Science, Maths & Technology

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An introduction to exoplanets

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# 4.1  Transit depth gives size

Last week you learned how to interpret transit light curves. By measuring the brightness of the star before and during the transit you can work out how much starlight is blocked by the planet, and therefore the relative sizes of the planet and the star. What you measure directly is the transit depth – the fractional amount of light missing (usually expressed as a percentage). Last week we worked out that the transit depth reveals the ratio of cross-sectional areas for the planet and the star, which is the same as the squared ratio of the planetary and stellar radii:

Equation label: (Equation 1)

We’ll briefly revisit the interactive application from last week that calculates transit depths for us and then you will be invited to have a go at using Equation 1 for yourself.

## Activity 1  The transit depth of a familiar planet

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Using this interactive application, set Rstar = 1.00 RSun and Rplanet = 1.00 RJ. You can also use the arrow keys on the keyboard to amend the values.

1. To which familiar planet do these sizes correspond?

These sizes are those of Jupiter and its star, the Sun.

2. What transit depth would an extraterrestrial astronomer see if they measured the brightness of the Sun while Jupiter passes exactly between them and the Sun?

(Remember that the transit depth is usually expressed as a percentage of starlight blocked.)