# Difference between revisions of "Solar System"

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Solar System
This event is an event held in the current season.

Type Earth Science
Category Study
Event Information
Latest Appearance 2019
Forum Threads
Forum Question Threads

Solar System is a Division B event that is set to return for the 2018 season with a focus on terrestrial planets and other rocky bodies in the solar system. In 2014 and 2015 the event focused on water and ice in the solar system, while in past years it addressed the Sun, planets and their satellites, comets, asteroids, the Oort Cloud, the Kuiper Belt, meteoroids, meteorites, and meteors.

## Year-Specific Information

This page focuses on the general competition of Solar System. However, the event sometimes involves a specific topic. These topics are presented in the table below.

Solar System Topics
Season Topic
2019 Terrestrial Bodies
2018
2014 Extraterrestrial Water
2015
2011 General Solar System (this page)
2010
2007
2006

## Origins of the Solar System

Our solar system was formed about 4.57 billion years ago in a nebula, the center of which was the protosun. Surrounding it were the materials that would be the planets, planetesimals. When the nebula was sent into a spinning motion (possibly by a large star), the heavier, rocky materials gravitated to the center, and the lighter gaseous materials fell to the outer solar system. In the inner solar system, the small planetesimals continued to gather more material becoming the 4 rocky terrestrial planets (Mercury, Venus, Earth, and Mars). In the outer solar system the rocky materials gathered to form planets, but the lighter gas materials were attracted by the gravity of the cores to form the Jovian (gaseous) planets(Jupiter, Saturn, Uranus, and Neptune). One group of planetesimals that never formed a planet was between Mars and Jupiter. They formed the asteroid belt. The leftover materials on the far edges of the solar system that did not form planets formed the Oort Cloud and Kuiper (kai-per) belt. These two bodies are the source of many comets, the dwarf planets Pluto, Ceres, Eris, Haumea, and Makemake, and the questionable planet or dwarf planet Sedna.

## Quick Guide to the Solar System

Planets and Basic Info
Planet Orbit Period Rotation Period Date Discovered Distance From Sun Radius Mass
Mercury 87.97 days 58.6 days Prehistory .39 AU 2,439.7 km $3.302*10^{23}$ kg
Venus 224.7 days 243 days Prehistory .72 AU 6,051.9 km $4.869*10^{24}$ kg
Earth 365.25 days 1 day Prehistory 1 AU 6,371.0 km $5.9742*10^{24}$ kg
Mars 686.98 days 1.03 days Prehistory 1.52 AU 3,389.5 km $6.4191*10^{24}$ kg
Jupiter 11.86 years 0.41 days Prehistory 5.2 AU 72,000 km $1.8987*10^{27}$ kg
Saturn 29.46 years .41667 days Prehistory 9.54 AU 60,268 km $5.6851*10^{26}$ kg
Uranus 84.01 years .71833 days March 13, 1781 19.18 AU 25,559 km $8.6849*10^{25}$ kg
Neptune 164.9 years .67125 days September 23, 1846 30.06 AU 24,764 km $1.0244*10^{26}$ kg

Use this chart to quickly compare some basic info for all the planets.

## Bodies of the Solar System

(Please note that all of the largest/smallest classifications of the planets do NOT include Pluto!)

### The Sun

Main article: Sun

This article is about the Sun. For Solar System event in general, see Solar System.
Diameter 1,392,000 km 1.989 1030 kg 3.846 1033 erg/s 74% Hydrogen 25% Helium (Trace amounts of other elements)

The Sun is the largest body in our solar system, and contains 99.8% of the mass. (Most of the remaining mass is held by Jupiter) Because it is a globe of gases, it rotates differently depending on the area. The equator takes around 25 days, whereas the polar regions take around 35 days (Earth days). It is 4.6 billion years old and is made up of layers, starting from the outside(with Temperatures), Corona(1,000,000 C), Transitive Region, Chromosphere, Photosphere(6,000 C), Convection Zone(1,000,000 C), Radiative Zone(2,000,000 C),and the Core(15,000,000 C).It produces heat from the fusion of hydrogen atoms. The heat is transferred by the process of convection, through the radiative and the convective zone, where it is radiated out through the photosphere and corona to the planets in the form of rays.

## Parts of the Sun

Parts of the Sun

### Core

The core is the densest part of the sun, with a density of 160 g/cm^3. That is ten times that of lead, 160 that of water, or about 250 billion atmospheres. Although, the temperature is 15 million kelvin, or 27 million degrees Fahrenheit, which keeps the core in a gaseous state. The core is where fusion takes place. Fusion is the process of small molecules combining to form larger ones, releasing energy. The force of gravity is so strong that it breaks down atoms into protons, neutrons, and electrons. Sometimes protons will combine with neutrons to form deuterons. If these deuterons combine with one more proton, they form a helium-3 isotope. Two of these combine to make a helium atom. The missing proton (remember that the helium-3 isotope has 1 neutron and 2 protons each) is released in the form of energy. Every second, about 700 million tons of hydrogen are converted to 695 million tons of helium and 5 million tons of energy in the form of gamma rays. The energy generated in the core of the Sun takes about 1 million years to reach the surface of the Sun.

### Radiative Zone

Surounding the core is the radiative zone. This area is hot and dense that energy from the core can radiate outwards through this area. Ions of helium and hydrogen emit photons, which then get absorbed by more hydrogen and helium isotopes. Basically, light energy bounces from particle to particle. Photons travel slowly this way. It can take 100,000 years for a single photon to exit the radiative zone, as they take a zigzag path to the convective zone, rather than a shorter, straight route.

### Convection Zone

The sun isn't hot enough in this outer layer to radiate through here, but instead the energy moves through here through convection, or the process of hotter things rising and cooler things falling. The solar plasma here heats up as you get closer to the core, rising, lets out the energy near the top, then falls again. The convection zone is from about 200,000 km deep to the "visible surface" of the Sun. It helps carry energy from the top of the radiation zone to the surface.

### Photosphere

The outermost part of the sun is called the photosphere. This is where the energy made in the core is finally released into the sun's atmosphere. This is the visible part of the sun. Sunspot are "cool" regions of the photosphere. Scientists believe that these darker, cooler patches of the photosphere may be responsible for climate change.

### Chromosphere

Above the photosphere, the chromosphere is almost transparent, but it can be seen as a thin red glow around the Sun during a Solar Eclipse. Solar flares also take place in the Chromosphere. The temperature here rises from about 4600 K at the part closest to the photosphere to about 8600 K at the outer edge. It is about 2,000 km deep.

### Transitive Region

Helium becomes fully ionized, which leads to worse heat conduction and skyrocketing temperatures. Also, this is where magnetic forces start to determine how solar structures look and act instead of gas pressure and fluid dynamics. Below the transitive region, solar emissions are largely in infrared, visible light, and near ultraviolet. At or above the trasitive region, emissions tend to be in far UV or X-rays.

### Corona

A type of plasma "atmosphere" that extends hundreds of kilometers into space. However, the corona is only visible during a total Solar Eclipse. The light that can be seen from the corona comes from light bouncing off free electrons (K-corona, "k" for kontinuierlich or continuous), light bouncing off dust particles (F-corona, "f" for Fraunhofer, a German physicist who discovered certain spectral lines in sunlight), and ions present in the corona's plasma (E-corona, "e" for emissions from the ions). The word corona is derived from the Latin word for "crown".

### Solar Features

Prominence- These large bright solar features extend outwards from the sun's photosphere into its corona, often in the shape of a loop. Typically, prominence loops are many times the size of the earth. They are more common at the height of the solar cycle, when the sun's magnetic field has become tangled and strong magnetic loops extend into its atmosphere, bringing ionized hydrogen gas with it. Prominences decrease in number as the magnetic field sorts itself out into simpler arrangements. They are made up of plasma, just like the corona (only much cooler), and evolve from day to day because of the instability of the magnetic field. The more stable prominences can last for several days or weeks while the more violent ones only last several hours. Some of them break up as the loop expands (the pressure of the hydrogen gas increases until it's enough to break through the field) and produce coronal mass ejections.

Filament- A prominence seen with the sun behind it rather than open space. Filaments appear darker than the rest of the sun.

Spicule- These small, jet-like eruptions on the sun's surface appear as dark streaks. Spicules only last a few minutes, but they eject material outwards into the corona at 20-30 km/s.

Flare- Flares are sudden, intense variations in the brightness of the sun- basically an explosion resulting from stored energy being released into space. They accelerate electrons, protons, and ions to almost light-speed and heat plasma to tens of millions of Kelvins. The resulting radiation is visible across the whole electromagnetic spectrum, from radio waves through visible light to gamma rays. They tend to occur near sunspots between opposing magnetic fields.

Sunspot- Sunspots are "cool" (about 3800 K) areas of the Sun's surface. They appear dark in comparison with the rest of the photosphere. No one really knows what causes sunspots, although it is thought that they result from complicated interactions within the Sun's magnetic field.

Solar Wind- A low density stream of photons and electrons that moves throughout the solar system and about 450 km/sec. During the minimum of the solar cycle, the solar wind flows much faster (750 km/sec) at the Sun's poles than it does closer to the equator. When the solar cycle is at its maximum, the solar wind flows at a much more constant speed. In the late 1600s, a period of low sunspot activity, called the Maunder Minimum, coincided with the "Little Ice Age" in Europe- a time of abnormally low temperatures.

Coronal Mass Ejection- Large bubbles of gases following magnetic field lines that are ejected from the sun over the course of several hours. They produce disturbances in the solar wind that can issues with telecommunications on Earth as well as other adverse effects. CMEs are often associated with solar flares and prominences.

Granules- Small convection cells of really hot solar gas on the surface of the sun.

## See Also

Solar System
Solar System/Inner Planets
Solar System/Outer Planets

## Links

### Inner Planets

Main article: Terrestrial Bodies.

The inner planets of our solar system are those between the Sun and the asteroid belt; Mercury, Venus, Earth, and Mars.

### Outer Planets

Main article: Outer Planets

This article is about the outer planets of our solar system. For Solar System event in general, see Solar System.

The inner planets of our solar system are those between the Sun and the asteroid belt: Mercury, Venus, Earth, and Mars. The outer planets are those beyond the asteroid belt: Jupiter, Saturn, Uranus, and Neptune.

## Jupiter

Jupiter is the fifth planet from the sun at a distance of 778.3 million km. It is also the largest planet with a radius of 71,492 km. Jupiter holds most of the non-solar mass in the solar system. It gives off more energy than it receives from the sun, so it is believed that Jupiter is a failed star. That means that Jupiter could have formed into a star, but was not under the right conditions. Contrary to popular belief, Jupiter has three small rings around it. These rings are made out of tiny particles. Its day is the shortest of the solar system's planets at about 10 hours, and its year is equal to 12 earth years. It has an atmosphere composed of hydrogen and helium. The outer layer is the thin visible cloud bands that we see this is also the zone that contains the circular storm known as the Great Red Spot, which has been churning for centuries. This is followed by a thick layer of liquid hydrogen. Beneath that is a nearly same size level of liquid hydrogen that, because of the pressure, behaves like a metal. Finally, beneath that is an iron-silicate core.

## Saturn

Saturn is the sixth planet from the sun, and it is 1.427 billion km away from the sun. It is also the second largest with a 60,330 km radius. Its day is only 10 hours and 40 minutes, and its year is about 30 Earth years. It is the least dense planet, and if placed in a large enough body of water, it would float. It has the largest and most spectacular ring system in the solar system. They have a diameter of 275,000 km, but they are only a few hundred meters thick. The rings are made up of particles that vary in size, from dust like particles, to the moons Janus and Epimetheus. Sheperd moons such as Atlas, Prometheus, and Pandora keep the rings in line. Saturn, like Jupiter, is made up of only Hydrogen and Helium, and gives off more energy than it receives. It is not as large as Jupiter, so it is not believed to have ever had the potential to be a star.

## Uranus

The pronunciation preferred by astronomers is /ˈjʊərənəs/ (yoor-uh-nuhs).
Uranus is the third largest planet (25,560 km radius), and the seventh planet from the sun (2.87 billion km away). It was the first planet discovered after prehistoric times, because it is so far away from Earth. It was discovered by William Herschel. Uranus is known for having its axis of rotation almost parallel to its plane of orbit. This gives it seasons that seem rather strange on Earth - 42 years of almost complete darkness followed by 42 years of consistent sunlight. Its 9-ring ring system is also parallel to its plane of orbit. These rings are different from those of Jupiter and Saturn, because they are more like hoops than rings of particles, and they have large gaps between them. Its day is about 18 hours long, and its year is 84 Earth years long. Its outer atmosphere is composed of hydrogen, helium, and methane, which gives it its blue green color. Beneath the outer layer is a layer of high pressure solid water, methane, and ammonia. Then, beneath that layer is a ball of rocky material that is very similar to Earth, but its surface is distorted by the dense inner ocean of water and methane.

## Neptune

The 8th planet from the sun, Neptune is 4.479 billion km away from the sun. It is the fourth largest planet with a radius of 24,765 km. It was discovered in 1846 after calculations in Uranus's orbit revealed that its motions were disturbed by a more distant planet. Its day is about 19 hours, and its year is 165 Earth years. The outer third of Neptune is made of hydrogen, helium, water, and methane, which, as on Uranus gives it a blue tint. The inner two thirds are made of molten rock, liquid water, liquid ammonia, and methane. Neptune's most apparent feature is a storm similar to the Great Red Spot, the Great Dark Spot. Here, winds can reach more than 1600 kph, making it one of the windiest places in the solar system.

### Dwarf Planets

The definition of a Dwarf Planet is a planet with enough of a gravitational pull to keep a spherical shape, but not strong enough to "clear the neighborhood", which means that any object that comes close to the planet, it either "pushes away" or "pulls into an orbit". In addition to that it cannot be a satellite of a non-stellar body.

#### Ceres

The largest object in the Asteroid Belt, containing 30% of its mass. When it was discovered in the early 1800s, Ceres was considered a planet, but was reclassified as an asteroid 50 years later. Since 2006, it has been considered a dwarf planet. Ceres orbits the Sun once every 4.6 Earth years and its day is about 9 hours.

#### Pluto

When it was still considered a planet, Pluto was the ninth planet from the Sun and the smallest planet. Very little is known about Pluto and its similarly sized moon, Charon (pronounced "karen"). It was discovered in 1930 by Clyde Tombaugh, and was the only planet discovered in the 20th century. It is a part of the Kuiper belt, and is one of many similar Kuiper Belt objects. The only thing we know about Pluto is that it has a highly eccentric orbit, which crosses Neptune’s orbit every 200 years or so, for 20 years. It also has two smaller moons, Nix and Hydra. It became a dwarf planet in 2006. In July 2011, a fourth satellite was discovered. It has yet to be named.

#### Eris

Eris is in the scattered disc, a region beyond the Kuiper Belt. Since Eris is larger than Pluto, its discovery led the IAU (International Astronomical Union) to define "planet" and reclassify Pluto as a dwarf planet. Its only known satellite is Dysnomia.

#### Haumea

An "egg-shaped" dwarf planet in the Kuiper belt. The odd shape is believed to come from a high rotational speed, which flattens the poles and creates a bulge around the equator. Haumea has a year of about 283 earth years. It also has two moons, Hi'iaka and Namaka. These are believed to be fragments of the KBO - early in its history, it was hit by something, breaking off Hi'iaka and Namaka.

#### Makemake

Makemake has no moons, making it unique among the larger Kuiper Belt objects. It orbits the sun every 310 years. Like most KBOs, it has an highly eccentric orbit - it's perihelion (distance closest to sun), is 38.5 AU(5,760,000,000 km). It's aphelion (distance farthest from sun), is 53.1 AU (7,939,000,000 km). It has a mass of 3 × 10^21 kg.

### Plutoids

To be considered a Plutoid, a dwarf planet must have a semi-major axis greater than that of Neptune. In other words, it must orbit outside of Neptune. Any Dwarf planet that orbits within Neptune is considered still considered a dwarf planet. As of right now, there are four official Plutoids. They are Pluto, Haumea, Makemake, and Eris.

### Plutoid Candidates

Some objects in the solar system are not officially considered dwarf planets or plutoids, but are large enough to be prime candidates for plutoid status.

#### Sedna

Sedna is a plutoid candidate with an orbit lasting about 11,518 Earth years. Its orbit is also highly eccentric, with a perihelion in the outer Kuiper Belt and an aphelion possibly in the inner Oort Cloud. Sedna's diameter is 995 miles, or about 1,600 kilometers. This object has no known natural satellites. Its discovery was mostly luck, as it was near its perihelion and at a (barely) detectable magnitude. Should it have been at the aphelion, it would remain unknown for thousands more years. This great distance is a potential reason that no natural satellites have been found; they would be way too dim.

### Small Solar System Bodies

Main article: Small Bodies

Some examples of small bodies in the Solar System include asteroids, meteoroids, and comets.

### Other Features of the Solar System

#### Oort Cloud

The Oort cloud is an immense cloud at the outer limits of the solar system. This is believed to be the farthest reaches of the Sun's gravitational pull that measurably affects other objects. This cloud is so vast that comets within it can be tens of millions of kilometers apart. It is believed that the cloud is denser along the elliptical plane. The estimated mass of all the bodies in the Oort cloud is about 40 times Earth's mass. These comets are easily influenced by other stars, and often a star that comes to close to another star's Oort cloud can fling these comets out into deep space or into the solar system. It is believed that this is where many of the comets and asteroids in our solar system originated from.

#### Kuiper Belt

The Kuiper belt is similar to the Asteroid belt. It lies beyond Neptune, about 30-50 AU from the Sun. It is believed that these are the remains of when the Solar System was first created. When the solar system was created, most space debris was condensed to form planets. The debris that did not form planets slowly drifted outwards to form the Kuiper Belt. NASA’s New Horiozons was the first spacecraft to actually visit an object in the Kuiper Belt. It flew by Pluto and its moons in July 2015. New Horizons is to fly past another Kuiper Belt object, 2014 MU69 on New Year's Eve 2018.

### Moons of the Solar System

Mercury: No moons

Venus: No moons

Earth's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Moon prehistory prehistory 384,400 3476 27.322
Mars' Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Deimos 1877 A. Hall 23,460 16x12x10 1.263
Phobos 1877 A. Hall 9,270 28x23x2 .319
Jupiter's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Callisto 1610 Galileo 188300 4800 16.689
Europa 1610 Galileo 670900 3126 3.551
Ganymede 1610 Galileo 1070000 5276 7.155
Io 1610 Galileo 421600 3629 1.769

There are 79 moons of Jupiter (53 named and 26 awaiting confirmation as of July 2018), but only the most famous ones are listed here.

Saturn's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Atlas 1980 R. Terrile 137640 37 0.602
Mimas 1789 Herschel 185520 392 .942
Enceladus 1789 Herschel 238020 444 1.370
Tethys 1684 G. Cassini 294660 1060 1.888
Dione 1684 Cassini 377400 1120 2.737
Rhea 1672 G. Cassini 527040 1520 4.518
Titan 1655 C. Huygens 1221850 5150 15.945
Hyperion 1848 Bond 1481100 410x260x220 21.277
Iapetus 1671 Cassini 3561300 1436 79.330
Phoebe 1898 Pickering 12952000 220 550.56

There are 60 moons and numerous moonlets of Saturn , but only the most famous ones are listed here.

Uranus's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Miranda 1948 G. P. Kuiper 129390 471.6 1.413
Ariel 1851 W. Lassell 191020 1157.8 2.520
Umbriel 1851 Lassell 266300 1169.4 4.144
Titania 1787 W. Herschel 435910 1576.8 8.706
Oberon 1787 Herschel 583520 1522.8 13.463

There are 27 moons of Uranus, but only the major ones, those massive enough for their surfaces to have collapsed into a spheroid, are listed here.

Neptune's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)1
Naiad 1989 Voyager 48227 66 0.294
Thalassa 1989 Voyager 50074 82 0.311
Despina 1989 Voyager 52526 150 0.335
Galatea 1989 Voyager 61953 176 0.429
Larissa 1981 H. J. Reitsema 73548 194 0.555
S/2004 N 1 2013 M. R. Showalter 105300 16-20 0.936
Proteus 1989 Voyager 117646 420 1.122
Triton 1846 W. Lassell 354759 2705.2 -5.877
Nereid 1949 G. Kuiper 5513818 340 360.13
Halimede 2002 M. J. Holman 16611000 62 -1879.08
Sao 2002 Holman 22228000 44 2912.72
Laomedeia 2002 Holman 23567000 42 3171.33
Psamathe 2003 S. S. Sheppard 48096000 40 -9074.30
Neso 2002 Holman 49285000 60 -9740.73

1Negative orbital periods indicate retrograde orbit.

Pluto's Moons
Name Year discovered Discoverer Distance from Planet (km) Diameter (km) Orbital Period (days)
Charon 1978 J. Christy 19571 1207 6.387
Styx 2012 M. R. Showalter 44448 10-25 20.1617
Nix 2005 H. A. Weaver 48675 137 42.856
Kerberos 2011 M. R. Showalter 59785 13-34 32.1
Hydra 2005 H. A. Weaver 64780 167 38.206

## Eclipses

### Lunar Eclipses

A type of eclipse that occurs when the Earth passes directly between the moon and sun, which means that the moon is in Earth's shadow. Since Earth is in the middle of the moon and sun, it must always be a full moon for a lunar eclipse to occur. There are several types of lunar eclipses:

Penumbral Eclipse- The moon passes through Earth's penumbra, causing its surface to darken slightly.

Total Penumbral Eclipse- The moon passes "exclusively" through Earth's penumbra. The area of the moon closest to the umbra can appear darker than the rest of it.

Partial Lunar Eclipse- A portion of the moon passes through Earth's umbra.

Total Lunar Eclipse

Total Lunar Eclipse- The whole moon passes through Earth's umbra. Totality can last up to 107 minutes, depending on the distance of the moon (at apogee, the moon's speed is slower, meaning a longer eclipse).

Selenehelion- Also known as a "horizontal eclipse", this is when the sun and the eclipsed moon can be seen at the same time. It can only occur right after sunrise or just before sunset. Technically, the moon and sun shouldn't be visible at the same time, but Earth's atmosphere refracts light and things near the horizon appear higher in the sky than they really are. The name is derived from the Greek goddess of the Moon (Selene) and their word for Sun, helios.

Total Solar Eclipse, not to scale.

### Solar Eclipses

The moon passes between the Earth and sun so that the sun's light is partially or completely blocked. Solar Eclipses can only occur during a new moon, when the moon is between the earth and the sun. However, since the moon's orbit around the earth is inclined at about 5°, solar eclipses can only happen when the moon's orbit crosses the ecliptic. There are four types of solar eclipses:

Total Eclipse- The sun is completely blocked by the moon. A total eclipse often happens near perigee because the moon is closer to the earth and its apparent size is larger. When earth is close to aphelion, total eclipses are also more likely to occur. The sun's disk is obscured and its corona is visible. Total eclipses are only visible from the path of totality in the moon's umbra.

Annular Eclipse- The sun and moon are in line, but the moon's apparent size is smaller than the sun because the moon is close to apogee. Annular eclipses are more likely to occur during earth's perihelion. The sun appears as a bright ring around the moon's outline. Annular eclipses are only visible in the antumbra.

Hybrid Eclipse- A hybrid eclipse is visible as a total eclipse from some places on earth and is visible as an annular eclipse from other places. This kind of eclipse is rare compared to the other kinds.

Partial Eclipse- The moon only obscures part of the sun. Partial eclipses can be seen from "a large part of earth" (the moon's penumbra) outside the path of totality for a total or annular eclipse. Some eclipses are only visible as a partial eclipse because the umbra passes above the poles.

## Important Laws

### Newton's Laws of Motion and Gravitational Attraction

#### Laws of motion

1. An object at rest stays at rest unless acted on by an outside force. An object in motion stays in motion unless acted on by an outside force.

2. F=ma. Force equals mass times acceleration.

3. For every action, there is an equal and opposite reaction.

#### Law of Gravitational Attraction

Every object attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of their distance.

$F = G \frac{m_1 m_2}{r^2}$

Where,

F is the magnitude of the gravitational force between the two point masses,

G is the gravitational constant,

m1 is the mass of the first point mass,

m2 is the mass of the second point mass, and

r is the distance between the two point masses.

### Kepler's Laws of Planetary Motion

Kepler's Laws are as follows:

1. The orbit of every planet is an ellipse with the sun at a focus.
2. A line joining a planet and the sun sweeps out equal areas during equal intervals of time.
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

See here for more info. Many of the pictures and diagrams used in this section are from here.

#### Law 1

The orbit of every planet is an ellipse with the sun at a focus.

To understand this law, you must first understand ellipses. You can think of an ellipse as a flatten circle, with two axes. There is the major axis, which is the longer one, and the minor axis, which is the shorter one. There are always two focuses, which are on the major axis. There is also a semi-major axis, which is half the major axis, and a semi-minor axis, or half the minor axis. The sum of the distance to both of the foci is constant.

What the law states is that the sun is at one of the foci, and the planet orbits around it in an ellipse. Most of the time the ellipse is close to a circle in shape, but is never a circle.

#### Law 2

A line joining a planet and the sun sweeps out equal areas during equal intervals of time. This one is harder to envision. So we've established that the orbit is elliptical, right? Two lines extending out of the sun will always have the same area, and the planet we are talking about will always travel this distance in equal time. Look at this picture:

Make sense now? The blue sections have the same area, and the Earth will travel the distance the blue area covers in the same time. So when the blue is wider, the Earth moves faster. The blue is wider closer to the sun, so the closer to the sun you are, the faster the planet will orbit around the sun.

#### Law 3

The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. This is purely math. $\frac{P_1^2}{P_2^2} = \frac{R_1^3}{R_2^3}$

So what this means is that these two fractions are equal. Remember in the first law, we defined the major and minor axes? The semi-major axis is half of the major axis. So that shows that the minor axis defines the orbital period! You can use this law to find either the semi-major axis, which can then be used to find the major axis, or the orbital period. Since $p^2 = a^3$, we can use the formula $p = a^{3/2}$ to find the orbital period, or $a = p^{2/3}$ to find the semi-major axis.

### Escape Velocity

Escape Velocity is the velocity something must reach in order to escape the gravitational pull of a planet. You can calculate the escape velocity using this formula: $E_v = \sqrt{\frac{2GM}{R}}$

Where, Ev is the escape velocity, M is the mass (in kg) of the planet, G is the gravitational constant (equal to $6.67\times10^{-11} \ {\rm N} \ {\rm m^{2}} \ {\rm kg^{-2}}$), and R is the radius of your planet in meters.

This is a strange form of measurement for a planet, so watch out. It can change your answer dramatically.

Here is a simple, easy to use Escape Velocity calculator made with Microsoft Excel. To use it, you fill in each box in a row with the mass and radius of your planet, respectively. You usually use meters for the radius, but this calculator converts it for you, so fill in the radius in kilometers. The first two boxes are used for the mass, so that a*10^b = mass, you would fill in "a" in the first box and "b" in the second box. Just look at the examples to figure out how to use it. Escape Velocity Calculator (.xls)

## Effects of Planets/Satellites

Tidal locking- when one side of an astronomical object always faces another astronomical body. For example, the Moon takes just as long to rotate one time as it does to revolve around Earth one time. Two objects of a similar size (like Pluto and Charon) may both become tidally locked to each other.

Shepherding- Where a moon orbits near the edge of a ring, using its gravitational pull to keep the ring's particles in a tight band and prevent them from spreading out too much.

Planet Shepherd Moons
Jupiter Metis, Adrastea, Amalthea, and Thebe
Saturn Pan, Daphnis, Atlas, Prometheus, Pandora, Aegaeon, and the "moonlets"
Uranus Cordelia and Ophelia

Resonance- A relationship in which the orbital period of one body is related to that of another by a simple integer fraction.

Resonance Astronomical Bodies
2:3 Neptune and Pluto (Neptune's orbital period is 2/3 that of Pluto)
1:2 Mimas and Tethys (Saturn's moons)
1:2 Enceladus and Dione (Saturn's moons)
3:4 Titan and Hyperion (Saturn's moons)
1:2:4 Io, Europa, and Ganymede (Jupiter's moons)*

*Eventually (in a few hundred million years), Io, Europa, Ganymede, and Callisto will be in a 1:2:4:8 resonance with these three moons. Callisto will orbit Jupiter once for every 2 Ganymede orbits, every 4 Europa orbits, or every 8 Io orbits.

Laplace Resonance- Where 3 or more astronomical bodies are in resonance with each other. The only known Laplace resonance is between Jupiter's moons Io, Europa, and Ganymede.

Trojans- A 1:1 resonance between two astronomical bodies where a minor planet or moon shares the same orbital path as a larger body but does not collide with it because it orbits 60° ahead of or behind the larger planet or moon (at the Lagrangian points L₄ or L₅). Mars, Jupiter, and Neptune each share their orbits with Trojan asteroids, while Saturn's moons have smaller Trojan moons (Telesto and Calypso share an orbit with Tethys, Helene and Polydeuces with Dione).

## Famous Astronomers

### Aristarchus

Aristarchus was an ancient Greek astronomer. He was the one to first put forward the idea of a heliocentric Solar System. After observing solar and lunar eclipses, he deduced correctly that the Solar System was heliocentric.

### Tycho Brahe

(1546-1601)

Tycho Brahe was a Danish astronomer that was famous for creating precise measurements of the planets, and also more than 700 stars. He discovered a supernova in 1572 near Cassiopeia. The king of Denmark was so impressed with this discovery that he funded a large observatory on the island of Ven. He also invented his own view of the Universe, the Tychonian System. In it, every planet but Earth orbited the Sun, and the Sun and Moon orbited the Earth.

### Galileo Galilei

(1564-1642)

Galileo Galilei was a very famous astronomer who is sometimes known as "the father of modern observational astronomy". His greatest astronomical achievements include discovering Jupiter's four largest satellites, observing and recording the phases of Venus, improving the design of the telescope, and greatly supporting the theory of a heliocentric solar system.

Galileo was born in Pisa, Italy, but moved to Florence at the age of 8. He later applied to the University of Pisa to get a medical degree, but his interests took a different course (no pun intended) and he ended up studying mathematics.

Aristotle's Universe

This upset the church, who then sentenced him to house arrest. He went blind (most likely from studying the sun), shortly before he died.

### Johannes Kepler

(1571-1630)

Johannes Kepler was a German astronomer most famous for developing the Kepler's Laws of Planetary Motion. He began to work on complex math formulas to explain planetary motion, which he mistakenly thought were circular in shape. Later, he became Tycho Brahe's assistant. Kepler and Tycho did not get along, however, and Tycho set Kepler to the task of understanding Mars' orbit. It was just this that allowed him to find the final piece in developing the Laws of Planetary Motion.

### Clyde Tombaugh

(1906-1997)

Clyde Tombaugh is credited for discovering Pluto. He began at home with a nine inch home-made telescope, and used this to draw pictures of Saturn and Jupiter. He sent the pictures to the Lowell Observatory, and was immediately offered a position. His goal was to discover the elusive "planet X", later to be renamed Pluto. Even after this great accomplishment, he went on to discover many more things such as comets, open clusters, and globular clusters.

### Nicholas Copernicus

(1473-1543)

Nicholas Copernicus was a Polish astronomer who developed the Copernicus theory, stating that the sun lies near the center of the Solar System, and the Earth revolves around it, rather than the other way around. This theory was not proven until Galileo, and not widely accepted for many more years. Later in life he went on to lecture in Rome about astronomy.

### Edmond Halley

(1656-1742)

Edmond Halley was a British astronomer who was the first to calculate a comet's orbit. He went to the University of Oxford where he studied the theories of Sir Issac Newton. He published a book in 1705 called Astronomiae Cometicae Synopsis (Synopsis on Cometary Astronomy). His theories were validated when a comet appeared in 1758, just as he predicted. The comet was named after him for his remarkable accuracy, and is now known as Halley's comet.

## Missions

For more information on each individual mission, see Solar System/Missions.

Many missions have been undertaken for the exploration and advancement of knowledge of the celestial bodies described above. In addition, many missions are currently being planned and prepared. Some of the most noteworthy are highlighted in the table below.

Missions
Name Start Date End Date Object(s) Objectives
Voyager 1 Sept 5, 1977 Continuing Jupiter and Saturn To explore the solar system. Ended up making groundbreaking discoveries about the moons of Jupiter and Saturn.
Voyager 2 Aug 20, 1977 Continuing Jupiter and Saturn To explore the solar system. It added to Voyager 1's discoveries by doing more flybys of the moons of Jupiter and Saturn.

## Helpful Tips

This event often contains many questions/tasks not listed on the event sheet, so you should study anything that could be interpreted as related to our solar system. If you do this (And have a decent reference book) you should be guaranteed to get a top ten finish. Also, make sure to check information posted on the site - it may be mistaken and/or outdated.

When making a note sheet, use One Note, since you can fit a lot of text and diagrams on one page, and you can easily use the clipping tool to copy and paste text from websites onto your note sheet.

## Links

Media:Solar_Study_Guide.pdf - Example Study Guide
The 9 planets
Views of the solar system
Soinc's solar system page
Information on famous astronomers
Kepler's Laws of Planetary Motion
Facts about the solar system
Notes on planet characteristics
Formation of Cycloidal Ridges on Europa