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The Ultimate Exploration: Searching for Another Earth in the Universe 終 極 探 索 ： 尋 找 第 二 個 地 球

The Ultimate Exploration: Searching for Another Earth in the Universe 終 極 探 索 ： 尋 找 第 二 個 地 球. 香港大學物理系 潘振聲 Chun Shing Jason Pun Department of Physics The University of Hong Kong. Outline. Random facts about planets History of planet search Recent developments What might come next?.

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The Ultimate Exploration: Searching for Another Earth in the Universe 終 極 探 索 ： 尋 找 第 二 個 地 球

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  1. The Ultimate Exploration: Searching for Another Earth in the Universe終 極 探 索 ： 尋 找 第 二 個 地 球 香港大學物理系 潘振聲 Chun Shing Jason Pun Department of Physics The University of Hong Kong

  2. Outline • Random facts about planets • History of planet search • Recent developments • What might come next? Hong Kong Space Museum Oct 29, 2005

  3. Planets of our solar system Hong Kong Space Museum Oct 29, 2005

  4. Random Facts about Planets • Important mass:Jupiter (biggest planet) mass 木星質量 MJ = 318 MEarth = 0.1% MSun • Important distance:天文單位 Astronomical Unit (AU) = Earth-Sun distance = 1.5x1011 m • Jupiter-Sun distance = 5.2 AU • Important time: Earth orbits around the Sun in 1 year; Jupiter tJ = 11.8 year Hong Kong Space Museum Oct 29, 2005

  5. Basic Properties of Planets • Main difference between planets and stars: • Stars (恆星): Shines by generating energy through fusion of hydrogen at core • Planets (行星): Shines by reflecting lights of the star • For our solar system, planets come in two main categories: • Terrestrial planets 類地行星: Mercury, Mars, Venus, Earth, (Pluto) • Gas giants 氣體巨行星: Saturn, Jupiter, Uranus, Neptune Hong Kong Space Museum Oct 29, 2005

  6. Basic Properties of Planets • Our solar system has a total of 9 major planets • Not so fast  • Trouble maker: Pluto • There can be many objects like Pluto!!! • Astronomers are still debating the definition of planets Hong Kong Space Museum Oct 29, 2005 Credit: The New York Times

  7. Just to make things more complicated…… • Stars: maximum mass ~100 MSun minimum mass ~ 0.08 MSun ~ 80 MJ • Brown dwarf: mass ~10-80 MJ • No Hydrogen fusion at core • Possible Deuterium fusion at core • Similar size at Jupiter but much heavier • Planets: maximum mass ~10 MJ minimum mass ~ 0.1% MJ ??? Hong Kong Space Museum Oct 29, 2005

  8. Planets around other stars • Why do we search for extrasolar planets 太陽系外行星? • Learn about solar system formation • Learn about solar system future • Learn about whether there are planets that supports life • Learn about whether there are planets that support intelligent life! Hong Kong Space Museum Oct 29, 2005

  9. Drake Equation (1961) • How many technologically advanced civilizations are out there in our Galaxy? N = N* fp nefl fifc fL N* = # of stars in the Galaxy fp = fraction of stars with planets ne = # of planets per solar system suitable for life fl = fraction of habitable planets actually with life fi = fraction where intelligent lifeforms evolves fc = fraction with species which communicate extraterrestrially fL = fraction of planet’s life where communicating species lives Hong Kong Space Museum Oct 29, 2005

  10. Planets around other stars • Difficulty: Planets are much dimmer than stars The Sun is 1010 times brighter than Earth in visible light. Credit: TPF/NASA Hong Kong Space Museum Oct 29, 2005

  11. Planets around other stars • Advantage: Laws of nature are universal ─ extrasolar planets must orbit in the same way Earth orbits the Sun Newton’s Law of Gravitation Two objects bounded by gravitation will rotate around the center of mass of the two objects. Hong Kong Space Museum Oct 29, 2005

  12. Method 1: Astrometry Method天體位置測量 • Search for evidence of planets by measuring the tiny wobbles of the star 1 Rsun Motion of the Sun around the center of mass of the solar system over a 65-year period Hong Kong Space Museum Oct 29, 2005

  13. Method 1: Astrometry Method • German astronomer Bessel used this method to identify the first binary companion • Data used: Images of Sirius 1833-1844 Sirius Sirius B Credit: McDonald Observatory Hong Kong Space Museum Oct 29, 2005

  14. Method 1: Astrometry Method • Precision astrometry difficult to do! • Images of stars don’t stay stationary due to the effect of our atmosphere • Twinkle twinkle little star…… Courtesy: Nick Strobel’s Astronomy Notes http://www.astronomynotes.com Real time images of g-Perseus 25 millisec exposure per frame 250 frames in total Courtesy: Photonics group at Imperial College Hong Kong Space Museum Oct 29, 2005

  15. The Story Begins… • US-Dutch astronomer Peter van de Kamp • Studied tiny wobble of Barnard’s star with a 24- inch refractor 1950 1997 Credit: Palomar Obs Credit: J. Schmidling Hong Kong Space Museum Oct 29, 2005 Peter van de Kamp

  16. First planet discovered! • 2400 photographs from 1937-1963 • 1963: Periodic motion due to a planet! • Planet mass: 1.6 MJ • Planet orbit: 24 year • Orbit size: 4.4 AU • Slight worry: Measured orbits is highly elliptical (e = 0.6) • BUT Jupiter has a very circular orbit… Size of the star image Hong Kong Space Museum Oct 29, 2005 Credit: Van de Kamp, 1963

  17. First planet discovered! • 1968: Van de Kamp 3000 photos • Mass 1.7 MJ, Orbit: 4.5AU, more elliptical orbit Hong Kong Space Museum Oct 29, 2005

  18. First planet discovered!! • 1968: Van de Kamp 3000 photos • Mass 1.7 MJ, Orbit: 4.5AU, more elliptical orbit • 1969: Van de Kamp Brilliant idea? • Two planets, both circular orbit on same plane • Masses 0.8 & 1.1 MJ, Orbits: 2.8 & 4.7AU Hong Kong Space Museum Oct 29, 2005

  19. First planet discovered!? • 1968: Van de Kamp 3000 photos • Mass 1.7 MJ, Orbit: 4.5AU, more elliptical orbit • 1969: Van de Kamp Brilliant idea? • Two planets, both circular orbit on same plane • Masses 0.8 & 1.1 MJ, Orbits: 2.8 & 4.7AU • 1972: D.C. Black (postdoc) Theoretical calculations • Two planets possible, but orbits not on same plane Hong Kong Space Museum Oct 29, 2005

  20. First planet discovered?? • 1968: Van de Kamp 3000 photos • Mass 1.7 MJ, Orbit: 4.5AU, more elliptical orbit • 1969: Van de Kamp Brilliant idea? • Two planets, both circular orbit on same plane • Masses 0.8 & 1.1 MJ, Orbits: 2.8 & 4.7AU • 1972: D.C. Black (postdoc) Theoretical calculations • Two planets possible, but orbits not on same plane • 1973: J.L.Hershey (colleague of Van de Kamp) • Found the same shift pattern for another star! • Likely systematic effect rather than coincidence Hong Kong Space Museum Oct 29, 2005

  21. First planet discovered! ’49: New equipment installed ’57: Lens adjustment • 2400 photographs from 1937-1963 • 1963: Periodic motion due to a planet! • Planet mass: 1.6 MJ • Planet orbit: 24 year • Orbit size: 4.4 AU • Slight worry: Measured orbits is highly elliptical (e = 0.6) • BUT Jupiter has a very circular orbit… Size of the star image Hong Kong Space Museum Oct 29, 2005 Credit: Van de Kamp, 1963

  22. First planet discovered!! • 1973: G. Gatewood & H. Eichhorn 241 photographs from 1916-1971 • Follow movement of Barnard’s star using a different telescope : NO SIGNAL OF PLANET OBSERVED Credit: Gatewood & Eichhorn 1973 Hong Kong Space Museum Oct 29, 2005

  23. Method 2: Radial Velocity 視向速度測量 • Measure tiny movements of stars caused by planets • Wavelengths of light slightly different when the star is moving away and towards the observer (Doppler Effect多普勒效應) • Jupiter: 12.5 ms-1 • Saturn: 4 ms-1 • Earth: 8 ms-1 藍移 紅移 Hong Kong Space Museum Oct 29, 2005

  24. Method 2: Radial Velocity • So what do you need to do? Measure wavelength of light very very accurately! • To measure wavelength of light: Spectroscopy (break down light into different energy components) Hong Kong Space Museum Oct 29, 2005

  25. Light: Electromagnetic waves Credit: Chaisson & McMillan: Astronomy Today Hong Kong Space Museum Oct 29, 2005

  26. Method 2: Radial Velocity • So what do you need to do? Measure wavelength of light very very accurately! • To measure wavelength of light: Spectroscopy (break down light into different energy components) • We need good standard wavelengh reference: • Need to be stable under different physical conditions • Need to provide many reference wavelengths (to reduce errors) • Need to be easy to use and easy to handle Hong Kong Space Museum Oct 29, 2005

  27. Method 2: Radial Velocity • Candidate: Gaseous lamps filled with simple elements or molecules! • An absorption spectrum will be generated Gaseous lamp Stars Credit: Chaisson & McMillan: Astronomy Today E.g. absorption spectrum of Sodium Hong Kong Space Museum Oct 29, 2005

  28. The Story Continues…… • 1977: Gordon Walker and his student Bruce Campbell used this technique to search for companions near sun-like stars • Telescope used: 3.6m diameter Canada-France-Hawaii Telescope • Gaseous lamp used: Hydrogen Floride • Poisonous, corrosive, and invisible! Gordon Walker Hong Kong Space Museum Oct 29, 2005

  29. The Story Continues…… Star only Star + HF Campbell, Walker, & Yang (1988) ApJ, 331, 902 Measure movement of Ca+ line Absorption lines of HF 61 Cygnus A Hong Kong Space Museum Oct 29, 2005

  30. The Story Continues…… • 1988: NO companions for the 14 sun-like stars nearby (Mass ~ 10-80 MJ; Period < 50 yr) • 1992: Bruce Campbell gave up astronomy and become a businessman / Velocity error: ~ 15 ms-1 Campbell, Walker, & Yang (1988) ApJ, 331, 902 Hong Kong Space Museum Oct 29, 2005

  31. The Story Continues… • Starting from mid-80s, Geoff Marcy and graduate student Paul Butler started planet searching program. • Use Iodine instead of HF (not as poisonous, with color & odor) • Many more lines! (need bigger & faster computers) • Velocity accuracy ~ 8-10 ms-1 • Monitor sun-like stars nearby at frequency few times a year Hong Kong Space Museum Oct 29, 2005

  32. Some good news finally! • 1988: David Latham announced object of lower limit mass 11 MJ around star HD114762 with orbit 84 days • Velocity of “planet”~ 600 ms-1 compared to 400 ms-1 accuracy of spectrograph • Invited Michel Mayor to verify results (accuracy ~ 300 ms-1 ) • Some problems: • Borderline mass between planet and brown dwarf… • Orbit radius implied only 0.4AU Hong Kong Space Museum Oct 29, 2005

  33. Some 3D complications • Important parameter that may affect the mass measured: the inclination angle of the orbit 軌道傾斜角 i • Orbital plane viewed face on: i = 0o • Assume planet moving at speed v • Radial speed (speed moving towards or away from us) observed = v sin 0o = 0 • Orbital plane viewed side on: i = 90o • Assume planet moving at speed v • Radial speed (speed moving towards or away from us) observed = v sin 90o = v • Orbital plane at inclination angle i→ radial speed observed = v sin i Hong Kong Space Museum Oct 29, 2005

  34. yr Some 3D complications • Define K = (lmeasured – ltheory)/ ltheory 公轉週期(P) 恆星質量(M*) 放射速度(K) 公轉軌道半徑(a) (含軌道傾斜性)行星質量(MP sin i) Inputs: Outputs: It turns out the planet around HD114762 is face on, i.e. i is very small, or, Mp = 11 MJ/sin i (LARGE!) Hong Kong Space Museum Oct 29, 2005

  35. Method 3: Timing method 計時法 • Unexpected: need to introduce pulsars 脈沖星 • Neutron starsare leftover from centers of supernova explosion • 10km diameter in size (~Hong Kong Island), but with mass of Sun • Pulsarsare rotating neutron stars with strongmagnetic field • Pulsars have very accurate periods (can be used as clocks) Credit: Mr Ko Shu Fung (HKU) Hong Kong Space Museum Oct 29, 2005

  36. Method 3: Timing Method • 1992: A. Wolszczan & D. Frail discovered small time variations between pulses of pulsar PSR B1257+12 • Later known to be caused by 3 planets • Discovered made at the Arecibo Radio Observatory Credit: Pulsar Planet Detection Hong Kong Space Museum Oct 29, 2005

  37. Arecibo Radio Observatory Diameter: 305m World’s largest radio dish Hong Kong Space Museum Oct 29, 2005

  38. The First Extrasolar planet! Surprises: • Not only do we have planets, we have a planetary system! • Planets discovered are few x MEarth orbiting close to the pulsar • Nobody expect planets to be found around pulsars! Artistic Impression of planet on PSR B1257+12 Hong Kong Space Museum Oct 29, 2005 Credit: Lynette Cook

  39. Method 2: Radial Velocity (II) • 1994: Michel Mayor and Didler Queloz started a new radial velocity search program using a new spectrograph • Use 1.93m telescope of the Haute-Provence Observatory (southern France) • Use Thorium-Argon lamp as wavelength reference (accuracy ~ 13 ms-1) • Had a 8-night observing run in August 1995 to follow up January data… Hong Kong Space Museum Oct 29, 2005

  40. Method 2: Radial Velocity (II) 飛馬51 Hong Kong Space Museum Oct 29, 2005

  41. First extrasolar planet around sun-like star! • Oct 6, 95: Announce planet around 51 Pegasi, a sun-like star • Variation up to 60 ms-1 observed! • Circular orbit • Confirmed by Marcy & Butler in Oct 19, 95 • Other major suspicions (rotation, pulsation, brown dwarf) seem unlikely sinusoidal curve Mayor & Queloz (1995) Nature, 378, 357 Hong Kong Space Museum Oct 29, 2005

  42. 47.9光年 太陽：G2V A STRANGE PLANET! 149個地球質量 MP sin i = aJ = 5.2 AU (10% of sun Mercury distance) tJ = 11.8 year Hong Kong Space Museum Oct 29, 2005

  43. 1995 is a good substellar year • Jun 95: First brown dwarf discovered (Palomar Pleiades 15) – lithium spectrum • Oct 95: First brown dwarf imaged (Gliese 229B) Hong Kong Space Museum Oct 29, 2005

  44. More Planets to come… • Jan 96: Marcy team announced discovery of 2 more stars with planets • 47 Ursae Majoris (47 UMa) • Normal Jupiter-like planets around normal sun-like stars!! (mPsin i = 2.4MJ, P = 3 years, a ~ 2 AU) • 2003: Announce discovery of a second planet, almost like Saturn (mPsin i = 0.8 MJ, P = 7 years, a ~ 3.7 AU) • A solar-system like planetary system Credit: Lynette Cook Hong Kong Space Museum Oct 29, 2005

  45. 2005 邵逸夫獎 US$500,000 US$500,000 Hong Kong Space Museum Oct 29, 2005

  46. Upsilon Andromedae (Ups And) • Multiple planets around a sun-like star • Solid lines (planets of Ups And – a: 0.7 MJ, 0.06AU;b: 1.9 MJ, 0.8AU;c: 3.8 MJ, 2.5AU) • Broken lines (Mercury, Venus, Earth, Mars) Radial velocity curve due to multiple planets Hong Kong Space Museum Oct 29, 2005

  47. More Planets to come… • Majority (over 100) of extrasolar planets discovered so far are with the radial velocity method • Best accuracy now: 3 ms-1; 11 kmh-1 • Method favors massive planets orbiting near the star Hong Kong Space Museum Oct 29, 2005

  48. Method 4: Periodic Transit 周期測光(掩食) • Look for tiny variation of star’s brightness when planet moves in front of it • Accuracy needed to observe Jupiter at ~30 light years away: 0.02 magnitude Hong Kong Space Museum Oct 29, 2005

  49. Method 4: Periodic Transit • Disadvantages: • Only works if viewed almost perfectly from the side (i ~ 90o) • For large and close-by planets only • Model dependent • ~10 planets discovered so far with this method Hong Kong Space Museum Oct 29, 2005

  50. Method 4: Periodic Transit HD 209485b: 0.67 MJ, 3.53day, 0.05AU Credit: STARE Credit: The Transits of Extrasolar Planets Network Hong Kong Space Museum Oct 29, 2005

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