Exoplanet Transit & Habitable Zone Lab
Investigate how astronomers detect exoplanets using the transit method. Observe how a planet crossing its star produces a measurable dip in brightness, apply Kepler's third law to find orbital periods, and calculate habitable zone boundaries for different star types.
Guided Experiment: Transit Depth and Planet Size
How does the planet-to-star radius ratio affect transit depth? Can you predict the depth before measuring it?
Write your hypothesis in the Lab Report panel, then click Next.
Transit Visualization
Controls
Transit Results
Data Table
(0 rows)| # | Trial | Star Type | Planet R (R⊕) | Distance (AU) | Transit Depth (%) | Period (days) | In HZ? |
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Reference Guide
Transit Method
When a planet passes in front of its host star as seen from Earth, it blocks a tiny fraction of the starlight. The fractional brightness drop equals the ratio of the planet's cross-sectional area to the star's cross-sectional area.
An Earth-size planet transiting a Sun-like star produces a dip of only 0.0084%, while a Jupiter-size planet produces roughly 1%. This is why large planets orbiting small stars are easiest to detect.
Kepler's Third Law
Kepler's third law relates a planet's orbital period to its distance from the star and the star's mass. For a planet orbiting at distance a (in AU) around a star of mass M (in solar masses), the period P (in years) satisfies this relation.
This means a planet at 1 AU around a Sun-mass star orbits in 1 year (365.25 days), while one at 0.05 AU completes an orbit in about 4 days.
Habitable Zone
The habitable zone (HZ) is the range of orbital distances where liquid water could exist on a planet's surface. The boundaries depend primarily on the star's luminosity.
For the Sun (L = 1 L☉), the HZ spans roughly 0.95 to 1.37 AU. A dim M-dwarf at 0.001 L☉ has its HZ at only 0.03 to 0.04 AU, making those planets easier to detect via transit.
Equilibrium Temperature
A planet's equilibrium temperature is the temperature it would reach if heated only by its star, with no atmosphere. It depends on the star's temperature and radius, the orbital distance, and the planet's Bond albedo (fraction of light reflected).
Earth's equilibrium temperature is about 255 K (−18 °C). The actual surface temperature of 288 K (15 °C) is higher because of the greenhouse effect, which this simplified formula does not include.