What the Solar System contains
The Solar System is the Sun and everything held in orbit by its gravity. The Sun contains more than 99% of all the mass in it, which is why everything else orbits the Sun rather than the other way round.
Eight planets orbit the Sun in nearly circular paths, in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four inner planets are small and rocky; the four outer ones are far larger and made mostly of gas and ice. Between the two groups lies the asteroid belt.
A moon is a natural satellite orbiting a planet rather than the Sun. Comets follow highly elliptical orbits that bring them close to the Sun and then take them far beyond the outermost planets. Asteroids are rocky bodies, most of them between Mars and Jupiter.
The Sun itself is a star: a ball of hot gas releasing energy by nuclear fusion in its core, where hydrogen nuclei join to form helium.
| Inner (rocky) planets | Outer (gas) planets | |
|---|---|---|
| Which | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Size | small | much larger |
| Made of | rock and metal | gas and ice |
| Orbit time | short — months to about 2 years | long — 12 to 165 years |
| Temperature | warmer, closer to the Sun | very cold |
Orbits and the force that holds them
A planet moves in a circle at very nearly constant speed, and yet it is accelerating the whole time. This is the point the examiners test, and it rests on velocity being a vector. The direction of motion is changing continuously, so the velocity is changing continuously, and a changing velocity is an acceleration.
That acceleration requires a resultant force directed towards the centre of the orbit. For a planet that force is the Sun's gravitational attraction; for a moon it is the gravity of its planet; for an artificial satellite it is the gravity of the Earth.
Gravity gets weaker with distance, so a planet far from the Sun is held more weakly and moves more slowly along its orbit. Combined with the much longer path it has to travel, this is why Neptune takes 165 years to go round once while Mercury takes 88 days.
The same reasoning applies to satellites. A satellite in a low orbit experiences a stronger gravitational field, needs a larger centripetal force, and must therefore travel faster and complete an orbit more quickly.
- v
- orbital speedm s⁻¹
- r
- orbital radiusm
- T
- orbital periods
A satellite orbits the Earth at a radius of 7.0 × 10⁶ m with a period of 97 minutes. Calculate its orbital speed.
- Convert the period:
97 × 60 = 5820 s.Seconds are needed for a speed in m s⁻¹. - Circumference
= 2πr = 2π × 7.0 × 10⁶ = 4.40 × 10⁷ m.One orbit is one full circle. v = 4.40 × 10⁷ / 5820.v = 7.6 × 10³ m s⁻¹, about 7.6 km per second.A realistic low-Earth-orbit speed.
7.6 × 10³ m s⁻¹
Gravity around a star behaves like the field drawn here, with one difference: it only ever attracts. The lines spread out with distance, which is why a distant planet is held more weakly and orbits more slowly.
Day, year and the seasons
Two separate motions produce the rhythms of life on Earth, and keeping them apart matters.
The Earth rotates on its own axis once every 24 hours, and that gives us day and night. The half facing the Sun is in daylight; the half turned away is in darkness. The Sun does not move across the sky — we turn beneath it.
The Earth also orbits the Sun once every 365¼ days, and that gives us the year. The extra quarter day is why we add a leap day every fourth year.
The seasons are not caused by the Earth being closer to the Sun at some times of year — a very common misconception, and one worth being able to reject. They are caused by the tilt of the Earth's axis, about 23.5° from the vertical. When a hemisphere is tilted towards the Sun, its days are longer and the sunlight strikes more directly, concentrating the energy on a smaller area. That is summer. Six months later the same hemisphere is tilted away, and it is winter.
The seasons are the tilt, not the distance
The Earth is actually slightly closer to the Sun during the northern winter. If distance caused the seasons, both hemispheres would have summer at the same time — and they do not. The tilt is the answer.
Key points
- The Sun holds more than 99% of the mass of the Solar System.
- Inner planets are small and rocky; outer planets are large, gaseous and cold.
- Orbiting bodies accelerate because their direction changes, not their speed.
- The centripetal force is gravity, directed towards the centre of the orbit.
- Day and night come from rotation; the seasons come from axial tilt.
Comets, asteroids and why orbits differ
Not everything in the Solar System travels the same kind of path, and comparing a comet with a planet is a standard exam question.
A planet follows a nearly circular orbit, so its distance from the Sun barely changes and its speed stays almost constant. A comet follows a highly elliptical orbit that takes it from well inside the inner Solar System to far beyond Neptune.
Because gravity is much stronger close to the Sun, a comet accelerates as it approaches and is moving fastest at its closest point. It then decelerates as it climbs away, and is slowest at the far end of its orbit — where it spends most of its time. This is why comets are visible for only a short period of each orbit.
As a comet nears the Sun, the ice in it vaporises and is blown outwards by the solar wind, forming the bright tail. The tail always points away from the Sun, not backwards along the comet's path — so on the outward journey the comet is travelling tail-first.
Asteroids are rocky bodies, most of them orbiting in a broad belt between Mars and Jupiter. They are thought to be material that Jupiter's gravity prevented from ever forming into a planet.
Key points
- The Sun holds more than 99% of the mass of the Solar System.
- Inner planets are small and rocky; outer planets are large, gaseous and cold.
- Orbiting bodies accelerate because their direction changes, not their speed.
- A comet moves fastest at its closest approach, where gravity is strongest.
- Day and night come from rotation; the seasons come from axial tilt.