the search for extra-solar planets p1y* frontiers of physics feb 2007 dr martin hendry dept of...

The Search The Search for Extra-for Extra-Solar PlanetsSolar Planets

P1Y*Frontiers of PhysicsFeb 2007

http://www.astro.gla.ac.uk/users/martin/teaching/

Dr Martin HendryDept of Physics and Astronomy

Extra-Solar Planets

One of the most active and exciting areas of astrophysics

Well over 200 exoplanets discovered since 1995

Extra-Solar Planets



Some important questionso How common are planets?How common are planets?

o How did planets form?How did planets form?

o Can we find Earth-like planets?Can we find Earth-like planets?

o Do they harbour life?Do they harbour life?

Extra-Solar Planets



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

Rigel

Betelgeuse

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

W104 26bol L

e.g. for the Sun

Stars radiate isotropically (equally in all directions) at distance r, luminosity spread

over surface area

24 r

(this gives rise to the Inverse-Square Law )

Planet, of radius R, at distance r from star

Intercepts a fraction of

2

2

2

24

r

R

r

Rf

SL

Planet, of radius R, at distance r from star

Intercepts a fraction of

Assume planet reflects all of this light

2

2

2

24

r

R

r

Rf

SL

2

S

P

2

r

R

L

L

Examples

Sun – Earth:

m104.6 6R

m105.1 11r 10

S

P 106.4 L

L

Examples

Sun – Earth:

Sun – Jupiter:

m104.6 6R

m105.1 11r 10

S

P 106.4 L

L

m102.7 7R

m108.7 11r 9

S

P 101.2 L

L

2nd problem:

Angular separation of star and exoplanet is tiny

Distance units

Astronomical Unit = mean Earth-Sun distance

m101.496A.U.1 11

2nd problem:

Angular separation of star and exoplanet is tiny

Distance units

Astronomical Unit = mean Earth-Sun distance

For interstellar distances: Light year

m101.496A.U.1 11

m10461.9yearlight 1 15

Star Planet

Earth

e.g. ‘Jupiter’ at 30 l.y. r

d

m108.2l.y.30 17d

m105.7A.U.5 11r

d

rtan

radians107.2 6

deg105.1 4

e.g. ‘Jupiter’ at 30 l.y.

m108.2l.y.30 17d

m105.7A.U.5 11r

d

rtan

radians107.2 6

deg105.1 4

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

Keck telescopeson Mauna Kea,Hawaii

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

Need OWL telescope:

100m mirror,planned for nextdecade?

‘Jupiter’ at 30 l.y.

100m


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 MotionKepler’s Laws of Planetary Motion

1) Planets orbit the Sun in an ellipse with the Sun at one focus



2) During a planet’s orbit around the Sun, equal areas are swept out in equal times


Kepler’s Laws, published 1609, 1619


2) During a planet’s orbit around the Sun, equal areas are swept out in equal times

3) The square of a planet’s orbital period is proportional to the cube of its mean distance from the Sun

Newton’s law of Universal Gravitation,Published in the Principia: 1684 - 1686

Newton’s gravitational force provided a physical explanation for Kepler’s laws

221

G r

mmGF

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

2211 rmrm

P

SSPS m

mrrrr 1




Centre of mass condition

2211 rmrm

P

SSPS m

mrrrr 1

e.g. ‘Jupiter’ at 30 l.y.

radiansd

rSS

kg109.1

kg100.227

30

P

S

m

m

deg105.1 7

The Sun’s “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 www.planetquest.jpl.nasa.gov/SIM/

Suppose line of sight is in orbital plane

Direction to Earth

Direction to Earth


Star has a periodic motion towards and away from Earth – radial velocity varies sinusoidally


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

hc

hE Planck’s constant

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 -

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

n = 1

n = 2

n = 4

(ground state)

n = 3

Lyman

Balmer

Paschen

Brackett

Ene

rgy

diff

eren

ce (

eV)

5

10

15

m = 2,3,4,… n = 1

n = 2m = 3,4,5,…

m = 4,5,6,… n = 3

m = 5,6,7,… n = 4Ionised 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.

Star

Laboratory

How large is the Doppler motion? Equating gravitational and circular accleration

For the planet:-

For the star:-

22

r

mmGrmF SP

PPC

22

r

mmGrmF SP

SSC

T

2

Angular velocity

Period of ‘wobble’

How large is the Doppler motion? Equating gravitational and circular accleration

For the planet:-

For the star:-

Adding:-

22

r

mmGrmF SP

PPC

2

2

r

mmGrr PS

SP

22

r

mmGrmF SP

SSC

T

2

Angular velocity

Period of ‘wobble’

SPS GmmmGT

rr

2

3232 4

SPS GmmmGT

rr

2

3232 4

Kepler’sThird Law

The square of a planet’s orbital period is proportionalto the cube of its mean distance from the Sun

SPS GmmmGT

rr

2

3232 4

Kepler’sThird Law

The square of a planet’s orbital period is proportionalto the cube of its mean distance from the Sun

e.g. Earth:

Jupiter:

A.U.1r year1T

A.U.2.5r 2

3

2

3

J

J

E

E

T

r

T

r years86.112.5 3 JT

Amplitude of star’s radial velocity:-

SS rvP

SPS

m

rmmr

)( From centre of

mass condition


SS rvP

SPS

m

rmmr

)( From centre of

mass condition

3

332 )(

P

SPSPS

m

rmmmmG


SS rvP

SPS

m

rmmr

)( From centre of

mass condition

3

332 )(

P

SPSPS

m

rmmmmG

2

v

2

v)( 32323 TmTmm

mG SSSPSP


SS rvP

SPS

m

rmmr

)( From centre of

mass condition

3

332 )(

P

SPSPS

m

rmmmmG

2

v

2

v)( 32323 TmTmm

mG SSSPSP

PSS mmT

G 3/23/1

2v

PSS mmT

G 3/23/1

2v

Examples

Jupiter:

-2-1311 skgm10673.6 G

kg100.2 30Sun m

kg109.1 27Jup m years86.11T

-1ms4.12v S

PSS mmT

G 3/23/1

2v

Examples

Jupiter:

Earth:

-2-1311 skgm10673.6 G

kg100.2 30Sun m

kg109.1 27Jup m years86.11T

-1ms4.12v S

kg100.6 24Earth m year1T

-1ms09.0v S

Are these Doppler shifts measurable?…

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

c

v

0


Doppler formula

Wavelength of light as measured in the laboratory

Change in wavelength

Radial velocity

Speed of light

Limits of current technology:

-1sm1v

c

v

0


Doppler formula

Wavelength of light as measured in the laboratory

Change in wavelength

Radial velocity

Speed of light

millionth3000

The Search The Search for Extra-for Extra-Solar PlanetsSolar Planets

P1Y*Frontiers of PhysicsFeb 2007

http://www.astro.gla.ac.uk/users/martin/teaching/

Dr Martin HendryDept of Physics and Astronomy

51 Peg – the first new planetDiscovered in 1995

Doppler amplitude

-1sm55v

51 Peg – the first new planetDiscovered in 1995

Doppler amplitude

How do we deduce planet’s data from this curve?

-1sm55v

PSS mmT

G 3/23/1

2v

We can observethese directly

We can infer this from spectrum

Stars of different colours have different surface temperatures

We can determine a star’s temperature from

its spectrum

Stars are good approximations to black

body radiatorsWien’s Law

The hotter the temperature, the shorter the wavelength at which the black body curve peaks

25000 10000 8000 6000 5000 4000 3000

Surface temperature (K)

O5 B0 A0 F0 G0 K0 M0 M8

Lum

inos

ity

(Sun

=1)

Spectral Type

1

102

104

106

10-2

10-4

-10

-5

0

+5

+10

+15

Abs

olut

e M

agni

tude

0.001 RS

0.01 RS

0.1 RS

1 RS

10 RS

Main Sequence

White dwarfs

Supergiants

1000 RS

100 RS

Giants

When we plot the temperature and luminosity of stars on a diagram most are found on theMain Sequence

25000 10000 8000 6000 5000 4000 3000

Surface temperature (K)

O5 B0 A0 F0 G0 K0 M0 M8

Lum

inos

ity

(Sun

=1)

Spectral Type

1

102

104

106

10-2

10-4

-10

-5

0

+5

+10

+15

Abs

olut

e M

agni

tude

. . . . .

. . . .

..

...

. ....

.... .. .. .

.....

..............

.........

... ....... ....

........

. .. .

...........

.. ..

..

............ .

..

....Regulus

Vega

Sirius A

Altair Sun

Sirius B

Procyon B

Barnard’sStar

Procyon A

..

.. ... Aldebaran

Mira

Pollux

Arcturus

RigelDeneb

Antares

Betelgeuse

Stars on theMain 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 theMain Sequence

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

Llo

gL

Sun

10

0 0.5 1.0-1

0

1

2

3

4

5

mlog mSun

10

L ~ m3.5

Summary: Doppler ‘Wobble’ method

Stellar spectru

m

Velocity of stellar

‘wobble’

Stellartemperature

Luminosity

Orbital period+

Orbital radius

Planet mass

From Kepler’s Third Law

Stellar mass +

Complications

Elliptical orbits

Complicates maths a bit, butotherwise 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’

obssinv iS

i

obssinvv iSS Pm

1sin i

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.

Change in brightness from a planetary transit

Brightness

Time

Star

Planet

Ignoring light from planet, and assuming star is uniformly bright:

Total brightness during transit

e.g. Sun:

Jupiter:

Earth:

Total brightness outside transit

2

2**

22** 1

S

PP

R

R

RB

RRB

m102.7 7Jup R

m100.7 8Sun R

Brightness change of ~1%

m104.6 6Earth R 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

2

2**

22** 1

S

PP

R

R

RB

RRB

m102.7 7Jup R

m100.7 8Sun R

Brightness change of ~1%

m104.6 6Earth R 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.

PR

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.

“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

GR passed GR passed the test!the test!


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

Gravitational lens

Lens’ gravity focuses the light of the background star on the Earth

So the background star briefly appears brighter



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



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



Sep 2005: 156 exoplanets



Sep 2005: 156 exoplanets

Up-to-date summary at

http://www.exoplanets.org

Now finding planets at larger orbital semimajor axis

What have we learned about exoplanets?Why larger semi-major axes now?

Kepler’s third law implies longer period, so requires monitoring for many years to determine ‘wobble’ precisely

PSS mmT

G 3/23/1

2v



Amplitude of wobble smaller (at fixed ); benefit of improved spectroscopic precision

PSS mmT

G 3/23/1

2v

Pm



Amplitude of wobble smaller (at fixed ); benefit of improved spectroscopic precision

PSS mmT

G 3/23/1

2v

Pm

Improving precision also now finding lower mass planets (and getting quite close to Earth mass planets)

i

mmP sin

9.5 EarthFor example:Third planet of GJ876 system

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

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

Artist’s impression of ‘Hot Jupiter’ orbiting

HD195019

‘Hot Jupiters’ produce Doppler wobbles of very large amplitude

e.g. Tau Boo:

-1ms474sinv iS

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

The nebula spins more rapidly as it collapses

Forming stars Forming stars and planets….and planets….

versusversus

Forming stars Forming stars and planets….and planets…. versusversus

As the nebula collapses a disk forms

Forming stars Forming stars and planets….and planets…. versusversus

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.

What have we learned about exoplanets?

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?

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


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)





2010


3. The Kepler mission (launch 2008?) will detect transits

of Earth-type planets, by observing the brightness dip of stars





2010




(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

www.superwasp.org

Transit detections by SuperWASP from 2006


detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will

continue to detect more massive planets


2010




(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

PmimP sin

Saturn mass planet in transit across HD149026

From Sato et al 2006

From Doppler wobble method From transit method

4. Gravitational microlensing satellite? Launch date ????

Could detect mars-mass planets


Jan 2006: ground-based detection of a 5 Earth-mass planet via microlensing

5. 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

5. 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, watercarbon dioxide in atmosphere

Simulated ‘Earth’ from 30 light years

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:-

Newton’s laws of motion and gravity Atomic spectroscopy Black body radiation






o By ~2020, there is a real prospect of finding not only Earth-like planets, but detecting signs of life on them.







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?…


the search for extra-solar planets p1y* frontiers of physics feb 2007 dr martin hendry dept of...

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