How the Sun Got its Name

A solar system refers to a star and all the objects that travel in orbit around it. Our solar system consists of the sun - our star - eight planets and their natural satellites (such as our moon); dwarf planets; asteroids and comets. Our solar system is located in an outward spiral of the Milky Way galaxy.
10 Need-to-Know Things About Our Solar System:
1. Our solar system is made up of the sun and everything that travels around it. This includes eight planets and their natural satellites such as Earth's moon; dwarf planets such as Pluto and Ceres; asteroids; comets and meteoroids

2. The sun is the center of our solar system. It contains almost all of the mass in our solar system and exerts a tremendous gravitational pull on planets and other bodies.
3. Our solar system formed about 4.6 billion years ago.
4. The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are called the terrestrial planets because they have solid, rocky surfaces.
5. Two of the outer planets beyond the orbit of Mars - Jupiter and Saturn - are known as gas giants; the more distant Uranus and Neptune are called ice giants.
6. Most of the known dwarf planets exist in an icy zone beyond Neptune called the Kuiper Belt, which is also the point of origin for many comets.
7. Many objects in our solar system have atmospheres, including planets, some dwarf planets and even a couple moons.
8. Our solar system is located in the Orion Arm of the Milky Way Galaxy. There are most likely billions of other solar systems in our galaxy. And there are billions of galaxies in the universe.
9. We measure distances in our solar system by Astronomical Units (AU). One AU is equal to the distance between the sun and the Earth, which is about 150 million km (93 million miles).
10. NASA's twin Voyager 1 and Voyager 2 spacecraft are the first spacecraft to explore the outer reaches of our solar system.

The Milky Way Galaxy - Home to Many Planets
An artist's impression of our home galaxy -- the Milky Way. Our solar system is one of billions in the galaxy. And the galaxy is one of billions in the Universe.
Credit: GSFC
The Moon and the Sun
Date: 7 Oct 2010
This image is a view of the sun captured by NASA's Solar Dynamics Observatory on 7 Oct. 2010, while partially obscured by the moon. A close look at the crisp horizon of the moon against the sun shows the outline of lunar mountains. A model of the moon from NASA's Lunar Reconnaissance Orbiter has been inserted into the picture, showing how perfectly the moon's true topology fits into the shadow observed by SDO.
Credit: NASA/SDO/LRO/GSFC

Sunspot Loops
Even a relatively quiet day on the Sun is busy. This ultraviolet image shows bright, glowing arcs of gas flowing around the sunspots.
Credit: GSFC

How the Sun Got its Name
The sun has many names in many cultures, all of them presumably pre-historic in their origins. The ancient Greeks called it Helios and the ancient Romans called it Sol, both of which derive from the same Proto-Indo-European term. Latin Sol developed as sole in Italian, sol in Portuguese and Spanish, and with the addition of an originally diminutive suffix, as soleil in French. Modern English sun evolved from the same Proto-Germanic form that today is Sonne in German and zon in Dutch, variously attested as sonne and sunne in Old and Middle English, with similar forms found in other ancient Germanic languages such as Old Norse, Old Saxon, Old High German and Gothic.
10 Need-to-Know Things About the Sun:
1. The sun is a star. A star does not have a solid surface, but is a ball of gas (92.1 percent hydrogen (H2) and 7.8 percent helium (He)) held together by its own gravity.

2. The sun is the center of our solar system and makes up 99.8% of the mass of the entire solar system.
3. If the sun were as tall as a typical front door, Earth would be about the size of a nickel.
4. Since the sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 days, but at its poles the sun rotates once on its axis every 36 Earth days.
5. The solar atmosphere (a thin layer of gases) is where we see features such as sunspots and solar flares on the sun.
6. The sun is orbited by eight planets, at least five dwarf planets, tens of thousands of asteroids, and hundreds of thousands to three trillion comets and icy bodies.
7. The sun does not have any rings.
8. Spacecraft are constantly increasing our understanding of the sun -- from Genesis (which collected samples of the solar wind and returned the particles to Earth) to SOHO, STEREO, THEMIS, and many more, which are examining the sun's features, its interior and how it interacts with our planet. .
9. Without the sun's intense energy there would be no life on Earth.
10. The temperature at the sun's core is about 15 million degrees Celsius (27 million degrees Fahrenheit).

Terrestrial Planet Interiors

Mercury
Mercury has an average density of 5430 kilograms per cubic meter, which is second only to Earth among all the planets. It is estimated that the planet Mercury, like Earth, has a ferrous core with a size equivalent to two-thirds to three-fourths that of the planet's overall radius. The core is believed to be composed of an iron-nickel alloy covered by a mantle and surface crust.

Venus
It is believed that the composition of the planet Venus is similar to that of Earth. The planet crust extends to around 10-30 kilometers below the surface, under which the mantle reaches to a depth of some 3000 kilometers. The planet core comprises a liquid iron-nickel alloy. Average planet density is 5240 kilograms per cubic meter.

Earth
The Earth comprises three separate layers: a crust, a mantle, and a core (in descending order from the surface). The crust thickness averages 30 kilometers for land masses and 5 kilometers for seabeds. The mantle extends from just below the crust to some 2900 kilometers deep. The core below the mantle begins at a depth of around 5100 kilometers, and comprises an outer core (liquid iron-nickel alloy) and inner core (solid iron-nickel alloy). The crust is composed mainly of granite in the case of land masses and basalt in the case of seabeds. The mantle is composed primarily of peridotite and high-pressure minerals. Average planet density is 5520 kilograms per cubic meter.

Mars
Mars is roughly one-half the diameter of Earth. Due to its small size, it is believed that the martian center has cooled. Geological structure is mainly rock and metal. The mantle below the crust comprises iron-oxide-rich silicate. The core is made up of an iron-nickel alloy and iron sulfide. Average planet density is 3930 kilograms per cubic meter.

Pluto
The structure of Pluto is not very well understood at present. Nevertheless, spectroscopic observation from Earth in the 1970s has revealed that the planet surface is covered with methane ice. Surface temperature is -230?C (-382?F), and the frozen methane exhibits a bright coloration. However, with the exception of the polar caps, the frozen methane surface is seen to change to a dark red when eclipsed by its moon Charon. Average planet density is 2060 kilograms per cubic meter. The low average density requires that the planet must be a mix of ice and rock.
Credit: NASA

Gas Giant Interiors

Jupiter
Jupiter's composition is mainly hydrogen and helium. In contrast to planetary bodies covered with a hard surface crust (the Earth, for example), the jovian surface is gaseous-liquid, rendering the boundary between the atmosphere and the planet itself almost indistinguishable. Below the roughly 1000-kilometer-thick atmosphere, a layer of liquid hydrogen extends to a depth of 20,000 kilometers. Even deeper, it is believed that there is a layer of liquid metallic hydrogen at a pressure of 3 million bars. The planet core is believed to comprise iron-nickel alloy, rock, etc., at a temperature estimated to exceed 20,000C.

Saturn
As with Jupiter, Saturn is mainly composed of hydrogen and helium and is observed to be of extremely low density. In fact, Saturn's mean density is only about two-thirds that of water. The Saturn atmosphere comprises, in descending order of altitude, a layer of ammonia, a layer of ammonium hydrogen sulfide, and a layer of ice. Below this, the saturnian surface is a stratum of liquid hydrogen (as in the case of Jupiter) underlain with a layer of liquid metallic hydrogen. It is believed that the liquid hydrogen layer of Saturn is thicker than that of Jupiter, while the liquid metallic hydrogen layer may be thinner. The planet's core is estimated to be composed of rock and ice.

Uranus
Uranus is gaseous in composition, mainly comprising hydrogen and helium as in the case of Jupiter and Saturn. The planet atmosphere is mostly hydrogen but also includes helium and methane. The planet core is estimated to be rock and ice encompassed by an outer layer of ice comprised of water, ammonium, and methane.

Neptune
The atmosphere of Neptune consists of mainly hydrogen, methane and helium, similar to Uranus. Below it is a liquid hydrogen layer including helium and methane. The lower layer is made up of the liquid hydrogen compounds oxygen and nitrogen. It is believed that the planet core comprises rock and ice. Neptune's average density, as well as the greatest proportion of core per planet size, is the greatest among all the gaseous planets.
Image Credit: Lunar and Planetary Institute
Credit: Lunar and Planetary Institute

10 Need-to-Know Things About Mercury:
1. Mercury is the smallest planet in our solar system - only slightly larger than the Earth's moon.
2. It is the closest planet to the sun at a distance of about 58 million km (36 million miles) or 0.39 AU.
3. One day on Mercury (the time it takes for Mercury to rotate or spin once) takes 59 Earth days. Mercury makes a complete orbit around the sun (a year in Mercury time) in just 88 Earth days.

4. Mercury is a rocky planet, also known as a terrestrial planet. Mercury has a solid, cratered surface, much like Earth's moon.
5. Mercury's thin atmosphere, or exosphere, is composed mostly of oxygen (O2), sodium (Na), hydrogen (H2), helium (He), and potassium (K). Atoms that are blasted off the surface by the solar wind and micrometeoroid impacts create Mercury's exosphere.
6. Mercury has no moons.
7. There are no rings around Mercury.
8. Only two spacecraft have visited this rocky planet: Mariner 10 in 1974-5 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011.
9. No evidence for life has been found on Mercury. Daytime Temperatures can reach 430 degrees Celsius (800 degrees Fahrenheit) and drop to -180 degrees Celsius (-290 degrees Fahrenheit) at night. It is unlikely life (as we know it) could survive on this planet.
10. Standing on on Mercury's surface at its closest point to the sun, the sun would appear more than three times larger than it does on Earth.

Double Ring Crater
Date: 14 Jan 2008
This scene was imaged by MESSENGER's Narrow Angle Camera (NAC) on the Mercury Dual Imaging System (MDIS) during the spacecraft's flyby of Mercury on 14 January 2008. The scene is part of a mosaic that covers a portion of the hemisphere not viewed by Mariner 10 during any of its three flybys (1974-1975). The surface of Mercury is revealed at a resolution of about 250 m/pixel (about 820 feet/pixel). For this image, the sun is illuminating the scene from the top and north is to the left.

The outer diameter of the large double ring crater at the center of the scene is about 260 km (about 160 miles). The crater appears to be filled with smooth plains material that may be volcanic in nature. Multiple chains of smaller secondary craters are also seen extending radially outward from the double ring crater. Double or multiple rings form in craters with very large diameters, often referred to as impact basins. On Mercury, double ring basins begin to form when the crater diameter exceeds about 200 km (about 125 miles); at such an onset diameter the inner rings are typically low, partial, or discontinuous. The transition diameter at which craters begin to form rings is not the same on all bodies and, although it depends primarily on the surface gravity of the planet or moon, the transition diameter can also reveal important information about the physical characteristics of surface materials. Studying impact craters, such as this one, in the more than 1200 images returned from this flyby provide clues to the physical properties of Mercury's surface and its geological history.

Last Update: 31 Mar 2011 (AMB)
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury's Caloris Basin
Date: 18 Jan 2008
The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 km and is seen in yellowish hues in this enhanced color mosaic.

The image data is from the 14 January 2008 flyby of the MESSENGER spacecraft, captured with the MDIS instrument. Orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled.

Last Update: 27 Jun 2011 (AMB)
Credit: NASA

10 Need-to-Know Things About Venus:
1. Venus is only a little smaller than Earth.
2. Venus is the second closest planet to the sun at a distance of about 108 million km (67 million miles) or 0.72 AU.

3. One day on Venus lasts as long as 243 Earth days (the time it takes for Venus to rotate or spin once). Venus makes a complete orbit around the sun (a year in Venusian time) in 225 Earth days.
4. Venus is a rocky planet, also known as a terrestrial planet. Venus' solid surface is a cratered and volcanic landscape.
5. Venus' thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
6. Venus has no moons.
7. There are no rings around Venus.
8. More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet's surface.
9. No evidence for life has been found on Venus. The planet's extreme high temperatures of almost 480 degrees Celsius (900 degrees Fahrenheit) make it seem an unlikely place for for life as we know it.
10. Venus spins backwards (retrograde rotation) when compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.

10 Need-to-Know Things About Earth:
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel.
2. Earth is the third planet from the sun at a distance of about 150 million km (93 million miles) or one AU.

3. One day on Earth takes 24 hours (this is the time it takes the Earth to rotate or spin once). Earth makes a complete orbit around the sun (a year in Earth time) in about 365 days.
4. Earth is a rocky planet, also known as a terrestrial planet, with a solid and dynamic surface of mountains, valleys, canyons, plains and so much more. What makes Earth different from the other terrestrial planets is that it is also an ocean planet: 70 percent of the Earth's surface is covered in oceans.
5. The Earth's atmosphere is made up of 78 percent nitrogen (N2), 21 percent oxygen (O2) and 1 percent other ingredients -- the perfect balance for us to breathe and live. Many planets have atmospheres, but only Earth's is breathable.
6. Earth has one moon. Another name for a moon is satellite.
7. Earth has no rings.
8. Many orbiting spacecraft study the Earth from above as a whole system and together aid in understanding our home planet.
9. Earth is the perfect place for life.
10. Earth's atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.

Moonrise
Date: 26 Jan 2003
A quarter moon is visible in this oblique view of Earth's horizon and airglow, recorded with a digital still camera on the final mission of the Space Shuttle Columbia.

Columbia's crew was killed on Feb. 1, 2003 when the shuttle broke up on re-entry into Earth's atmosphere.
Credit: NASA

10 Need-to-Know Things About Earth's Moon:
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and the moon would the size of a green pea.
2. The moon is Earth's satellite and orbits the Earth at a distance of about 384 thousand km (239 thousand miles) or 0.00257 AU.

3. The moon makes a complete orbit around Earth in 27 Earth days and rotates or spins at that same rate, or in that same amount of time. This causes the moon to keep the same side or face towards Earth during the course of its orbit.
4. The moon is a rocky, solid-surface body, with much of its surface cratered and pitted from impacts.
5. The moon has a very thin and tenuous (weak) atmosphere, called an exosphere.
6. The moon has no moons.
7. The moon has no rings.
8. More than 100 spacecraft been launched to explore the moon. It is the only celestial a body beyond Earth that has been visited by human beings (The Apollo Program).
9. The moon's weak atmosphere and its lack of liquid water cannot support life as we know it.
10. Surface features that create the face known as the "Man in the moon" are impact basins on the moon that are filled with dark basalt rocks

Linear Gullies inside Russell Crater, Mars
Several types of downhill flow features have been observed on Mars. This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter is an example of a type called "linear gullies." Linear gullies are characterized by relatively constant width and by raised banks or levees along the sides. Unlike gullies caused by water-lubricated flows on Earth and possibly on Mars, they don't have aprons of debris at the downhill end of the channel. The grooves shown here, on the side of a large sand dune inside Russell Crater, are the longest linear gullies known, extending almost 2 km (1.2 miles) down this dune slope.New research points to chunks of frozen carbon dioxide, commonly called "dry ice," creating linear gullies by gliding down sandy slopes on cushions of carbon-dioxide gas sublimating from the dry ice. Linear gullies are on mid-latitude sandy slopes, where the ground is covered with carbon-dioxide frost in Martian winter. Before-and-after pairs of HiRISE images indicate that the linear gullies are formed during early spring. Some linear gullies -- such as the ones in the magnified section of this image shown as Figure 1 -- have pits at the downhill end that could be caused by a block of dry ice ending its slide and resting in place as it sublimates away.
This image is a portion of the HiRISE exposure catalogued as PSP_001440_1255, taken on 16 Nov. 2006, at 54.25 degrees south latitude, 12.92 degrees east longitude.
The University of Arizona Lunar and Planetary Laboratory operates HiRISE, which was built by Ball Aerospace & Technologies Corp., Boulder, Colo. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate in Washington. Lockheed Martin Space Systems, Denver, built the orbiter.
Credit: NASA/JPL-Caltech/University of Arizona

10 Need-to-Know Things about Mars
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, and Mars would be about as big as an aspirin tablet.
2. Mars orbits our sun, a star. Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU.
3. One day on Mars takes just a little over 24 hours (the time it takes for Mars to rotate or spin once). Mars makes a complete orbit around the sun (a year in Martian time) in 687 Earth days.
4. Mars is a rocky planet, also known as a terrestrial planet. Mars' solid surface has been altered by volcanoes, impacts, crustal movement, and atmospheric effects such as dust storms.
5. Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).
6. Mars has two moons named Phobos and Deimos.
7. There are no rings around Mars.
8. More than 40 spacecraft have been launched for Mars, from flybys and orbiters to rovers and landers that touched surface of the Red Planet. The first true Mars mission success was Mariner 4 in 1965.
9. At this time in the planet's history, Mars' surface cannot support life as we know it. A key science goal is determining Mars' past and future potential for life.
10. Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil -- and the dusty atmosphere -- to look red

The Asteroid Belt
Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across, less than half the diameter of our Moon.

The asteroid belt lies in the region between Mars and Jupiter. The Trojan asteroids lie in Jupiter's orbit, in two distinct regions in front of and behind the planet.

Image Credit: Lunar and Planetary Institute
Credit: Lunar and Planetary Institute

Looking Down on a Shooting Star
Date: 13 Aug 2011
This astronaut photograph, taken from the International Space Station while over China (approximately 400 kilometers to the northwest of Beijing), provides the unusual perspective of looking down on a meteor as it passes through the atmosphere. The image was taken on August 13, 2011, during the Perseid Meteor Shower that occurs every August.
Credit: NASA

10 Need-to-Know Things About Meteors and Meteorites:
1. Meteoroids become meteors -- or shooting stars -- when they interact with a planet's atmosphere and cause a streak of light in the sky. Debris that makes it to the surface of a planet from meteoroids are called meteorites.
2. Meteorites may vary in size from tiny grains to large boulders. One of the largest meteorite found on Earth is the Hoba meteorite from southwest Africa, which weighs roughly 54,000 kg (119,000 pounds).
3. Meteor showers are usually named after a star or constellation which is close to the radiant (the position from which the meteor appears to come).

4. Meteors and meteorites begin as meteoroids, which are little chunks of rock and debris in space.
5. Most meteorites are either iron, stony or stony-iron.
6. Meteorites may look very much like Earth rocks, or they may have a burned appearance. Some may have depressioned (thumbprint-like), roughened or smooth exteriors.
7. Many of the meteor showers are associated with comets. The Leonids are associated with comet Tempel-Tuttle; Aquarids and Orionids with comet Halley, and the Taurids with comet Encke.
8. When comets come around the sun, they leave a dusty trail. Every year the Earth passes through the comet trails, which allows the debris to enter our atmosphere where it burns up and creates fiery and colorful streaks (meteors) in the sky.
9. Leonid MAC (an airborne mission that took flight during the years 1998 - 2002) studied the interaction of meteoroids with the Earth's atmosphere.
10. Meteoroids, meteors and meteorites cannot support life. However, they may have provided the Earth with a source of amino acids: the building blocks of life.

10 Need-to-Know Things About Jupiter:
1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Jupiter would be about as big as a basketball.

2. Jupiter orbits our sun, a star. Jupiter is the fifth planet from the sun at a distance of about 778 million km (484 million miles) or 5.2 AU.
3. One day on Jupiter takes about 10 hours (the time it takes for Jupiter to rotate or spin once). Jupiter makes a complete orbit around the sun (a year in Jovian time) in about 12 Earth years (4,333 Earth days).
4. Jupiter is a gas-giant planet and therefore does not have a solid surface. However, it is predicted that Jupiter has an inner, solid core about the size of the Earth.
5. Jupiter's atmosphere is made up mostly of hydrogen (H2) and helium (He).
6. Jupiter has 50 known moons, with an additional 17 moons awaiting confirmation of their discovery -- that is a total of 67 moons.
7. Jupiter has a faint ring system that was discovered in 1979 by the Voyager 2 mission.
8. Many missions have visited Jupiter and its system of moons. The Juno mission will arrive at Jupiter in 2016.
9. Jupiter cannot support life as we know it. However, some of Jupiter's moons have oceans underneath their crusts that might support life.
10. Jupiter's Great Red Spot is a gigantic storm (about the size of two to three Earths) that has been raging for hundreds of years.

10 Need-to-Know Things About Saturn:
1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Saturn would be about as big as a basketball.

2. Saturn orbits our sun, a star. Saturn is the sixth planet from the sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU.
3. One day on Saturn takes 10.7 hours (the time it takes for Saturn to rotate or spin once). Saturn makes a complete orbit around the sun (a year in Saturnian time) in 29 Earth years.
4. Saturn is a gas-giant planet and does not have a solid surface.
5. Saturn's atmosphere is made up mostly of hydrogen (H2) and helium (He).
6. Saturn has 53 known moons with an additional 9 moons awaiting confirmation of their discovery.
7. Saturn has the most spectacular ring system of all our solar system's planets. It is made up of seven rings with several gaps and divisions between them.
8. Five missions have been sent to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings.
9. Saturn cannot support life as we know it. However, some of Saturn's moons have conditions that might support life.
10. When Galileo Galilei looked at Saturn through a telescope in the 1600s, he noticed strange objects on each side of the planet and drew in his notes a triple-bodied planet system and then later a planet with arms or handles. The handles turned out to be the rings of Saturn.

High above Saturn
Date: 10 Oct 2013
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. It was made by amateur image processor and Cassini fan Gordan Ugarkovic. This image has not been geometrically corrected for shifts in the spacecraft perspective and still has some camera artifacts.The mosaic was created from 12 image footprints with red, blue and green filters from Cassini's imaging science subsystem. Ugarkovic used full color sets for 11 of the footprints and red and blue images for one footprint.
Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic

Uranus' Rings on Edge
Date: 14 Aug 2007
NASA's Hubble Space Telescope captures a rare view of the entire ring system of the planet Uranus, tilted edge-on to Earth.

The rings were photographed with Hubble's Wide Field Planetary Camera 2 on 14 August 2007.
The edge-on rings appear as spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the HST photo (a small amount of residual glare appears as a fan-shaped image artifact, along with an edge between the exposure for the inner and outer rings).

A much shorter color exposure of the planet has been photo-composited to show its size and position relative to the ring plane. Earthbound astronomers only see the rings' edge every 42 years as the planet follows a leisurely 84-year orbit about the sun. However, the last time the rings were tilted edge-on to Earth astronomers didn't even know they existed. The fainter outer rings appear in the 2003 Hubble Space Telescope images, but were not noticed there until they were seen in the 2005 images and the previous ones were analyzed more carefully. Uranus has a total of 13 dusty rings.

Last Update: 15 September 2011 (AMB)
Credit: NASA/ESA/STScI

10 Need-to-Know Things About Uranus:
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and Uranus would be about as big as a baseball.

2. Uranus orbits our sun, a star. Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU.
3. One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). Uranus makes a complete orbit around the sun (a year in Uranian time) in about 84 Earth years.
4. Uranus is a gas giant and therefore does not have a solid surface. The bulk (80 percent or more) of the mass of Uranus is made up of a hot dense fluid of "icy" materials (water (H2O), methane (CH4). and ammonia (NH3)), above a small rocky core.
5. Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4).
6. Uranus has 27 moons. Uranus' moons are named after characters from the works of William Shakespeare and Alexander Pope.
7. Uranus has faint rings. The inner rings are narrow and dark and the outer rings are brightly colored.
8. Voyager 2 is the only spacecraft to have visited Uranus.
9. Uranus cannot support life as we know it.
10. Like Venus, Uranus has a retrograde rotation (east to west). Unlike any of the other planets, Uranus rotates on its side, which means it spins horizontally.

10 Need-to-Know Things About Neptune:
1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Neptune would be about as big as a baseball.

2. Neptune orbits our sun, a star. Neptune is the eighth planet from the sun at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU.
3. One day on Neptune takes about 16 hours (the time it takes for Neptune to rotate or spin once). Neptune makes a complete orbit around the sun (a year in Neptunian time) in about 165 Earth years (60,190 Earth days).
4. Like the other gas giants, Neptune does not have a solid surface. Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
5. Neptune's atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
6. Neptune has 13 moons. Neptune's moons are named after various sea gods and nymphs in Greek mythology.
7. Neptune has six rings.
8. Voyager 2 is the only spacecraft to have visited Neptune.
9. Neptune cannot support life as we know it.
10. At times during the course of Neptune's orbit, dwarf planet Pluto is actually closer to the sun, and us, than Neptune. This is due to the unusual elliptical (egg) shape of Pluto's orbit
Neptune's Rings
Date: 24 Aug 1989
This wide-angle Voyager 2 image, taken through the camera's clear filter, is the first to show Neptune's rings in detail.
The two main rings, about 53,000 km (33,000 miles) and 63,000 km (39,000 miles) from Neptune, are 5 to 10 times brighter than in earlier images. The difference is due to lighting and viewing geometry. In approach images, the rings were seen in light scattered backward toward the spacecraft at a 15-degree phase angle.
However, this image was taken at a 135-degree phase angle as Voyager left the planet. That geometry is ideal for detecting microscopic particles that forward-scatter light preferentially.
The fact that Neptune's rings are so much brighter at that angle means the particle-size distribution is quite different from most of Uranus' and Saturn's rings, which contain fewer dust-size grains.
This image was obtained when Voyager was 1.1 million km (683,000 miles) from Neptune. Exposure time was 111seconds. Credit: NASA/JPL

10 Need-to-Know Things About Dwarf Planets:
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planets Pluto and Eris, for would each be about the size of the head of a pin.

2. Dwarf planets orbit our sun, a star. Most are located in the Kuiper Belt, a region of icy objects beyond the orbit of Neptune. Pluto, one of the largest and most famous dwarf planets, is about 5.9 billion km (3.7 billion miles) or 39.48 AU away from the sun. Dwarf planet Ceres is in the main asteroid belt between Mars and Jupiter.
3. Days and years vary on dwarf planets. One day on Ceres, for example, takes about nine hours (the time it takes for Ceres to rotate or spin once). Ceres makes a complete orbit around the sun (a year in Ceresian time) in about 4.60 Earth years.
4. Dwarf planets are solid rocky and/or icy bodies, The amount rock vs. ice depends on their location in the solar system.
5. Many, but not all dwarf planets have moons.
6. There are no known rings around dwarf planets.
7. Dwarf planets Pluto and Eris have tenuous (thin) atmospheres that expand when they come closer to the sun and collapse as they move farther away.
8. The first mission to a dwarf planet is Dawn. The first mission to the Kuiper Belt is New Horizons.
9. Dwarf planets cannot support life as we know it.
10. Pluto was considered a planet until 2006. The discovery of a similar-sized worlds deeper in the distant Kuiper Belt sparked a debate that resulted in a new official definition of a planet that did not include Pluto.

10 Need-to-Know Things About Comets:
1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, dwarf planet Pluto would be the size of a head of a pin and the largest Kuiper Belt comet (about 100 km across, which is about one twentieth the size of Pluto) would only be about the size of a grain of dust.

2. Short-period comets (comets that orbit the sun in less than 200 years) reside in the icy region known as the Kuiper Belt beyond the orbit of Neptune from about 30 to 55 AU. Long-period comets (comets with long, unpredictable orbits) originate in the far-off reaches of the Oort Cloud, which is five thousand to 100 thousand AUs from the sun.
3. Days on comets vary. One day on comet Halley varies between 2.2 to 7.4 Earth days (the time it takes for comet Halley to rotate or spin once). Comet Halley makes a complete orbit around the sun (a year in this comet's time) in 76 Earth years.
4. Comets are cosmic snowballs of frozen gases, rock and dust.
5. A comet warms up as it nears the sun and develops an atmosphere, or coma. The coma may be hundreds of thousands of kilometers in diameter.
6. Comets do not have moons.
7. Comets do not have rings.
8. More than 20 missions have explored comets from a variety of viewpoints.
9. Comets may not be able to support life themselves, but they may have brought water and organic compounds -- the building blocks of life -- through collisions with Earth and other bodies in our solar system.
10. Comet Halley makes an appearance in the Bayeux Tapestry from the year 1066, which chronicles the overthrow of King Harold by William the Conqueror at the Battle of Hastings.

Sources

http://solarsystem.nasa.gov/planets/profile.cfm http://solarsystem.nasa.gov/planets/