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

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

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

Hong Kong Space Museum Oct 29, Random Facts about Planets Important mass:Jupiter (biggest planet) mass 木星質量 M J = 318 M Earth = 0.1% M Sun Important distance: 天文單位 Astronomical Unit (AU) = Earth-Sun distance = 1.5x10 11 m Jupiter-Sun distance = 5.2 AU Important time: Earth orbits around the Sun in 1 year; Jupiter t J = 11.8 year

Hong Kong Space Museum Oct 29, 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, 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 Credit: The New York Times

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

Hong Kong Space Museum Oct 29, 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, Drake Equation (1961) How many technologically advanced civilizations are out there in our Galaxy? N = N * f p n e f l f i f c f L N * = # of stars in the Galaxy f p = fraction of stars with planets n e = # of planets per solar system suitable for life f l = fraction of habitable planets actually with life f i = fraction where intelligent lifeforms evolves f c = fraction with species which communicate extraterrestrially f L = fraction of planet’s life where communicating species lives

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

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

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

Hong Kong Space Museum Oct 29, Courtesy: Nick Strobel’s Astronomy Notes 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: Photonics group at Imperial College Real time images of  -Perseus 25 millisec exposure per frame 250 frames in total

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

Hong Kong Space Museum Oct 29, First planet discovered! Credit: Van de Kamp, photographs from : Periodic motion due to a planet! Planet mass: 1.6 M J 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, First planet discovered! 1968: Van de Kamp 3000 photos –Mass 1.7 M J, Orbit: 4.5AU, more elliptical orbit

Hong Kong Space Museum Oct 29, First planet discovered!! 1968: Van de Kamp 3000 photos –Mass 1.7 M J, 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 M J, Orbits: 2.8 & 4.7AU

Hong Kong Space Museum Oct 29, First planet discovered!? 1968: Van de Kamp 3000 photos –Mass 1.7 M J, 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 M J, 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, First planet discovered?? 1968: Van de Kamp 3000 photos –Mass 1.7 M J, 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 M J, 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, First planet discovered! Credit: Van de Kamp, photographs from : Periodic motion due to a planet! Planet mass: 1.6 M J 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 ’49: New equipment installed ’57: Lens adjustment

Hong Kong Space Museum Oct 29, First planet discovered!! 1973: G. Gatewood & H. Eichhorn 241 photographs from 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, 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, 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, Light: Electromagnetic waves Credit: Chaisson & McMillan: Astronomy Today

Hong Kong Space Museum Oct 29, 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: 1.Need to be stable under different physical conditions 2.Need to provide many reference wavelengths (to reduce errors) 3.Need to be easy to use and easy to handle

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

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

Hong Kong Space Museum Oct 29, The Story Continues…… 1988: NO companions for the 14 sun-like stars nearby (Mass ~ M J ; 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, 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, Some good news finally! 1988: David Latham announced object of lower limit mass 11 M J around star HD 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, 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 = 0 o –Assume planet moving at speed v –Radial speed (speed moving towards or away from us) observed = v sin 0 o = 0 Orbital plane viewed side on: i = 90 o –Assume planet moving at speed v –Radial speed (speed moving towards or away from us) observed = v sin 90 o = v Orbital plane at inclination angle i → radial speed observed = v sin i

Hong Kong Space Museum Oct 29, 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 (M P sin i) Some 3D complications Define K = ( measured – theory )/ theory yr Inputs: Outputs: It turns out the planet around HD is face on, i.e. i is very small, or, M p = 11 M J /sin i (LARGE!)

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

Hong Kong Space Museum Oct 29, Method 3: Timing Method 1992: A. Wolszczan & D. Frail discovered small time variations between pulses of pulsar PSR B 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, Arecibo Radio Observatory Diameter: 305m World’s largest radio dish

Hong Kong Space Museum Oct 29, The First Extrasolar planet! Surprises: –Not only do we have planets, we have a planetary system! –Planets discovered are few x M Earth orbiting close to the pulsar –Nobody expect planets to be found around pulsars! Credit: Lynette Cook Artistic Impression of planet on PSR B

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

Hong Kong Space Museum Oct 29, 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 First extrasolar planet around sun-like star! Mayor & Queloz (1995) Nature, 378, 357 sinusoidal curve

Hong Kong Space Museum Oct 29, 光年 太陽： G2V 149 個地球質量 A STRANGE PLANET! t J = 11.8 year a J = 5.2 AU (10% of sun Mercury distance) M P sin i =

Hong Kong Space Museum Oct 29, 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, Gliese 229B Methane present in the spectrum, just like Jupiter Surface temperature 600 o C, 1000 o C cooler than surface of the coolest normal stars

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

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

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

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

Hong Kong Space Museum Oct 29, Method 4: Periodic Transit Credit: The Transits of Extrasolar Planets Network Credit: STARE HD b: 0.67 M J, 3.53day, 0.05AU

Hong Kong Space Museum Oct 29, Method 5: Gravitational Microlensing 微引力透鏡 According to General Relativity, light can be bent by massive objects Light from distant stars can be magnified if it passes through a planet, similar to the way light can be magnified through a lens If there is a planet, there will be additional magnification!

Hong Kong Space Museum Oct 29, Method 5: Gravitational Microlensing 2003: First planet with 1.3m Warsaw Telescope in Chile OGLE 2003-BLG- 235/MOA 2003-BLG-53: 1.5 M J, 3.0AU Advantage: Exact mass of planet determined, not lower limit Disadvantage: –Lensing very rare –Cannot repeat observations Credit: Optical Gravitational Lensing Experiment

Hong Kong Space Museum Oct 29, Method 5: Gravitational Microlensing Credit: Optical Gravitational Lensing Experiment

Hong Kong Space Museum Oct 29, Search for Extrasolar Planets 資料來源： The Extrasolar Planets Encyclopaedia

Hong Kong Space Museum Oct 29, Extrasolar Planets Family 數量 -- 目錄 (Extrasolar Planets Catalog) 截止 05 年 9 月 16 日： 圍繞主序星公轉的 ESP ： –144 個行星系統 (planetary systems) –168 顆行星 –18 個多重行星系統 (multiple planet systems) 5 顆軌道不明的 ESP 2 顆圍繞脈沖星公轉的 ESP 8 顆未被証實或具爭議性的 ESP 1 顆「星團 ESP 」

Hong Kong Space Museum Oct 29, Latest News: Hubble Space Telescope Found star to be off center from the surrounding round of dust, caused by a planet

Hong Kong Space Museum Oct 29, Latest News: Spitzer Space Telescope Planetary transit in infrared More light contrast in IR compared to visible light Visible lightInfrared

Hong Kong Space Museum Oct 29, Special Extrasolar Planets 16 Cygni B Triple star system (two sun-like & a red-dwarf) Highly elliptical orbit 16 Cygni B Planet & Moon I Credit: Lynette Cook

Hong Kong Space Museum Oct 29, Special Extrasolar Planets Credit: Anglo-Australian Planet Search Program HD70642: Best solar system analog yet discovered Planet: 2.0 M J, P=6.1 yr, a = 3.3AU

Hong Kong Space Museum Oct 29, Special Extrasolar Planets Oldest planet known to exist (~13 billion years old, formed ~1 billion yrs after bigbang) Strange: Planet (2.5 Jupiter mass) rotating with neutron star and white dwarf

Hong Kong Space Museum Oct 29, Method 6: Search for Extraterrestrial Intelligence (SETI) Over 4.6 million people have signed up to contribute 1.6 million hour of computer time to analyze data collected. Credit:

Hong Kong Space Museum Oct 29, Plans for the Future Kepler: Small satellite to look for Earth size planets 0.95m diameter telescope to monitor 100,000 stars Photometric accuracy: magitude 50 Earth mass planets in 4 years lifetime Launch date: June

Hong Kong Space Museum Oct 29, Plans for the Future Space Interferometry Mission (SIM) 10m baseline optical interferometer in space Actually image the small movement of stars (1  as) Find earth mass planets Launch date: 2009?

Hong Kong Space Museum Oct 29, Bigger Goals How to find planets that can be habitable 可棲息 ? What do we look for?

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

Hong Kong Space Museum Oct 29, Bigger Goals How to find planets that can be habitable 可棲息 ? What do we look for? Hint: Look in infrared

Hong Kong Space Museum Oct 29, Bigger Goals Terrestrial Planet Finder (TPF) 2015? search for 200 of such planets Look for atmospheric signature of habitable planets Presence of O 3 means planet may be feasible for lifeforms to survive

Hong Kong Space Museum Oct 29, Bigger Goals Allen Telescope Array (ATA) m radio dishes Look for radio signal from extra-terrestrial intelligence life forms Privately funded Online in antennae now operational Your name on a dish for (only) US$400,000

Hong Kong Space Museum Oct 29, Search for Extraterrestrial Intelligence (SETI) Arthur C. Clarke C.B.E. (author of science fiction 2001: A Space Odyssey) “(SETI) represents the highest possible form of exploration, and when we cease to explore, we will cease to be human.” 《 2001 太空漫遊》小說作家克拉克： 「尋找外太空智慧體是人類最高層次的歷險；此歷險惟在人類 不再是宇宙智者時才會終止 。」

Hong Kong Space Museum Oct 29, Happy Birthday!

Hong Kong Space Museum Oct 29, Acknowledgments and References Mr Chan Wen Ling Mr So Chu Wing Thank you!