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 outline
• Lecture 1
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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
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–
–
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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
Blue: radial velocity, Green: transiting, Red: microlensing,
Orange: direct imaging, Yellow: pulsar timing
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• When a foreground star passes in front of a background star, light
from the background star is bent by the gravitational field of a
foreground lens to create distorted, multiple and/or brightened
images
– Consequence of general relativity
• The milli-arcsecond separation between multiple images is too
small to be resolved by modern telescopes. The combined light of
all images is instead observed as a single image of the source
• The brightness of the combined image is a function of the projected
separation of the source and lens on the sky, and changes as the
source, lens and observer move relative to one another
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• If the lens is a single, isolated object, the lightcurve of the background
source is simple, smooth and symmetric.
• The background star appears to brighten and then dim as the
projected separation between the source and lens first decreases and
then increases.
• For sources and microlenses in our own Galaxy, a typical timescale
for the detectable rise and fall of the apparent brightness of the source
star is weeks to months.
• The basic shape is the same regardless of the relative path the source
takes on the sky; the amplitude of the the lightcurve is determined by
the minimum angular separation between the lens and source in units
of the Einstein radius, ie θLS/θE .
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• If the star has planets, the magnification pattern experienced by a
background source is no longer circularly symmetric on the sky
• The combined gravitational field of the star and planet can create strong
deviations in the lensing pattern, called caustics
– This means that the changes in the lightcurve of the background source can
be quite dramatic if it does happen to cross the planet-affected area, even
for Earth-sized planets.
– In the diagram, the red patches are the caustics and P indicates the position
of the planet
• Because the planet has a gravitational mass that is much smaller than
that of the lensing star, the percentage of the lensing pattern area
influenced by the planet will be relatively small.
– This means that the probability that the source will cross the planet-affected
area is low, and thus the chance of detecting a planet by microlensing is
also low,
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• Beginning in the 1990s, millions
of stars have been monitored
every night in search of the few
that are microlensed at that time
– 30 planets found
• Microlensing gives the mass ratio
between the planet and its parent
star, q=Mp/M∗ , and the angular
separation between the planet
and star on the sky at the time of
the lensing event, θ∗,p/θE , in units
of the Einstein ring radius
– M* is obtained from the spectral
type of the lensing star
– The star’s proper motion gives
the time to cross the Einstein ring
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• Advantages
– Can detect Earth-size
planets across Galaxy
– Can detect planets in other
galaxies
• Disadvantages
– Must monitor millions of
stars constantly (eg in
galactic bulge)
– Lensing event never
repeats
– Star too far away to study
planet again
Dr. Matt Burleigh
3677: Life in the Universe
Astrometry
• The motion of a star around the centre
of mass of a star-planet system can be
detected by repeatedly measuring the
position of the star on the sky
• The amplitude of the motion in micro
arseconds (10-6 arcsec) is given by:
-1
æ q öæ a öæ d ö
DQ = 0.5ç -3 ÷ç
֍
÷
è 10 øè 5AU øè 10 pc ø
• Where q=(Mpl/M*), a = semi-major axis
of the orbit in AU and d = distance to
star in pc
• For Jupiter at 5AU, the amplitude of the
Sun’s motion as seen from another star
is ~5x10-4 arcsec (right)
• GAIA will discover ~10,000 Jupiters at
1-4AU around stars up to 200pc away
Dr. Matt Burleigh
3677: Life in the Universe
Other methods
• Pulsation timing:
– Many stars, like white dwarfs, have
very stable pulsation modes. The
presence of a planet will be
revealed in anomalous timings, just
as with pulsar planets
• Eclipse timing:
– Close, eclipsing binary systems
can also reveal the presence of
planets through anomalous timings
of the expected eclipses
– A good example is the close
eclipsing white dwarf + red dwarfs
binary NN Ser, which appears to
have 2 Jupiter mass planets
orbiting it
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
• In 2008 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
• Over 100 WD within 20pc
– At 10pc a separation of 100AU = 10” on sky
– Planets should be located well away from the host
white dwarf
•
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
Log(L/Lsun)
PN ejected
to WD
Thermal Pulse begins
4
AGB
2
He
Helium Flash
C+O
RGB
H
0
He
WD
cooling
4.2
3.8
3.4
Log Teff (K)
Dr. Matt Burleigh
3677: Life in the Universe
The end of our solar system
Dr. Matt Burleigh
3677: Life in the Universe
The end of our solar system
• The inner planets, Mercury, Venus and probably Earth, will be destroyed
by the expanding red giant
– As a red giant (actually, asymptotic giant), the Sun’s radius will be ~1AU
• Mars, the asteroids and outer gas giants will survive
• As the red giant loses mass when it evolves to the planetary nebula
stage, the outer planets orbits evolve outwards by factor:
• Jeans (1924):
M MS
M WD
• Where MMS and MWD are the main sequence and white dwarf masses in solar mass
units (Msun)
• Note: the relationship between a star’s mass and a white dwarf’s mass is
given by:
MWD = 0.12M MS + 0.36
• This is called the “initial-final mass relation”, and is derived from observations of white
dwarfs in clusters (Casewell et al., 2009, MNRAS, 395, 1795)
Dr. Matt Burleigh
3677: Life in the Universe
Spitzer 4.5micron image
GJ3483 (LTT3059 / WD0806-661)
130” /
2500AU
I maybe a planet…
or a brown dwarf
Dr. Matt Burleigh
3677: Life in the Universe
I am the
white dwarf
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Proper motion
WD
Companion
• PM error +/-25mas/yr
Dr. Matt Burleigh
3677: Life in the Universe
Calculating the planet’s mass
•
How can we estimate the mass of a directly imaged planet?
– Planets of identical mass are assumed to be born with identical temperatures, &
cool with age (no nuclear burning in core)
– Thus by measuring their brightness, and estimating the host star’s age &
distance, we can use a theoretical “evolutionary model” to convert the brightness
to a mass!
•
Method:
– (1) measure it’s brightness from the image
– (2) determine the star’s distance (eg from it’s spectral type if its main sequence,
or better still from its parallax)
– (3) convert the star’s apparent mag to absolute mag
– (4) estimate the star’s age (eg from it’s rotation period, or if it belongs to an open
cluster or coeval moving group)
– (5) compare the absolute mag to evolutionary model predicted masses and
luminosities for the correct age
•
Caveats:
– Ages of main sequence stars are notoriously difficult to measure
– There is no guarantee that two planets of the same age and mass will have the
same atmospheric chemistry, structure and temperature
– Evolutionary models are only as good as the input physics and assumptions, and
are particularly poor at predicting masses at very young ages (few million years)
Dr. Matt Burleigh
3677: Life in the Universe
Calculating the planet’s mass:
example: GJ3483b
•
Measure apparent magnitude of object in Spitzer’s 4.5micron filter (“Band 2”)
– Find m = 16.75
•
•
We know the distance d to the white dwarf star from its parallax (it’s 19.2parsecs
away)
So we can convert the apparent mag to an absolute mag
– Absolute mag M is magnitude at 10pc
– Use m – M = 5 log d – 5
(Pogson’s equation)
– Find M = 15.33
•
•
We also know how old the white dwarf is from its temperature (white dwarfs cool
steadily with time) – 2Gyr
Look up a theoretical model which predicts brightness of gas giant planets at
different ages
•
•
•
•
•
t (Gyr) = 2.00
-------------------------------------------------------------------------------M/Ms Teff L/Ls gIRS Blue IRS red Band1 Band2 Band3 Band4
0.0080 370. -6.73 4.29 18.25 18.74 18.93 15.44 16.99 18.16
0.0090 391. -6.64 4.35 17.99 18.55 18.65 15.20 16.76 17.87
•
Jupiter is about 0.001M/Ms, so our planet GJ3483b is between 8-9 times the
mass of Jupiter!
Dr. Matt Burleigh
3677: Life in the Universe
What did the original system look
like?
• The white dwarf GJ3483 has a mass of 0.58MSun
– From the intial-final mass relation
•
MWD = 0.12M MS + 0.36
– The progenitor main sequence star had a mass of
1.83MSun (a late A or early F star)
The white dwarf and planet are separated by
130”
– At 19.2pc, 130” = 2500AU
• (Note 1AU = 1” at a distance of 1pc)
– Using the Jean’s relation between the initial and
final separations the planet was originally located
at a separation: 2500AU / (MMS/MWD) = 790AU
– Still a very large solar system!
Dr. Matt Burleigh
3677: Life in the Universe