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PH709
4
Extrasolar Planets
Professor Michael Smith
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The extrasolar planet population
NEW STATISTICS. DO DWARFS HARBOUR PLANETS? Is the disc
massive enough?
Paper: Observed Properties of Exoplanets: Masses, Orbits, and
Metallicities
Marcy, Butler et al…….1987-2007
Large Stellar Sample 1330 Nearby FGKM Stars
Star Selection Criteria:
• Vmag < 10 mag
• No Close Binaries
Age > 2 Gyr
(~2000 stars total with Mayor et al. )
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Extrasolar Planets
Professor Michael Smith
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Ongoing 18-year survey of 1330 FGKM type stars at Lick,
Keck, and the Anglo-Australian Telescopes that offers both
uniform Doppler precision (3 m s-1) and long duration.
The 104 planets detected in this survey have minimum masses (M
sin i) as low as 6 M Earth, orbiting between 0.02 and 6 AU
EXAMPLE. GJ 849 (M3.5 V) 8.8pc 0.36 M
We report precise Doppler measurements of GJ 849 (M3.5 V) that
reveal the presence of a planet with a minimum mass of 0.82 MJup in
a 5.16 yr orbit. (Butler et al 1006, PASP , 118 , 1685).
At a = 2.35 AU, GJ 849b is
1. the first Doppler-detected planet discovered around an M
dwarf orbiting beyond 0.21 AU, and
2. is only the second Jupiter-mass planet discovered around a
star less massive than 0.5 Msolar.
This detection brings to 4 the number of M stars known to harbour
planets. We find: Giant planets within 2.5 AU are ~3 times more
common around GK stars than around M stars.
PH709
Extrasolar Planets
Professor Michael Smith
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Due to GJ 849's proximity of 8.8 pc, the planet's angular
separation is 0.27", making this system a prime target for highresolution imaging using adaptive optics and future space-borne
missions such as the Space Interferometry Mission PlanetQuest.
STATISTICS. DO GIANT STARS HARBOUR PLANETS?
Yes, a few, but there are no planets orbiting closer than 0.6 AU.
(Close-in, Jovian planets are relatively easy to detect using radial
velocities because of the larger amplitude they induce and the
increased number of orbit cycles per observing time baseline.)
It appears that stellar mass has a dramatic effect on the semimajor
axis distribution of planets. However, it is not clear whether this
effect is a reflection of the process of planet formation and
migration, or instead related to the effects of the evolution of the
host stars.
Different Classes of Object found:
1. Giant Planets or Brown Dwarfs in Unclosed, Long-Period Orbits:
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Extrasolar Planets
Professor Michael Smith
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2. Examples of Jupiter-mass/Saturn mass Planets Detected by RV:
3. Sub-Saturn mass:
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Extrasolar Planets
Professor Michael Smith
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Detectable mass increases with semi-major axis: a selection effect:
STATISTICS: Eccentricity:
Planet orbits have significant eccentricity. The median orbital
eccentricity is <e >= 0.25, and even planets beyond 3 AU reside in
eccentric orbits, suggesting that the circular orbits in our Solar
System are unusual.
The eccentricity of an orbit is how much it varies from a perfect
circle. A stable orbit can have an eccentricity anywhere from a
perfect circle with an eccentricity of 0, up to a highly elliptical orbit
with an eccentricity up to (but not including) 1. If an orbit had an
eccentricity of 1, it would be parabolic and escape from the system.
If it were larger than 1, it would be hyperbolic and also escape from
the system.
PH709
Extrasolar Planets
With k=1 for an ellipse:
Professor Michael Smith
e = (da – dp)/(da + dp)
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Extrasolar Planets
Professor Michael Smith
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Most extrasolar planets reside in non-circular orbits. Of the 90
extrasolar planets that reside beyond 0.15 AU, their average orbital
PH709
Extrasolar Planets
Professor Michael Smith
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eccentricity is 0.32. These planets have a wide scatter in e,
including some planets with very large e.
In contrast, planets orbiting within 0.1 AU of their host star all reside
have close to circular orbits. All detected planets with semi-major
axis < 0.07 AU have low e. This is similar to binary stars, and is
likely due to tidal circularization.
Earth's eccentricity is 0.017, while Jupiter's is 0.094.
In our solar system, the planet with the largest eccentricity is Pluto
at 0.244, and Mercury with 0.205.
The planet with the lowest eccentricity is Venus with 0.007.
Unless there is some gravitational tugging (such as with the
Galilean Satellites) that keeps an orbit eccentric, orbits will usually
circularize with time.
About 10% of the planets found so far have an eccentricity of nearly
0. About 15% have an eccentricity smaller than Earth's, and over
25% have an eccentricity smaller than Jupiter's. 45% are smaller
than Mercury's eccentricity, and 50% are lower than Pluto's.
The other half have very eccentric orbits; this means that,
throughout their years, they come very close to and very far from
their parent star.
This will create wide temperature swings, and for any life like
Earth's, this would make survival quite difficult, if not
impossible.
Theories: explanations uncertain
Most proposed mechanisms invoke gravitationally scattering or
perturbations of planets by other planets, perhaps in resonances, or
by interactions with the protoplanetary disk.
Beyond 0.1 AU, the distribution of eccentricities appears essentially
uniform between 0 and 0.8. For most Doppler surveys, sensitivity is
not a strong function of eccentricity for 0 < e < 0.8 and a < 3 AU.
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Extrasolar Planets
Professor Michael Smith
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This plot represents results from many surveys, and so is drawn
from an inhomogeneous sample.
Distribution of Eccentricity:
Eccentricity v. planet mass
Figure: the 130 known nearby exoplanets with M sin i < 13 MJup, excluding
those for which a < 0.1 AU, i.e., those planets which may have been tidally
circularized. High-mass exoplanets (M sin i > 5MJup) have a slightly higher
median eccentricity than lower-mass exoplanets. The completeness of Doppler
surveys increases with M sin i and is generally insensitive to eccentricity.
PH709
Extrasolar Planets
Professor Michael Smith
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Ignoring the hot Jupiters, no obvious correlation between planet
mass and eccentricity.
Planets within 0.1 AU are presumably tidally circularized.
Note that the distance of closest approach is a(1-e), and that the
effect of tidal torques scales as separation d-6. The very eccentric
planet around HD80606 (a = 0.438 AU, e = 0.93, a(1-e) = 0.03 AU)
may pose some problems for tidal circularization theory.
STATISTICS: Minimum mass as a function of semimajor axis:
Nothing very striking in these plots: Accessible region of mp
- a space is fully occupied by detected planets
Get rid of the log (Mj) :
Minimum mass as a function of semimajor axis for the known
nearby exoplanets. Doppler surveys are generally incomplete for
exoplanets with a > 3 AU, low-mass planets (M sin i < 1MJup) beyond
1 AU, and very low-mass planets
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Extrasolar Planets
Professor Michael Smith
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(M sin i < 0.1MJup) everywhere. This plot represents results from
many surveys, and so is drawn from an inhomogeneous sample.
STATISTICS: THE HISTOGRAMS
Results from radial velocity searches
Massive planets exist at small orbital radii.
Closest-in planet is at a = 0.035 AU (cf Mercury at ~ 0.4 AU). Less
than 10 solar radii!
EXAMPLES:
Best-fit orbit to the radial velocities measured at Keck Observatory
for HD 11964,
with P = 5.8yr, e ~ 0, and M sin i = 0.6MJup.
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Extrasolar Planets
Professor Michael Smith
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Best-fit orbit to the radial velocities measured at Keck Observatory
for HD 66428, with P = 5.4yr, e = 0.5, and M sin i = 3MJup.
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Extrasolar Planets
Professor Michael Smith
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Account for this by considering only planets with masses large
enough to be detectable at any a < 2.7 AU.
-> dN / dlog(a) rises steeply with orbital radius
Orbital distance distribution of the 197 known nearby exoplanets with
0.03 <a < 10 in logarithmic distance bins.
Planets with a > 3AU have periods comparable to or longer than the
length of most Doppler surveys, so the distribution is incomplete beyond
that distance.
STATISTICS: Location
Strong selection effect in favour of detecting planets at small orbital
radii, arising from:
- lower mass planets can be detected there
- mass function rises to smaller masses
The distribution of semi-major axes rises from 0.3 – 3.0 AU (dN/d
log a) and extrapolation suggests that about 12% of the FGK stars
harbour gas-giant exoplanets within 20 AU.
The occurrence of “hot Jupiters” within 0.1 AU of FGK stars is 1.2 ±
0.2%.
Among stars with one planet, 14% have at least one additional
planet, occasionally locked in resonances.
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Extrasolar Planets
Professor Michael Smith
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Planet fraction among ~ solar-type stars exceeds 7%. Most are
beyond 1 AU:
STATISTICS. IS THERE A PERIOD VALLEY?
A Pile-up:
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Extrasolar Planets
Professor Michael Smith
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Distribution of periods among the known nearby “hot
Jupiters”. There is a clear “pile-up” of planets with orbital
periods near 3 days.
The "period valley" observed around Sun-like stars, marked by a
deficit of planets with periods ranging from roughly 10 to 100
days, a sharp increase in the number of detected planets beyond
1 AU and a pile-up near P = 3 day.
Implies that the currently detected planet fraction ~7% is likely to
be a substantial underestimate of the actual fraction of stars with
massive planets.
STATISTICS: THE MASS DISTRIBUTION
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Extrasolar Planets
Professor Michael Smith
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15-25% of solar-type stars may have planets with masses
0.2 MJ < mp < 10 MJ.
MJ sin i Observed mass function increases to smaller Mp:
Note: the brown dwarf desert!
The mass distribution shows a dramatic decrease in the number of
planets at high masses, a decrease that is roughly characterized by
a power law, dN/dM ~ M-1.25.
STATISTICS: Metallicity distribution of stars with and
without planets
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Extrasolar Planets
Professor Michael Smith
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Here NFe and NH is the number of iron and hydrogen atoms per unit
of volume respectively
Left plot: metallicity of stars with planets (shaded histogram)
compared to a sample of stars with no evidence for planets (open
histogram) (data from Santos, Israelian & Mayor, 2001)
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Extrasolar Planets
Professor Michael Smith
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Host star metallicity: Planets are preferentially found around stars
with enhanced metal abundance.
Cause or effect? High metal abundance could:
(a) Reflect a higher abundance in the material which formed
the star + protoplanetary disc, making planet formation more
likely.
(b) Result from the star swallowing planets or planetesimals
subsequent to planets forming. If the convection zone is fairly
shallow, this can apparently enrich the star with metals even if
the primordial material had Solar abundance.
DO THESE PLANETS FORM FROM DISCS?
The core-accretion model of planet formation is supported by
four observations:
1) The mass distribution rises toward the lowest detectable
masses, dN/dM ~ M -1.0.
2) Stellar metallicity correlates strongly with the presence of
planets. Sample exhibits a planet-metallicity correlation at all
stellar masses; this argues that the high metallicities of stars
with planets is not likely due to convective envelope
"pollution." BUT: Neptune-mass planets form preferentially
around metal-poor stars, in contrast to stars with giant planets
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Extrasolar Planets
Professor Michael Smith
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3) One planet (1.3 M Sat) has a massive rocky core, M Core ≈ 70 M
Earth.
4) A super-Earth of about 7 M Earth has been discovered. Four
very low mass planets have been detected (20 earth masses).
Detailed pattern of abundances can distinguish these possibilities,
but results currently still controversial.
SUMMARY
The core-accretion model of planet formation is supported by four
observations: 1) The mass distribution rises toward the lowest
detectable masses, dN/dM =M-1.0. 2) Stellar metallicity
correlates strongly with the presence of planets. 3) One planet (1.3
MSat) has a massive rocky core, MCore ≈ 70 MEarth. 4) A super-Earth
of ˜ 7 MEarth has been discovered.
BUT HOW …. do
Cluster issue: Lack of transits in 47 Tuc
A long HST observation monitored ~34,000 stars in the globular
cluster 47 Tuc looking for planetary transits.
Locally: 1% of stars have hot Jupiters
~ 10% of those show transits
 Expect 0.001 x 34,000 ~ few tens of planets
None were detected. Possible explanations:
• Low metallicity in cluster prevented planet formation
• Cluster environment destroyed discs before planets formed
• Stellar fly-bys ejected planets from bound orbits
All of these seem plausible - make different predictions for other
clusters.
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Extrasolar Planets
Professor Michael Smith
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The lack of detections in the uncrowded outer regions of both
clusters indicates that stellar metallicity is the dominant factor
inhibiting Hot Jupiter formation in the cluster environment.
Microlensing Statistics:
Constraint from monitoring of 43 microlensing events.
Typically, the lenses are low mass stars.
At most 1/3 of 0.3 Solar mass stars have Jupiter mass planets between 1.5
AU and 4 AU.
Currently consistent with the numbers seen in radial velocity
searches.
massive planets end up so close to their stars?