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?
Hong Kong Space Museum Oct
29, 2005
2
Planets of our solar system
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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
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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
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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
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Credit: The New York Times
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 ???
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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!
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Drake Equation (1961)
• How many technologically advanced
civilizations are out there in our Galaxy?
N = N* fp ne fl fi fc 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
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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
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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.
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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
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Museum
Oct
29, 2005
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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
McDonald
HongCredit:
Kong Space
MuseumObservatory
Oct
29, 2005
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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:
Hong Kong Space Museum
Oct Photonics group at Imperial College
14
29, 2005
The Story Begins…
• US-Dutch astronomer
Peter van de Kamp
• Studied tiny wobble of
Barnard’s star with a 24inch refractor
1950
Credit: Palomar Obs
1997
Credit: J. Schmidling
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29, 2005
Peter van de Kamp
15
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 Credit:
Oct
Van de Kamp, 1963
29, 2005
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First planet discovered!
• 1968: Van de Kamp 3000 photos
– Mass 1.7 MJ, Orbit: 4.5AU, more elliptical orbit
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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
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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
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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
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First
planet
discovered!
’49: New
’57: Lens
equipment installed
adjustment
• Slight worry: Measured orbits
is highly elliptical (e = 0.6)
• BUT Jupiter has a very circular
orbit…
• 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
Size of the
star image
Hong Kong Space Museum Credit:
Oct
Van de Kamp, 1963
29, 2005
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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
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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
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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)
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Light: Electromagnetic waves
Chaisson & McMillan: 25
Hong Kong Space Museum Credit:
Oct
Astronomy Today
29, 2005
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
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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
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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-FranceHawaii Telescope
• Gaseous lamp used:
Hydrogen Floride
• Poisonous, corrosive, and
invisible!
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29, 2005
Gordon Walker
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The Story Continues……
Star only
Star + HF
Measure
movement
of Ca+ line
Absorption lines of HF
Campbell, Walker,
& Yang (1988) ApJ,
331, 902
61 Cygnus A
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The Story Continues……
Velocity error: ~ 15 ms-1
Campbell, Walker, &
Yang (1988) ApJ, 331,
902
• 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
/
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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
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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
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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
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Some 3D complications
• Define K = (lmeasured – ltheory)/ ltheory
 a 
P

 1AU 
2/3
 P 

K  28.4  
 1yr 
1 / 2
Inputs:
 M* 


 M sun  yr
1 / 3
公轉週期(P)
恆星質量(M*)
放射速度(K)
2 / 3
 M p sin i  M * 


 ms1
 M J  M sun  Outputs: 公轉軌道半徑(a)
(含軌道傾斜性)行星質量
It turns out the planet around
(MP sin i)
HD114762 is face on, i.e. i is very
small, or, Mp = 11 MJ/sin
i (LARGE!)
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29, 2005
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
Credit: Mr Ko Shu Fung (HKU)
used as clocks) Hong Kong Space Museum Oct
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29, 2005
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
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Arecibo Radio Observatory
Diameter: 305m
World’s largest radio dish
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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
Credit: Lynette Cook
29, 2005
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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…
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Method 2: Radial Velocity (II)
飛馬51
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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
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sinusoidal curve
Mayor & Queloz (1995)
Nature, 378, 357
41
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
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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)
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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,
Credit: Lynette Cook
P = 7 years, a ~ 3.7 AU)
• A solar-system like
planetary system Hong Kong Space Museum Oct
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2005 邵逸夫獎
US$500,000
US$500,000
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29, 2005
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Upsilon Andromedae (Ups And)
• Multiple planets around a sunlike 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
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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
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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
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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
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Method 4: Periodic Transit
HD 209485b: 0.67 MJ, 3.53day, 0.05AU
Credit: STARE
Credit: The Transits of Extrasolar Planets Network
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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!
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Method 5: Gravitational
Microlensing
• 2003: First planet with
1.3m Warsaw Telescope
in Chile
• OGLE 2003-BLG235/MOA 2003-BLG-53:
1.5 MJ, 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
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Method 5: Gravitational
Microlensing
Credit: Optical Gravitational Lensing Experiment
Hong Kong Space Museum Oct
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Search for Extrasolar Planets
資料來源：The Extrasolar Planets
Encyclopaedia
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Extrasolar Planets Family
• 數量 -- 目錄 (Extrasolar Planets Catalog)截止05年9月16日：
http://vo.obspm.fr/exoplanetes/encyclo/catalog.php
• 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」
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Method 6: Search for Extraterrestrial
Intelligence (SETI)
Credit: SETI@home
Over 4.6 million people have signed up to
contribute 1.6 million hour of computer time
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to analyze data collected.
Plans for the Future
• Kepler: Small satellite to
look for Earth size
planets
• 0.95m diameter
telescope to monitor
100,000 stars
• Photometric accuracy:
0.00005 magitude
• 50 Earth mass planets in
4 years lifetime
• Launch date: 2006
2008 June
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Plans for the Future
Space Interferometry Mission (SIM)
• 10m baseline optical
interferometer in space
• Actually image the small
movement of stars (1 mas)
• Find earth mass planets
• Launch date: 2009?Hong Kong Space Museum Oct
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Bigger Goals
How to find planets that can be
habitable 可棲息? What do we
look for?
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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
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Bigger Goals
How to find planets that can be
habitable 可棲息? What do we
look for?
Hint: Look in infrared
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Bigger Goals
Look for atmospheric signature of habitable
planets
Presence of O3 means
planet may be feasible for
lifeforms to survive
Terrestrial Planet Finder (TPF) 2015?
Hong Kong Space Museum Oct
search for 200 of
such
planets
29, 2005
68
Bigger Goals
Allen Telescope Array (ATA)
• 350 6.1m radio dishes
• Look for radio signal
from extra-terrestrial
intelligence life forms
• Privately funded
• Online in 2005
2004
• 40 antennae now
operational
• Your name on a dish
for (only) US$400,000
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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 。」
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Happy
Birthday!
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Acknowledgments and References
•
•
•
•
•
•
Mr Chan Wen Ling
Mr So Chu Wing
http://www.exoplanets.org
http://spacescience.nasa.gov
http://www.physics.hku.hk/~astro/
http://setiathome.ssl.berkeley.edu/
Thank you!
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