The Search for Extra-Solar Planets

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

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

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

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?

1. How can we detect extra-solar planets?
Planets don’t shine by themselves; they just reflect light from their parent star Exoplanets are very faint

Planets don’t 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)

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

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

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

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

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

Examples Sun – Earth:

Examples Sun – Earth: Sun – Jupiter:

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

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

e.g. ‘Jupiter’ at 30 l.y. Star Planet Earth

e.g. ‘Jupiter’ at 30 l.y.

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

Exoplanets are ‘drowned out’ by their parent star
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? 100m ‘Jupiter’ at 30 l.y.

They cause their parent star to ‘wobble’, as they orbit their common centre of gravity

They cause their parent star to ‘wobble’, as they orbit their common centre of gravity Johannes Kepler Isaac Newton

Kepler’s Laws of Planetary Motion
Planets orbit the Sun in an ellipse with the Sun at one focus

Planets orbit the Sun in an ellipse with the Sun at one focus During a planet’s orbit around the Sun, equal areas are swept out in equal times

Planets orbit the Sun in an ellipse with the Sun at one focus During a planet’s orbit around the Sun, equal areas are swept out in equal times The square of a planet’s orbital period is proportional to the cube of its mean distance from the Sun Kepler’s Laws, published 1609, 1619

Newton’s gravitational force provided a physical explanation for Kepler’s laws
Newton’s law of Universal Gravitation, Published in the Principia:

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 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 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?… e.g. ‘Jupiter’ at 30 l.y.

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

Suppose line of sight is in orbital plane
Direction to Earth

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

Can detect motion from shifts in spectral lines
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

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 Planck’s constant

Absorption e - e - Absorption spectrum
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

Absorption e - 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

Emission e - e - Emission spectrum
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

Emission e - 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

Hydrogen Spectral Line Series
15 Brackett Ionised above 13.6 eV m = 5,6,7,… n = 4 n = 4 n = 3 Paschen m = 4,5,6,… n = 3 n = 2 10 Balmer m = 3,4,5,… n = 2 Energy difference (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. 5 n = 1 Lyman (ground state) m = 2,3,4,… n = 1

Star Laboratory

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

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

The square of a planet’s orbital period is proportional
Kepler’s Third Law The square of a planet’s orbital period is proportional to the cube of its mean distance from the Sun

The square of a planet’s orbital period is proportional
Kepler’s Third Law The square of a planet’s orbital period is proportional to the cube of its mean distance from the Sun e.g. Earth: Jupiter:

Amplitude of star’s radial velocity:-
From centre of mass condition

Examples Jupiter:

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

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

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

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

Dr Martin Hendry Dept of Physics and Astronomy P1Y* Frontiers of Physics Feb 2007

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

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

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

When we plot the temperature and luminosity of stars on a diagram most are found on the
Main Sequence Surface temperature (K) 106 -10 100 RS 1000 RS 10 RS Supergiants 104 1 RS -5 102 Giants 0.1 RS Luminosity (Sun=1) Main Sequence Absolute Magnitude 1 +5 0.01 RS 10-2 +10 0.001 RS White dwarfs 10-4 +15 O5 B A F G K M M8 Spectral Type

Main Sequence turn hydrogen into helium.
When we plot the temperature and luminosity of stars on a diagram most are found on the Main Sequence Surface temperature (K) . . . 106 . -10 . Deneb . . . . Rigel . . . . Betelgeuse . . . . . 104 Antares . -5 . . . . . . . . . Arcturus . . . . . . . Aldebaran . . . . . . . Regulus . . . . . . . . . . . . 102 Vega . . . . . . . Sirius A . Mira . . . Stars on the Main Sequence turn hydrogen into helium. Stars like the Sun can do this for about ten billion years . . . Pollux . Procyon A . . . . Luminosity (Sun=1) Altair . . Sun Absolute Magnitude 1 . . . +5 . . . . . . . . . . . . . . . 10-2 . +10 . . . . . . . . . . . . . . . . Barnard’s Star . . . Sirius B . . . . . 10-4 . +15 Procyon B . . . . . . O5 B A F G K M M8 Spectral Type

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 LSun 10 0.5 1.0 -1 1 2 3 4 5 m mSun L ~ m 3.5

Velocity of stellar ‘wobble’
Summary: Doppler ‘Wobble’ method Stellar spectrum Velocity of stellar ‘wobble’ Stellar temperature Luminosity Orbital period + Orbital radius Planet mass From Kepler’s Third Law Stellar mass

( as ) 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’

Transit of Mercury: May 7th 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 star’s disk along our line of sight. Transit of Mercury: May 7th 2003

Change in brightness from a planetary transit
Star Planet Time

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%

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 planet’s orbit, we can use the width of brightness dip to relate , via Kepler’s 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.

Another method for finding planets is gravitational lensing
The physics behind this method is based on Einstein’s 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 Eddington’s 1919 observations of a total solar eclipse.

The physics behind this method is based on Einstein’s 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 Eddington’s 1919 observations of a total solar eclipse. GR passed the test! “He was one of the finest people I have ever known…but he didn’t 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

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.

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

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

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

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

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!

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

Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Sep 2005: 156 exoplanets

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

Why larger semi-major axes now? Kepler’s third law implies longer period, so requires monitoring for many years to determine ‘wobble’ precisely

Why larger semi-major axes now? Kepler’s 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

Why larger semi-major axes now? Kepler’s 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

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

Artist’s impression of ‘Hot Jupiter’ orbiting HD195019
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 ‘Hot Jupiters’ produce Doppler wobbles of very large amplitude e.g. Tau Boo: Artist’s impression of ‘Hot Jupiter’ orbiting HD195019

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

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

Pressure versus Gravity As the nebula collapses a disk forms

Pressure versus Gravity Lumps in the disk form planets

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.

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.

Looking to the Future 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 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 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 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 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) The Kepler mission (launch 2008?) will detect transits of Earth-type planets, by observing the brightness dip of stars

Looking to the Future 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 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) 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)

Transit Detection by OGLE III program in 2003

Transit detections by SuperWASP from 2006

Looking to the Future 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 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) 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) Note that (2) and (3) permit measurement of the orbital inclination Can determine and not just

Saturn mass planet in transit across HD149026

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

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

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

Simulated ‘Earth’ from 30 light years
Looking to the Future } NASA: Terrestrial Planet Finder ESA: Darwin ~ 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

The field is still in its infancy, but there are exciting times ahead In about 10 years more than 200 planets already discovered

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

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

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

The Search for Extra-Solar Planets

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