The Kepler Mission: 20% of all Stars in the Milky Way Have Earth like Planets! Kepler Spacecraft Can we believe this result? What techniques and data were used to derive this important result? 1
How to Find Planets Around Other Stars By direct imaging take a picture and see one! Hard because planets are dim By looking for varying Doppler shifts in a star s spectrum Relatively easy By looking for varying light output from a star due to transits ( eclipses ) by a planet Not as easy as Doppler shifts but can yield other information about the planet By looking for variations in a star s position Very hard, mostly superceeded by Doppler shifts By looking for gravitational lensing events Relatively easy but yields only statistical information about planets 2
Suppose you want to learn how many stars have planets. What elements are important in designing an experiment to learn this? -- already know that we might need to look at a number of stars as the first searches found no planets at all -- which planet finding technique will be best: ideally a technique that would be efficient (for example, observe many stars at once) and would find planets down to Earth size and no matter how they are arranged around their parent star 3
Planet Transits 4
Planet Transits Look for periodic changes in the brightness of a star as a planet passes across the face of a star. This requires very accurate photometry but is not impossible. Consider a K2V star like epsilon Eri, a star with evidence for a planet and dust around the star: Luminosity=.3xL Sun = 1.17x10 26 watts R of star: T = 5000 K Radius of Jupiter = 3.55x10 7 meters Kepler mission is based on this technique. 5
Kepler Mission Point a large field camera at a region of the Milky Way with lots of stars Take pictures of each star every 30 minutes for four years (the mission ended this spring when the pointing system failed) Mission measured ~100,000 stars Measurements limited by square root of the number of photons from the star that is they are very accurate 2.2 Megapixel module 21 modules for a total of 47 Megapixels 6
Transit Data A C B A. the period of recurrence of the transit gives the length of the planet s year, P in Kepler s 3 rd Law B. the duration of the transit gives the inclination angle (angle to line of sight) C. the fractional change in brightness of the star gives the area of the planet as compared to that of the star 7
Inclination Angle Which is just the star diameter divided by the diameter of the planet s orbit! 8
Translating Measurements into Planet Properties To be able to do most of these calculations, it is important to know the properties of the parent star need its mass, radius, and temperature which all come from knowing its spectral type Planet s year (or period) P gives the planet s distance from its star using Kepler s 3 rd Law. Knowing the distance from the star and the star s temperature, we can compute the planet s temperature (equivalent to determining whether the planet is in the habitable zone) Planet s radius lets us determine whether it is similar in size to Earth If we also have data from spectroscopy, we can measure the planet s mass and then by combining mass and radius, we know its density which clinch whether the planet is earth like or not [ the planets measured in the next homework have masses estimated by comparison to the variation in planet properties with distance from the sun in the solar system because most Kepler discovered stars don t have spectra] The Kepler mission looked for transits from 100,000 stars. By counting how many stars have transits indicating earth like planets and correcting for how many are missed because of wrong inclination angles, the total fraction of stars with earth like planets is computed = 20%! 9
Transit Spectroscopy If an instrument+telescope combination is sufficiently stable, spectra of a transiting planet s atmosphere can be obtained. Both emission spectra and transmission spectra can be obtained. HST and Spitzer have produced such data. This is likely to be a very powerful technique for JWST. 10