Reminders! Website: http://starsarestellar.blogspot.com/ Lectures 1-9 are available for download as study aids. Reading: You should have Chapters 1-8 done, so that you have all the necessary reading done for the midterm. Homework: Homework #2 is due TODAY. Homework #3 is due next Wednesday, June 17th. Homework #3 is short--might be a good time to concentrate on your projects!!! I should have Homework #2 solutions up soon to help you study for the midterm. Other Worlds Today’s Lecture: Chapter 9, pages 184-209 • Detecting extra-solar planets Doppler Shifts Transits Microlensing • Properties of extra-solar planets Hot Jupiters Extra-solar planets • It’s a huge technical challenge to ﬁnd extrasolar planets! The parent star completely outshines the planet, making direction detection next to impossible. • Doppler effect: Look for periodic “wobble” of a star (sinusoidal change in its radial velocity) due to the motion of a planet around it. We can estimate the mass of the planet using Kepler’s third law. (Important note: You only get the minimum mass of the planet this way, since you don’t know the inclination of the orbit. In other words, the measured radial velocity of the star may be only part of its total orbital velocity.) Extra-solar planets (cont.) • October 1995: Breakthrough in optical search for extra-solar planets -- a planet found by a Swiss team around star 51 Pegasi! (called 51 Pegasi b) • 51 Pegasi b is weird! Half the mass of Jupiter and a period of only 4.2 days!!! Conﬁrmed by Geoff Marcy (UC Berkeley) and Paul Butler at Lick Observatory. They subsequently found > 100 more planets (mostly at Lick and Keck) Over 350 extra-solar planets are now known! Lots of selection biases • Most of these stars are (barely) visible to the naked eye. Distance ~ 50 ly • Technology at optical wavelengths is not yet good enough (usually) to detect Earth-sized planets • We preferentially ﬁnd very small orbits, high eccentricities, and massive planets because these produce the greatest Doppler shift. • Most of these planets are unlike anything that we see in our Solar System, forcing us to rethink our understanding of how the Solar System formed. Other methods for discovery • Transits: The planet passes in front of its parent star, blocking the star’s light a little (making it dimmer). Advantage: We can measure the planet’s relative radius and probe its atmosphere. • Microlensing: As the star and its planet pass in front of a background star, they magnify the more distant star (light bending from General Relativity - Einstein). Extra magniﬁcation indicates the presence of a the star. Advantage: the planet can be further away from the star! Planetary transit: light curve (star brightness vs time) The first observed transit HD 209458 with Hubble Space Telescope 1.5% Dip! Measure a planet’s radius Consider a star with radius Rstar and planet Rplanet. Before the planet passes in front of the star, the area of the star seen is Area of star = π(Rstar)2 When the planet passes in front, we instead see an area Area seen = π(Rstar)2 - π(Rplanet)2 The fractional change in area seen is therefore (Area of star - Area seen)/(Area of star) = (Rplanet / Rstar)2 Example: If the luminosity changes by 2% or 0.02, then the planet must have a radius that is (0.02)1/2 = 0.14 of the star’s radius. Notable Extra-solar Planets! • OGLE-2005-BLG-390Lb: First planet found using microlensing. Mass of 5.5MEarth and distance of 2.6 AU. • HD 209458 b and HD 189733 b: First spectra of extrasolar planets. Found water vapor and other mysterious absorption lines. Also first transiting planet. • TrES-4: Lowest density planet ever at 0.2 g/cm3! The same density of balsa wood! What makes the planet so bloated? • HD 189733 b: The first organic molecule found in a planet’s atmosphere (methane). What is making methane? • Fomalhaut b: The first direct image of an extrasolar planet. Mass of 3MJupiter. Found due to effects on a debris disk. • Gliese 581 e: The smallest mass at 1.9MEarth and distance of 0.03 AU.