Detection of exoplanets (A-level only): Typical light curve.

The idea

A typical exoplanet transit light curve is a graph of relative stellar flux or apparent brightness against time. It shows a flat baseline, a gradual ingress as the planet's disc begins to cross the star, a flat-bottomed dip while the planet is fully in front of the star, a gradual egress as it leaves, and a return to baseline. The depth of the dip indicates how much of the star's light is blocked and therefore relates to the ratio of planet radius to star radius. The time between successive dips is the orbital period, while the width of the dip depends on orbital speed and the chord crossed. Real curves include noise and small variations, so astronomers look for a repeating, consistent shape rather than a single dip.

What earns the marks

• The vertical axis represents relative flux or apparent brightness, often normalised to 1.0 outside transit, and the horizontal axis represents time.
• The baseline is the unobscured stellar flux, and a transit appears as a temporary decrease below this baseline.
• Ingress and egress are the sloping parts of the curve where the planet's disc partially overlaps the star's disc.
• The flat bottom occurs while the planet is fully within the stellar disc, and its depth gives the fraction of starlight blocked.
• The depth is approximately equal to the square of the ratio of planet radius to star radius, so a larger or deeper transit indicates a larger planet relative to its star.
• The interval between repeated dips is the orbital period, and the width of the dip depends on the orbital speed and the path the planet takes across the stellar disc.
• A real light curve contains noise and may show slight variations, so a planet detection requires a periodic, repeatable signal rather than one isolated dip.

Common mistakes and how to put them right

• Reading the depth as the planet's diameter rather than a ratio of areas.

  Put it right by relating depth to the square of the radius ratio.

• Assuming the dip is perfectly rectangular.

  Put it right by including gradual ingress and egress as the planet's disc partially overlaps the star.

• Treating a single dip as conclusive.

  Put it right by requiring a regular, repeating pattern consistent with an orbiting planet.

• Confusing the time between dips with the duration of one dip.

  Put it right by distinguishing orbital period from transit duration.

Exam tips

• Label the axes, baseline, ingress, flat bottom, egress and period when sketching a typical light curve.
• Use the depth to compare planet sizes only when comparing transits of the same star, and explain that depth is a ratio of areas.
• Explain that repeated dips at regular intervals provide stronger evidence than a single dip because they confirm a periodic orbit.
• Keep descriptions of the curve consistent with the physics: the planet blocks a fraction of the star's light, so the flux falls and then recovers.