Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Perihelion 46.00 107.5 147.09 206.62 740.52 1352.55 2741.30 4444.45 (106 km) Mean Orbital Distance 57.91 108.2 149.60 227.92 778.57 1433.53 2872.46 4495.06 (106 km) Aphelion 69.82 108.9 152.10 249.23 816.62 1514.50 3003.62 4545.67 (106 km) Average Orbital 47.87 35 29.78 24.13 13.07 9.69 6.81 5.43 Velocity (km/s) Orbital Inclination 7.00° 3.4° 0.0° 1.850° 1.304° 2.485° 0.772° 1.769° (from Earth's Orbit) Orbital Eccentricity 0.2056 0.007 0.0167 0.0935 0.0489 0.0565 0.0457 0.0113 Equatorial Radius 2439.7 6051.8 6378.1 3397 71,492 60,268 25,559 24,764 (km) Polar Radius 2439.7 6051.8 6,356.8 3375 66,854 54,364 24,973 24,341 (Km) Volume 6.083 92.843 108.321 16.318 143,128 82,713 6833 6254 (1010 km3 Ellipticity 0.0000 0.000 0.00335 0.00648 0.06487 0.09796 0.02293 0.01708 Axial Tilt 0.01° 177.4° 23.45° 25.19° 3.13° 26.73° 97.77° 28.32° (from Earth's North) Mass 0.3302 4.87 5.9736 0.64185 1898.6 568.46 86.832 102.43 (1024 kg) Density 5.427 5.243 5.515 3.933 1.326 0.687 1.27 1.638 (water = 1) Escape Velocity 4.3 10.36 11.19 5.03 59.5 35.5 21.3 23.5 (km/s) Gravity 3.70 8.802 9.78 3.716 23.1 9.0 8.7 11 (m/s2 Surface Pressure ≈10-15 92 1.014 0.000636 N/A N/A N/A N/A (bars) Sidereal Rotation 1407.6 -5832.5 23.9345 24.6229 9.9250 10.656 -17.24 16.11 Period (hours) Length of Day (hours) 4222.6 2802 24 24.6597 9.9259 10.656 17.24 16.11 Tropical Orbital Period 87.968 224.7 365.256 686.980 4330.595 10,746 30,588 59,799.9 (days) Solar Irradiance 9126.6 2613.9 1367.6 589.2 50.50 14.90 3.71 1.51 (W/m2) Black-Body Temperature 442.5 231.7 254.3 210.1 110.0 81.1 58.2 46.6 (o K) Average Surface 167° 464° 15° -65° -110° -140° -195° -200° Temperature(Celsius) Number of Moons 0 0 1 2 63 47 27 13 Facts and Figures of Our Planets
Dwarf Planets Overview On August 24, 2006, the International Astronomical Union (IAU) voted to designate a new class of planets - the "Dwarf Planets," of which Pluto was the prototype. At this point in time, there is no official set definition of what a dwarf planet is, other than it does not meet the third criterion for planetary status: 1. The body orbits the Sun. 2. The body has enough mass that it has gravitationally collapsed into a spherical shape. 3. The body has not cleared its orbit, asserting orbital dominance. The not in the third part is key to what a dwarf planet is, and it is why Pluto is considered to be a dwarf planet, since it has not cleared its path of Neptune. Two other bodies were granted dwarf planet status on that day -- Ceres, the (formerly) largest asteroid, and Eris, the so-far largest-known body outside of Neptune's orbit. In time, more small solar system bodies may be granted dwarf planet status, though since there is no quantitative definition, this category is likely to remain a subject of debate.
Data for the Dwarf Planets. Ceres Pluto Eris Perihelion 380.612 4436.82 5650 (106km) Mean Orbital Distance 413.715 5906.38 10,120 (106km) Aphelion 446.818 7375.93 14,600 (106km) Average Orbital Velocity 17.882 4.72 3.436 (km/s) Orbital Inclination 10.587° 17.16° 44.187° (from Earth's Orbit) Orbital Eccentricity 0.080 0.2488 0.44177 Equatorial Radius 975 1195 2400 ± 100 (km) Polar Radius 909 1195 2400 ± 100 (km) Volume 0.20 0.715 ~5.8 (1010 Mass 0.00095 0.0125 ? (1024 Density (water=1) 2.08 1.75 ? Escape Velocity 0.51 1.1 ? (km/s) Gravity 0.27 0.6 ? (m/s2) Surface Pressure 0.27 0.6 ? (bars) Length of Day 9.074 153.2820 > 8? (hours) Albedo 0.113 0.4-0.6 0.86 ± 0.07 Surface Temperature -106° -225° ~-240° (Celsius) Number of Moons 0 3 1 Rings? No No No Absolute Magnitude 3.34 -1.12 ± 0.01
Where They Are. The vast majority of asteroids are grouped in the asteroid belt, which is more like a loose grouping than a belt, and lies between 1.8 and 4.5 A.U.(1 A.U. is the average distance between Earthand the Sun) from the sun - between the orbits of Mars and Jupiter. The asteroids are so small and far away that they appear as faint stars, if they even appear at all; no asteroid is bright enough to be seen without some optical aid, except Ceres (see below) on its closest approach to Earth. The asteroid belt is usually thought of as a defined region where asteroids abound (reference many science fiction movies, with spaceships flying in and out, dodging debris). This is actually very unlike the asteroid belt -- the region is so vast that asteroids are usually hundreds of thousands of kilometers from their closest neighbor. If the asteroid belt were taken to be the width of its densest region (3.2-1.8 A.U.), and it were considered flat, it would have an area of about 6•1017 km2. Even if there are 1000 times more asteroids than we know of today, then on average each asteroid would have over one million km2 to itself. But, there are asteroids all over the solar system -- they are not just confined to the belt. Their location only indicates what kind of orbit they have, as most asteroids look pretty much the same. Other than belt asteroids, there are several other classifications of asteroids, based upon their location and orbit in the solar system: • Amors asteroids are those that cross Mars' orbit; there are approximately known 1540 Amors. • Apollos are asteroids that cross Earth's orbits, and are sometimes called "Earth Grazers." There are approximately 2113 known Apollos. • Atens asteroids are those whose orbits lie completely inside of Earth's. Only about 333 of these are known. • Going in the other direction, there are also asteroids on the other side of the belt. Centaur asteroids are those that lie between 5.5 and 25 A.U. from the sun; there are approximately 85 Centaur asteroids known. • There are also asteroids that lie in the Lagrange 4 and 5 points of some planets. These are called Trojan asteroids. Mars has 6 known, Jupiter has over 1619 known, and Neptune has 1 known. • Kuiper Belt objects, AKA Trans-Neptunian Objects (TNOs), are asteroids / comets that lie beyond Neptune's orbit. There are approximately 1109 known TNOs. There are also many asteroids that gallivant around the solar system on highly elliptical orbits like comets. Of note, the asteroid Icarus, when closest to the sun, lies within Mercury's orbit - it comes close enough to the sun that Relativity must be taken into account in order to accurately predict its orbit. Another example is Hildago, whose closest approach is between the orbit of Mars and Earth, and its farthest from the sun between Saturn and Uranus. It is estimated that the main belt alone contains well over 1 million asteroids. The total number in the solar system is estimated as much higher, especially once TNOs are considered. What They Are The first asteroid was discovered in 1801 by Giuseppe Piazzi of Italy. He named it "Ceres" after the Roman goddess of grain. Ceres is the largest known asteroid at approximately 950 km in diameter, and it lies in the belt of asteroids between Mars and Jupiter (see the above section) at an average distance from the sun of 2.6 A.U. Ever since, asteroids have received an official designation of a number (starting with Ceres of number "1"), and most larger ones have received a name based in Roman mythology. If they have a name, then they are usually referred to with the number then the name, such as 951 Gaspra. Currently, asteroids are also referred to by the International Astronomical Union (the only official body that can name astronomical objects) as Minor planets. Asteroids range in size from dust particles to many miles across. Most current theories hold that asteroids are bits and pieces left over from the formation of the solar system. They are also formed from other asteroids as they collide and break apart, as comets disintegrate, or even when the outer moons of the larger planets collide. Past theories have suggested that the asteroids are remnants of a planet that was destroyed early in the solar system's history. However, that theory is no longer held in much regard, for if all of the asteroids in the belt were combined, they would form a body less than 1500 km (932 miles) in diameter -- less than half the size of Earth’s moon, and so there is not enough material to make a planet. Asteroids are made of rock and metal. They are mainly grouped into three categories: Stony, Iron-Nickel, and a mixture of the two. Most asteroids that we know about (92.8%) fall into the first category, and are made of Silicates. 5.7% are Iron-Nickel. The balance form the third type. Despite their relative abundance, stony asteroids that have fallen to Earth are the hardest to find because they look like terrestrial rocks and they weather much faster than the metallic ones. Asteroids have a confusing system of nomenclature, especially when they are on Earth. While still in orbit, they are asteroids. Once they enter the atmosphere, they are called Meteors, and once they land, they are termed meteorites. Interesting Facts and Features. Asteroids are too small to be spherical in shape. Instead, they are usually ellipsoids, but some are dumbbell- haped, and others form even stranger ones. Asteroids bare a tale of the violence of the solar system; the larger ones have many sizeable craters pockmarking their surface. One of the most surprising features of asteroids is that several have been observed to have moons of their own. The first asteroid to be observed with a moon was 243 Ida (58 x 23 km); it's moon is called Dactyl, and measures approximately 1.2 x 1.392 x 1.6 km. It is now estimated that between 10-30% of asteroids have moons. As previously stated, if all the asteroids in the belt were combined into one, it would form a body less than 1500 km in diameter. Noting the immense size of Ceres, it comprises over 1/3 the total suspected mass of the belt (2.3 x 1021 kg). 26 known asteroids are larger than 200 km. We probably know 99% of the asteroids that are greater than 100 km, and there are probably literally millions of asteroids that are greater than 1 km in diameter. Over 300,000 asteroids have been found. When asteroids break apart, the pieces don't always fly off in random directions. Sometimes, they will continue in the same orbit as the original asteroid. When several asteroids are seen in relatively the same place and traveling along similar orbits, they are called orbital families. Asteroids.
Some Famous Asteroids. Asteroid Diameter Mass Rotation Period Distance from Sun Orbital Period (km) (1015kg (hours) (A.U.) (years) 1 Ceres 960 x 932 870,000 9.075 2.767 4.60 2 Pallas 570 x 525 x 482 318,000 7.811 2.774 4.61 3 Juno 240 20,000 7.210 2.669 4.36 4 Vesta 530 300,000 5.342 2.362 3.63 45 Eugenia 226 6,100 5.699 2.721 4.49 140 Siwa 103 1,500 18.5 2.734 4.51 243 Ida 58 x 23 100 4.633 2.861 4.84 433 Eros 33 x 13 x 13 6.69 5.270 1.458 1.76 951 Gaspra 19 x 12 x 11 10 7.042 2.209 3.29 1862 Apollo 1.6 0.002 3.063 1.471 1.81 2060 Chiron 180 4,000 5.9 13.633 50.7
Overview. For almost as long as history has been recorded, six planets have been known: Mercury, Venus, Earth, Mars, Jupiter, and Saturn, and they orbited Earth in that order outwards. This view persisted for thousands of years, and people thought that Saturn was the edge of the solar system. The discovery of Uranus in modern times pushed the edge even further, and then Neptune and Pluto expanded the width of the solar system to twice its previously "known" size. The discovery of Pluto prompted more speculation that there could be other planets out there, and the existence of a "Planet X" is still passionately believed to exist by a small group of people. Some people mark the edge with Pluto, and others believe that the edge exists at Planet X. The whole concept of an "edge" is somewhat inaccurate as far as the solar system is concerned, for there is no physical boundary to it - there is no wall past which there's a sign that says, "Solar System Ends Here." There are, however, specific regions of space that include outlying members of our solar system, and a region beyond which the Sun can no longer hold any influence. These locations are what will be discussed here. Kuiper Belt. G. Kuiper proposed that there exists a large belt of comets approximately 30-50 A.U.from the Sun (starting near Pluto's orbit). This region is believed to be the source of many short-period comets. Modern views of the Kuiper Belt hold that it is the source of short-period comets, and that they are nudged from their normally stable orbits by the outer solar system's planets, collisions with each other, or by passing stars. The Kuiper Belt is believed to hold many small objects, but also many large objects - several larger than Pluto. This is a mixed bag for Planet X proponents. If a giant Planet X exists, then there should be no Kuiper Belt because the Kuiper Belt would have prevented a giant planet from forming due to impacts and gravitational dispersion. But, if Pluto is considered a planet, and anything larger than Pluto is considered a planet, then the Kuiper Belt may hold many Planet Xs. The first direct evidence that the Kuiper Belt exists was the discovery of the first Kuiper Belt Object (KBO). It was found in 1992, and is called 1992 QB1. Since then, nearly 1000 KBOs have been found, and many of them several hundred kilometers in diameter. Oort Cloud. Another outer region of the solar system is the Oort Cloud, a theoretical cloud of comets that surrounds the solar system out to a distance of two light-years (11.36 trillion miles). The Oort Cloud was first proposed by Jan Oort in 1950. This "land of comets" contains comet-forming nuclei left over from the formation of the solar system. It is currently thought that this is the location where all comets originate, and it is the current origin of long-period comets. The way they enter the inner solar system is by gravitational pushes usually caused by a passing star. The Oort Cloud is a generally accepted theory, though no member of the Oort cloud has been conclusively found. The object "Sedna" that was announced on March 15, 2004, is a possible member of the inner Oort Cloud. Its orbit takes it to within 90 A.U., but as far out as about 900 A.U. from the Sun. It is believed to be about 75% Pluto's size, with a radius of 950±150 km. It is the coldest object in the solar system with a surface temperature of -240 °C (33.15 °K), and it is also a very red object - the reddest in the solar system other than Mars. It was originally believed that Sedna had a moon, but that was based upon an erroneous measurement of its "day," which was more accurately determined in 2005 to be about 10 hours instead of 20 Earth days. Sedna is the farthest known solar system object, and it has a period of 10,500 years. It is still, however, not representative of the Oort Cloud. Sedna is still well within what was believed to be the inner boundary of the Oort Cloud. For this reason, the Oort Cloud is still hypothetical, with no direct evidence that it exists. But the search is still on. Heliopause. Another valid boundary region for the "edge" of the solar system is the heliopause. This is the region of space where the sun's solar wind meets that of other stars. It is a fluctuating boundary that is estimated to be approximately 17.6 billion miles (120 A.U.) away. Note that this is within the Oort Cloud. The two Voyager spacecraft should cross this region within the next years as they "leave" the solar system to become the first terran interstellar travelers. Home page. Astronomy home page. Go to Solar System Pictures and Data. Previous page. Next page. The Edge of the Solar System.
