[History of Astronomy]
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The Minor Planets
Properties of selected minor planets
In addition to these major planets, there are thousands of Minor Planets. The first to be discovered (beginning in 1801) are the Asteroids (also called Planetoids). These orbit in the inner solar system mainly between the orbits of Mars and Jupiter. Examples are Ceres and Vesta.
The tables below describe various properties of selected minor planets. Below each table are explanations of the terms used. The Earth's Moon is shown for comparison.
(Earth = 1)
(Earth = 0)
|The Moon||n/a||149.6||1.0||1.00 year||n/a||n/a||n/a|
|Eros||NE Asteroid||218.1||1.458||1.76 years||24.36||10.83°||0.2229|
|Vesta||MB Asteroid||353.4||2.362||3.63 years||19.34||7.14°||0.0895|
|Ceres||MB Asteroid||413.9||2.767||4.6 years||17.882||10.58°||0.0789|
|Pallas||MB Asteroid||415.0||2.774||4.61 years||17.65||34.84°||0.2299|
Some asteroids orbit close in to the Sun among the inner planets. Some even approach the orbit of the Earth. These are called Near Earth Asteroids (NE Asteroids).
A group of about a hundred asteroids revolve around the Sun in the same orbit as Jupiter keeping either 60 degrees ahead or 60 degrees behind the giant planet. These positions in a planet's orbit are called Lagrange Positions. They can hold small asteroids in stable orbits as long as the object, Jupiter and the Sun form an equalateral triangle and the object is small compared to the other two bodies in the triangle. These asteroids are called Trojans. Achilles occupies the position 60 degrees preceeding Jupiter. Recently, Mars and Neptune have been found to have a small number of Trojan asteroids.
Another group orbit amongst the outer planets. These are called Centaurs.
Beyond Neptune orbit a large number of icy bodies called Kuiper Belt Objects (KBOs). All are smaller than the Earth's Moon but the larger ones are bigger than the asteroids. Beyond the Kuiper Belt is a region called the Oort Cloud which contains comets. Sedna lies at the inner region of the Oort Cloud.
Plutinos are KBOs that have orbits similar in size to Pluto's and have a 3:2 resonance with Neptune. That means that every time Neptune goes around the Sun three times, Pluto and the Plutinos revolve around the Sun twice.
The second compares a planet's distance to the Sun to the Earth's distance. This unit is called the Astronomical Unit (written AU).
The NE Asteroids have distances close to the Earth's distance from the Sun (around 1 AU). The MB Asteroids orbit between 2 and 4 AUs as they lie between Mars (distance 1.5 AU) and Jupiter (5.2 AU). The Trojan asteroids orbit at roughly the same distance from the Sun as Jupiter (5.2 AU).
Pluto and the Plutinos have distances around 39 AU. The other KPOs are beyond this distance (between 40 and 70 AU). Sedna is so far away (over 500 AU) that some astronomers consider it a new type of object (an Inner Oort Cloud Object).
Kuiper Belt Objects (like Pluto) have orbits that are more elliptical. Pluto's orbit is so eccentric that it sometimes moves closer to the Sun than Neptune. There is no danger of a collision because Pluto's orbit is so highly inclined. Other Kuiper Belt objects have similar orbits. Sedna has an extremely elliptical orbit.
(Moon = 1)
|The Moon||3,474.8||1.0||27.32166 days||1.5424°|
|Eros||33 × 13 × 13||0.009 × 0.003 × 0.003||5.270 hours|
|Vesta||578 × 560 × 458||0.166 × 0.161 × 0.132||5.342 hours||29°|
|Ceres||960 × 932||0.276 × 0.268||9.075 hours|
|Pallas||570 × 525 × 482||0.164 × 0.151 × 0.139||7.811 hours||~60°|
|Orcus||840 × 1880||0.242 × 0.541||?|
|Haumea||1,960 × 1,518 × 996||0.564 × 0.437 × 0.287||3.9154 hours|
|Makemake||1,600 × 2,000||0.460 × 0.576||? hours|
|Eris||2,400||0.691||~ 8 hours|
One column gives diameters in kilometres, the other relative to the Moon. All Minor Planets have diameters smaller than the Moon (which is smaller than any of the major planets).
Minor planets typically rotate quickly in a few hours. Pluto has a slow rotation period because of the presence of a satellite which slows the planet's rotation over time. The Moon's rotation has been slowed by the Earth.
An axial tilt of more than 90° implies that the planet rotates in a retrograde direction.
(Moon = 1)
(×103 kg m-3)
(Moon = 1)
(Moon = 1)
|Eros||9.797 × 10-8||2.40||0.0036||0.01||0.004|
|Vesta||3.67 × 10-3||3.40||0.136||0.35||0.147|
|Pallas||2.99 × 10-3||2.80||0.111||0.32||0.134|
|Achilles||3.54 × 10-5||2.0||0.023||0.072||0.030|
|Chiron||3.40 × 10-5||2.0?||0.025||0.07||0.029|
|Orcus||8.84 × 10-3||2.0?||0.216||0.5?||0.2?|
|Varuna||8.03 × 10-3||1?||0.093||0.39||0.163|
The mass of bodies with satellites can be measured by observing the motions of the satellites and applying Kepler's Law.
The mass of Pluto is so low that many astronomers do not consider it a major planet. Recently other Pluto-sized bodies have been found in the distant part of the Solar System. These are the Kuiper Belt Objects.
All minor planets have a smaller mass than the Moon.
The density of a body is its mass divided by its volume. The units are kilograms per cubic meter.
The Surface Gravity of a planet is proportional to the planet's mass and inversely proportional to the square of the planet's radius.
(Earth = 1)
|The Moon||1367.6||1.00||12||-153° to 107°|
|Eros||16||-150° to 100°|
|Vesta||38||-130° to -60°|
|Ceres||10||-130° to -38°|
|Eris||86||-248° to -232°|
|The Moon||n/a||31' 5.16''||-12.74||Grey|
|Pluto||366.73||0.06 - 0.11||+13.7||Yellow|
If the body is at its closest to the Earth, the Synodic Period describes how long the body will take to get back to the same position relative to the Earth. The closer the object is to the Earth, the longer its Synodic Period.
Pluto travels around the Sun so slowly that it takes the Earth a little over a year to orbit the Sun and catch up to it.
In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.54 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. Most of the naked eye planets also have negative magnitudes. The faintest stars visible to the naked eye are sixth magnitude. The asteroid Vesta is occasionally on the limit of naked eye visiblity but other objects cannot be seen without optical aid.
The brightness of an object as seen from Earth depends on the closeness of a planet to the Sun (more light to reflect), the albedo (how much light is reflected) and the distance between the body and the Earth.
|Name||Composition of Atmosphere||Discovery|
|Pallas||None||Germany 1802||0 ?|
|Chiron||Cometary coma||USA 1977||0|
|Pluto||CH4 N2 (trace amounts)||USA 1930||3|
|Makemake||CH4 (trace)||USA 2005||0|
|2002 AW197||None||USA 2002||0|