OCR Specification focus:
‘Define planets, planetary satellites, comets, solar systems, galaxies and the universe.’
A planet is a body orbiting a star; a planetary satellite orbits a planet; comets are icy bodies on elongated orbits; solar systems contain stars and their objects; galaxies contain stellar systems; the universe includes all space and matter.
Astronomical objects vary enormously in scale, composition, and behaviour, yet all follow the same physical laws. Understanding their definitions provides the foundation for studying stellar and cosmological processes.
Astronomical Objects and Their Characteristics
Planets
A planet is a major type of astronomical body whose properties are essential for understanding the structure of a solar system.
Planet: A celestial object that orbits a star, has sufficient mass for its gravity to make it roughly spherical, and has cleared its orbital neighbourhood.
Planets do not produce their own light through nuclear fusion; instead, they reflect light from their parent star. Because of this, their brightness and appearance vary depending on distance, size, and reflective properties.
Key features of planets include:
Stable orbits around a central star.
Differentiated internal structure, often including cores, mantles, and crusts.
Broad compositional categories, such as rocky terrestrial planets and gaseous or icy giants.
Planets may possess additional bodies, such as satellites, which form complex orbital systems influencing tides, rotation, and long-term stability.
Planetary Satellites
A planetary satellite is commonly known as a moon, although the term also includes artificial satellites when used in a general astronomical sense.
Planetary Satellite: A natural body that orbits a planet rather than a star.
Planetary satellites vary dramatically in size and composition. Some are geologically active, while others are frozen remnants of early solar system formation. They contribute to planetary system dynamics through:
Tidal interactions, affecting planetary rotation and internal heating.
Orbital resonances, which can stabilise or destabilise other satellites.
Reflective and radio properties, assisting astronomers in remote sensing.
Natural satellites give insight into the processes of planetary formation, capture, and evolutionary history.
Comets
A comet is an icy, volatile-rich body originating mainly from the outskirts of a solar system.
Comet: A small, icy astronomical object that orbits a star along an elongated path and forms a glowing coma and tail when near the star.
Comets provide valuable information about early stellar system material because they preserve unaltered primitive substances.
Diagram showing the structure of a comet, including its nucleus, coma, and distinct dust and ion tails. The ion tail points away from the Sun, while the dust tail curves along the comet’s orbital path. This schematic is representative and not to scale. Source.
When heated near a star, sublimation produces:
A bright coma, created by escaping gas and dust.
Two distinct tails: the ion tail and the dust tail.
A surrounding halo of ejected material, known as the comet’s atmosphere.
Their highly eccentric orbits can extend far beyond the main planetary region, linking inner system behaviour with outer reservoirs such as the Kuiper Belt and Oort Cloud.
Larger Astronomical Systems
Solar Systems
Solar System: A star and all natural objects gravitationally bound to it, including planets, satellites, asteroids, comets, and interplanetary dust.
A solar system forms from a rotating cloud of gas and dust known as a nebula. Over time, gravitational forces organise matter into distinct components. Typical features include:
A central star, generating energy through nuclear fusion.
Major and minor orbital objects, such as planets, dwarf planets, asteroids, and comets.
Interplanetary material, including dust and radiation fields.
This structure illustrates how gravity governs motion and how angular momentum shapes orbital architecture.
Galaxies
A galaxy represents the next major level of cosmic organisation, containing vast numbers of stellar systems.
Hubble image of an edge-on spiral galaxy showing its thin disc, bright bulge, and a dark dust lane along the midplane. This real observation illustrates the large-scale structure typical of spiral galaxies. No labels are included, keeping the focus on morphology. Source.
Galaxy: A massive collection of stars, stellar remnants, interstellar gas, dust, and dark matter, all bound together by gravity.
Galaxies display immense diversity:
Spiral galaxies, characterised by rotating discs and spiral arms.
Elliptical galaxies, ranging from nearly spherical to elongated structures.
Irregular galaxies, lacking clear symmetry or shape.
Within galaxies, stars form, evolve, and interact with surrounding gas and dust. Galaxies also house dark matter haloes, which dominate their gravitational dynamics despite being invisible.
The Universe
The universe encompasses all astronomical structures and physical laws.
Universe: The totality of space, time, matter, and energy, including all galaxies and the cosmic web that connects them.
The universe contains billions of galaxies arranged in clusters, superclusters, and filamentary structures. Observations of cosmic expansion, background radiation, and large-scale structure help physicists develop models describing its origin and evolution.
The scale of the universe enables astronomers to contextualise smaller systems, connecting local observations of stars and planets to the global processes governing cosmic evolution.
FAQ
The distinction relies on the object's ability to clear its orbital neighbourhood. While both are spherical and orbit a star, dwarf planets share their orbital zone with other bodies.
Observationally, astronomers examine orbital dynamics, gravitational influence on surrounding debris, and long-term simulations of orbital stability.
They also compare brightness and mass estimates to determine whether an object is gravitationally dominant in its region.
Formation pathways are inferred from a satellite’s orbit, composition, and dynamical behaviour.
Satellites forming in situ tend to have near-circular, prograde orbits aligned with the planet’s equatorial plane.
Captured satellites often have irregular, highly inclined, or retrograde orbits.
Compositional differences, such as unexpected volatile content, can also indicate capture from an external region of the solar system.
Comet tails form from different physical processes.
The ion tail forms when ultraviolet radiation ionises gas in the coma, which is then swept away by the solar wind, producing a straight tail pointing directly away from the Sun.
The dust tail consists of heavier dust particles pushed outward by radiation pressure, curving along the comet’s orbital path.
Differences in particle mass and the nature of the forces acting on them create visibly separate structures.
Long-term stability depends on gravitational interactions between objects and the distribution of mass throughout the system.
Key influences include:
Orbital resonances between planets and satellites
Perturbations from passing stars or interstellar clouds
The mass and activity of the central star
The presence of large planets that can eject smaller bodies
Stable systems maintain configurations where perturbations remain small relative to orbital energy.
Classification relies on analysing the galaxy’s shape, brightness profile, and distribution of stellar populations.
For distant galaxies, resolution limits require alternative indicators such as:
Light concentration and symmetry
Colour, which hints at star formation activity
Spectral features that indicate rotation and gas content
Even with limited data, these patterns allow astronomers to differentiate between spiral, elliptical, and irregular types.
Practice Questions
Question 1 (3 marks)
Define each of the following astronomical objects:
(a) planet
(b) planetary satellite
(c) galaxy
Mark Scheme:
(a) Planet
A celestial object that orbits a star (1)
Has sufficient mass for its gravity to make it roughly spherical (1)
Has cleared its orbital neighbourhood (accept: dominant in its orbit) (1)
(max 1 mark for part a)
(b) Planetary satellite
A natural object that orbits a planet rather than a star (1)
(c) Galaxy
A large collection of stars, gas, dust, and dark matter (1)
All bound together by gravity (1)
Question 2 (6 marks)
Astronomical systems exist on a vast range of scales, from individual bodies to structures containing billions of stars.
(a) Describe the structure of a solar system and explain how gravity determines the motion of objects within it.
(b) Explain what the universe is and outline how galaxies fit into its overall structure.
Mark Scheme:
(a) Solar system structure and gravity
A solar system consists of a star and all objects gravitationally bound to it (1)
Includes planets, planetary satellites, asteroids, comets, and dust (1)
Gravity provides the centripetal force keeping planets in orbit around the star (1)
Orbits are stable paths resulting from the balance between gravitational attraction and the bodies’ motion (1)
(b) The universe and galaxies
The universe is the totality of all space, time, matter, and energy (1)
Galaxies are large-scale structures within the universe containing billions of stars (1)
Galaxies form clusters, superclusters, and filamentary structures in the large-scale cosmic web (1)
