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The Universe
and the Solar
System
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The Solar System
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Stars and galaxies
The Universe is made up of stars: huge massive bodies that
produce light in their core and are grouped together in
galaxies.
The Milky Way is the galaxy where our Solar System is
located.
Around the galaxies, there are often planets and satellites,
which do not produce light.
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The Solar System /1
The Solar System consists of the Sun and all the entities that
feel its gravitational pull and move around it: planets,
satellites, dwarf planets, asteroids and comets.
Their distance from the Sun is expressed in astronomical
units (AU): 1 AU = average distance Earth-Sun.
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The Solar System /2
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The Sun is a star and produces light
The Sun produces light by
thermonuclear fusion which
takes place in its core.
There, temperatures reach 15
million degrees and matter is in
the plasma state.
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Thermonuclear fusion reactions
4 nuclei of H
(1 proton each)
H
H
1 nucleus of He
(2 protons, 2 neutrons)
H
H
He
+ energy
During a thermonuclear fusion reaction, two or more
atomic nuclei join, forming a more complex nucleus with a
mass which is smaller than the sum of the initial masses.
The lost mass is transformed into energy, in accordance
with the equation E = mc2.
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Light is energy
Light is a combination of
electromagnetic radiation
of different frequencies
and wavelengths, that
transport energy in the
form of photons.
Light moves at a speed of about 300 000 km/s.
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The planets
around our Sun
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Terrestrial planets /1
In addition to Earth, the terrestrial planets are Mercury,
Venus and Mars.
They are small and rocky, but they have high densities. In
fact they are mainly composed of metals and heavy elements
such as silicon and iron.
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Terrestrial planets /2
Mercury
Radius (km)
2439
Density (g/cm3)
5.4
Period of rotation (days)
59
Period of revolution (days) 88
Average distance from the
58
Sun (millions of km)
Number of satellites
0
The surface of Mercury
resembles the surface of the
Moon: rugged, covered with
dust and cratered due to the
impact of meteorites.
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Terrestrial planets /3
Venus
Radius (km)
6052
Density (g/cm3)
5.2
Period of rotation (days)
243
Period of revolution (days)
255
Atmosphere component
CO2
Average distance from the
Sun (millions of km)
108
Number of satellites
0
Venus is covered by thick
clouds of sulfuric acid which
reflect sunlight, making the
planet extremely bright. It has
volcanoes and impact craters.
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Terrestrial planets /4
Mars
Radius (km)
3396
Density (g/cm3)
3.9
Period of rotation (days)
24.4
Period of revolution (days)
689
Atmosphere component
CO2
Average distance from the
Sun (millions of km)
229
Number of satellites
2
On Mars there are strong
winds (up to 200 km/h) that can
create violent sand storms and
shape the landscape. Ancient
riverbeds are also visible.
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Jovian planets /1
The Jovian planets are Jupiter, Saturn, Uranus and
Neptune.
They are very large, do not have solid rocky crusts and
display a remarkably thick atmosphere made of light gasses.
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Jovian planets /2
Jupiter
Radius (km)
70850
Density (g/cm3)
1.4
Period of rotation (hours)
9.92
Period of revolution (years) 12
Atmosphere component
H, He
Average distance from the
Sun (millions of km)
778
Number of satellites
64
Jupiter’s atmosphere is full of
clouds in the form of equatorial
stripes due to the planet’s high
speed of rotation.
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Jovian planets /3
Saturn
Radius (km)
60000
Density (g/cm3)
0.7
Period of rotation (hours)
10.24
Period of revolution
(years)
30
Atmosphere component
H, He
Average distance from
the Sun (millions of km)
1427
Number of satellites
62
The system of concentric rings of
Saturn has a range of over
200,000 km with a width of only
20 m. The rings are made of
fragments of ice and solid matter.
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Jovian planets /4
Uranus
Radius (km)
25400
Density (g/cm3)
1.3
Period of rotation (hours)
15.6
Period of revolution
(years)
84
Atmosphere component
H, He,
methane
Average distance from
the Sun (millions of km)
2870
Number of satellites
27
Uranus has a system of
rings as well, although it is
far less complex and bright
when compared to Saturn.
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Jovian planets /5
Neptune
Radius (km)
24300
Density (g/cm3)
1.8
Period of rotation (hours)
18.5
Period of revolution (years) 165
Atmosphere component
H, He,
methane
Average distance from the
Sun (millions of km)
4497
Number of satellites
13
Neptune’s atmosphere is
dense and filled with clouds
and its composition is similar
of that of Uranus, as it
contains hydrogen, helium
and methane.
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Minor bodies in the Solar System
The dwarf planets, like Pluto, have small masses and are
spherical.
Asteroids are rocky bodies with an irregular shape found
mainly between Mars and Jupiter or at the edge of the Solar
System.
Comets come from the Oort Cloud and have a core of dust
and ice which partially transforms into gas as it approaches
the Sun.
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Movements of
the planets
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Rotation and the day
The motion of a planet around its axis is called rotation.
Planet
Rotation
Mercury
59 days
Venus
243 days
Earth
24 hours
Mars
~ 24 hours
Jupiter
10 hours
Saturn
10 h 40’
Uranus
11 hours
Neptune
16 hours
The rotational period is a day.
Each planet has a
characteristic day-length
which is independent of its
distance from the Sun. Earth’s
rotational period is
approximately 24 hours.
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Revolution and Kepler’s Laws /1
Kepler’s first law states that planets orbit the Sun in an
ellipse, with the Sun at one focus.
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Revolution and Kepler’s Laws /2
The speed of the planets varies between a maximum at
perihelion and a minimum at aphelion. Each planet revolves
around the Sun at a different average speed which decreases
with increasing distance from the Sun.
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How can we explain Kepler’s Laws?
Fg = G M1 x M2
d2
The law of Universal Gravitation
states that any two bodies attract with
a force directly proportional to the
product of their masses and inversely
proportional to the square of the
distance between them.
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