Learning outcomes
- Interpret tables and graphs of planetary data.
- Compare orbital distance, period, density, surface temperature and gravitational field strength.
- Identify broad trends and exceptions.
- Distinguish correlation from causation.
- Use data to support written conclusions.
4.1 Reading astronomical tables
Planetary data may be presented using different units, such as kilometres, astronomical units, days or years. Before comparing values, check headings, powers of ten and units. A planet with 5.2 AU is more distant from the Sun than one at 0.72 AU, even if the table also gives a much smaller numerical value in another column.
Questions commonly ask for a comparison, a trend or an exception. Quote relevant data rather than merely saying ‘it is bigger’. A strong response states both quantities and uses units where appropriate.
4.2 Orbital distance and period
A clear Solar System trend is that planets farther from the Sun have longer orbital periods. Their paths are longer and their average orbital speeds are lower. The relationship is not a simple direct proportion, so doubling distance does not double period.
A graph of period against distance curves upward. If asked to describe the graph, state that period increases as orbital distance increases and mention that the rate of increase also changes if visible.
4.3 Density and composition
The inner rocky planets generally have higher mean densities than the outer giant planets. The inner planets contain larger proportions of rock and metal, while the outer planets contain more low-density gases and ices. Density is mass divided by volume, so a very massive planet can still have a modest mean density if its volume is enormous.
Do not assume that the largest planet has the greatest density. Jupiter is the largest planet but Earth has a higher mean density because of its different composition.
4.4 Surface temperature
Surface temperature usually decreases with increasing distance from the Sun because less solar energy is received per square metre. However, atmospheric composition can produce exceptions. Venus is hotter than Mercury at its surface because its dense atmosphere causes an intense greenhouse effect.
The word ‘usually’ matters. A data question may deliberately include an exception and ask for a possible explanation. Use the data first, then introduce a physical explanation.
4.5 Gravitational field strength
Surface gravitational field strength depends strongly on a planet’s mass and also on how far the surface is from the centre. The syllabus explicitly requires knowledge that it depends on the planet’s mass, but table interpretation may reveal that size also affects the result.
When comparing values, do not claim that a more massive planet must always have proportionally greater surface field strength. A large radius places the surface farther from the mass concentration and weakens the field at the surface.
Worked examples
Describing a trend
From Earth to Neptune, orbital distance increases and orbital period also increases. Neptune is about 30 times farther from the Sun than Earth and takes about 165 years to orbit.
Finding an exception
Venus is farther from the Sun than Mercury but has a higher surface temperature; its dense carbon dioxide atmosphere traps thermal energy.
Practical focus
Investigation or modelling activity
Plot selected planetary data on graph paper or a spreadsheet. Use orbital distance on the horizontal axis and period or temperature on the vertical axis. Draw an appropriate smooth trend rather than joining every point with straight segments. Identify any anomalous point and suggest a physical reason.
Examination guidance
- Read column headings and powers of ten carefully.
- Support comparisons with data.
- Use “increases” or “decreases” rather than vague phrases.
- Do not force a straight-line relationship when the plotted trend is curved.
- Identify exceptions instead of ignoring them.
Check your understanding
- What broad trend links orbital distance and period?
- Why can a large planet have a relatively low density?
- Which planet is a common temperature exception and why?
- Does surface field strength depend only on mass?
Answers
- Planets farther from the Sun generally have longer orbital periods.
- Its volume may be very large and it may contain low-density gases or ices.
- Venus; its dense atmosphere causes strong greenhouse heating.
- No. Mass is important, and distance from the planet’s centre also affects the field at the surface.
