www.LewisDartnell.com Detecting and characterising exoplanets Astronomers have used several different techniques for discovering planets orbiting other stars - called extrasolar planets or exoplanets. Although in rare cases you can take a photograph of the tiny dot of the planet itself [like you might with Mercury or Neptune in our own solar system], most planets have been detected indirectly - by the effect they have on the light shining from their host star. The two most successful techniques are called the transit method and the radial velocity method. Transit method If the orientation of the other solar system just so happens to line up with our line-ofsight from Earth [as would be expected of a percentage of planetary systems by chance], the orbiting planets will pass across the disk of its star. This is exactly what happens with Mercury and Venus within our own solar system. Fig.1 star planet _ orbital path Our telescopes aren't able to actually see an image like Fig.1, but they can measure the drop in brightness of the star as the planet passes in front of it and blocks some of the starlight. So if you plot the measured brightness of a star over time, and see periodic dips in this 'lightcurve', you can infer the presence of an unseen exoplanet. Fig.2 Intensity ('56) 12 a Time (hours) www.LewisDartnell.com 1. How long did this exoplanet transit event last? 2. The 'depth' of a transit event is how much the starlight intensity dims by. What is the depth of the transit in the lightcurve shown in Fig.2? 3. The depth of a transit tells astronomers some very important information about the exoplanet. Look back at Fig.1, depicting the transit. What determines how much the starlight dims by? 4. The radius of the star being observed in Fig.2 is 5x10B m. Think carefully about how this relates to the answer for Q3, and calculate the radius of the transiting planet. 5. The radius ofjupiter is 7.0){107 m. Compare the size ofthis exoplanet to that of Jupiter. If you watch the star for long enough, the same planet will transit across again and then again. 1 | 'I' | | I Intensity (%) 6 D l l l | 3 4 5 6 7 Time (days) 6. What information do you learn about the planet in its solar system by looking at the time interval between transits in Fig.3