Download Extrasolar planets Topics to be covered Planets and brown dwarfs

3/25/2013
Topics to be covered
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Extrasolar planets
Astronomy 9601
12.1 Physics and sizes
12.2 Detecting extrasolar planets
12.3 Observations of exoplanets
12.4 Exoplanet statistics
12.5 Planets and Life
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Planets and brown dwarfs
What is a planet? What is a star?
• A star of mass less than 8%
of the Sun (80x Jupiter’s
mass) will never grow hot
Steady luminosity due to H burning enough in its core to fuse
hydrogen
• This is used as the boundary
between true stars and very
large gas planets
• Objects
Obj t below
b l
this
thi mass are
called brown dwarfs
• The boundary between BD
and planet is more
controversial
Luminosity “bump” due to shortlived deuterium burning
• The composition of Jupiter closely
resembles that of the Sun: who’s to say
that Jupiter is not simply a “failed star”
rather than a planet?
• The discovery of low-mass binary stars
would be interesting, but (perhaps) not as
exciting as discovering new “true” planets.
• Is there a natural boundary between
planets and stars?
– some argue it should be
based on formation
– other choose 0.013 solar
masses=13 Mj as the
boundary, as objects below
this mass will never reach
even deuterium fusion
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Nelson et al., 1986, AJ, 311, 226
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Pulsar planets
Artist’s conception of the planet
orbiting pulsar PSR B1257+12
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• In 1992, Wolszczan and Frail announced the discovery of a multi‐
planet planetary system around the millisecond pulsar PSR 1257+12 (an earlier announcement had been retracted).
• These were the first two extrasolar planets confirmed to be discovered, and thus the first multi‐planet extrasolar planetary system discovered, and the first pulsar planets discovered
• However, these objects are not in planetary systems as we usually 6
think of them
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Worlds Beyond Our Sun
Unseen Companions
• In 1995 a team of Swiss
astronomers discovered
the first planet (in a nonpulsar system) outside
our solar system, orbiting
a sun-like
sun like star called 51
Pegasi.
• Further discoveries bring
the grand total of known
extrasolar planets to 861
(as of March 2013) and
counting.
This artist's concept shows the
Neptune-sized extrasolar planet
circling the star Gliese 436.
• Curiously enough,
most extrasolar
planets remain
unseen
• They are usually
detected by indirect
means, though their
effects on their parent
star.
Artist's rendition of the star 51 Pegasi and
its planetary companion 51 Pegasi B.
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The “first confirmed” image of an exoplanet:
Obstacles to Direct Detection
• Direct detection is the only way to tell what these planets
are made of and whether there's water or oxygen in their
atmospheres.
• But most known exoplanets are impossible to see with
current technology
• Two reasons why:
– known exoplanets are too dim
GQ Lupi & Planetary Companion
• Jupiter, for example, is more than a billion times fainter than
the Sun. However it could easily be seen at large distances
except for…
– known exoplanets orbit too close to their parent stars
• most known exoplanets have orbits smaller than that of
Mercury
"It's like trying to see a firefly next to a searchlight from across town."
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21 Mj, 100 AU orbit. Imaged by ESO’s VLT,
then HST and Subaru confirmed (early Apr 2005)
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Astrometry
Detection methods: Astrometry
• STEPS (Stellar Planet Survey)
detected periodic proper motion
of VB 10, a nearby brown dwarf.
• VB 10b is approximately 6 Jupiter
masses, with a period of 9
months.
• No sign of planet when examined
with other techniques: busted!
• oldest method,
used since 1943
• the wobble
induced in the
plane-of-sky
motion of the star
by planets is
measured by
accurately
observing its
position over time
• 1 detection
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Astrometry:Difficulties
•Example: The Sun
wobbles by about its
diameter, mostly due to
Jupiter.
•At 30 light-years, this
would
o ld prod
produce
ce an
apparent motion of less
than 1 milliarcsecond.
• Typical good groundbased observing
conditions produce
positions with accuracies
below but around 1 arcsecond.
Detection methods: Pulsar planets
• Pulsar planets are planets that are found orbiting pulsars
– Pulsars are rapidly rotating neutron stars.
Apparent motion of Sun from 30 ly
• Pulsar planets are discovered through radio pulsar
timing measurements, to detect anomalies in the
pulsation period. Any bodies orbiting the pulsar will
cause regular
l changes
h
iin itits pulsation.
l ti
Si
Since pulsars
l
normally rotate at near-constant speed, any changes can
easily be detected with the help of precise timing
measurements.
• The first ever planets discovered around another star,
were discovered around a pulsar in 1992 by Wolszczan
and Frail around PSR 1257+12. Some uncertainty
initially surrounded this due to an earlier retraction of a
planet detection around PSR 1829-10
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5 of the 12 known pulsar planet systems
PSR 1257+12
Pulsar
• Pulsar located 2630 light years away
• These were the first extrasolar planets ever discovered
• Pulsar mass 0.3 Msun, rotational period 0.0062 seconds
Fi t l t
First planet
Mass
Orbit distance
Orbit period
PSR B1620-26 c
planet
2.5 Jupiters
23 AU
100yr
V391 Peg
b
3.2 Jupiters
1.7 AU
1170 days
PSR 1257+12
a
0.02 Earths
0.19 AU
25 days
b
4.3 Earths
0.36 AU
66 days
c
3.9 Earths
0.46 AU
98 days
Mass (ME)
a (AU)
Period (days)
e
d
0.0004 Earths
2.7 AU
3.5 years
0 020
0.020
0 19
0.19
25 26
25.26
00
0.0
QS Vir
b
6.4 Jupiters
4.2 AU
7.9 years
HW Vir
b
19.2 Jupiters
16 years
c
8.5 Jupiters
332 days
Second planet
4.3
0.36
66.54
0.02
Third planet
3.9
0.46
98.21
0.025
– possible small fourth object has an upper mass limit of 0.2 MPluto
and an upper size of R < 1000km.
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•Since neutron stars are formed after the violent death of massive stars
(supernovae), it was not expected that planets could survive in such a
system.
•Its now thought that the planets are either the remnant cores of giant
planets that were able to weather the supernova, or later accretion
products of supernova debris.
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Detection methods: Transits
The Observational Challenge
The fraction of stars expected to have transits is:
f = fs fMS fCEGP pt
fs
= fraction of stars that are single
= fraction of those on the main sequence
fMS
fCEGP = fraction of those that have a close-in planet
= fraction of those with an inclination to transit
pt
•
•
•
•
• Planets observed at inclinations (measured with respect
to the plane of the sky) near 90o will pass in front of
(“transit”) their host stars, dimming the light of the star.
This may be detectable by high-precision photometry.
= 0.5
= 0.5
= 0.01
= 0.1
Need to look at 4000 stars to find 1 that transits.
Need to sample often compared to transit duration.
Need 1% accuracy for a 3s detection of a 2 hour transit.
Need to look on sky for at least 1 orbital period.
Requires 1,000,000 15-minute samples
with 1% accuracy to detect one transit.
•Note that the planet is invisible, being unresolved, only
the brightness variation in the star is seen.
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Transits
Transits
• Advantages
• Assuming
– Easy. Can be done with small, cheap telescopes
– The whole planet passes in front of the star
– And ignoring limb darkening of the star as negligible
• WASP, STARE, numerous others
• Th
Then th
the d
depth
th off th
the eclipse
li
iis simply
i l th
the ratio
ti
of the planetary and stellar disk areas:
2
2
f = light flux
⎛ Rp ⎞
Δf πR p
⎜
⎟
=
=
2
⎜
⎟
f* πR*
⎝ R* ⎠
• We measure the change in brightness, and
estimate the stellar radius from the spectral type
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– Possible to detect low mass planets, including “Earths”,
especially from space (Kepler mission
mission, launched Mar
2009)
• Disadvantages
– Probability of seeing a transit is low
• Need to observe many stars simultaneously
– Easy to confuse with binary/triple systems
– Needs radial velocity measurements for confirmation,
masses
• Has found 294 exoplanets in 238 systems so far
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(March 2013)
Kepler (transits)
• OGLE-TR-10: Konacki et al. 2004
• 0.57Mj, 1.24Rj, P=3.1days
With a total of 95 mega-pixels of CCDs Kepler is
capable of observing over 100,000 stars all at once and
measuring their brightness to an accuracy of better than
1 part in 100,000.
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Kepler Orrery
Detection methods: microlensing
• If the geometry
is correct, a
planet can
actually produce
a brightening
(rather than a
dimming) of a
background star
(not the parent
star) through
gravitational
microlensing.
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First detection: OGLE
2003 BLG-235
Microlensing
• Microlensing has some
disadvantages
Analysis of
the light
curve reveals
second object
in lens with
.4%
4% of mass
of the other
– model-dependent
– only see the planet once
• 17,000 light years away, in the constellation Sagittarius.
• The planet, orbiting a red dwarf parent star, is most likely
one-and-a-half times bigger than Jupiter.
• The planet and star are three times farther apart than
Earth and the Sun.
• Together, they magnify a farther, background star some
24,000 light years away, near the Milky Way center.
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• However,, it is the “best”
technique for finding
smaller planets, farther
from their star
– ie. more Earth-like planets
than RV technique (next)
• 18 detections so far
(Mar/2013)
OGLE 2005-BLG-390 (Artist’s
impression): Five Earth mass
planet on a 10 yr orbit around a red
dwarf star. First (probably) icy
exoplanet found (25 Jan 2006)
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Stellar Doppler shifts
Detection methods: radial velocity
Observe the period P
• Most of the planets known to
date were discovered using the
“Doppler shift” or “radial velocity”
method.
• A planet's gravity pulls its host
star back and forth during its
orbit. This causes the light we
receive to be "blueshifted" and
"redshifted".
• Although the Doppler signals are
enough to convince us that
extrasolar planets exist, these
exoplanets are not seen directly.
• (~502 detections as of March
2013)
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r3 =
GM * 2
P
4π 2
Assume a circular orbit
(initially) to find planet
velocityy
K
V p = GM * / r
P
From conservation of
momentum, determine Mp
M p = M *V* / V p
Assume a mass for the star (from spectral
type) to compute Mp sin i (K = V*sin i)
M p sin i = M * K / V p
(i = inclination of orbital
plane to line of sight)
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51 Pegasi b
Eccentricity
• By looking more closely at the shape of the
curve, the eccentricity of the planet’s orbit can
be determined.
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• First planet discovered around a
sun-like star outside of the solar
system
• Radial velocity method
• Detection from regular velocity
changes in the star's spectral lines of
around
d 70 metres
t
per second
d
• Semi-major axis 0.052 AU (circular)
• Orbital period
4.23077 d
• Mass >0.468 ± 0.007 MJ
• Greater radius than Jupiter despite
its lower mass
• Superheated 700 K atmosphere
• It is the prototypical ”hot Jupiter”
• Orbital migration to present position?
Artist’s conception
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Observational challenges
Direct Detection
• Requires highprecision
repeatable
spectroscopic
measurements
of Doppler
shifts to ~ 1m/s
accuracy
• Most sensitive
to massive
planets near
the star (“hot
Jupiters”)
• To understand extrasolar planets,
we really need their light
• None of the radial velocity planets
can be imaged with current
technology
– Planet is too faint and too close to the star
• Solution: Remove the starlight
(adaptive optics, coronagraphy,
interferometry)
• To optimize the contrast between Above: Gliese 229B – brown
planet and star, one observes red dwarf companion to nearby M
dwarf
dwarfs, brown dwarfs & white
dwarfs, and chooses a wavelength
band that favours the planet
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Planet brightness vs age
The Adaptive Optics Difference
• Images of the planet Neptune from the W.M. Keck
observatory in Hawaii. Keck comprises two telescopes,
each with a primary mirror 10 m in diameter. Support
staff have recently installed an AO system on Keck II.
• The left-hand image is what you normally see using
Keck II. The right-hand image was taken after the AO
system was turned on.
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Solid lines Burrows 1997 models, dashed lines
Burrows 2002 models
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The “first” image of an exoplanet
•Gas giant planets are
hotter when they form,
and cool over time.
•Hot Jupiters emit more
strongly in the thermal IR
than more distant gas
giants.
•Jupiter
Jupiter is 109 times
fainter than the Sun in
the visible, but only 106
times fainter in the
thermal IR
•Young Jupiters and hot
Jupiters may be only 104
times fainter than their
stars in the IR
Models assume evolution in isolation: no additional
heating source or reflection component
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The “first confirmed” image of an exoplanet:
GQ Lupi & Planetary Companion
• 2M1207 parent “star” is a
brown dwarf
– 10Myr old (young)
– in an association of newly
formed stars
• Planet
– mass =5Mj
• determined from model of
spectrum of companion=
uncertainty!
– radius = 1.5 Rj
– 41 AU from the star
• Chauvin et al. 2004, A&A,
425, L29
Imaged with NACO (an adaptive optics instrument)
on ESO’s Very Large Telescope (VLT) Sep 2004.
Odd orbit means only confirmed after common
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proper motion confirmed (mid-Apr 2005)
21 Mj, 100 AU orbit. Imaged by ESO’s VLT,
then HST and Subaru confirmed (early Apr 2005)
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Michael Perryman, 2012, Astrobiology 12, 928.
Caution!
• AB Dor: nearby, young (~50
million years, 15pc) red
dwarf
• Brown dwarf companion
• In this case, the mass could
also be measured from direct
observations of orbit over
titime
• 2.5x more massive than
spectral models predict (90
MJ vs 36 MJ)
• So the planet is “just” a
brown dwarf /
• Masses measured by
applying models to
luminosities, ages and
distances may be underestimated by > factor 2
Close, Nature, 2005, 433, 286
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Exotic systems: PSR B1620-26c
Scorecard (Mar. 13, 2013): 861 •
•
•
•
•
•
•
Radial velocity: 501 planets in 389 systems
Transits: 294 planets in 238 systems.
Pulsar planets: 15 planets in 12 systems
Microlensing: 18 planets in 16 systems
Direct imaging: 32 planets in 28 systems
Astrometry: 1 planet Past scorecards
Apr 7 2006: 194
(SETI: nil)
Mar 13 2008: 278
Nov 25 2009: 404
Nov 7 2011: 697
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HD 209458b
Exotic systems: HD 209458b
„
• Spectroscopic radial velocity studies first revealed the presence of a
planet around HD 209458 on November 5, 1999
• 1.7% drop in HD 209458's brightness was measured, which was
later confirmed as being due to a transit. Each transit lasts about
three hours, and about 1.5% of the star's face is covered by the
planet during the transit
• Semi-major axis
0.045 AU (circular)
• Orbital period
3.52474541d
• Inclination
86.1 ± 0.1°
• Mass
0.69 ± 0.05 MJ
• Radius
1.32 ± 0.05 RJ
• Density
370 kg/m³
• Temperature
1,130 ± 150 K
• Probably a gas giant
Artist’s conception
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„
„
„
Envelope of hydrogen, carbon and
oxygen around the planet that
reaches a temperature of 10,000 K
The heavier carbon and oxygen
atoms are being blown off of the
planet by the extreme
"hydrodynamic
y
y
drag"
g created by
y its
evaporating hydrogen atmosphere
The hydrogen tail streaming off of
the planet is 200,000 kilometers
long
Measured by differential
spectroscopy during transit by HST
in UV (Vidal-Madjar et al 2004)
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Metallicity:
Orbit size distribution
Max about 6 AU
• Since most
planets
detected by
RV, there are a
lot of massive
planets near
their stars
• This
Thi
preponderance
is a selection
effect no doubt,
but how do the
ones we see
form?
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
– only ~3% of stars with 1/3rd of the Sun’s metallicity have planets
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The problem: hot Jupiters
Possible solution: planetary migration
Additional problem: why do the planets stop their
migration before falling into the star?
Mass distribution
• In our SS, the giant
planets form far
from the Sun as
the core-accretion
model requires that
they form a core
(including a lot of
ice) that reaches
10-20 Earth
masses before
they can accrete
gas
• However, many
large exoplanets
orbit very close to
their star
• This is perhaps the
outstanding
problem in the
study of extrasolar
planets.
• Super-Jupiters
(M>several
MJup) are not
common
• Implications
for planet
formation
theories?
• Or only exist
in numbers at
large
separation
that haven’t
yet been
detected?
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Cumming (2004)
Jupiter
Lines are 50% and 99%
detection thresholds for RV
surveys for 5 observations per
year for 3, 6 and 12 yrs.
• Length of surveys limits
distances planets have
been found from stars.
Normally one would like
to observe a planet for at
least one orbital period
(for RV and transit
methods)
• Earliest surveys started
1989
• Jupiter (5 AU from Sun)
takes 12 yrs to orbit Sun
– would only just have
been discovered
• Saturn takes 30 years
- would possibly remain
undetected
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Low-mass planets
• Low-mass
planets are not
easily detected
by RV
technique.
• Smallest
(except for
pulsar planets)
is α Cen B b
(radial v) at
0.00355 MJ ~
1.1 ME
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Habitable zone
• For a planet to be Earth-like in the sense of having life, it
likely must have a “moderate temperature”
– liquid water
– organic molecules stable
– energy available
• Ignoring geothermal heat, this likely means an
appropriate distance from its parent star
What about Earth-like planets?
„ The “appropriate” region
(which may be as simply and
vaguely defined as: “where
liquid water can exist”) is
called the “habitable zone” or
HZ
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Location of the Habitable Zone
Habitable zones around other stars
• In practice the location of the Habitable Zone depends
on the details of the planet itself, and possibly the
planet’s recent history
– an “ice ball” may be harder to warm up
• By examining the Earth’s climate under different
( q
water)) HZ stretches
received solar fluxes,, the (liquid
from about 0.95 to 1.4 AU
• 0.99 to 1.7 AU: Kopparapu et al. (Feb 2013)
Case
Inner limit (AU)
Standard model
0.95 (0.99)
Outer limit (AU)
1.37 (1.70)
Mars‐sized planet
0.98 (1.035)
1.49 (1.72)
10x Earth mass planet
0.91 (0.94)
1.29 (1.67)
Kasting et al 1993 (Kopparapu et al. 2013)
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Continuously Habitable Zone
(CHZ)
Habitable zones and biomarkers
• Additionally, a star will
typically increase in
luminosity throughout
its lifetime, moving
the HZ.
• If the zone moves too
much, there is no
“continously”
habitable zone (CHZ)
Luminosity evolution of the Sun (Kasting et al 1993)
• Brighter stars have
wider HZ’s further out,
while low-mass stars
have narrow HZ’s
huddled near them.
• This makes the HZ
harder to hit for the
(common) faint stars
• High mass stars have
shorter lifetimes: so
their larger HZ’s might
be counteracted by
HZs for two different luminosity stars.
the fact they die
Stars between 0.7 and 1.5 solar
before life can
masses might live long enough for life
evolve?
to develop and have HZs far enough
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from the star.
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• Though many exoplanet systems are seen to contain “hot
Jupiters” near their stars, they could contain as-yet
undetected low-mass planets in their HZ
– if they were not previously cleared out by migration
• Some HJ’s that are within the HZ could harbour moons
with more Earth-like properties.
• 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
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The End
<snip>
Earthshine spectrum with some features that might indicate life-bearing planets
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