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Extra-Solar Planets
Planetary Physics
Summer Term 2004
Lecture 10
Or: How to get on the Big
Breakfast Show...
Extra-Solar Planets
Since October 1995, more than 120 planets
have been detected around more than 100
stars like the Sun - why the sudden success?
 None has been seen directly: all found by
‘wobble’ of parent star caused by orbiting
planet - how?
 None is like the Earth: all have masses
more like Jupiter - why?
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Extra-Solar Planets
Definitions:
 Star: > 0.08 solar masses (80 jupiter masses),
powered by H fusion; formed by gravitational
collapse, occur singly or in multiples
 Brown Dwarf: 13 jup < mass < 80 jup, some
initial D fusion, then fade; formation and
occurrence as normal stars
 Planets: < 13 jup, no nuclear power source;
formed by accretion from protoplanetary disc
left behind from formation of parent star
In these HST
pictures we
see disks
around stars,
but also jets of
material being
blown out
again,
especially in
the lowest
picture (the
‘Rotten Egg’
nebula).
Planets form
from the
disks.
Extra-Solar Planets
Methods of detection:
Astrometry - long history, no detections
Radial velocity surveys - very successful
Brightness variations
– planet passes in front of star: drop in brightness
– gravitational lensing of star by planet: increase in
brightness - only way to detect earth-mass planets
Interferometry - ground and space missions
Extra-Solar Planets
Astrometry:
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Very precise measurements of position of parent star,
relative to inertial frame, repeated over many years
Centre of mass of system moves in straight line
If planet exists, star’s motion will have slight
‘wobble’ about CoM motion - can find size and
period of orbit, and mass of planet
Massive planets in wide, long-period orbits give
largest wobble; nearby stars are best, but effects still
too small to measure with present equipment
Extra-Solar Planets
Radial velocity surveys:
 Very precise measurements of radial
velocity of parent star, using doppler effect
 Single star will have constant velocity, but if
companions are present their orbital
motions will be reflected in motion of star
about CoM - look for periodic oscillations
in radial velocity
Extra-Solar Planets
Limits of radial velocity measurements:
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Can find period, eccentricity and MP sin i, where i
is inclination of orbit to line of sight (just measure
the radial velocity, = V1 sin i, where V1 is the
orbital motion of the star – see Lecture 5)
Can not measure i : need transits for this
Motion largest for large masses, short periods
Limit set by oscillations in star’s size (2-3 m/s)
Earth mass in 1-yr orbit gives only 0.1 m/s
Extra-Solar Planets
Gravitational lensing:
 Parent star (‘lens’) focuses light of
background star as it passes in front of it:
Observer
Star
Lens
(and planet)
time
 Planet causes blip in slow light variation
Extra-Solar Planets
History I:
 1950 - 1970: various false astrometric
claims of Jupiter size companions, e.g. to
Barnard’s star (van de Kamp)
 1980s: pioneering radial velocity survey by
Campbell & Walker - precision of 13 m/s,
but no detections (small sample - only 21
stars)
Extra-Solar Planets
History II:
 Late 1980s, early 1990s: several large, high
precision RV surveys began - Marcy &
Butler (Lick), Mayor & Queloz (OHP),
Cochran & Hatzes (McDonald)
 1995 October 6: first extrasolar planet
round a ‘normal’ star announced by Swiss
team (2 found earlier around a pulsar)
Marcy and Butler found this ‘wobble’ in the motion of the star
51 Pegasi, and were able to find a period of just over 4 days.
This shows the same observations, but now with all the
different orbits superimposed to show the variation better.
Extra-Solar Planets
What did we expect?
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small mass planets (Earth-like) close to star:
radiation should evaporate most abundant element,
hydrogen, leaving small rocky planets
massive gaseous planets (Jupiter-like: mainly
hydrogen) far enough out to avoid evaporation - so
expect long periods
circular orbits, as in solar system
stars like the Sun
Extra-Solar Planets
What did we find?
 massive planets close to star!
 many eccentric orbits!
 no earth-like planets
 stars generally richer in heavy elements
(such as iron) than the Sun
Of the more than 100 found so far, at least 10 of
the early one were around targets suggested by
Kevin Apps (Sussex u.g.)
Extra-Solar Planets
Kevin’s involvement:
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November/December 1997 - e-mailed Marcy
requesting list of 300 targets for Keck 10-m, found
30 unsuitable and suggested 30 replacements
Became responsible for generating new targets (in
1999, nearly 400 of the 900 targets were his) and for
searching the literature for their properties
April 1999: went to San Francisco to observe with a
Lick Observatory telescope
August 1999: observed on Hawaii (Keck)
May do PhD with Marcy from 2004
Extra-Solar Planets
Current results:
 122 planets with masses 0.12 to 16.9 jup
 41 ‘hot Jupiters’
(Jupiter mass, close to star: a < 0.4 AU)
 57 ‘Jupiter analogues’
(Jupiter mass, far from star: P > 1 yr)
 70 ‘eccentric’ planets
(high eccentricity orbits: e > 0.2)
 13 systems with multiple planets (2 have 3)
The first 22 planets were very close to their parent stars
Three planets around Upsilon
Andromedae: first ‘solar system’
Semi-major axes of planetary orbits
35
Number of planets
30
25
20
15
10
5
0
0
1
2
3
4
5
6
Semi-major axis (in AU)
The majority of the planets found so far have orbital radii less than
1 AU: 35% of them are less than 0.4 AU, although there is a
growing number with larger radius, including one ‘beyond Jupiter’.
Mass distribution for extra-solar planets
25
Number of planets
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
M sin i (in Jupiter masses)
The majority of planets (~60%) have masses between 1 and 8
Jupiter masses, but there are now many known (~30%) with
masses less than that of Jupiter (down to 0.12 jup: ~0.4 sat). But
only lower limits are known without knowing i: needs transits.
7 November 1999: first-ever planetary transit
observed for sun-like star HD 209458 by
Henry, Marcy, Butler and Vogt
The radial
velocity group of
Marcy, Butler
and Vogt found
this evidence of a
wobble in the
motion of the star
and alerted Greg
Henry to observe
its brightness.
This is the 0.8m Automatic
Photoelectric Telescope of
the Tennessee State
University’s Fairborn
Observatory in southern
Arizona, which was used
by Greg Henry to observe
the planetary transit.
An attempt to verify the
result on November 14 was
clouded out, but other
groups later verified it.
Average of four HST transit observations
Three more transits have now been detected, in the
microlensing surveys, and one genuine microlensing
event:
One of the three transit events….
….and the microlensing event
Extra-Solar Planets
The Future:
 Continuing radial velocity searches (lower mass
planets? more ‘solar systems’?)
 Transits – now four cases
 Gravitational lensing
 Ground-based astrometry
 Direct imaging? Difficult from the ground
 Space-based imaging (e.g. DARWIN)
 Space-based astrometry (e.g. GAIA)
An artist’s impression
of the proposed
6-spacecraft Darwin
mission, due to fly in
about 2010. Each craft
contains a large IR
telescope, and they are
spread over distances
of 40 to 500 metres.
Extra-Solar Planets
Where next?
Who knows!