Scale of the solar system

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Lesson notes

Overview of the Solar System

The **Solar System** is the Sun and all objects gravitationally bound to it, including eight planets, dwarf planets, moons, asteroids, comets, and dust.
The Sun contains **99.86%** of the system's total mass, making it the dominant gravitational body.
The Solar System formed about **4.6 billion years ago** from the collapse of a molecular cloud.
It is located in the **Milky Way Galaxy** and is an isolated single-star planetary system.

The Sun

The Sun is a **star** that produces energy by fusing hydrogen into helium in its core.
Energy radiates from the Sun's **photosphere**, creating the **heliosphere**—a bubble of charged particles.
The Sun's gravity holds all planets and other objects in orbit.

The Planets: Inner (Terrestrial) vs. Outer (Gas & Ice Giants)

The **inner planets** (Mercury, Venus, Earth, Mars) are **terrestrial**—small, rocky, and close to the Sun.
The **outer planets** (Jupiter, Saturn, Uranus, Neptune) are much larger, composed mostly of gas and ice, and lie beyond the **frost line** at about 5 AU.
Jupiter and Saturn are **gas giants**; Uranus and Neptune are **ice giants**.
Earth and Mars orbit within the Sun's **habitable zone**, where liquid water can exist.

Dwarf Planets and Small Bodies

**Dwarf planets** (e.g., Pluto, Ceres, Eris) orbit the Sun but do not dominate their orbits.
The **asteroid belt** between Mars and Jupiter contains rocky bodies like Ceres.
Beyond Neptune lie the **Kuiper belt** and **scattered disc**, home to many dwarf planets and comets.
The **Oort cloud** is a distant, spherical shell of icy bodies, source of long-period comets.

Scale: Sizes and Distances

Planet sizes vary dramatically: Earth's diameter is ~12,700 km; Jupiter's is ~143,000 km (11 times Earth's).
Distances are measured in **astronomical units (AU)**—1 AU = average Earth–Sun distance (~150 million km).
Mercury is 0.39 AU from the Sun; Neptune is 30.1 AU.
The Solar System's edge (Hill sphere) extends up to ~230,000 AU (3.6 light-years).

Orbits and Motions

All planets orbit the Sun in the same direction (counterclockwise as viewed from above Earth's north pole).
Orbital periods increase with distance: Mercury ~88 days, Earth 1 year, Neptune ~165 years.
Planets closer to the Sun move faster in their orbits (Kepler's laws).
Most planets rotate on their axes; Venus rotates backwards (retrograde).

Moons and Other Satellites

Many planets have **moons** (natural satellites) that orbit them.
Jupiter's moon **Ganymede** and Saturn's moon **Titan** are larger than Mercury.
Earth's Moon is the fifth-largest moon in the Solar System.
Moons can be rocky, icy, or a mix; some have atmospheres or subsurface oceans.

The Interplanetary Medium and Heliosphere

The **solar wind**—a stream of charged particles—flows from the Sun, filling the heliosphere.
The **heliopause** (~70–90 AU) is where the solar wind meets the interstellar medium.
Beyond the heliopause lies interstellar space; the Oort cloud extends far beyond.

Order of planets from the Sun (not to scale). The inner terrestrial planets are small and rocky; the outer gas/ice giants are much larger and farther apart.

Relative sizes of the Sun and planets. The Sun dwarfs even Jupiter, the largest planet.

Average distances from the Sun in AU. The gap between Mars and Jupiter contains the asteroid belt.

Major regions of the Solar System: inner, asteroid belt, outer, Kuiper belt, and Oort cloud.

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Practice questions

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1.What percentage of the Solar System's total mass does the Sun account for?
Easy
- A99.86%
- B86.99%
- C99.14%
- D90.86%
2.Which of the following is the correct order of the terrestrial planets from closest to farthest from the Sun?
Easy
- AMercury, Venus, Earth, Mars
- BVenus, Mercury, Earth, Mars
- CMercury, Earth, Venus, Mars
- DMercury, Venus, Mars, Earth
3.Beyond what distance from the Sun is the frost line located?
Medium
- A5 AU
- B1 AU
- C10 AU
- D0.5 AU
4.Which two planets together possess nearly 90% of the non-stellar mass of the Solar System?
Hard
- AJupiter and Saturn
- BJupiter and Neptune
- CSaturn and Uranus
- DUranus and Neptune
5.Which of the following is NOT a dwarf planet according to the International Astronomical Union?
Medium
- APluto
- BCeres
- CEris
- DTitan
6.About how many years ago did the Solar System form?
Hard
- A4.6 billion years ago
- B4.6 million years ago
- C4.6 trillion years ago
- D460 million years ago
7.What is the name of the region beyond Neptune that contains many dwarf planets and other objects?
Easy
- AKuiper belt
- BAsteroid belt
- COort cloud
- DScattered disc
8.Which of the following is the largest moon in the Solar System?
Medium
- AGanymede
- BTitan
- CEarth's Moon
- DIo

Revision notes

## Scale of the Solar System - **Solar System**: Gravitationally bound system of the Sun and objects that orbit it. The Sun contains **99.86%** of the total mass. - **Formation**: Formed ~4.6 billion years ago from a collapsing molecular cloud. The Sun formed at the center; planets formed by accretion from a protoplanetary disc. - **Planets**: Eight planets in order from the Sun: - **Inner (terrestrial) planets**: Mercury, Venus, Earth, Mars – small, rocky. - **Outer planets**: Jupiter, Saturn (gas giants), Uranus, Neptune (ice giants). - Jupiter and Saturn have ~90% of the non-stellar mass. - **Dwarf planets**: Ceres (asteroid belt), Pluto, Eris, Makemake, Haumea, and others (Orcus, Quaoar, Gonggong, Sedna). They orbit the Sun but do not dominate their orbits. - **Small bodies**: Asteroids (mostly in asteroid belt between Mars and Jupiter), comets, meteoroids, interplanetary dust. - **Moons**: Natural satellites orbiting planets. Largest moons (Ganymede, Titan) are larger than Mercury. - **Scale distances**: - **Astronomical Unit (AU)**: Average Earth–Sun distance (~150 million km). Used to measure distances in the solar system. - **Frost line** at ~5 AU separates inner and outer solar system. - **Heliopause** at ~70–90 AU (boundary where solar wind meets interstellar medium). - **Oort cloud** extends from ~2,000 AU to ~100,000 AU (source of long-period comets). - **Hill sphere** (gravitational influence of Sun) ends at ~178,000–227,000 AU. - Nearest star, Proxima Centauri, is ~269,000 AU away. - **Key concept**: The solar system is mostly empty space; planets are tiny compared to distances between them. Models often use scale to visualize sizes and distances.

Astronomical Unit (AU): The average distance from Earth to the Sun, about 150 million kilometers. Used to measure distances within the solar system.
Terrestrial planets: The four inner planets (Mercury, Venus, Earth, Mars) that are rocky and relatively small.
Gas giants: Jupiter and Saturn, the largest planets, composed mostly of hydrogen and helium.
Ice giants: Uranus and Neptune, composed of heavier elements like water, methane, and ammonia ices.
Dwarf planet: A celestial body that orbits the Sun, has enough mass for a round shape, but has not cleared its orbital neighborhood of other debris.
Asteroid belt: Region between Mars and Jupiter containing many small rocky bodies (asteroids) and the dwarf planet Ceres.
Kuiper belt: Region beyond Neptune containing icy bodies and dwarf planets like Pluto.
Oort cloud: A spherical shell of icy objects surrounding the solar system at distances from about 2,000 to 100,000 AU; source of long-period comets.
Heliosphere: The bubble-like region of space dominated by the solar wind, extending to about 70–90 AU.
Protoplanetary disc: A rotating disk of gas and dust around a young star from which planets form by accretion.

Flashcards

What percentage of the Solar System's total mass does the Sun account for?

The Sun accounts for 99.86% of the Solar System's total mass.

List the four terrestrial planets in order from the Sun.

Mercury, Venus, Earth, Mars.

What is the frost line and how far from the Sun is it?

The frost line is the distance from the Sun beyond which volatile compounds can condense into ice. It is located at about 5 astronomical units (AU).

Name the two gas giants and the two ice giants in the outer Solar System.

Gas giants: Jupiter and Saturn. Ice giants: Uranus and Neptune.

Which two planets orbit within the Sun's habitable zone?

Earth and Mars orbit within the Sun's habitable zone.

What is an astronomical unit (AU)?

An astronomical unit (AU) is the average distance from Earth to the Sun, about 93 million miles or 150 million kilometers.

What are dwarf planets?

Dwarf planets are objects of planetary mass that directly orbit the Sun but do not dominate their orbit (they share it with other bodies).

List the five dwarf planets recognized by the International Astronomical Union.

Ceres, Pluto, Eris, Makemake, and Haumea.

Where is the asteroid belt located?

The asteroid belt is located between the orbits of Mars and Jupiter.

What are trans-Neptunian objects?

Trans-Neptunian objects are small Solar System bodies that orbit the Sun at a greater distance than Neptune, including many dwarf planets in the Kuiper belt and scattered disc.

Which two moons are larger than the planet Mercury?

Ganymede (moon of Jupiter) and Titan (moon of Saturn) are larger than Mercury.

What is the heliosphere?

The heliosphere is a bubble-like region of space dominated by the solar wind, extending far beyond the planets.

What is the heliopause?

The heliopause is the boundary where the solar wind is halted by the interstellar medium, at about 70–90 AU from the Sun.

What is the Oort cloud?

The Oort cloud is a theorized spherical region of icy bodies surrounding the Solar System at distances from about 2,000 AU to 200,000 AU, the source of long-period comets.

Approximately how far away is the closest star, Proxima Centauri, in AU?

Proxima Centauri is about 269,000 AU away.

How old is the Solar System?

The Solar System formed about 4.6 billion years ago.

What process formed the Sun and planets?

The Solar System formed from the gravitational collapse of a region within a large molecular cloud, creating a protoplanetary disc from which the Sun and planets accreted.

What is the interplanetary medium?

The interplanetary medium consists of solar wind plasma, dust, gas, and cosmic rays that fill the space between Solar System bodies.

What is the Hill sphere of the Solar System?

The Hill sphere is the region where the Sun's gravitational influence dominates, extending to about 178,000–227,000 AU.

What are small Solar System bodies?

Small Solar System bodies include asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds.

How much of the non-stellar mass do Jupiter and Saturn contain?

Jupiter and Saturn contain nearly 90% of the non-stellar mass of the Solar System.

What is the Local Bubble?

The Local Bubble is a relatively small region of the Milky Way, about 1,000 light-years wide, that contains the Solar System and nearby stars.

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