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The Search for Extra-Solar Planets P1Y* Frontiers of Physics Feb 2007 Dr Martin Hendry Dept of Physics.

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Presentation on theme: "The Search for Extra-Solar Planets P1Y* Frontiers of Physics Feb 2007 Dr Martin Hendry Dept of Physics."— Presentation transcript:

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2 The Search for Extra-Solar Planets P1Y* Frontiers of Physics Feb Dr Martin Hendry Dept of Physics and Astronomy

3 Extra-Solar Planets One of the most active and exciting areas of astrophysics Well over 200 exoplanets discovered since 1995

4 Extra-Solar Planets One of the most active and exciting areas of astrophysics Well over 200 exoplanets discovered since 1995 Some important questions o How common are planets? o How did planets form? o Can we find Earth-like planets? o Do they harbour life?

5 Extra-Solar Planets One of the most active and exciting areas of astrophysics Well over 200 exoplanets discovered since 1995 What we are going to cover How can we detect extra-solar planets? What can we learn about them?

6 1. How can we detect extra-solar planets? Planets dont shine by themselves; they just reflect light from their parent star Exoplanets are very faint

7 1. How can we detect extra-solar planets? Planets dont shine by themselves; they just reflect light from their parent star Exoplanets are very faint We measure the intrinsic brightness of a planet or star by its luminosity Luminosity, L (watts)

8 Luminosity varies with wavelength (see later) e.g. consider Rigel and Betelgeuse in Orion Rigel Betelgeuse

9 Luminosity varies with wavelength (see later) e.g. consider Rigel and Betelgeuse in Orion Adding up (integrating) L at all wavelengths Rigel Betelgeuse Bolometric luminosity e.g. for the Sun

10 Stars radiate isotropically (equally in all directions) at distance r, luminosity spread over surface area (this gives rise to the Inverse-Square Law )

11 Planet, of radius R, at distance r from star Intercepts a fraction of

12 Planet, of radius R, at distance r from star Intercepts a fraction of Assume planet reflects all of this light

13 Examples Sun – Earth:

14 Examples Sun – Earth: Sun – Jupiter:

15 2nd problem: Angular separation of star and exoplanet is tiny Distance units Astronomical Unit = mean Earth-Sun distance

16 2nd problem: Angular separation of star and exoplanet is tiny Distance units Astronomical Unit = mean Earth-Sun distance For interstellar distances: Light year

17 Star Planet Earth e.g. Jupiter at 30 l.y.

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19 Exoplanets are drowned out by their parent star. Impossible to image directly with current telescopes (~10m mirrors) Keck telescopes on Mauna Kea, Hawaii

20 Exoplanets are drowned out by their parent star. Impossible to image directly with current telescopes (~10m mirrors) Need OWL telescope: 100m mirror, planned for next decade? Jupiter at 30 l.y. 100m

21 1. How can we detect extra-solar planets? They cause their parent star to wobble, as they orbit their common centre of gravity

22 1. How can we detect extra-solar planets? They cause their parent star to wobble, as they orbit their common centre of gravity Johannes Kepler Isaac Newton

23 Keplers Laws of Planetary Motion 1) Planets orbit the Sun in an ellipse with the Sun at one focus

24 Keplers Laws of Planetary Motion 1) Planets orbit the Sun in an ellipse with the Sun at one focus 2)During a planets orbit around the Sun, equal areas are swept out in equal times

25 Keplers Laws of Planetary Motion 1) Planets orbit the Sun in an ellipse with the Sun at one focus 2)During a planets orbit around the Sun, equal areas are swept out in equal times

26 Keplers Laws of Planetary Motion Keplers Laws, published 1609, ) Planets orbit the Sun in an ellipse with the Sun at one focus 2)During a planets orbit around the Sun, equal areas are swept out in equal times 3)The square of a planets orbital period is proportional to the cube of its mean distance from the Sun

27 Keplers Laws of Planetary Motion Keplers Laws, published 1609, ) Planets orbit the Sun in an ellipse with the Sun at one focus 2)During a planets orbit around the Sun, equal areas are swept out in equal times 3)The square of a planets orbital period is proportional to the cube of its mean distance from the Sun

28 Newtons law of Universal Gravitation, Published in the Principia: Newtons gravitational force provided a physical explanation for Keplers laws

29 Star + planet in circular orbit about centre of mass, to line of sight

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31 Can see star wobble, even when planet is unseen. But how large is the wobble?…

32 Star + planet in circular orbit about centre of mass, to line of sight Can see star wobble, even when planet is unseen. But how large is the wobble?… Centre of mass condition

33 Star + planet in circular orbit about centre of mass, to line of sight Can see star wobble, even when planet is unseen. But how large is the wobble?… Centre of mass condition e.g. Jupiter at 30 l.y.

34 The Suns wobble, mainly due to Jupiter, seen from 30 light years away = width of a 5p piece in Baghdad! Detectable routinely with SIM Planet Quest (launch date 2010?) but not currently See

35 Suppose line of sight is in orbital plane Direction to Earth

36 Suppose line of sight is in orbital plane Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally

37 Suppose line of sight is in orbital plane Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally Detectable via the Doppler Effect Can detect motion from shifts in spectral lines

38 Spectral lines arise when electrons change energy level inside atoms. This occurs when atoms absorb or emit light energy. Since electron energies are quantised, spectral lines occur at precisely defined wavelengths Plancks constant

39 Absorption e - Electron absorbs photon of the precise energy required to jump to higher level. Light of this energy (wavelength) is missing from the continuous spectrum from a cool gas Absorption spectrum

40 Absorption e - Electron absorbs photon of the precise energy required to jump to higher level. Light of this energy (wavelength) is missing from the continuous spectrum from a cool gas

41 Emission e - Electron jumps down to lower energy level, and emits photon of energy equal to the difference between the energy levels. Light of this energy (wavelength) appears in the spectrum from a hot gas Emission spectrum

42 Emission e - Electron jumps down to lower energy level, and emits photon of energy equal to the difference between the energy levels. Light of this energy (wavelength) appears in the spectrum from a hot gas

43 Hydrogen Spectral Line Series n = 1 n = 2 n = 4 (ground state) n = 3 Lyman Balmer Paschen Brackett Energy difference (eV) m = 2,3,4,…n = 1 n = 2m = 3,4,5,… m = 4,5,6,…n = 3 m = 5,6,7,…n = 4 Ionised above 13.6 eV Shown here are downward transitions, from higher to lower energy levels, which produce emission lines. The corresponding upward transition of the same difference in energy would produce an absorption line with the same wavelength.

44 Star Laboratory

45 Star Laboratory

46 How large is the Doppler motion? Equating gravitational and circular accleration For the planet:- For the star:- Angular velocity Period of wobble

47 How large is the Doppler motion? Equating gravitational and circular accleration For the planet:- For the star:- Angular velocity Period of wobble

48 How large is the Doppler motion? Equating gravitational and circular accleration For the planet:- For the star:- Adding:- Angular velocity Period of wobble

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50 Keplers Third Law The square of a planets orbital period is proportional to the cube of its mean distance from the Sun

51 Keplers Third Law The square of a planets orbital period is proportional to the cube of its mean distance from the Sun e.g. Earth: Jupiter:

52 Amplitude of stars radial velocity:- From centre of mass condition

53 Amplitude of stars radial velocity:- From centre of mass condition

54 Amplitude of stars radial velocity:- From centre of mass condition

55 Amplitude of stars radial velocity:- From centre of mass condition

56 Examples Jupiter:

57 Examples Jupiter: Earth: Are these Doppler shifts measurable?…

58 Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours

59 Doppler formula Wavelength of light as measured in the laboratory Change in wavelength Radial velocity Speed of light

60 Limits of current technology: Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours Doppler formula Wavelength of light as measured in the laboratory Change in wavelength Radial velocity Speed of light

61 The Search for Extra-Solar Planets P1Y* Frontiers of Physics Feb Dr Martin Hendry Dept of Physics and Astronomy

62 51 Peg – the first new planet Discovered in 1995 Doppler amplitude

63 51 Peg – the first new planet Discovered in 1995 Doppler amplitude How do we deduce planets data from this curve? We can observe these directly We can infer this from spectrum

64 Stars of different colours have different surface temperatures We can determine a stars temperature from its spectrum Stars are good approximations to black body radiators Wiens Law The hotter the temperature, the shorter the wavelength at which the black body curve peaks

65 Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type Absolute Magnitude R S 0.01 R S 0.1 R S 1 R S 10 R S Main Sequence White dwarfs Supergiants 1000 R S 100 R S Giants When we plot the temperature and luminosity of stars on a diagram most are found on the Main Sequence

66 Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type Absolute Magnitude Regulus Vega Sirius A AltairSun Sirius B Procyon B Barnards Star Procyon A Aldebaran Mira Pollux Arcturus Rigel Deneb Antares Betelgeuse Stars on the Main Sequence turn hydrogen into helium. Stars like the Sun can do this for about ten billion years When we plot the temperature and luminosity of stars on a diagram most are found on the Main Sequence

67 Main sequence stars obey an approximate mass– luminosity relation We can, in turn, estimate the mass of a star from our estimate of its luminosity L log L Sun m log m Sun 10 L ~ m 3.5

68 Summary: Doppler Wobble method Stellar spectrum Velocity of stellar wobble Stellar temperature Luminosity Orbital period + Orbital radius Planet mass From Keplers Third Law Stellar mass +

69 Complications Elliptical orbits Complicates maths a bit, but otherwise straightforward radius semi-major axis Orbital plane inclined to line of sight We measure only If is unknown, then we obtain a lower limit to ( as ) Multiple planet systems Again, complicated, but exciting opportunity (e.g. Upsilon Andromedae) Stellar pulsations Can confuse signal from planetary wobble

70 Transit of Mercury: May 7 th 2003 In recent years a growing number of exoplanets have been detected via transits = temporary drop in brightness of parent star as the planet crosses the stars disk along our line of sight.

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72 Change in brightness from a planetary transit Brightness Time Star Planet

73 Ignoring light from planet, and assuming star is uniformly bright: Total brightness during transit e.g.Sun: Jupiter: Earth: Total brightness outside transit Brightness change of ~1% Brightness change of ~0.008%

74 Ignoring light from planet, and assuming star is uniformly bright: Total brightness during transit e.g.Sun: Jupiter: Earth: Total brightness outside transit Brightness change of ~1% Brightness change of ~0.008% If we know the period of the planets orbit, we can use the width of brightness dip to relate, via Keplers laws, to the mass of the star. So, if we observe both a transit and a Doppler wobble for the same planet, we can constrain the mass and radius of both the planet and its parent star.

75 Another method for finding planets is gravitational lensing The physics behind this method is based on Einsteins General Theory of Relativity, which predicts that gravity bends light, because gravity causes spacetime to be curved. This was one of the first experiments to test GR: Arthur Eddingtons 1919 observations of a total solar eclipse.

76 Another method for finding planets is gravitational lensing The physics behind this method is based on Einsteins General Theory of Relativity, which predicts that gravity bends light, because gravity causes spacetime to be curved. This was one of the first experiments to test GR: Arthur Eddingtons 1919 observations of a total solar eclipse. He was one of the finest people I have ever known…but he didnt really understand physics because, during the eclipse of 1919 he stayed up all night to see if it would confirm the bending of light by the gravitational field. If he had really understood general relativity he would have gone to bed the way I did Einstein, on Max Planck GR passed the test!

77 Another method for finding planets is gravitational lensing If some massive object passes between us and a background light source, it can bend and focus the light from the source, producing multiple, distorted images.

78 Another method for finding planets is gravitational lensing If some massive object passes between us and a background light source, it can bend and focus the light from the source, producing multiple, distorted images.

79 Another method for finding planets is gravitational lensing If some massive object passes between us and a background light source, it can bend and focus the light from the source, producing multiple, distorted images.

80 Another method for finding planets is gravitational lensing If some massive object passes between us and a background light source, it can bend and focus the light from the source, producing multiple, distorted images. Multiple images of the same background quasar, lensed by a foreground spiral galaxy

81 Even if the multiple images are too close together to be resolved separately, they will still make the background source appear (temporarily) brighter.

82 Background stars Gravitational lens Lens gravity focuses the light of the background star on the Earth So the background star briefly appears brighter

83 Even if the multiple images are too close together to be resolved separately, they will still make the background source appear (temporarily) brighter. We call this case gravitational microlensing. We can plot a light curve showing how the brightness of the background source changes with time. Time The shape of the curve tells about the mass and position of the object which does the lensing

84 Even if the multiple images are too close together to be resolved separately, they will still make the background source appear (temporarily) brighter. We call this case gravitational microlensing. We can plot a light curve showing how the brightness of the background source changes with time. If the lensing star has a planet which also passes exactly between us and the background source, then the light curve will show a second peak. Even low mass planets can produce a high peak (but for a short time, and we only observe it once…) Could in principle detect Earth mass planets!

85 What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets

86 What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Sep 2005: 156 exoplanets

87 What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Sep 2005: 156 exoplanets Up-to-date summary at Now finding planets at larger orbital semimajor axis

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89 What have we learned about exoplanets? Why larger semi-major axes now? Keplers third law implies longer period, so requires monitoring for many years to determine wobble precisely

90 What have we learned about exoplanets? Why larger semi-major axes now? Keplers third law implies longer period, so requires monitoring for many years to determine wobble precisely Amplitude of wobble smaller (at fixed ); benefit of improved spectroscopic precision

91 What have we learned about exoplanets? Why larger semi-major axes now? Keplers third law implies longer period, so requires monitoring for many years to determine wobble precisely Amplitude of wobble smaller (at fixed ); benefit of improved spectroscopic precision Improving precision also now finding lower mass planets (and getting quite close to Earth mass planets) For example: Third planet of GJ876 system

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95 What have we learned about exoplanets? Discovery of many Hot Jupiters: Massive planets with orbits closer to their star than Mercury is to the Sun Very likely to be gas giants, but with surface temperatures of several thousand degrees. Mercury

96 What have we learned about exoplanets? Discovery of many Hot Jupiters: Massive planets with orbits closer to their star than Mercury is to the Sun Very likely to be gas giants, but with surface temperatures of several thousand degrees. Mercury Artists impression of Hot Jupiter orbiting HD Hot Jupiters produce Doppler wobbles of very large amplitude e.g. Tau Boo:

97 Existence of Hot Jupiters is a challenge for theories of star and planet formation:- Star forms from gravitational collapse of gas cloud. Angular momentum conservation proto-planetary disk Orion Nebula

98 Existence of Hot Jupiters is a challenge for theories of star and planet formation:- Star forms from gravitational collapse of gas cloud. Angular momentum conservation proto-planetary disk Orion Nebula

99 Existence of Hot Jupiters is a challenge for theories of star and planet formation:- Star forms from gravitational collapse of gas cloud. Angular momentum conservation proto-planetary disk Orion Nebula

100 The nebula spins more rapidly as it collapses Forming stars and planets…. versus

101 versus As the nebula collapses a disk forms

102 Forming stars and planets…. versus Lumps in the disk form planets

103 Computer modelling indicates that a Hot Jupiter could not form so close to its star and maintain a stable orbit Current theory is that Hot Jupiters formed further out in the protoplanetary disk, and migrated inwards due to tidal interactions with the disk material during its early evolution. What have we learned about exoplanets?

104 Computer modelling indicates that a Hot Jupiter could not form so close to its star and maintain a stable orbit Current theory is that Hot Jupiters formed further out in the protoplanetary disk, and migrated inwards due to tidal interactions with the disk material during its early evolution. How common are Hot Jupiters? We need to observe more planetary systems before we can answer this. Their common initial detection was partly because they give such a large Doppler wobble. As sensitivity increases, and lower mass planets are found, the statistics on planetary systems will improve. What have we learned about exoplanets?

105 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets Looking to the Future

106 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets 2. The position wobble (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after 2010 (done with HST in Dec 2002 for a 2 x Jupiter-mass planet) Looking to the Future

107 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets 2. The position wobble (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after 2010 (done with HST in Dec 2002 for a 2 x Jupiter-mass planet) 3. The Kepler mission (launch 2008?) will detect transits of Earth-type planets, by observing the brightness dip of stars Looking to the Future

108 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets 2. The position wobble (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after 2010 (done with HST in Dec 2002 for a 2 x Jupiter-mass planet) 3. The Kepler mission (launch 2008?) will detect transits of Earth-type planets, by observing the brightness dip of stars (already done in 2000 with Keck, and now becoming routine for Jupiter mass planets, e.g. from OGLE and SuperWASP) Looking to the Future

109 Transit Detection by OGLE III program in 2003

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111 Transit detections by SuperWASP from 2006

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113 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets 2. The position wobble (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after 2010 (done with HST in Dec 2002 for a 2 x Jupiter-mass planet) 3. The Kepler mission (launch 2008?) will detect transits of Earth-type planets, by observing the brightness dip of stars (already done in 2000 with Keck, and now becoming routine for Jupiter mass planets, e.g. from OGLE and SuperWASP) Looking to the Future Note that (2) and (3) permit measurement of the orbital inclination Can determine and not just

114 Saturn mass planet in transit across HD149026

115 From Sato et al 2006 From Doppler wobble method From transit method

116 4. Gravitational microlensing satellite? Launch date ???? Could detect mars-mass planets Looking to the Future Jan 2006: ground-based detection of a 5 Earth-mass planet via microlensing

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119 5. NASA: Terrestrial Planet Finder ESA: Darwin Looking to the Future } ~ 2015 launch??? These missions plan to use nulling interferometry to blot out the light of the parent star, revealing Earth-mass planets

120 5. NASA: Terrestrial Planet Finder ESA: Darwin Looking to the Future } ~ 2015 launch??? These missions plan to use nulling interferometry to blot out the light of the parent star, revealing Earth-mass planets Follow-up spectroscopy would search for signatures of life:- Spectral lines of oxygen, water carbon dioxide in atmosphere Simulated Earth from 30 light years

121 The Search for Extra-Solar Planets o The field is still in its infancy, but there are exciting times ahead o In about 10 years more than 200 planets already discovered

122 The Search for Extra-Solar Planets o The field is still in its infancy, but there are exciting times ahead o In about 10 years more than 200 planets already discovered o The Doppler method ultimately will not discover Earth-like planets, but other techniques planned for the next 15 years will o Search methods are solidly based on well-understood fundamental physics:- Newtons laws of motion and gravity Atomic spectroscopy Black body radiation

123 The Search for Extra-Solar Planets o The field is still in its infancy, but there are exciting times ahead o In about 10 years more than 200 planets already discovered o The Doppler method ultimately will not discover Earth-like planets, but other techniques planned for the next 15 years will o Search methods are solidly based on well-understood fundamental physics:- o By ~2020, there is a real prospect of finding not only Earth-like planets, but detecting signs of life on them. Newtons laws of motion and gravity Atomic spectroscopy Black body radiation

124 The Search for Extra-Solar Planets o The field is still in its infancy, but there are exciting times ahead o In about 10 years more than 200 planets already discovered o The Doppler method ultimately will not discover Earth-like planets, but other techniques planned for the next 15 years will o Search methods are solidly based on well-understood fundamental physics:- o By ~2020, there is a real prospect of finding not only Earth-like planets, but detecting signs of life on them. What (or who) will we find?… Newtons laws of motion and gravity Atomic spectroscopy Black body radiation


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