What is in the solar system, and what keeps the planets in orbit?
Describe the structure of the solar system (the Sun, terrestrial and gas-giant planets, and small bodies), explain that orbits are elliptical (Kepler), and explain how gravity and inertia keep planets in orbit (Virginia 2018 Earth Science SOL ES.2 and ES.11).
A SOL-level answer on the solar system for the Virginia Earth Science EOC: the Sun and the inner terrestrial versus outer gas-giant planets, asteroids, comets and other small bodies, the nebular hypothesis, Kepler's elliptical orbits, and how gravity and inertia combine to keep planets orbiting, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standards ES.2 and ES.11 ask you to describe the solar system (its structure and contents), to know that orbits are elliptical (Kepler), and to explain how gravity and inertia keep planets in orbit. The EOC tests this with planet-classification items (terrestrial versus gas giant), orbit-shape items (Kepler's first law), and force items (what keeps a planet in orbit). It sets up the larger scales of stars and the universe in the next topics.
The structure of the solar system
How the solar system formed
The leading explanation is the nebular hypothesis: the solar system formed about 4.6 billion years ago from a giant, slowly spinning cloud of gas and dust (a nebula). As it collapsed under gravity, most material gathered in the center to form the Sun, while the rest flattened into a disk where the planets grew by accretion. The hot inner region kept only rock and metal (the small rocky planets), while the cold outer region kept ices and gases too (the gas giants), which is why the inner and outer planets differ.
Kepler's laws of orbits
The most-tested point is the first law: orbits are ellipses, not perfect circles, with the Sun at one focus.
Gravity and inertia keep planets in orbit
Try this
Q1. State Kepler's first law of planetary motion. [1]
- Cue. Planets orbit the Sun in ellipses with the Sun at one focus.
Q2. Explain why the inner planets are rocky while the outer planets are gas giants. [2]
- Cue. The inner solar system was too hot for ices and gases to remain, so only rock and metal collected there (small rocky planets); the cold outer region kept gases and ices, forming the large gas giants.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksWhat is the shape of the orbit of a planet around the Sun? (A) a perfect circle. (B) an ellipse with the Sun at one focus. (C) a square. (D) a straight line.Show worked answer →
A 1-point multiple-choice item on orbits.
The correct answer is B. Kepler's first law states that planets orbit the Sun in ellipses (slightly flattened circles) with the Sun at one focus. The orbits are not perfect circles (A); a planet's distance from the Sun varies a little over its orbit. Squares (C) and straight lines (D) are not orbital shapes.
The test rewards Kepler's first law: elliptical orbits with the Sun at one focus.
VA Earth Science SOL 2024 (style)2 marksA planet stays in orbit around the Sun rather than flying off into space or falling into the Sun. (a) Name the force that keeps the planet in orbit. (b) Explain how that force and the planet's motion (inertia) combine to produce a curved orbit.Show worked answer →
A 2-point item on gravity and orbits.
(a) 1 point: gravity (the gravitational attraction between the Sun and the planet) keeps the planet in orbit.
(b) 1 point: the planet's inertia keeps it moving forward in a straight line, while the Sun's gravity continuously pulls it toward the Sun; the balance of the forward motion and the inward pull bends the straight-line motion into a closed, curved (elliptical) orbit instead of letting it fly off or fall straight in.
Markers reward naming gravity in (a) and the combination of inertia (straight-line motion) and gravity (inward pull) producing a curved orbit in (b).
Related dot points
- Explain how Earth's rotation causes day and night, how the Moon's orbit causes its phases, and how the alignment of the Sun, Earth and Moon causes solar and lunar eclipses (Virginia 2018 Earth Science SOL ES.11).
A SOL-level answer on the Earth-Moon-Sun system for the Virginia Earth Science EOC: how rotation causes day and night, the cause and sequence of Moon phases, why eclipses happen only at certain alignments, the difference between a solar and a lunar eclipse, and why eclipses are not monthly, with worked exam questions.
- Explain that Earth's seasons are caused by the tilt of its axis (about 23.5 degrees) as it revolves around the Sun, which changes the directness of sunlight and the length of daylight (Virginia 2018 Earth Science SOL ES.11).
A SOL-level answer on the seasons for the Virginia Earth Science EOC: why the tilt of Earth's axis, not its distance from the Sun, causes the seasons, how the directness of sunlight (insolation) and daylight length change, the solstices and equinoxes, and why the hemispheres have opposite seasons, with worked exam questions.
- Explain that stars produce energy by nuclear fusion, describe the Hertzsprung-Russell diagram and the relationship between a star's color, temperature and luminosity, and outline the life cycle of a star (Virginia 2018 Earth Science SOL ES.12).
A SOL-level answer on stars for the Virginia Earth Science EOC: how stars produce energy by nuclear fusion, what the Hertzsprung-Russell diagram shows about color, temperature and luminosity, the main sequence, and how a star's life cycle depends on its mass, with worked exam questions.
- Describe galaxies and the scale of the universe, explain the Big Bang theory and its evidence (redshift and the cosmic microwave background), and outline how the electromagnetic spectrum and telescopes are used to study space (Virginia 2018 Earth Science SOL ES.12).
A SOL-level answer on the universe for the Virginia Earth Science EOC: galaxies and their types, the vast scale measured in light-years, the Big Bang theory and its evidence (the redshift of distant galaxies and the cosmic microwave background), and how the electromagnetic spectrum and telescopes let us study space, with worked exam questions.
- Describe how wind generates ocean waves and the parts of a wave, and explain that tides are caused by the gravitational pull of the Moon and Sun, including spring and neap tides (Virginia 2018 Earth Science SOL ES.10 and ES.11).
A SOL-level answer on waves and tides for the Virginia Earth Science EOC: how wind makes waves, the parts of a wave (crest, trough, wavelength, height) and what fetch controls, why tides are caused by the gravity of the Moon (and Sun), the daily pattern of two high and two low tides, and spring versus neap tides, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)