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Searching for planets around OTHER stars… BETA Pictoris (1984) – the first hint at Stars with Planetary Disks? BETA Pictoris (2006) – a solar system in the making? BETA Pictoris – now confirmed with a Jupiter-sized planet ESA image from 2012: Dust is like that in our solar system! Inferring planets that aren’t seen! Detecting Planets around OTHER stars! Orbiting Telescopes: KEPLER COROT Hubble Ground Observations WASP Why are extrasolar planets difficult to detect? • Sun-like stars are billions of times brighter than any light reflected from planets. • June 2016 – direct image of a planet orbiting another star (a LONG way away!) • A 27,000-year orbit! Why are extrasolar planets difficult to detect? • Planets are EXTREMELY close to their stars, relative to distance from us to the star. – Like being in San Francisco & trying to see a pinhead 15 meters from a grapefruit … – … in Washington, D.C. So given how bright stars are, how CAN we detect planets around other stars? Planet Detection • Indirect: Look at the star… – Measurements of stellar properties revealing effects of the star’s orbiting planets • Direct: Look for the planet – Pictures or spectra of the planets themselves Indirect Detection through Observation • Stars hold onto planets with gravity, but… • Planets tug on their stars, too! • We can see the stars move in space over time… • but they don’t move much! Example from our Solar System: Indirect Detection through Astrometry • Sun & Jupiter orbit around common center of mass. • Sun wobbles around center of mass with same period as Jupiter (~12 years) Astrometric detection • We can detect planets by measuring change in star's position on sky. • However, tiny motions are very difficult to measure (~ 0.001 arcsecond). Combined Gravitational Tugs • Sun's motion around solar system's center of mass depends on tugs from all planets. • Astronomers around other stars measuring this motion could determine masses and orbits of all planets! Indirect Detection through Gravitation • A faster way to detect presence of planets • Use the stars “wobble” revealed through spectra • Angle of solar system matters! Doppler Technique • Measuring a star's Doppler shift can tell us its motion toward & away from us. • Current techniques can measure motions as small as 1 m/s (walking speed!). 1st Extrasolar Planet Detected (1995) • The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance. First Extrasolar Planet • Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period. • Mass ~ ½ of Jupiter! • But very short period means that planet has small orbit – not where ice could form! Another Indirect Method: Transits and Eclipses • A transit is when a planet crosses in front of a star. Another Indirect Method: Transits and Eclipses • A transit is when a planet crosses in front of a star. Another Indirect Method: Transits and Eclipses • Reduces star's apparent brightness & tells radius. • Sometimes an eclipse can also be detected Kepler • NASA's Kepler mission launched in 2008 to begin looking for transiting planets. • Designed to measure 0.008% decline in brightness when an Earth-mass planet eclipses a Sun-like star. Kepler Can you see a gnat flying in front of a car headlight Planet Detection Challenges Indirect Planet Detection Challenges – Astrometry: • takes lots of TIME – Doppler Shift: • detects LARGE planets NEAR to stars – Transits: • needs LUCK, & detects lots of “False Positives” Doppler Method Challenges • Most detected planets have orbits smaller than Jupiter’s. • Planets at greater distances are harder to detect with Doppler technique. Transit Method Challenges • Lots of things cause starlight to change other than a planet’s transit! – Variable stars – Eclipsing binaries – Brown dwarfs • 2015 paper: up to 50% of Kepler’s finds are “false positives? • 2016 Study: ~ 30% confirmed as planets The Process of Science • Observation: TMR-1 “detected” in 1998 The Process of Science • Observation: TMR-1 “detected” in 1998 • Hypothesis: It is a planet, connected by a disk to a double (binary star) The Process of Science • Observation: TMR-1 “detected” in 1998 • Hypothesis: It is a planet ? • Critical Tests: Spectra The Process of Science • Observation: TMR-1 “detected” in 1998 • Hypothesis: It is a planet ? • Critical Tests: Spectra • Result: A background star! The Process of Science • We can be fooled! • “Brown Dwarfs” exist • “Super-Jupiters” Still LOTS of planets detected in 5+ years by Kepler Orbits & sizes of extrasolar planets from Kepler • Results from Kepler through 2015 indicate that planets are common, and small planets greatly outnumber large planets! Masses & sizes of extrasolar planets Direct Detection? Direct Detection? How do extrasolar planets compare with our solar system? Upsilon Andromeda has multiple planets! Do we need to modify our theory of solar system formation? Do we need to modify our theory of solar system formation? Revisiting the Nebular Theory • The nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU). • The discovery of hot Jupiters has forced reexamination of nebular theory. • Planetary migration or gravitational encounters may explain hot Jupiters. Planetary Migration • A young planet’s motion can create waves in a planetforming disk. • Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward. Hot Jupiters Gravitational Encounters • Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit. • Multiple close encounters with smaller planetesimals can also cause inward migration. Modifying the Nebular Theory • Observations of extrasolar planets have shown that the nebular theory was incomplete. • Effects like planet migration and gravitational encounters might be more important than previously thought. Are planetary systems like ours common? What have we learned? • Do we need to modify our theory of solar system formation? – Original nebular theory cannot account for the existence of hot Jupiters. – Planetary migration or gravitational encounters may explain how Jupiter-like planets moved inward. • Are planetary systems like ours common? – The answer is coming soon… Planets: Common or Rare? • One in ten stars examined so far have turned out to have planets. • The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques. What properties of extrasolar planets can we measure? Calculating density • Using mass, determined using Doppler technique, & size, determined using transit technique, density can be calculated. Other Planet-Hunting Strategies • Gravitational Lensing: Mass bends light in a special way when a star with planets passes in front of another star. Other Planet-Hunting Strategies • Features in Dust Disks: Gaps, waves, or ripples in disks of dusty gas around stars can indicate presence of planets.