ASVAB · General Science · 6 min read
Astronomy and the Solar System
- Identify the components of our solar system, including the Sun, inner terrestrial planets, outer giant planets, and smaller celestial bodies.
- Explain the mechanics of Earth's motions, including day and night, the cause of the seasons, lunar phases, eclipses, and ocean tides.
- Describe the fundamental characteristics of stars, deep-space objects, and galaxies, including their structures and classifications.
The Sun and the Solar System
The solar system consists of the Sun, eight planets, dwarf planets, moons, asteroids, and comets. At the center sits the Sun, which is a star made mainly of hydrogen and helium in the form of hot plasma. Inside the Sun's core, a process called nuclear fusion converts hydrogen nuclei into helium. This reaction releases massive amounts of light and heat, powering our entire solar system.
Moving outward from the Sun, the eight planets follow a specific order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Scientists divide these into two distinct groups. The four inner planets, Mercury, Venus, Earth, and Mars, are called terrestrial planets because they have solid, rocky surfaces. Mercury is the smallest planet and the closest to the Sun. Venus is the hottest planet in the solar system because of a runaway greenhouse effect caused by its dense carbon dioxide atmosphere. Earth supports life, and Mars is known as the Red Planet because its surface contains high amounts of iron oxide, commonly known as rust.
Beyond Mars lie the outer planets, which are much larger. Jupiter and Saturn are classified as gas giants, while Uranus and Neptune are ice giants. Jupiter is the largest planet in our solar system and possesses a giant storm known as the Great Red Spot. Saturn is best known for its extensive and prominent ring system made primarily of ice and rock particles. Uranus has an unusual orientation; it rotates on its side with an axial tilt greater than ninety degrees. Neptune is the farthest planet from the Sun, and methane in its atmosphere absorbs red light, helping give Neptune a blue appearance.
Smaller objects also travel through our solar system. The asteroid belt is a region of rocky debris located between the orbits of Mars and Jupiter. Comets are bodies of ice, dust, and rock that can form a glowing coma and tails as they approach the Sun, with their tails generally extending away from the Sun. A meteoroid is a small rocky or metallic object in space; when it enters Earth's atmosphere, the streak of light produced is called a meteor, and if a piece reaches the ground, it is called a meteorite. Beyond Neptune lies the Kuiper Belt, where Pluto is classified as a dwarf planet.
| Category | Planets Included | Key Characteristics |
|---|---|---|
| Terrestrial Planets | Mercury, Venus, Earth, Mars | Inner planets with solid, rocky surfaces; Mercury is smallest; Venus is hottest due to carbon dioxide. |
| Gas Giants | Jupiter, Saturn | Massive outer planets; Jupiter features the Great Red Spot; Saturn features prominent rings of ice and rock. |
| Ice Giants | Uranus, Neptune | Outer planets; Uranus rotates on its side (tilt over 90 degrees); Neptune has atmospheric methane absorbing red light. |
Common mistakes
- Thinking a light-year is a unit of time because it contains the word 'year.' On the ASVAB, remember that a light-year is strictly a unit of astronomical distance.
- Believing red stars are hotter than blue stars. In astronomy, blue stars have the highest surface temperatures and red stars are the coolest.
- Assuming Earth's seasons are caused by changing distance from the Sun. Seasons are caused by the 23.5-degree tilt of Earth's axis, not perihelion or aphelion.
- Confusing meteors with meteorites. A meteor is only the streak of light in the atmosphere; a meteorite is the physical rock that survives and reaches the ground.
Key points
- The Sun produces energy via nuclear fusion of hydrogen into helium; the solar system contains eight planets split into inner terrestrial planets and outer gas and ice giants.
- Earth rotates in roughly 24 hours (day and night) and revolves in approximately 365.25 days; its 23.5-degree axial tilt causes the seasons, solstices, and equinoxes.
- The Moon reflects sunlight through an eight-phase cycle over roughly 29.5 days; solar eclipses happen at new moon and lunar eclipses at full moon.
- The gravitational pull of the Moon and Sun creates ocean tides, producing spring tides during new and full moons and neap tides during quarter moons.
- Star colors indicate surface temperature (blue is hottest, red is coolest), and stellar brightness depends on luminosity and distance.
- Galaxies are classified as spiral, elliptical, or irregular; our solar system resides in the Milky Way, a barred spiral galaxy.
Practice this section
Explained ASVAB General Science questions, no account needed. ASVAB General Science practice test
The full lesson continues with 3 more sections with exam tips, audio in English and Spanish, 5 Check yourself questions and 20 flashcards.
- 1 month
$38.99$19.49 - 3 months
$113.99$56.99 - Lifetime
$233.99$116.99
One payment, no subscription.
See ASVAB plansAlready a member? Log in
Sample lesson: Cells, Genetics and Heredity25 practice questions on this exam
Each ASVAB practice test has 25 original questions with the answers explained. No account needed.
Open the ASVAB practice testsOriginal lesson written for ASVAB practice · updated October 30, 2026. Exam names are trademarks of their owners; tests.global is not affiliated with them.