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What do we know about
extrasolar planets?
Neill Reid
STScI
The big questions
Over 145 extrasolar planets in more than 125 stellar systems
have been discovered since 1995
-this includes several multiplanet systems
What are the properties of those planetary systems?
What is the likely frequency of planetary systems in the
Milky Way Galaxy?
Outline
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A brief introduction to the Milky Way
How to find planets
What do we know about extrasolar planetary systems
The Milky Way
Spiral galaxy:
halo – old metal-poor population
near-spherical, R~80,000 l.y.
disk – flattened, rotating system
radius ~ 40,000 l.y.
Sun lies ~27,000 l.y. from Centre
bulge – central condensation
radius ~ 3000 l.y.
The astronomer’s periodic table
In astronomy,
metals = everything but H, He
Metals are formed by stars
Up to Fe by fusion
Above Fe, supernovae
 elements (O, Mg, Ti, etc) are
enhanced in stars with low
overall metal abundance
 capture elements formed
preferentially by massive
supernovae  enhanced 
indicates early formation
The Halo
First stars to form in the Milky Way during the initial gravitational collapse
near spherical distribution with no net rotation – “pressure supported”
halo stars have metal content from 0.001 to 10% that of the Sun
May be an inner halo (initial collapse) + an outer halo (accreted satellite galaxies)
Halo contributes less than 1% of the total mass of the Galaxy
The disk
The disk is a flattened, rotating population
Formed by dissipational collapse
~10% total mass of Milky Way
All current star formation located in
Galactic disk
Approximately half gas/dust & half stars
Building the Milky Way
Big Bang, t~ 13 Gyrs
Density perturbations (regions with enhanced dark matter density)
start to collapse shortly after initial expansion
3. First stars in the Milky Way form during early stages of collapse at
t~ 12 Gyrs – globulars, halo stars and central bulge
4. Galactic disk forms at t~ 10-11 Gyrs; conservation of angular
momentum gives a flattened, rotating system; high gas density,
socontinuing star formation
5. Merger with major satellite at t~ 9-10 Gyrs; initial disk is heated to
form a “thick disk” – hiatus in star formation
6. Disk reforms at t~ 8-9 Gyrs and star formation resumes
7. Continued accretion of small satellite galaxies as Milky Way builds
its outer halo
Total mass ~ few x 1011 solar masses:
<1% halo+bulge;
10% disk;
~90% dark matter
1.
2.
The Solar Neighbourhood
The Sun lies in a relatively low
density region, diameter ~300 l.y.
Average stellar separation ~ 3 l.y.
Almost all stars known to have
planets lie within 100 l.y. of the
Sun
Planet formation (classical)
1.
2.
3.
4.
5.
Collapse of protostellar core in
giant molecular cloud
Central star surrounded by
protoplanetary disk of gas and dust
Fragmentation within the disk leads
to formation of planetismals, then
planetary embryos, then planetary
cores.
Cores beyond the ice line (~ 4 AU)
accrete further material to become
gas giants (time scale < 107 years);
rocky terrestrial planets in inner
Solar system
Gas and dust clears (save for
residual zodiacal dust) to leave
current Solar System
Making planets
Considerable evidence for dusty
protoplantary disks around young
stars
Gas & dust to age ~10 Myrs
Residual dust disks to ~100 Myrs
Zodiacal dust in older stars (Sun)
The Solar System
Dimensions:
Inner solar system, r < 4 AU
Terrestrial planets and the
asteroid belt: rocks
Masses < 0.3% MJ
Gas giants, 5 < r < 30 AU
Jupiter to Neptune
Masses: 7% MJ to 1 MJ
Kuiper belt, 30 < r < 120 AU
Pluto and beyond: icy bodies
Masses < 0.05% MJ
Oort comet cloud, r ~ 105 AU
Extrasolar expectations
Terrestrial planets at
small radii; gas giants
beyond the “snow line”
Planets on low eccentricity
coplanar orbits 
Pluto has highest inclination
at ~ 17o.
Finding planets: Doppler surveys
Star+planets orbit
common centre of
gravity
Detect reflex
motion of primary
Requires high
resolution (Jupiter
induces ~12 m/s
velocity in Sun)
Require bright
target stars (< 9th
magnitude even with
Keck 10-metre)
Doppler planets
Main limitation – only measure
projected mass, semi-major
axis
Unexpected planets: Hot Jupiters
First planetary discovery confounded
expectations:
Instead of jovian analogue, 51 Peg b
is a gas giant, orbiting around its
parent star with a period of 4.2 days
Orbit lies well within Mercury’s in
the Solar System
Gas giants form by accreting ices
(water ice, CO2, etc) onto a rocky
core – but ices can’t exist within the
“ice line”  ~ 4-5 AU
How can a gas giant form where the
ambient temperature is ~1300K?
Hot Jupiters: migration
Answer: hot jupiters don’t form at
their present radii:
formation occurs beyond the
ice line
subsequent migration through
the disk due to angular
momentum exchange with the
disk i.e. energy loss in viscous
disk
Multiple systems
Several systems have more than one
known planet
At least 15 with 2 planets
Lowest mass planet detected so far
has M~30 Earth masses
Finding planets: Transits
Main alternative to Doppler surveys
Use photometric monitoring to search
for periodic variations indicating an
eclipsing planet
Advantage  known inclination
Disadvantage  need high precision
Jupiter gives ~1% dip; Earth ~0.01%
HD 209458
Planet detected by
Doppler motions  a hot
Jupiter
Photometry revealed that
the orbital inclination is
low enough that the
planet transits the primary
star.
 Mass = 0.69 Mjupiter
HST observations show
Radius ~ 1.34 Rjupiter
Sodium detected in the
planetary atmosphere
Other transit surveys
Photometric surveys need to cover tens of thousands of stars per night
with millimagnitude accuracy to optimise chances of detecting a
handful of transiting systems
CCDs are detector of choice (relatively large format + reliable data)
Mounted on moderate-sized telescopes (~1 metre), can achieve field
of view of a few square degrees (~10 times the size of the Moon)
Target stars are much fainter than Doppler surveys, and therefore at
larger distances
e.g. TrES-1 ~12th magnitude
~100 times fainter than typical Doppler and d~600 l.y.
OGLE Galactic Bulge surveys – 15th-17th magnitude, d ~ 4000 l.y.
Different types of stars in different environments
 also fainter, so more difficult to get detailed observations, but
Radii are closer to 1.05-1.1 Rjupiter
Future: KEPLER satellite will observe 100,000 stars with sufficient
photometric accuracy to detect terrestrial transits
Astrometric searches
Planetary companions also
produce a `wobble’ in the parent
star’s position on the sky
As viewed from ~30 l.y., Jupiter
causes the Sun to move by 1
milliarcsecond (< 10-6 diameter of
the full moon)
requires extremely precise
positional measurement
No planets detected to date (but
HST verification of 2 systems)
Future: space missions, notably
SIM Planetquest
Microlensing
Detect planets by their ability to
focus light gravitationally
Planet produces short-period
spike in microlensing light curve
One candidate discovered to date
 probably a jovian-mass planet
around an M dwarf halfway
beftween us and the Galactic
Centre
Advantage: sampling different
environments
Disadvantage: Detailed follow-up
observations impossible
Current statistics
• 143 planets in 125 ‘normal’ planetary systems
• 5 discovered in transit surveys
• 1 discovered through gravitational microlensing
• Remaining 119 systems from radial velocity surveys
• Lowest mass planet: Gl 436b, M~0.067 MJ or ~21 ME –
Neptunian mass companion of a nearby M dwarfs
• Longest period systems: P ~ 8 years, a ~ 4.2 AU
What stars have planets?
Most stars with detected planets are
similar to the Sun –
Masses ~ 0.8 to 1.2 Msun
Most are H-burning main sequence
stars – some giants and 2 lowmass M dwarfs
 Mainly reflects biases in radial
velocity surveys – we’re looking
for planetary systems like our own
(and solar type stars are good
targets for Doppler surveys)
Planets or brown dwarfs?
Early planetary detections
attracted some skepticism
 are they just very lowmass stars and brown
dwarfs in low inclination
orbits?
No !
Solar type stars show
evidence for a ‘brown
dwarf desert’ at small
separations
 Planets are distinct
from stellar/BD
companions
The mass distribution
Mass distribution of known planets rises sharply in number as mass falls
below ~20 Mjupiter; flattens at ~4 Mjupiter (M  M-1); and declines below 1
Mjupiter (almost certainly a selection effect).
Planetary orbits
Solar system planets have
low eccentricity
Extrasolar planets show
much wider range of e, save
at short period (tidal
circularisation)
Probably reflects dynamical
interactions in multi-planet
systems
 Solar system may be
slightly unusual
Planets and metals
Planets are much more
common around metal-rich
stars
Not unexpected, since
planets have higher metal
content than the parent stars
Even so, ~5% of sun-like stars
have planetary companions
 ~1.1 million sub-like stars
with planets at Galactic radii
from 24,000 to 30,000 l.y.
Habitability
The conventional Habitable Zone:
Ambient temperature allows liquid water on planetary surface
– ~0.8 to 1.5 AU for the Sun, Earth-like atmosphere
– Depends on temperature of central star & planetary atmosphere
Ambient temperatures
Planetary temperatures depend
on:
1. Temperature of central star
2. Distance from central star
3. Planetary atmosphere
1+2  sub-stellar temperature
~40 known planets spend at
least part of their orbit
within the habitable zone
of their primary
All of these planets are gas
giants – but what about
satellites?
Summary
1. ~150 planets around ~135 stars currently known
2. Most are companions of stars within the immediate
Solar Neighbourhood
3. Metal-rich stars are significantly more likely to have
jovian-mass planetary companions. Are lower-mass
planets more common at lower [m/H]?
4. Despite this trend, planetary companions to solar-like
stars are not rare  ~5% locally
A place in the Sun