About the Exoplanet Transit Method
This simulation reproduces how astronomers detect distant planets photometrically: as a planet crosses the disc of its star, it blocks a small fraction of the light and the measured flux dips. The light curve is computed from the analytic occultation geometry of two overlapping circles (a simplified Mandel & Agol approach), so the central transit depth is approximately the squared radius ratio, ΔF/F ≈ (Rₚ/R★)², with a linear limb-darkening correction applied.
The five sliders set planet radius (in stellar radii), orbital radius, inclination, the linear limb-darkening coefficient u, and playback speed. Together they reshape the depth, duration, and curvature of the modelled light curve. This is the very technique used by the Kepler and TESS space telescopes, which discovered thousands of worlds, including small rocky planets, by monitoring tiny periodic brightness dips of distant stars.
Frequently Asked Questions
What is the transit method?
The transit method detects a planet by watching for the small, periodic dip in a star's brightness that occurs each time the planet passes in front of it. By measuring how deep, how long, and how regular those dips are, astronomers infer the planet's size and orbit. It is the technique behind missions such as Kepler and TESS.
How deep is a transit dip?
The fractional drop in flux at mid-transit is roughly the square of the planet-to-star radius ratio, ΔF/F ≈ (Rₚ/R★)². A Jupiter-sized planet across a Sun-like star produces about a 1% dip, while an Earth-sized planet gives only around 0.008%. The simulation reports this as the "Max depth" statistic.
What do the sliders control?
Planet radius sets the depth of the dip, orbital radius scales the period via Kepler's third law and changes transit duration, and inclination shifts the impact parameter so transits become grazing or vanish below about 70 degrees. Limb darkening u shapes the bottom of the curve, and orbital speed only changes the animation rate.
What is limb darkening and why does it matter?
Limb darkening means a star's disc looks brighter at its centre than near its edge, because we see deeper, hotter layers there. A uniform disc would give a flat-bottomed transit, but limb darkening rounds and curves the bottom of the light curve. The slider u sets the strength of this effect in the linear model used here.
What is the impact parameter?
The impact parameter b is the minimum sky-projected distance between the planet's path and the centre of the star, measured in stellar radii. It depends on inclination: an edge-on orbit (90 degrees) gives b near zero and a central transit, while higher b produces a shorter, shallower, more grazing transit. Above b = 1 + Rₚ/R★ no transit occurs at all.
Is this simulation physically accurate?
It captures the real geometry well: the occultation depth comes from the analytic overlap area of two circles, and a linear limb-darkening factor is applied. It is a teaching-grade simplification, however. The orbit is treated as circular, the period uses a scaled form of Kepler's third law, and it does not model stellar noise, eccentricity, or the full non-linear limb-darkening laws used in professional fitting.
How does transit duration relate to the orbit?
For a central transit, the duration scales with the time the planet takes to cross the stellar diameter, so wider orbits and slower planets give longer transits. The simulation estimates the duration from the chord length set by the impact parameter and the orbital geometry, then converts it to hours using the scaled period. Grazing transits are noticeably shorter.
Why must the inclination be near 90 degrees?
A transit only happens if the planet's orbital plane is aligned closely enough with our line of sight for it to cross the stellar disc. That is why the inclination slider is limited to between 70 and 90 degrees. Most randomly oriented planetary systems never transit from our viewpoint, which is why transit surveys monitor enormous numbers of stars.
What can the transit method not tell you?
The transit depth gives the planet's radius but not its mass, so density and composition stay unknown from transits alone. To get mass, astronomers combine transits with the radial-velocity method, which measures the star's Doppler wobble. Together the two methods yield radius, mass, and therefore bulk density, distinguishing rocky worlds from gas giants.
Can I reproduce a real planet with the presets?
Yes. The preset buttons load parameter sets approximating an Earth-like planet, a hot Jupiter, a mini-Neptune, a grazing transit, and Kepler-7b, an inflated hot Jupiter. Selecting one updates the sliders and redraws the light curve so you can compare how different planet sizes, orbits, and inclinations change the depth and shape of the signal.