles) thick.



  • Mercury
  • Mercury Planet Cores
  • Mercury Symbol
Orbit Specifications
Semi-major axis 5.791 x 107 km
0.387 AU
Eccentricity 0.21
Orbital Period 87.969 d
Physical Characteristics Specifications
Mean radius 2,439.7 km
0.3829 Earths
Mass 3.30 x 1023 kg
0.055 Earths
Mean density 5.427 g/cm3
Surface gravity 3.7 m/s2
0.38 g
Axial tilt 2.11°

The second planet from the Sun, Venus, is almost a twin to Earth in size and mass. With a surface temperature 462° C, even lead will melt on Venus. The atmosphere is 96% carbon dioxide and has a pressure 92 times that of Earth. Venus is the hottest planet.

The thick atmosphere of Venus makes observations of the surface with visible light impossible. The surface of Venus was mapped using radar by the Magellan probe. This map shows 167 volcanoes over 100 km across and almost 1000 impact craters, all younger than about 600 million years.
The surface of Venus gets totally recycled every few hundred million years. Interior heat builds up and melts the entire crust over a period of about 100 million years.

While all the other planets rotate counter-clockwise on their axis, Venus rotates clockwise with a day of 243 Earth days. This reverse rotation was caused by a huge impact that reversed the planet's spin and gave it a very long day. On Venus the Sun rises in the west and sets in the east.

The orbit of Venus is inclined slightly relative to Earth's orbit, so it usually does not cross the face of the Sun as seen from Earth. Venus transits occur every 120 years or so, the last being June 5, 2012. The next transit is not until Dec 10, 2117.

Because it has an orbit interior to Earth, Venus goes through phases just like the Moon. Discovery of these phases by Galileo in 1610 proved that the Sun was the center of the Solar System.

  • Venus
  • Venus
  • Venus Symbol
Orbit Specifications
Semi-major axis 1.08 x 108 km
0.728 AU
Eccentricity 0.0067
Orbital Period 224.7 d
Physical Characteristics Specifications
Mean radius 6051.8 km
0.9499 Earths
Mass 4.8676 x 1024 kg
0.815 Earths
Mean density 5.243 g/cm3
Surface gravity 8.87 m/s2
0.904 g
Axial tilt 177.36°

Earth is the third planet from the Sun and the only planet currently known to harbor life. It has a molten core that generates a protective magnetic field that shields against the solar wind and flares. Earth is the only planet that has surface water.

Tectonic plates float on the hot magma just below the solid rocky crust of the Earth. These plates make up the continents of Earth and have shifted over time, creating mountain ranges and causing earthquakes. For example, Africa and South America were once adjacent, but are now separated by the Atlantic Ocean.

The Moon is the largest moon in the solar system relative to its planet. Overall, our Moon is similar to the magma of the Earth. It was formed by a gigantic impact with another planet early in the solar system that blasted magma and crust into orbit around the Earth. That material pulled together via gravity and the Moon was formed. The Moon stabilizes the tilt of the Earth, helping the development of life.

Rocks returned from the Apollo missions date the Moon to 4.5 billion years. This is consistent with primordial meteorites that date the Solar System to 4.6 billion years.

Water on Earth was delivered by impacts of comets and icy asteroids in the early era of the Solar System. There is evidence that the amino acids found in comets may have provided the building blocks for early life on Earth.

  • Earth
  • Moon
  • Earth Symbol
Orbit Specifications
Semi-major axis 1.496 x 108 km
1.00000261 AU
Eccentricity 0.012
Orbital Period 365.2563 d
Physical Characteristics Specifications
Mean radius 6,371 km
0.533 Earths
Mass 5.97219 x 1024 kg
3.0 x 10-6 Suns
Mean density 5.515 g/cm3
Surface gravity 9.780327 m/s2
0.99732 g
Axial tilt 23° 26' 21.4119''
Moon Specifications
Radius 1,737.4 km
Mass 7.349 x 1022 kg
Semi-major axis 384,400 km
Orbital period 27 d 7 h 43.7 m

Named after the Roman god of war, the red appearance of Mars is caused by iron oxide that covers much of its surface. The fourth planet shows volcanoes, valleys, deserts and polar ice caps much like those on Earth. A day on Mars is almost the same as Earth: 24h 37m 22s.

Channels on the surface of Mars are very similar to river channels on Earth. This suggests that liquid water once flowed on the surface of Mars when it had a thicker atmosphere. Today, much of the Martian atmosphere has been eroded away by the solar wind, and water can no longer exist on the surface due to the low atmospheric pressure. Large amounts of water ice are thought to be trapped in a thick permafrost layer below the surface. Radar data from satellites support this concept.

Mars is also home to the largest volcano in the Solar System – Olympus Mons. This shield volcano would cover the state of Arizona, and is so tall it extends out of the Martian atmosphere. With a pressure suit, you could hike from the surface of Mars to space! Now that's a field trip.

For years people thought Mars was inhabited. Percival Lowell build an observatory in Flagstaff, AZ to study Mars and wrote many books about life on Mars. H.G. Wells wrote War of the Worlds, a book about a Martian invasion. A radio play in 1938 based on the book caused widespread panic. Today we hope to find bacteria on Mars.

Mars is the most explored of the planets. Currently the Curiosity rover is the largest on Mars. Several orbiters are also active around the planet and more are on their way.

  • Mars
  • Mars Closeup
  • Mars Symbol
Orbit Specifications
Semi-major axis 2.28 x 108 km
1.5237 AU
Eccentricity 0.09
Orbital Period 686.971 days
Physical Characteristics Specifications
Mean radius 3,390 km
0.533 Earths
Mass 6.4185 x 1023 kg
0.107 Earths
Mean density 3.93 g/cm3
Surface gravity 3.71 m/s2
0.38 g
Axial tilt 25.19°

Asteroid Classifications (NASA)

Main asteroid belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.

 Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan's tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.

Near-Earth asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids - those that could pose a threat to Earth.



  • Asteroid Belt
Orbit Specifications
Semi-major axis 2.4921 x 10?? km
1.5237 AU
Eccentricity 0.093315
Orbital Period 686.971 d
Physical Characteristics Specifications
Mean radius 3389 km
0.533 Earths
Mass 6.4185 x 1023 kg
0.107 Earths
Mean density 3.9335 g/cm??
Surface gravity 3.711 m/s2
0.376 g
Axial tilt 25.19??

Jupiter is the largest planet in our Solar System. Jupiter is so large that all the other planets combined do not add up to the mass of Jupiter. All we can see are the tops of the clouds that circle the planet. Chemicals interact with UV from the Sun to create the bands on Jupiter.

Due to its rapid rotation and liquid metallic hydrogen core, Jupiter has an intense magnetic field. As the three inner moons rotate through this magnetic field, they generate electricity causing bright aurora at Jupiter's poles. In the picture at right, the bright spot to the left is caused by Io. This also creates radio signals we can detect on Earth.

Since Jupiter formed just outside the frost line in the early Solar System, it was in position to pick up much of the hydrogen and helium driven out of the inner system by the heat of the Sun. Jupiter's gravity creates pressures high enough in the core for the hydrogen to act like a metal and conduct electricity.
The Great Red Spot is a cyclonic storm larger than Earth. It has existed for centuries. This storm rises 8 km above the surrounding clouds. You can see it in the picture above. The Spot does not rotate at the same rate as Jupiter itself.

Jupiter has 67 moons, but the ones we can see at Henize are those discovered by Galileo. Io is a volcanic moon while Europa has an ocean of liquid water beneath its surface – and possibly life. Ganymede is the largest moon in the Solar System, and Callisto is a frozen ice ball covered with craters.

  • Jupiter
  • Jupiter Atmosphere
  • Jupiter Planet Views
  • Jupiter Symbol
Orbit Specifications
Semi-major axis 7.79 x 108 km
5.2 AU
Eccentricity 0.049
Orbital Period 11.86 years
Physical Characteristics Specifications
Mean radius 69,900 km
11.209 Earths
Mass 1.9 x 1027 kg
317.8 Earths
Mean density 1.33 g/cm3
Surface gravity 24.79 m/s2
2.53 g
Axial tilt 3.1°

Possibly the most dramatic view in the Solar System, the sixth planet sports rings that can be viewed with a small telescope. It is also the least dense of the planets – less dense than water. Due to its rapid rotation and lower gravity, Saturn is a flattened sphere that bulges in the middle.

The rings of Saturn are very thin. This picture from the Cassini probe shows the shadows of the rings while edge-on the rings are almost invisible. The rings average only 20 m in thickness. This image also shows a storm that has gone completely around the planet. The large gap in Saturn's rings is called the Cassini division. There are many separate rings. Recently, a new moon was discovered forming from the ring material.

When Galileo first observed the rings of Saturn, he called them “ears”. Due to the tilt of Saturn, sometimes the rings are edge-on to Earth and cannot be seen.

Saturn has at least 150 moons. Titan is the largest moon and one we can see from Henize. Titan is larger than Mercury and has an atmosphere that is mostly nitrogen with a little methane. Lakes of liquid hydrocarbons have been found on the surface of Titan. The Huygens lander reached Titan in 2005.

The moon Enceladus has liquid water beneath an ice crust. Geysers of water have been observed to shoot into space. Material in these geysers show organic compounds that suggests life exists in a large ocean below the surface.

Like Jupiter, Saturn has metallic hydrogen in its interior. The lower gravity does not compress it as much, however. The interior of Saturn is hot – 11,700° C – and Saturn radiates 2.5 times the heat it receives from the Sun.

  • Saturn
  • Saturn Cloesup
  • Saturn Symbol
Orbit Specifications
Semi-major axis 1.43 x 109 km
9.582 AU
Eccentricity 0.056
Orbital Period 29.46 years
Sidereal rotation 10.57 h
Physical Characteristics Specifications
Mean radius 58,232 km
9.449 Earths
Mass 5.68 x 1026 kg
95.152 Earths
Mean density 0.687 g/cm3
Surface gravity 10.44 m/s2
1.065 g
Axial tilt 26.73°

Discovered as a planet by William Hershel in 1781, Uranus can be seen with the naked eye, but was always thought to be a star due to its very long orbital period. This discovery helped make Hershel famous and provided a annual stipend of £200 from King George III.

The composition of Uranus is different from Jupiter and Saturn. While the two large planets are gas giants mostly made of hydrogen and helium, Uranus and Neptune are ice giants with a mantle of compressed water, ammonia and methane ice with an atmosphere of hydrogen.
While we can view Uranus from Henize, it is a boring blue-green ball in the eyepiece.

Uranus has an extreme axial tilt. This causes wind speeds on Uranus to reach 900 km/h during the solstice as energy flows from the hemisphere facing the Sun to the unilluminated hemisphere. Still, the atmosphere of Uranus showed almost no features during the Voyager 2 flyby in 1986. Voyager 2 also discovered a faint ring system about Uranus. In near-infrared, the Hubble can image faint cloud bands in the atmosphere.

Research at the Lawrence Livermore Lab suggests that pressures deep with within Uranus may result in an ocean of liquid diamond at the base of the mantle as carbon atoms form the tetrahedral arrangement we see in diamonds.

While Uranus has 27 known moons, none are particularly large. They also have quite dark surfaces and cannot be seen from Henize. The names of the moons come from characters in works by Shakespeare and Alexander Pope.

  • Uranus
  • Uranus Diagram
  • Uranus Symbol
Orbit Specifications
Semi-major axis 2.88 x 109 km
19.229 AU
Eccentricity 0.047
Orbital Period 84.02 yr
Sidereal Rotation 17.24 hr retrograde
Physical Characteristics Specifications
Mean radius 25,362 km
4.007 Earths
Mass 8.681 x 1025 kg
14.536 Earths
Mean density 1.27 g/cm3
Surface gravity 8.69 m/s2
0.886 g
Axial tilt 97.77°

Neptune is the farthest planet in the Solar System. It is an ice giant almost identical to Uranus in mass, size and composition. It appears as a tiny blue dot in the telescopes at Henize. It has been visited by only one spacecraft as Voyager 2 passed by in 1989.

The discovery of Neptune was a triumph of science. It was determined that the orbit of Uranus was not exactly as expected. Sometimes Uranus was ahead in its orbit, sometimes it lagged. Calculations by the French mathematician Urbain Le Verrier predicted the position of the mystery planet. Those calculations were sent to the Berlin Observatory and on the evening of Sep 23, 1846, Johanne Galle found Neptune within an hour. The planet is named for the Roman god of the sea./

In 2011, Henize celebrated the one year anniversary of the discovery of Neptune. One Neptunian year, of course. Could you imagine a school year on Neptune?

Unlike its twin, Uranus, Neptune has active and visible weather patterns. The Great Dark Spot on Neptune is driven by winds of up to 2,100 km/h. Due to its distance from the Sun, the cloud tops of Neptune are only -218° C. Methane in the upper atmosphere gives Neptune its blue color.

he gravity of Neptune affects the objects in the Kuiper Belt beyond Neptune. The dwarf planet Pluto, for example, is locked into a 2:3 orbital resonance with Neptune, so even though Pluto crosses Neptune's obit, they can never collide.

We continue to discover objects in the Kuiper Belt. Plutoids such as Eris, Sedna, and Makemake are all leftovers from the formation of our Solar System when it formed from a cloud of gas and dust 4.5 billion years ago.

  • Neptune
  • Neptune Symbol
Orbit Specifications
Semi-major axis 4.46 x 109 km
30.07 AU
Eccentricity 0.087
Orbital Period 164.79 yr
Sidereal rotation 16.11 h
Physical Characteristics Specifications
Mean radius 24,622 km
3.883 Earths
Mass 1.0243 x 1026 kg
17.147 Earths
Mean density 1.638 g/cm3
Surface gravity 11.15 m/s2
1.14 g
Axial tilt 28.32°


Pluto is classified as a dwarf planet and is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system. These icy, rocky bodies are called Kuiper Belt objects or trans-neptunian objects.

Pluto is about two-thirds the diameter of Earth's moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's moon. Pluto is more massive than Ceres -- the dwarf planet that resides in the asteroid belt between Mars and Jupiter -- by a factor of 14.

Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the sun. (One AU is the mean distance between Earth and the sun: about 93 million miles or 150 million kilometers.) From 1979 to 1999, Pluto was actually closer to the sun than Neptune, and in 1989, Pluto came to within 29.8 AU of the sun, providing rare opportunities to study this small, cold, distant world.

Pluto has a very large moon that is almost half its size named Charon, which was discovered in 1978. This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 12,200 miles (19,640 kilometers).
  • Pluto
  • Pluto Symbol
Orbit Specifications
Semi-major axis 5.9 x 109 km
39.4 AU
Eccentricity 0.25
Orbital Period 247.92 years
Physical Characteristics Specifications
Mean radius 1,185 km
0.186 Earths
Mass 1.3 x 1022 kg
0.002 Earths
Mean density 1.86 g/cm3
Surface gravity 0.66 m/s2
0.07 g
Axial tilt 122.5°
Last Updated: 12/14/23