Astronomy: Exoplanet Transit Light Curves
Finding planets by measuring tiny changes in starlight
Astronomy: Exoplanet Transit Light Curves
Finding planets by measuring tiny changes in starlight
Astronomy - Grade 9-12
- 1
A star has a normal brightness of 1.000 in relative flux. During a transit, its brightness drops to 0.990. What is the transit depth as a percent?
Transit depth equals the fractional drop in brightness.
The transit depth is 1.0%. This is found by subtracting 0.990 from 1.000 to get 0.010, then multiplying by 100 to convert to a percent. - 2
A light curve shows a repeating dip in brightness every 12 days. What does the 12-day interval most likely represent?
The 12-day interval most likely represents the orbital period of the planet. Each dip happens when the planet passes in front of the star once per orbit. - 3
A planet blocks 0.25% of its star's light during transit. Using the relationship transit depth = (planet radius / star radius)^2, find the planet radius as a fraction of the star radius.
Convert the percent to a decimal before taking the square root.
The planet radius is 0.05 times the star radius. The transit depth as a decimal is 0.0025, and the square root of 0.0025 is 0.05. - 4
Two planets orbit stars of the same size. Planet A causes a 1% dip in brightness, while Planet B causes a 0.01% dip. Which planet has the larger radius, and how can you tell?
Planet A has the larger radius because it blocks a larger fraction of its star's light. Since transit depth depends on the square of the planet-to-star radius ratio, a deeper dip means a larger planet when the stars are the same size. - 5
A transit begins at hour 5 and ends at hour 9 on a light curve. What is the transit duration?
Duration is the end time minus the start time.
The transit duration is 4 hours. This is found by subtracting the start time, 5 hours, from the end time, 9 hours. - 6
A star has a radius of 700,000 km. An exoplanet has a radius equal to 0.10 times the star's radius. What is the planet's radius in kilometers?
The planet's radius is 70,000 km. This is calculated by multiplying 700,000 km by 0.10. - 7
A light curve has a shallow dip, then a deeper dip, and the pattern repeats. What is one possible explanation for this pattern?
Different planet sizes can create different transit depths.
One possible explanation is that two different planets are transiting the same star. The deeper dip could be caused by a larger planet, while the shallower dip could be caused by a smaller planet. - 8
Explain why the transit method only finds some exoplanets, not all exoplanets around distant stars.
The transit method only works when a planet's orbit is lined up so that the planet passes in front of the star from our point of view. If the orbit is tilted too much, the planet will not block the star's light as seen from Earth. - 9
A planet causes a transit depth of 4%. What is the planet radius as a fraction of the star radius?
Use radius ratio = square root of transit depth.
The planet radius is 0.20 times the star radius. The transit depth is 0.04 as a decimal, and the square root of 0.04 is 0.20. - 10
A student says, "A deeper transit always means a planet is closer to its star." Explain why this statement is not correct.
The statement is not correct because transit depth mainly depends on the size of the planet compared with the size of the star. A deeper transit usually means the planet blocks more light, which indicates a larger planet-to-star size ratio, not necessarily a closer orbit. - 11
A star's brightness measurements are: 1.000, 1.000, 0.997, 0.994, 0.997, 1.000, 1.000. At what point in this sequence is the planet most centered in front of the star?
Look for the minimum flux value.
The planet is most centered in front of the star at the fourth measurement, where the brightness is 0.994. This is the lowest brightness value, so the planet is blocking the most light at that time. - 12
Describe two features of a light curve that astronomers can use to learn about an exoplanet.
Astronomers can use the depth of the dip to estimate the planet's size compared with the star. They can also use the time between repeated dips to determine the planet's orbital period. - 13
A transit dip occurs at day 3, day 18, and day 33. What is the planet's orbital period, and when would you predict the next transit?
Find the difference between consecutive transit times, then add it to the last transit time.
The planet's orbital period is 15 days because the dips are 15 days apart. The next transit would be predicted at day 48. - 14
A small star and a large star are transited by identical planets. Which star will show a larger percent drop in brightness, and why?
The small star will show a larger percent drop in brightness. The identical planet covers a larger fraction of the disk of a small star than it covers of a large star. - 15
A light curve shows one unusually low data point, but no repeated dips in later observations. Give one reason astronomers should be cautious before claiming they found an exoplanet.
Scientific claims are stronger when a pattern repeats and can be confirmed.
Astronomers should be cautious because a single low data point could be caused by measurement error, star spots, an instrument problem, or another event. A real transiting exoplanet should usually produce repeated dips at regular intervals.