Transcript 1. How can we detect extra-solar planets?

The Search for
Extra-Solar
Planets
Dr Martin Hendry
Dept of Physics and Astronomy
Extra-Solar Planets
 One of the most active and
exciting areas of astrophysics
 About 150 exoplanets discovered
since 1995
Extra-Solar Planets
 One of the most active and
exciting areas of astrophysics
 About 150 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
2nd problem:
Angular separation of star and exoplanet is tiny
Distance units
Astronomical Unit = mean Earth-Sun distance
1 A.U.  1.496 10 m
11
For interstellar distances: Light year
1 light year  9.46110 m
15
e.g. ‘Jupiter’ at 30 l.y.
Star
r
Planet
d  30 l.y.  2.8 10 m
17
r  5 A.U.  7.5 10 m
11
r

d
d
  2.7 10 radians
6
4
 1.5 10 deg

Earth
e.g. ‘Jupiter’ at 30 l.y.
d  30 l.y.  2.8 10 m
17
r  5 A.U.  7.5 10 m
11
r

d
  2.7 10 radians
6
4
 1.5 10 deg
Exoplanets are ‘drowned out’ by their parent
star. Impossible to image directly with current
telescopes (~10m mirrors)
Keck telescopes
on Mauna Kea,
Hawaii
1. How can we detect extra-solar planets?
 They cause their parent star to ‘wobble’, as
they orbit their common centre of gravity
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
1. How can we detect extra-solar planets?
 They cause their parent star to ‘wobble’, as
they orbit their common centre of gravity
1. How can we detect extra-solar planets?
 They cause their parent star to ‘wobble’, as
they orbit their common centre of gravity
Star + planet in circular
orbit about centre of
mass,  to line of sight
Star + planet in circular
orbit about centre of
mass,  to line of sight
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?…
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
m1r1  m2r2
 mS 

r  rS  rP  rS 1 
 mP 
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
m1r1  m2r2
e.g. ‘Jupiter’ at 30 l.y.
mS  2.0 1030 kg
mP  1.9 1027 kg
rS
 S  radians
d
7
 1.5 10 deg
Detectable routinely
with SIM
(launch date 2009)
but not currently
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.
Direction
to Earth
Suppose line of sight is in
orbital plane
Detectable via the
Doppler Effect
Star has a periodic motion
towards and away from
Earth – radial velocity
varies
Can detect motion from shifts in spectral lines
Star
Laboratory
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

v

0 c
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

v

0 c
Wavelength of light as
measured in the laboratory
Limits of current technology:

0
Speed
of light
 300 millionth

v  1 ms -1
51 Peg – the first new planet
Discovered in 1995
Doppler ‘wobble’
v  55 ms
-1
51 Peg – the first new planet
Discovered in 1995
Doppler ‘wobble’
v  55 ms
-1
How do we deduce planet’s
data from this curve?
 2 G 
2 / 3
vS  
m
mP

S
 T 
1/ 3
We can observe
these directly
We can infer this
from spectrum
When we plot the
temperature and
luminosity of stars
on a diagram most
are found on the
Main Sequence
Surface temperature (K)
25000
106
10000
8000 6000
.
5000 4000 3000
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Deneb
-10
Rigel
Betelgeuse
Antares
Luminosity (Sun=1)
Arcturus
102
Aldebaran
Regulus
Vega
Procyon A
Altair
10-2
Pollux
Sun
Procyon B
O5 B0
+5
+10
Barnard’s
Star
Sirius B
10-4
0
Mira
Sirius A
1
-5
A0
F0
G0
Spectral Type
K0
M0
M8
+15
Absolute Magnitude
104
Stars on the
Main Sequence
turn hydrogen
into helium.
Stars like the
Sun can do this
for about ten
billion years
Main sequence stars obey
an approximate mass–
luminosity relation
5
L~m
4
We can, in turn,
estimate the mass
of a star from our
estimate of its
luminosity
3
L
log10 L
Sun

3.5
2
1
0
-1
0
0.5
m
log10 m
Sun
1.0
Summary: Doppler ‘Wobble’ method
Stellar
spectrum
Stellar
temperature
Luminosity
Velocity of
stellar ‘wobble’
+
Stellar
mass
Orbital radius
Planet mass
+
Orbital period
From Kepler’s
Third Law
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
Brightness
Star
Planet
Time
Ignoring light from planet, and assuming star is uniformly bright:
Total brightness during transit
Total brightness outside transit
e.g.


B*  R  R
B*  R*2
2
*
2
P

 RP 
 1   
 RS 
2
Sun:
RSun  7.0  108 m
Jupiter:
RJup  7.2 107 m

Brightness change of ~1%
REarth  6.4 106 m

Brightness change of ~0.008%
Earth:
What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000: 29 exoplanets
What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000: 29 exoplanets
Nov 2005: ~150 exoplanets
What have we learned about exoplanets?
Highly active, and rapidly changing, field
Aug 2000: 29 exoplanets
Up-to-date summary at
http://www.exoplanets.org
Now finding planets at larger
orbital semimajor axis
Nov 2005: ~150 exoplanets
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
Looking to the Future
4.
NASA: Terrestrial Planet Finder
ESA: Darwin
}
~ 2015 launch
These missions plan to use interferometry to ‘blot out’ the
light of the parent star, revealing Earth-mass planets
Looking to the Future
4.
NASA: Terrestrial Planet Finder
ESA: Darwin
}
~ 2015 launch
Spectroscopy will search for signatures of life:Spectral lines of oxygen, water
carbon dioxide in atmosphere?
Simulated ‘Earth’ from 30 light years
The Search for Extra-Solar Planets
What (or who) will we find?…