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DEPARTMENT OF PHYSICS AND ASTRONOMY
3677 Life in the Universe:
Extra-solar planets
Dr. Matt Burleigh
www.star.le.ac.uk/mrb1/lectures.html
Course 3677 Life in Universe 2013/2014 Academic Year
Course Given by Prof. Mark Sims and Dr. Matt Burleigh
Topics: Life in Universe and Extra-Solar Planets
Lecture Dates and Lecturer
Actual
Lecture
Number
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13
Lecture by
Topic
Locatio
n
Time
Date
Nominal Course
Order 2013/14
M.R. Sims
M. Burleigh
M. Burleigh
M. Burleigh
M. Burleigh
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M. Burleigh
Both
Life in Universe
Extra-Solar Planets
Extra-Solar Planets
Extra-Solar Planets
Extra-Solar Planets
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Extra-Solar Planets
Continuous
Assessment Answers
Revision Lectures
Phys A
KE LT2
Phys A
Phys A
KE LT2
Phys A
Phys A
KE LT2
Phys B
KE LT2
Phys B
KE LT2
Phys B
1300
1300
1300
0900
1300
1300
0900
1300
1100
1300
1100
1300
1100
5/11
8/11
12/11
13/11
15/11
19/11
20/11
22/11
27/11
29/11
4/12
6/12
11/12
1
8
9
10
11
2
3
4
5
6
7
12
13
R1
R2
M. Burleigh
M.R. Sims
Phys D
Phys B
0900
0900
7/5/14
15/5/14
Please note course is not in nominal order due to availability of Lecturers
Course has been extensively revised from previous years as Prof. Raine is no longer teaching part of
the course, consequently exam format has changed to 4 questions two short (20 marks each), two
long (30 marks each) all compulsory.
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 1
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–
–
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Definition of a planet
A little history
Pulsar planets
Doppler “wobble” (radial velocity) technique
• Lecture 2
– Transiting planets
– Transit search projects
– Detecting the atmospheres of transiting planets:
secondary eclipses & transmission spectroscopy
– Transit timing variations
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 3
–
–
–
–
Microlensing
Direct Imaging
Other methods: astrometry, eclipse timing
Planets around evolved stars
• Lecture 4
– Statistics: mass and orbital distributions, incidence of solar
systems, etc.
– Hot Jupiters
– Super-Earths
– Planetary formation
– Planetary atmospheres
– The host stars
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 5
– The quest for an Earth-like planet
– Habitable zones
– Results from the Kepler mission
• How common are rocky planets?
• Amazing solar systems
– Biomarkers
– Future telescopes and space missions
Dr. Matt Burleigh
3677: Life in the Universe
Useful numbers
•
•
•
•
RSun = 6.995x108m
Rjup = 6.9961x107m ~ 0.1RSun
Rnep = 2.4622x107m ~ 4Rearth
Rearth = 6.371x106m ~ 0.1Rjup ~ 0.01RSun
•
•
•
•
MSun= 1.989x1030kg
Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth
Mnep= 1.02x1026kg ~ 5x10-5MSun ~ 0.05Mjup = 17.15Mearth
Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup
• 1AU = 1.496x1011m
• 1 day = 86400s
Dr. Matt Burleigh
3677: Life in the Universe
Exoplanet count 10/11/13
(exoplanet.eu)
• 1039 confirmed planets
– In 787 planetary systems
–
–
–
–
–
173 multi-planet systems
873 radial velocity detected planets
425 transiting planets
41 directly imaged
“Confirmed” = have “measured” masses
• Unexpected population with periods
of <1 to ~4 days: “hot Jupiters”
• Planets with orbits like Jupiter
discovered (eg 55 Cancri d)
• Smallest planets:
– Kepler-20e: 0.87Rearth ,
– Alpha Cen Bb M sin i > 1.1Mearth
Dr. Matt Burleigh
3677: Life in the Universe
Hit 1000 exoplanet mark
Transiting planets in blue
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Eccentricity of exoplanet orbits
Solar systems with highly eccentric planets may be bad news for life
Dr. Matt Burleigh
3677: Life in the Universe
Extra-solar planet period distribution
• Notice the “pile-up”
at periods of <1 to ~4
days / 0.04-0.05AU
• The most distant
planets discovered
by radial velocities so
far are at 5-6AU
• Imaging surveys
finding very wide
(>10AU) orbit planets
• Orange are “hot
Jupiters”
• Yellow is Jupitermass in Jupiter-like
orbits
Dr. Matt Burleigh
3677: Life in the Universe
Selection effects
• Astronomical surveys tend to have built in biases
• These “selection effects” must be understood before
we can interpret results
– The Doppler Wobble method is most sensitive to massive,
close-in planets, as is the Transit method
– Imaging surveys sensitive to massive planets in very wide orbits
(>10AU)
• These methods are not yet sensitive to planets as small
as Earth, even close-in
• As orbital period increases, the Doppler Wobble method
becomes insensitive to planets less massive than
Jupiter
• The length of time that the DW surveys have been
active (since 1989) sets the upper orbital period limit
– But imaging surveys can find the widest planets
Dr. Matt Burleigh
3677: Life in the Universe
“Hot Jupiter” planets
• Doppler Wobble and transit surveys
find many gas giants in orbits of <1 to
~4 days
– cf Mercury’s orbit is 80 days
• These survey methods are biased
towards finding them
– Larger Doppler Wobble signal
– Greater probability of transit
• These planets are heated to >1000oF
on “day” side
– And are “tidally locked” like the Moon
– Causes extreme weather conditions
Dr. Matt Burleigh
3677: Life in the Universe
Extra-solar planet mass distribution
• Mass distribution peaks at 12 x mass of Jupiter
• Lowest mass confirmed
planet so far: Alpha Cen Bb
M sin i =1.1xMEarth
• Super-Jupiters (>few MJup)
are not common
– Implications for planet
formation theories?
– Or only exist in number at
large separation?
– Or exist around massive stars?
Dr. Matt Burleigh
3677: Life in the Universe
A continuum of planet mass
1’000’000
Red box indicates “Super-Earths”
Dr. Matt Burleigh
3677: Life in the Universe
Transiting planets in blue
Red box indicates “Super-Earths”
Dr. Matt Burleigh
3677: Life in the Universe
Super-Earths
• In the solar
system, there is
no planet with a
mass and radius
between that of
Earth and
Neptune/Uranus
• But we see
many such
exoplanets
• What are they?
Gas giants,
terrestrial, or
something else?
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
What are exoplanets
made of?
?
?
?
hydrogen/helium envelope
thin atmosphere
ice mantle/volatile envelope
solid core (rocks+metals)
Dr. Matt Burleigh
3677: Life in the Universe
?
What are exoplanets
made of?
telluric
super-Earths?
gas dwarfs?
?
ocean planets?
mini Neptunes?
hydrogen/helium envelope
thin atmosphere
ice mantle/volatile envelope
solid core (rocks+metals)
Dr. Matt Burleigh
3677: Life in the Universe
What are exoplanets
made of?
telluric
super-Earths?
gas dwarfs?
?
HD
149026b
ocean planets?
mini Neptunes?
hydrogen/helium envelope
thin atmosphere
ice mantle/volatile envelope
solid core (rocks+metals)
Dr. Matt Burleigh
3677: Life in the Universe
How common are gas giants?
• Radial velocity surveys
– ~10% of FGK stars have gas
giants between 0.02AU and
5AU
– At least 20% have gas giants
in wider orbits
• Known population will grow as
radial velocity surveys cover
longer periods, & direct imaging
improves
– <0.1% have Hot Jupiters
• Hot Jupiters are easy to discover,
but in fact are rare
• How many have
Earths…..?
Dr. Matt Burleigh
3677: Life in the Universe
What about the stars themselves?
• Surveys began by targeting sun-like
stars (spectral types F, G and K)
• Now extended to M dwarfs (<1 Msun) and
subgiants (>1.5Msun)
– Subgiants are the descendants of A stars
• Incidence of planets is greatest for late F
stars
– F7-9V > GV > KV > MV
• More massive stars tend to have more
massive planets
Dr. Matt Burleigh
3677: Life in the Universe
Metallicity
The abundance of elements
heavier than He relative to the Sun
• Overall, ~10% of solar-like stars have radial velocity –detected Jupiters
• But if we take metallicity into account:
– >20% of stars with 3x the metal content of the Sun have gas giants
– ~3% of stars with 1/3rd of the Sun’s metallicity have gas giants
•
Does this result imply that planets more easily form in metal-rich environments?
–
–
Possibly true for gas giants
But Kepler results suggest super-Earths & terrestrial planets equally common around lower metallicity
stars!
Dr. Matt Burleigh
3677: Life in the Universe
Planet formation
scenarios
• There are two main models which have been proposed to
• describe the formation of the extra-solar planets:
– (I) Planets form from dust which agglomerates into cores which then
accrete gas from a disc.
– (II) A gravitational instability in a protostellar disc creates a number of
giant planets.
• Both models have trouble reproducing both the observed
distribution of extra-solar planets and the solar-system.
Dr. Matt Burleigh
3677: Life in the Universe
Accretion onto cores
•
•
•
•
•
•
•
Planetary cores form through
the agglomeration of dust into
grains, pebbles, rocks and
planetesimals within a gaseous
disc
At the smallest scale (<1 cm)
cohesion occurs by nongravitational forces e.g.
chemical processes.
On the largest scale (>1 km)
gravitational attraction will
dominate.
On intermediate scales the
process is poorly understood.
These planetesimals coalesce
to form planetary cores
The most massive cores
accrete gas to form the giant
planets
Planet formation occurs over
107 yrs.
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational instability
• A gravitational instability requires a sudden
change in disc properties on a timescale
less than the dynamical timescale of the
disc.
• Planet formation occurs on a timescale of
1000 yrs.
• A number of planets in eccentric orbits may
be formed.
• Sudden change in disc properties could be
achieved by cooling or by a dynamical
interaction.
• Simulations show a large number of planets
form from a single disc.
• Only produces gaseous planets – rocky
(terrestrial) planets are not formed.
• Is not applicable to the solar system.
• Could explain the directly imaged HR8799
system
Dr. Matt Burleigh
3677: Life in the Universe
Where do the hot Jupiters come from?
•
•
No element will condense within ~0.1AU of a star
since T>1000K
Planets most likely form beyond the “ice-line”, the
distance at which ice forms
– More solids available for building planets
– Distance depends on mass and conditions of protoplanetary disk, but generally >1AU
•
Hot Jupiters currently at ~0.03-0.04AU cannot have
formed there
– Migration: Planets migrate inwards and stop when
disk is finally cleared
•
If migration time < disk lifetime
– Planets fall into star
– Excess of planets at 0.03-0.04AU is evidence of a
stopping mechanism
• tides? magnetic cavities? mass transfer?
•
Large planets will migrate more slowly
– Explanation for lack of super-Jupiters in close orbits
Dr. Matt Burleigh
3677: Life in the Universe