1. 1.4 Our Solar Neighbourhood
In Section 1.3, you learned that stars are born in stellar nurseries called nebulae.
The formation of our solar system (the Sun and nine planets) occurred the same way.
The “protoplanet hypothesis” is a model for explaining the birth of solar systems.
The process can be described in three steps:
1. A cloud of gas and dust in space begins swirling.
2. Most of the material (more than 90%) accumulates in the centre, forming the Sun.
3. The remaining material accumulates in smaller clumps circling the centre. These form the planets.

2. 1.4 Our Solar Neighbourhood
Figure 1.27 The three stages in the formation of a solar system, according to the protoplanet
hypothesis (sometimes known as the “nebular theory”)

3. 1.4 Our Solar Neighbourhood
THE SUN
After telescopes were invented (Hans Lipprshey) it wasn’t long before filters were designed to allow observers to gaze directly at the Sun.
Satellites have offered an even closer look.
The Sun is almost 110 times wider than Earth.
If the Sun were a hollow ball, almost a million Earths would be required to fill it.
Surface temperature is about 5500°C,
Core temperature is close to °C.

4. 1.4 Our Solar Neighbourhood

5. 1.4 Our Solar Neighbourhood
THE PLANETS
Every planet has its own unique features and characteristics.
The solar system can be divided into two distinct planetary groups:
the inner planets, also called terrestrial, or Earth-like, planets;
the outer planets, or Jovian (in reference to Jupiter), planets.
The terrestrial planets tend to be smaller, rockier in composition, and closer to the Sun than the Jovian planets.
The Jovian planets are large and gaseous and are located great distances from the Sun.
Technology has enabled us to learn a lot about our nearest neighbours in space.
All the planets except Pluto have been visited by orbiting space
probes. Mars and Venus have had robots land on their surface.

6. 1.4 Our Solar Neighbourhood
Mercury
Closest planet to the Sun.
No atmosphere
No protection from the bombardment of meteroids, asteroids, and comets.
Temperatures on Mercury vary from over 400°C on the sunny side to –180°C on the dark side.

7. 1.4 Our Solar Neighbourhood
Venus
Similar to Earth in diameter, mass, and gravity, and is often called Earth’s twin.
Temperatures can be over 450°C—hot enough to melt lead.
The permanent clouds are made of carbon dioxide, and it often rains sulfuric acid (the same acid found in a car battery).
Russians landed a probe on Venus in 1982, but it only lasted there for 57 min.
In 1991, Spacecraft Magellan mapped Venus using radio waves (radar).
It found huge canyons, extinct volcanoes, and ancient lava flows.
Venus is one of the only planets in the solar system to rotate from east to west—opposite to the other six.

8. 1.4 Our Solar Neighbourhood
Earth
It is the only planet where water exists in all three phases: solid, liquid, and gas.
Seventy percent of the planet’s surface is covered in water.
Earth is one of the few places in our solar system that has active volcanism.

9. 1.4 Our Solar Neighbourhood
Mars
the “red planet,”
Iron oxides on the planet’s surface.
Mars has two polar ice caps.
The atmosphere is carbon dioxide.
Temperatures at Mars’s equator can reach 16°C in the summer.
Like Venus and Earth, Mars has canyons, valleys, and extinct volcanoes.
Mars also has two small moons, Phobos and Deimos.

10. 1.4 Our Solar Neighbourhood
Jupiter
16 moons.
Jupiter is the largest of all the planets
Contains more than twice the mass of all the other planets combined.
It is a gas giant (hydrogen and helium)
If Jupiter were only 10 times larger it may have formed into a star.
The Great Red Spot on Jupiter is a huge storm in its atmosphere.
Jupiter has three very thin rings.

11. 1.4 Our Solar Neighbourhood
Saturn
Galileo saw Saturn’s rings with his primitive telescope in 1610,
Voyager 1 and 2 flew by Saturn in 1980 and 1981, respectively. In late 2004, the Cassini spacecraft will reach Saturn and drop a probe onto Titan, the largest of the planet’s 19 moons.
second largest planet in our solar system.
most distinctive ring system.
Over a thousand rings exist, hydrogen and helium.
wind speeds at 1800 km/h.

12. 1.4 Our Solar Neighbourhood
Uranus
Voyager 2 has given us most of our close-up information about Uranus, last sending data back to Earth in 1986.
its axis of rotation is tilted (rolls like a barrel)
Another gas giant: hydrogen and helium.
Methane gives the planet a distinctive blue colour.
Uranus has a large ring system, and 17 moons.

13. 1.4 Our Solar Neighbourhood
Neptune
Another gas giant composed of hydrogen, helium, and methane,
Neptune (like Uranus) is bluish in colour.
Very little of the Sun’s energy reaches the eighth planet. Neptune gives off about 3 times more energy than it receives.
It boasts the fastest wind speeds in the solar system, at 2500 km/h. Like all the other gas giants, Neptune has its own ring system, as well as eight moons.

14. 1.4 Our Solar Neighbourhood
Pluto
Hubble Space Telescope.
Planet or not?
It is a frozen ball of methane smaller than our moon.
Pluto’s orbit is raised 17.2° from the plane of the other planets and is more elliptical than that of other planets.
Like Venus and Uranus, Pluto rotates from east to west.
Between 1979 and 1999, Pluto was closer to the Sun than Neptune.
Some astronomers believe that Pluto and its moon, Charon, are comets captured by the Sun’s gravity

15. 1.4 Other Bodies Comets Comets, often described as “dirty snowballs,”
are objects made up of dust and ice that travel through space.
Their long tails and bright glow only
Comets spend most of their time slowly orbiting in the outer reaches of the solar system.
Halley’s comet, which is visible from Earth every 76 years. The last time it was seen was in 1986.

16. 1.4 Other Bodies Meteoroids, Meteors, and Meteorites
Small pieces of rocks flying through space with no particular path are called meteoroids.
Meteoroids can be as small as a grain of sand or as
large as a car.
When one gets pulled into the atmosphere by Earth’s gravity, the heat of atmospheric friction causes it to give off light and it is known as a meteor.
These are the so-called “shooting stars”.
If a meteor lasts long enough to hit Earth’s surface, it is called a meteorite.

17. 1.4 TRACKING OBJECTS IN THE SOLAR SYSTEM
The paths of the major bodies in the solar system are ellipses.
Because astronomers understand the nature and geometry of elliptical orbits, as well as of celestial motion, they now understand the paths of planets and their moons.
This means that a variety of events can be accurately predicted, including both solar and lunar eclipses
Figure 1.33 During a total solar eclipse, the Sun’s corona is visible.

18. Homework
Check & Reflect Q 1- 9 Page 400
Due next class