## Step 2 - The Size of the Exoplanet

The **depth **of the exoplanet transit is equivalent to the ratio of the area of the planet’s disc and the area of the star’s disc. By measuring the transit’s depth from the transit light curve and knowing the stellar radius (R_{s}) you can determine the **exoplanet’s radius** (R_{p}).

Transit depth (%) ≈ \frac{\pi.{R_p}^2}{\pi.{R_s}^2} x 100

Watch the video to learn more, complete your calculations and then check your solutions with our expert. When you’re ready to continue to the next step, return to this page and click “*continue the investigation*“.

## Watch the video on exoplanet size:

Subtitles are available (automatically generated by YouTube) – select your language using the YouTube player controls.

## Ready for the KELT-3b solution?

Have you measured the size of KELT-3b? Check below to see whether your results match our expert’s solution for determining the size of KELT-3b.

Let’s now analyse KELT-3b data as an example.

###### Figure 1: KELT-3b data from Cheops with the transit light curve best fit model from *allesfitter*.

The radius of the star KELT-3 is known and provided in the case file:

R_s = 1.70 R_\text{Sun}

By analysing the Cheops data we can measure the transit depth to be approximately 0.9\% (Figure 1).

Using the equation:

R_p = \sqrt{{R_s}^2 \text{x} \frac{\text{transit depth}}{100}} = \sqrt{{1.70}^2 \text{x} \frac{0.9}{100}} = 0.161 R_\text{Sun}

Converting to Earth radii units:

R_p = 0.161 \text{x} 109 = 17.5 R_\text{Earth}.

**How does your estimate of the size of the exoplanet compare to the best model fit value?**

## Step 2 Completed!

Your Investigation Progress

40%

Have you analysed the Cheops data and determined the size of your exoplanet? If yes, you can continue your investigation into the exoplanet’s properties with Step 3 – the orbital period and distance of an exoplanet!