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DEPARTMENT OF PHYSICS AND ASTRONOMY
Life in the Universe:
Extra-solar planets
Dr. Matt Burleigh
www.star.le.ac.uk/~mbu
3677 Timetable
• Today and Friday 10am: MB Extrasolar
planets
• Then Derek Raine (Origins of Life)
• Then Mark Sims (Life in the solar system)
Dr. Matt Burleigh
3677: Life in the Universe
Contents
• Methods for detection
– Doppler “wobble”
– Transits
– Direct Imaging
• Characterisation
– Statistics
– Implications for formation scenarios
Dr. Matt Burleigh
3677: Life in the Universe
Useful reading / web sites
• Extra-solar planets encyclopaedia
• California & Carnegie Planets Search
• How stuff works planet-hunting page
– Includes lots of animations & graphics
• JPL planet finding page
– Look at the science & multimedia gallery pages
Dr. Matt Burleigh
3677: Life in the Universe
What is a planet?
• International Astronomical Union definition –
– An object orbiting a star
• But see later this lecture…
– Too small for dueterium fusion to occur
• Less than 13 times the mass of Jupiter
– Formation mechanism?
• Forms from a circumstellar disk
– Lower mass limit – IAU decided that Pluto should
be downgraded!
Dr. Matt Burleigh
3677: Life in the Universe
A brief history of extra-solar planets
• In the 16th century the Italian philosopher
Giordano Bruno said that the fixed stars are really
suns like our own, with planets going round them
• 1991 Radio astronomers Alex Wolszczan & Dale
Frail discovered planets around a pulsar
PSR1257+12
– Variations in arrival times of pulses suggests
presence of three or more planets
– Planets probably formed from debris left after
supernova explosion
• 1995 Planet found around nearby Sun-like star 51
Peg by Swiss astronomers Michel Mayor & Didier
Queloz using the “Doppler Wobble” method
– Most successful detection method by far, but other
methods like transits are now very successful
– 502 exoplanets in total found to date by all methods
– ~100 found since I gave this lecture last year
Dr. Matt Burleigh
3677: Life in the Universe
Planet Hunting: The Radial Velocity Technique
(“Doppler Wobble”)
• Star + planet orbit common centre of
gravity
• As star moves towards observer,
wavelength of light shortens (is blueshifted)
• Light red-shifted as star moves away
• Measure:
Dl / le = (l0-le) / le = vr / c
lo=observed wavelength, le=emitted wavelength
468 planets detected by Doppler
Wobble inc. 47 multiple systems
Dr. Matt Burleigh
3677: Life in the Universe
M* from spectral type
Dr. Matt Burleigh
3677: Life in the Universe
Doppler Wobble Method: Summary
• Precision of current surveys is now 1m/s:
– Jupiter causes Sun’s velocity to vary by 12.5m/s
– All nearby, bright Sun-like stars are good targets
• Lots of lines in spectra, relatively inactive
– Smallest planet found by this method is ~2Mearth
• Length of surveys limits distances planets
have been found from stars
– Earliest surveys started 1989
– Jupiter (5AU from Sun) takes 12 yrs to orbit Sun
– Saturn takes 30 years
• Would remain undetected
• Do not see planet directly
Dr. Matt Burleigh
3677: Life in the Universe
Doppler Wobble Method: Summary
• Since measure K (= v* sin i), not v* directly, only know
mass in terms of the orbital inclination i
• Therefore only know the planet’s minimum mass, M sin i
– If i=90o (eclipsing or transiting) then know mass exactly
Orbital
plane
i=900
Orbital
plane
i0
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• Planets observed at inclinations near 90o will transit their host stars
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• Assuming
– The whole planet passes in front of the star
– And ignoring limb darkening as negligible
• Then the depth of the eclipse is simply the ratio of the planetary and
stellar disk areas:
– i.e. Df / f* = pRp2 / pR*2 = (Rp / R*)2
• We measure the change in magnitude Dm, and obtain the stellar
radius from the spectral type
– Hence by converting to flux we can measure the planet’s radius
– Rem. Dm = mtransit – m* = 2.5 log (f* / ftransit)
• (smaller number means brighter)
Dr. Matt Burleigh
3677: Life in the Universe
Transits
Example: first known transiting planet HD209458b

–
–
–
–
–
–
Dm = 0.017 mags
So (f* / ftransit) = 1.0158, i.e. Df=1.58%
From the spectral type (G0) R=1.15Rsun
So using Df / f* = (Rp / R*)2 and setting f*=100%
Find Rp=0.145Rsun
Since Rsun=9.73RJ then
Rp = 1.41RJ
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• HD209458b more:
– From Doppler wobble method know M sin i = 0.62MJ
– Transiting, hence assume i=90o, so M=0.62MJ
– Density = 0.29 g/cm3
• c.f. Saturn 0.69 g/cm3
– HD209458b is a gas giant!
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• For an edge-on orbit, transit duration is
given by:
 Dt = (PR*) / (pa)
• Where P=period in days, a=semi-major axis of
orbit
• Probability of transit (for random orbit)
–
–
–
–
Ptransit= R* / a
For Earth (P=1yr, a=1AU), Ptransit=0.5%
But for close, “hot” Jupiters, Ptransit=10%
Of course, relative probability of detecting
Earths is lower since would have to observe
for up to 1 year
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• Advantages
– Easy. Can be done with small, cheap telescopes
• E.g. WASP
– Possible to detect low mass planets, including “Earths”,
especially from space (Kepler mission, 2008)
• Disadvantages
– Probability of seeing a transit is low
• Need to observe many stars simultaneously
– Easy to confuse with starspots, binary/triple systems
– Needs radial velocity measurements for confirmation, masses
Dr. Matt Burleigh
3677: Life in the Universe
Super WASP
• Wide Angle Search for Planets
(by transit method)
• First telescope located in La
Palma, second in South Africa
• Operations started May ‘04
• Data stored and processed at
Leicester
• >40 new planets detected!
• www.superwasp.org
• www.wasp.le.ac.uk
Dr. Matt Burleigh
3677: Life in the Universe
Super WASP
• SuperWASP monitors about 1/4
of the sky from each site
• That means millions of stars,
every night!
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection
• Imaging = spectroscopy = physics:
composition & structure
• Difficult
• Why?
– Stars like the Sun are billions of times brighter than
planets
– Planets and stars lie very close together on the sky
• At 10pc Jupiter and the Sun are separated by 0.5”
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection
• Problem 1:
– Stars bright, planets faint
• Solution:
– Block starlight with a coronagraph
• Problem 2:
– Earth’s atmosphere distorts starlight, reduces
resolution
• Solution:
– Adaptive optics, Interferometry – difficult,
expensive
– Or look around very young and/or intrinsically faint
stars (not Sun-like)
Dr. Matt Burleigh
3677: Life in the Universe
First directly imaged planet?
• 2M1207 in TW Hya
association
• Discovered at ESO VLT in
Chile
• 25Mjup Brown dwarf + 5Mjup
“planet”
• Distance ~55pc
• Very young cluster ~10M
years
• Physical separation ~55AU
• A brown dwarf is a failed
star
– Can this really be called a
planet?
– Formation mechanism may
be crucial!
Dr. Matt Burleigh
3677: Life in the Universe
First directly imaged planetary system
• Last year 3 planets imaged
around the star HR8799
• 130 light years away (40pc)
• Three planets at 24, 38 and 68AU
separation
– In comparison, Jupiter is at 5AU
and Neptune at 30AU
• Masses of 7Mjup, 10Mjup and
10Mjup
• Young: 60Myr
– Earth is ~4.5Gyr
Dr. Matt Burleigh
3677: Life in the Universe
Fomalhaut (alpha Piscis Austrini)
• One of the brightest stars in
the southern sky
• Long known to have a dusty
debris disk
• Shape of disk suggested
presence of planet
• 2Mjup planet imaged by HST
inside disk
• 200Myr old
• Like early solar system
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection: White Dwarfs
• White dwarfs are the end state of stars like the Sun
– What will happen to the solar system in the future?
• WDs are 1,000-10,000 times fainter than Sun-like
stars
– contrast problem reduced
•
Outer planets should survive evolution of Sun to
white dwarf stage, and migrate outwards
– more easily resolved
•
Over 100 WD within 20pc
– At 10pc a separation of 100AU = 10” on sky
•
At Leicester we are searching for planets around
nearby WD with 8m telescopes and the Spitzer
space telescope
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection: White Dwarfs
• No planets yet, just brown dwarfs
– Currently limited to finding planets >5Mjup
– Not very common
– May have to wait for JWST
• But have found some WDs are
surrounded by dust and gas disks
– Remains of small rocky planets and
asteroids that strayed too close to WD
– Ripped apart by tidal forces
– Can study composition of extra-solar
terrestrial bodies!
Dr. Matt Burleigh
3677: Life in the Universe
What we know about
extra-solar planets
•
•
•
•
502 planets now found
52 multiple systems
106 transiting planets
Unexpected population
with periods of 2-4 days:
“hot Jupiters”
• Planets with orbits like
Jupiter discovered
(eg 55 Cancri d)
• Smallest planet:
CoRoT-7b - 1.7Rearth
Dr. Matt Burleigh
3677: Life in the Universe
Extra-solar planet period distribution
• Notice the “pile-up”
at periods of 2-4
days / 0.04-0.05AU
• The most distant
planets discovered
by radial velocities
so far are at 5-6AU
• Imaging surveys
finding very wide
orbit planets
Dr. Matt Burleigh
3677: Life in the Universe
“Hot Jupiter” planets
• Doppler Wobble and transit surveys
find many gas giants in orbits of 2-4
days
– cf Mercury’s orbit is 80 days
• Surveys 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 planet so far:
5.5xMEarth
• 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
What we know about extra-solar planets
Eccentricity vs semi-major
axis
:
- large distribution of e
(same as close binary stars)
extra-solar planets
solar system planets
Dr. Matt Burleigh
observational bias
most extra-solar planets are
- in much more eccentric orbits
than the giant planets in the
solar system
- planets close to the star are
tidally circularized
- but some planets in circular
orbits do exist far away from star
- the planets in our own system
have small eccentricities ie
STABLE
3677: Life in the Universe
Results of the Planet Hunting surveys
• Of 2000 stars surveyed
– 5% have gas giants between
0.02AU and 5AU
– 10% may have gas giants in
wider orbits
– <1% have Hot Jupiters
• 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
• Stars that host planets appear to be on
average more metal-rich
• 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, ~5% 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 planets
– ~3% of stars with 1/3rd of the Sun’s metallicity have planets
•
Does this result imply that planets more easily form in metal-rich environments?
– If so, then maybe planet hunters should be targeting metal-rich stars
– Especially if we are looking for rocky planets
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 non-gravitational 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.
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
Hunting for Earth-like planets
• Pace of planet discoveries will continue to increase in next
few years
• Radial velocity and direct imaging surveys will reveal outer
giant planets with long periods like our own Solar System
• Transit surveys will reveal small planets in close orbits to
their suns
• But the greatest goal is the detection of other Earths
Dr. Matt Burleigh
3677: Life in the Universe
Towards other Earths
Telescope
Method
Date
Corot (Fr)
Transits
2007
Kepler (NASA)
Transits
2008
GAIA (ESA)
Astrometry
2012
SIM (NASA)
Astrometry
2015 (?)
Plato (ESA)
Transits
2017
Darwin (ESA)
Imaging
2025+ (?)
42m E-ELT
Imaging
2018
Dr. Matt Burleigh
3677: Life in the Universe
Kepler
•
•
•
•
•
Searching for Earths by transit
method
Launched last year by NASA
Aims to find an Earth around a Sunlike star in a one year orbit
Need three transits to confirm
So mission lasts at least three
years…
Dr. Matt Burleigh
3677: Life in the Universe
Towards Other Earths: Habitable Zones
• Habitable zone defined as where liquid water exists
• Changes in extent and distance from star according to star’s
spectral type (ie temperature)
Dr. Matt Burleigh
3677: Life in the Universe
Towards Other Earths: Biomarkers
• So we find a planet
with the same mass as
Earth, and in the
habitable zone:
– How can we tell it
harbours life?
• Search for biomarkers
– Water
– Ozone
– Albedo
Dr. Matt Burleigh
3677: Life in the Universe