Download The Solar System

The Solar System
Physics 217
February 13, 2011
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Introduction
The term ’The Solar System’ refers to the sun and all the objects orbiting
around it.
Our Sun is just one of ∼100 Billion stars in our galaxy, ’The Milky Way’,
and the Milky Way is just one of ∼100 Billion galaxies in the Universe.
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But for now lets focus on just the Solar System. The Solar system can
be divided into three main regions:
• The innermost region region containing the Sun and the small, terrestrial and rocky planets
• The Gas giants. Large balls of gas orbited by their own systems of
moons
• The outer limits, inhabited by large empty wastelands with a scattering
of icy worlds both large and small.
The Solar system is, as the name suggests, dominated by the Sun which
contains 99.86% of the mass of the Solar System, mostly in the form of
ionised Hydrogen and Helium. The gas giants account for about 90% of the
remaining mass with a tiny fraction left for the terrestrial planets.
To illustrate how small the planets are in comparison to the Sun imagine
if the Sun were modelled as a beach ball, Jupiter, the largest planet would be
a golf ball about 150 metres away and earth would be smaller than a pea and
about 20m away.
A few things to note about the Solar System:
• The orbits are roughly circular, apart from Mercury and Pluto.
• The orbits are mostly coplanar, i.e. the planets all orbit in the same
plane.
• Most of the movements and spins are in the same direction.
• It obeys Keplers Laws
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The Sun
The Sun is the nearest star to Earth and the centre of our Solar system. It
is a large ball of gas (74% Hydrogen, 24% Helium, 2% heavier elements). It
has a diameter of about 1,392,000 km, about 109 times that of Earth, and
its mass is about 2 × 1030 kilograms, 330,000 times that of Earth. The Sun
is about 150,000,000 km from the earth, which is the equivalent of about
light-seconds.
The Sun, like most stars shines incredibly brightly, the energy it emits
is generated by nuclear fusion in the center of the Suns core. This fusion
is combining Hydrogen atoms(protons) to form Helium atoms. The energy
release from these reactions in released in the form of photons which after
moving slowly to the surface of the sun are eventually converted into the
photons we see on earth. The Suns surface temperature is ∼6000K and
therefore the spectrum is that of 6000K blackbody radiator.
Sunspots, the Solar Wind and Aurorae
The sun often has dark spots on its surface known as sunspots.
We often see loops of gas pulled away from the surface of the sun, these
are caused by magnetic fields. The gas that is pulled away cools and thus
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emits less light than the gas at the surface These cooler loops of gas thus
appear as dark spots on the surface of the sun.
Sometimes gas pulled from the surface of the sun travels all the way to
earth this is known as the solar wind. The charged particles from the sun
can disrupt satellite communications on earth as well as power grids.
As these charged particles reach the earth the earth’s magnetic field channels charged particles from the sun onto the earth’s magnetic poles. As the
particles crash into the the atmosphere, light is emitted. This hazy light
called the aurora, can be seen near from high latitude regions.
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The Moon
The moon is Earth’s only natural satellite. It is a cold, dry orb with a surface that is riddled with craters and strewn with rocks and dust (This dust
is called regolith). The moon has no atmosphere. Recent lunar missions
indicate that there is some frozen ice at the poles.
The same side of the moon always faces the Earth as it is in what is called
synchronous rotation. The far side of the moon was first observed by humans
in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and
photographed it. Neil Armstrong and Buzz Aldrin (on NASA’s Apollo 11
mission, which also included Michael Collins) were the first people to walk
on the moon, on July 20, 1969.
If you were standing on the moon, the sky would always appear dark,
even during the daytime. This is because the moon has no atmosphere.
Also, from any spot on the moon (except on the far side of the moon where
you cannot see the Earth), the Earth would always be in the same place in
the sky; the phase of the Earth changes and the Earth rotates, displaying
various continents.
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The Moon’s Orbit
The moon is about 384,000 km from Earth on average. At its closest approach (the lunar perigee) the moon is 356,410 km from the Earth. At its
farthest approach (its apogee) the moon is 406,700 km from the Earth.
The moon revolves around the Earth in about one month (27 days 8
hours). It rotates around its own axis in the same amount of time. The same
side of the moon always faces the Earth; it is in a synchronous rotation with
the Earth.
The Moon’s orbit is expanding over time as it slows down (the Earth is
also slowing down as it loses energy). For example, a billion years ago, the
Moon was much closer to the Earth (roughly 200,000 kilometers) and took
only 20 days to orbit the Earth. Also, one Earth ’day’ was about 18 hours
long (instead of our 24 hour day). The tides on Earth were also much
stronger since the moon was closer to the Earth.
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The Inner Planets
Our solar system consists of the Sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is
the center of our solar system; the planets, their moons, a belt of asteroids,
comets, and other rocks and gas orbit the sun.
The eight planets that orbit the sun are (in order from the sun): Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Another large body
is Pluto, now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These
objects all orbit the sun in roughly circular orbits that lie in the same plane,
the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17
from the ecliptic).
The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune,
Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the dwarf
planets). Jupiter is so big that all the other planets could fit inside it.
The Inner Planets vs. the Outer Planets
The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun).
• The inner planets are: Mercury, Venus, Earth, and Mars. They are
relatively small, composed mostly of rock, and have few or no moons.
• The outer planets include: Jupiter, Saturn, Uranus, Neptune. They
are mostly huge, mostly gaseous, ringed, and have many moons.
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Mercury
Mercury is the smallest planet and also the closest to the Sun, it takes about
88 Earth days to orbit the Sun. Mercury is quite bright when viewed from
earth but it is not very easy to see as it is seldom very far from the Sun.
Compared to the other planets very little is known about Mercury. The
first of two spacecraft to visit the planet was Mariner 10, which mapped about
45% of the planets surface from 1974 to 1975. The second is the Messenger
spacecraft, which mapped a further 30% during its flyby of January 14, 2008.
Messenger’s last flyby took place in September 2009 and it is scheduled to
attain orbit around Mercury in 2011, where it will begin mapping the rest of
the planet.
Mercury looks very similar to the Moon and is heavily cratered with some
regions of smooth plains. It also has no natural satellites and no substantial
atmosphere. However, unlike the Moon, it has a large iron core, which generates a small magnetic field. Surface temperatures range from about 90 to
700 K (183 C to 427 C).
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Venus
Venus is the second planet from the Sun, its orbit takes about 225 Earth
days. After the Moon it is the brightest natural object in the night sky.
Venus reaches its maximum brightness shortly before sunrise or shortly after
sunset, which is why it has been known as the Morning Star or Evening Star.
Venus is classified as a terrestrial planet and it is sometimes called Earth’s
”sister planet” due to the similar size, gravity, and composition. Venus is
covered with an thick layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. Venus has the
densest atmosphere of all the terrestrial planets in our solar system, consisting mostly of carbon dioxide. Venus’s surface is a dusty dry desertscape with
many slab-like rocks, periodically refreshed by volcanism. The atmospheric
pressure at the planet’s surface is 92 times that of the Earth.
The Venusian surface was a subject of speculation until some of its secrets
were revealed by planetary science in the twentieth century. It was mapped
in detail by Project Magellan in 199091. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate that there
have been some recent eruptions. The planet has few impact craters, demonstrating that the surface is relatively young, approximately 300600 million
years old.
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Earth
Earth is the third planet from the Sun and the densest of the eight planets in
the Solar System. It is also the largest of the Solar System’s four terrestrial
planets.
Home to millions of species including humans, Earth is currently the only
astronomical body where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years.
Earth’s outer surface is divided into several rigid segments, or tectonic
plates, that migrate across the surface over periods of many millions of years.
About 71% of the surface is covered with salt water oceans, the remainder
consisting of continents and islands which together have many lakes and other
sources of water contributing to the hydrosphere.
Earth interacts with other objects in space, especially the Sun and the
Moon. At present, Earth orbits the Sun once for every roughly 366.26 times
it rotates about its axis, which is equal to 365.26 solar days, or one sidereal
year.
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Mars
Mars is the fourth planet from the Sun in the Solar System. It is often described as the ”Red Planet”, as the iron oxide prevalent on its surface gives
it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere,
having surface features reminiscent both of the impact craters of the Moon
and the volcanoes, valleys, deserts, and polar ice caps of Earth.
The rotational period and seasonal cycles of Mars are likewise similar to
those of Earth, as is the tilt that produces the seasons. Mars is the site of
Olympus Mons, the highest known mountain within the Solar System, and
of Valles Marineris, the largest canyon.
Until the first flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquid water on the planet’s surface. Geological
evidence gathered by unmanned missions suggest that Mars once had largescale water coverage on its surface, while small geyser-like water flows may
have occurred during the past decade. In 2005, radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. The
Phoenix lander directly sampled water ice in shallow Martian soil on July
31, 2008.
Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids. Mars is currently host to
three functional orbiting spacecraft: Mars Odyssey, Mars Express, and the
Mars Reconnaissance Orbiter. On the surface are the two Mars Exploration
Rovers (Spirit and Opportunity) and several inert landers and rovers, both
successful and unsuccessful. The Phoenix lander completed its mission on
the surface in 2008. Mars can easily be seen from Earth with the naked eye.
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The Gas Giants
A gas giant (sometimes also known as a Jovian planet after the planet Jupiter,
or giant planet) is a large planet that is not primarily composed of rock or
other solid matter. There are four gas giants in our Solar System: Jupiter,
Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.
Planets above 10 Earth masses are termed giant planets. Below 10 Earth
masses they are called super earths or, sometimes probably more accurately
for the higher mass examples, ”Gas Dwarfs”. Objects large enough to start
deuterium fusion (above 13 Jupiter masses) are called brown dwarfs and
these occupy the mass range between that of large gas giant planets and the
lowest mass stars. The 13 Jupiter-mass cutoff is a rule of thumb rather than
something of precise physical significance.
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Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the
Solar System. It is a gas giant with a mass slightly less than one-thousandth
of the Sun but is two and a half times the mass of all the other planets in
our Solar System combined. Jupiter is classified as a gas giant along with
Saturn, Uranus and Neptune. Together, these four planets are sometimes
referred to as the Jovian planets.
The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans
named the planet after the Roman god Jupiter. When viewed from Earth,
Jupiter is the third-brightest object in the night sky after the Moon and
Venus.
Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. The outer
atmosphere is divided into several bands at different latitudes, resulting in
turbulence and storms along their boundaries. A prominent result is the
Great Red Spot, a giant storm that is known to have existed since at least
the 17th century when it was first seen by telescope. Surrounding the planet
is a faint planetary ring system and a powerful magnetosphere. There are
also at least 63 moons, including the four large moons called the Galilean
moons that were first discovered by Galileo Galilei in 1610. Ganymede, the
largest of these moons, has a diameter greater than that of the planet Mercury.
Jupiter has been explored on several occasions by robotic spacecraft, most
notably during the early Pioneer and Voyager flyby missions and later by the
Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound
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New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed. Future targets for exploration in the
Jovian system include the possible ice-covered liquid ocean on the moon Europa.
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the
Solar System, after Jupiter. Saturn is named after the Roman god Saturn.
Saturn has an average radius about 9 times larger than the Earth’s. While
only 1/8 the average density of Earth, due to its larger volume, Saturn’s mass
is just over 95 times greater than Earth’s.
Because of Saturn’s large mass and resulting gravitation, the conditions
produced on Saturn are extreme if compared to Earth. The interior of Saturn
is probably composed of a core of iron, nickel, silicon and oxygen compounds,
surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and finally, an outer gaseous layer. Wind
speeds on Saturn can reach 1,800 km/h, significantly faster than those on
Jupiter.
Saturn has nine rings, consisting mostly of ice particles with a smaller
amount of rocky debris and dust. 62 known moons orbit the planet; fiftythree are officially named. This is not counting hundreds of ”moonlets”
within the rings. Titan, Saturn’s largest and the Solar System’s second
largest moon (after Jupiter’s Ganymede), is larger than the planet Mercury
and is the only moon in the Solar System to possess a significant atmosphere.
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Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary
radius and fourth-largest planetary mass in the Solar System. Though it is
visible to the naked eye like the five classical planets, it was never recognized
as a planet by ancient observers because of its dimness and slow orbit. Sir
William Herschel announced its discovery on March 13, 1781, expanding the
known boundaries of the Solar System for the first time in modern history.
Uranus was also the first planet discovered with a telescope.
Uranus is similar in composition to Neptune, and both are of different
chemical composition than the larger gas giants Jupiter and Saturn. Uranus’s
atmosphere, while similar to Jupiter and Saturn’s in its primary composition
of hydrogen and helium, contains more ”ices” such as water, ammonia and
methane, along with traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (224
◦
C). It has a complex, layered cloud structure, with water thought to make
up the lowest clouds, and methane thought to make up the uppermost layer
of clouds. In contrast the interior of Uranus is mainly composed of ices and
rock.
Like the other giant planets, Uranus has a ring system, a magnetosphere,
and numerous moons. The Uranian system has a unique configuration among
the planets because its axis of rotation is tilted sideways, nearly into the plane
of its revolution about the Sun. As such, its north and south poles lie where
most other planets have their equators. Seen from Earth, Uranus’s rings can
sometimes appear to circle the planet like an archery target and its moons
revolve around it like the hands of a clock. The wind speeds on Uranus can
reach 900 km/h.
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Neptune
Neptune is the eighth and farthest planet from the Sun in our Solar System.
Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth
and is slightly more massive than Uranus. On average, Neptune orbits the
Sun at a distance of 30.1 AU, approximately 30 times the Earth-Sun distance.
Discovered on September 23, 1846, Neptune was the first planet found by
mathematical prediction rather than by empirical observation. Unexpected
changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit
was subject to gravitational perturbation by an unknown planet. Neptune
was subsequently observed by Johann Galle within a degree of the position
predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered
shortly thereafter, though none of the planet’s remaining 12 moons were located telescopically until the 20th century. Neptune has been visited by only
one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants Jupiter and Saturn.
Neptune’s atmosphere, while similar to Jupiter’s and Saturn’s in that it is
composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of ”ices” such as
water, ammonia and methane. The interior of Neptune, like that of Uranus,
is primarily composed of ices and rock. Traces of methane in the outermost
regions in part account for the planet’s blue appearance.
In contrast to the relatively featureless atmosphere of Uranus, Neptune’s
atmosphere is notable for its active and visible weather patterns. At the time
of the 1989 Voyager 2 flyby, for example, the planet’s southern hemisphere
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possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter.
These weather patterns are driven by the strongest sustained winds of any
planet in the Solar System, with recorded wind speeds as high as 2,100 km/h.
Because of its great distance from the Sun, Neptune’s outer atmosphere is one
of the coldest places in the Solar System, with temperatures at its cloud tops
approaching 218 ◦ C (55 K). Temperatures at the planet’s centre, however,
are approximately 5,400 K (5,000 C). Neptune has a faint and fragmented
ring system, which may have been detected during the 1960s but was only
indisputably confirmed in 1989 by Voyager 2.
Asteroids and Comets
Asteroids
Asteroids are a class of small Solar System bodies in orbit around the Sun.
The term ”asteroid” was historically applied to any astronomical object orbiting the Sun that was not observed to have the characteristics of an active
comet or a planet, but it has increasingly come to particularly refer to the
small rocky and metallic bodies of the inner Solar System and out to the
orbit of Jupiter. As small objects in the outer Solar System have begun to
be discovered their observed composition differs from the objects historically
termed asteroids.
There are millions of asteroids, and like most other small Solar System
bodies the asteroids are thought to be remnants of planetesimals, material
within the young Sun’s solar nebula that have not grown large enough to form
planets. The large majority of known asteroids orbit in the main asteroid
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belt between the orbits of Mars and Jupiter, however many different orbital
families exist with significant populations including Jupiter Trojans and nearEarth asteroids.
Comets
A comet is an icy small Solar System body that, when close enough to the
Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are
themselves loose collections of ice, dust, and small rocky particles, ranging
from a few hundred meters to tens of kilometers across.
Comets have a wide range of orbital periods, ranging from a few years to
hundreds of thousands of years. Short-period comets originate in the Kuiper
belt, or its associated scattered disc, which lie beyond the orbit of Neptune.
Longer-period comets are thought to originate in the Oort Cloud, a spherical
cloud of icy bodies in the outer Solar System. Long-period comets plunge
towards the Sun from the Oort Cloud because of gravitational perturbations
caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass
once through the inner Solar System before being thrown out into interstellar
space along hyperbolic trajectories.
Comets are distinguished from asteroids by the presence of a coma or a
tail. However, extinct comets that have passed close to the Sun many times
have lost nearly all of their volatile ices and dust and may come to resemble
small asteroids.
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As of May 2010 there are a reported 3,976 known comets. This number
is steadily increasing. However, this represents only a tiny fraction of the
total potential comet population: the reservoir of comet-like bodies in the
outer solar system may number one trillion. The number visible to the
naked eye averages roughly one per year, though many of these are faint
and unspectacular.
Extrasolar Planets
An extrasolar planet, or exoplanet, is a planet outside the Solar System. As
of February 7 2011, astronomers have announced the confirmed detection of
529 such planets. There are at least another 1,235 planet candidates awaiting confirmation by more detailed investigations, including 54 that may be
in the ”Habitable Zone”.
Many exoplanets have been detected through radial velocity observations
and other indirect methods rather than actual imaging. Most are giant planets thought to resemble Jupiter although this is mainly because they are
easier to detect.
It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. The true proportion
may be much higher. It follows that billions of exoplanets must exist in our
own galaxy alone.
Extrasolar planets became an object of scientific investigation in the nineteenth century. Many astronomers supposed that they existed, but there was
no way of knowing how common they were or how similar they might be to
the planets of our solar system. The first confirmed detection was made in
1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting
a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b,
was found in a four-day orbit around the nearby G-type star 51 Pegasi. The
frequency of detections has tended to increase on an annual basis since then.
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