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Class events: week 12
Goals: learn about extrasolar planets
Methods of detection
Planets observed
Towards detecting life
Solar system creation theories
The Rare Earth Hypothesis
Extra readings:
http://en.wikipedia.org/wiki/Extrasolar_planet
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Challenges in detection
Visual detections of planets are difficult because their
photons are swamped by the central star.
L /LEarth=1.5×109
L/LJupiter=4.1×108
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Direct observations
Imagery of the object itself
This has been achieved using speckle interferometry or other
modern methods for only a few planets.
Spectral data of the object
While brightness differences of suns and their planets are huge
(i.e., 109× difference for Jupiter vs. our Sun in optical), they can be
less overwhelming in infrared (i.e., a 105× difference in IR).
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Astrometric detections
Looking for tiny shifts in stellar position
Seeking planets because of their gravitational
influence on the central star is possible, but difficult
because of the mass difference.
With some algebra…
For Earth-Sun system, X* = 3×10-6 a.u., 450 km, 0.00065 R
For Jupiter-Sun system, X* = 1.0 R.
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The Doppler effect and radial velocity
Waves emitted by approaching [receding] objects are shifted to
shorter [longer] wavelengths.
This is called the Doppler effect, with blueshifts and redshifts.
By analyzing the Doppler
shifts of photons, the
line-of-sight component
of an object’s velocity can
be measured.
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Radial velocity detections of planets
Look for Doppler shifts exhibited by the central star in a planetary
system.
– Highly effective (pre-Kepler, the vast majority of exoplanets
were found this way, including 51 Pegasi, the first star with an
exoplanet discovered).
– Asymmetries in the stellar motions can indicate orbital
parameters such as eccentricities, as in 70 Virginis.
– In some cases, even multiple planet systems can be analyzed.
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Doppler detections
HOWEVER…
Orbital tilts mean we only measure some, and not all, of the orbital
velocity.
Therefore, we only measure a portion of the Doppler shift from the
planet, and the star may be getting yanked about harder than we know.
This method only gives us a lower limit for the planets.
(The value is distorted by cos θ.)
Fortunately, while we cannot correct a single planet’s mass for this
effect, on average, it is not too bad for a sample of exoplanets:
Only a 33% chance of a planet being more than 2× the inferred mass;
Only a 13% chance of a planet being more than 5× the inferred mass;
Only a 6% chance of a planet being more than 10× the inferred mass;
BUT a 0.6% chance of a planet being more than 100× the inferred mass!
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Transit detections
Looking for stellar eclipses...
– This method is effective only if the orbital plane is closely
aligned with the Earth.
– This alignment does not have to be as highly coincidental for
cases where the planet is very close to the star.
– Jupiter would cause a 1.1% brightness drop for the Sun.
– The Earth would cause a 0.008% drop for the Sun.
Many stars have brightness variations that exceed this.
Therefore, the job is to look for highly periodic brightness
changes.
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Stellar transits detected by Kepler
Kepler was launched in 2009.
10.5° square field of view
150,000 stars, every 30 minutes!
As of Feb 2014…
– About 1800 planet confirmed candidates;
– About 1800 planet confirmed candidates;
– 23% Jupiter to super-Jupiters (6-22 REarth);
– 40% Neptune-sized (2-6 REarth);
– 26% super-Earth (1.25-2 REarth);
– 10% ≈ Earth-sized (R< 1.25 REarth).
–
–

Most (76%) are Neptune-sized or smaller.
Many are within the habitable zone.
(More at http://exoplanet.eu)
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Gravitational lensing
General relativity shows us that gravity can bend beams of light.
One manifestation of this is to make stars wink brightly, as their
light is focused towards us.
OGLE-2005-BLG-390Lb: 5.5 MEarth, T~50 K, 2.1-4 a.u. from a (red dwarf?) star.
Detected at a range of about 7000 parsecs!
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Cancelled future missions
Space Interferometry Mission (SIM Lite)
The Flagship for exoplanet research.
Launch date: ~2015 (but oft postponed).
Dec 2010—cancelled.
Terrestrial Planet Finder
Possibly a multiple mirror, and upscaled Hubble?
Launch dates 2014, 2020?
Currently in budgetary purgatory (postponed indefinitely).
The ESA analog “Darwin” is similarly dead.
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The planetary zoo
Many of the planets detected are huge, and very close to their stars.
The most extreme of these are more massive than Jupiter, but are
closer than about 0.05 a.u.
(Mercury is at 0.4 a.u.).
These are called hot Jupiters.
Smaller versions are called
hot Neptunes
Our detection methods would
tend to preferentially detect
these planets.
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The planetary zoo
WASP-12b
An extreme Hot Jupiter.
1.4 MJupiter
1.74 RJupiter
1.09 day orbital period
Home star: G
Surface T: 2500K
It will be vaporized in
about 10 million years.
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The planetary zoo
HD 96167b
An “eccentric Jupiter.”
0.68 MJupiter
498 day orbital period
Home star: G5
e=0.710
7% of all systems have
eccentric Jupiters.
They are more common
than hot Jupiters!
It is unlikely that other planets can share the system with an eccentric Jupiter!
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The planetary zoo
HD 189733b
The azure planet
1.16 MJupiter
2.2 day orbital period
Home star: K1-2 V
Despite being a hot Jupiter, its color has been
measured as being deep blue.
Spectroscopy has detected atmospheric
molecule information!
K, Na, CO2, H2O, O2, CH4
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The planetary zoo
Kepler-10b (hot super-Earth)
One of the first rocky planets verified.
4.55 MEarth
1.39 REarth
0.84 day orbital period
Home star: G
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The planetary zoo
Gliese 1214b (super-Earth)
Based upon its mass and radius, estimates can be made about its composition and
structure.
Its spectrum has been detected and is
featureless—one explanation is that its
atmosphere is water-steamy.
Its overall composition may be
25% rock, 75% water.
6.36 MEarth
2.69 REarth
1.58 day orbital period
Home star: M
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The planetary zoo
Habitable super-Earth
Kepler-22b
11-30 MEarth
2.4 REarth
(g=2-3gEarth)
289 day orbital period
Home star: G5
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The planetary zoo
Very Earthlike
Gliese 581d
~6.04 Mearth
66 day orbital period
Home star: M2.5
Triple planet system
HD 85512b
~3.50 MEarth
54 day orbital period
Home star: K5
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The planetary zoo: Earthlike and in the habitable zone!
Kepler 186 f
~1.13 Rearth
130 day orbital period
Home star: M1
Five-planet system
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The planetary zoo
Kepler-20e, KOI-961: The smallest planets
detected so far.
Kepler-20e KOI-961
0.4-1.7 Mearth Sub-Earth?
0.87 REarth
6.1 day orbital period 0.45, 1.2, 1.9 day
Home star: G8
M star
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The planetary zoo
Planets in binary/multiple star systems.
Kepler-16 (AB)b
0.33 MJupiter
0.74 RJupiter
228.78 day orbital period
Home star: K, M
Alpha Cen Bb
1.13 MEarth
3.23 day orbital period
Home star: K1
This case is one where the planet orbits a
single star, which is in a multiple system
with a G2 and M5 star.
The azure planet is in a similar double
system
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The planetary zoo
HD 10180 - A planetary system around a G1V star
HD10180b(?) 1.4 MEarth 1.18d
HD10180c
13.1 MEarth
HD10180d
11.8 MEarth
HD10180e
25.1 Mearth
HD10180f
23.9 MEarth
HD10180g
21.4 MEarth
HD10180h(?)
63.6 MEarth
5.76d
16.36d
49.74d
122.76d
601.20d
2222.0d
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The planetary zoo
Gliese 667- A complicated system
Gliese 667A (K3V, 0.12 LSun) orbits Gliese 667B (K5V, 0.05 LSun) in 42 y
Gliese 667C (M1V, 0.014 LSun) orbits the pair in xx days
Gliese 667Cb
Gliese 667Ch(?)
Gliese 667Cc
Gliese 667Cf
Gliese 667Ce
Gliese 667Cd
Gliese 667Cg(?)
4-7 MEarth
 1-3 MEarth
 3-5 MEarth
 2-4 Mearth
 1-4 MEarth
 3-7 MEarth
 3-8 MEarth
7.2d
~17d
28.1d (Habitable zone)
39.1d (Habitable zone)
62.3d (Habitable zone)
91.6d
256d
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The planetary zoo
PSR J1719-1438b (the diamond planet)
Formerly a red giant star, and then a white dwarf in a binary. (Its
companion already converted itself into a pulsar.)
The pulsar blew away nearly all the white dwarf star, and the
remaining residual carbon-rich core is now considered a
“diamond planet.”
1.02 MJupiter
0.4 RJupiter (4 REarth)
2.18 h orbital period
Home star: pulsar
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Some (soft) planetary stats
Estimates of planetary numbers still varies widely from team to team. However,
all are suggesting that planets are common…
Analyses of Kepler data suggest that stars in the galaxy have, on average, 1.6
planets. Therefore, about 160 billion planets exist in the galaxy.
500 million of these planets may orbit within the habitable zone.
1.4—2.7% of all sunlike star systems are expected to have an Earthlike planet
within the habitable zone.
Oversized planets, orbiting in the habitable zone, may have habitable moons!
Planets in unbound orbits may number in the trillions (1012)!
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Detecting exoplanetary life
The heat is on for detecting Earth-size planets in habitable zones...
If one is found, how could Earthbound scientists look for
exoplanetary life?
Look for oxygen, methane, or other suspicious compounds in the
atmosphere. So far, we have detected atmospheric K, Na, CO2, H2O,
O2, CH4 in the azure planet and others.
Like the Martian meteorite ALH84001, however, evidence would
have to be very, very strong.
Turning the tables…these lines of evidence are present in abundance
in the Earth’s atmosphere. Curious alien astronomers that point their
telescopes towards Earth would easily detect our signatures of life…
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Hot Jupiters and solar system theories
Recall our theory of solar system formation.
Hydrogen planets would not form close to the central star, because the
proto-planetary disk would have been so hot that hydrogen, helium,
and hydrogen-rich compounds would have been in gas from.
This is why we have terrestrial planets close to the Sun, and Jovian
planets far from the Sun.
Hot Jupiters do not fit into our model of having terrestrial planets close
to the star, and jovian planets far from the star.
Is our notion of planetary formation wrong?
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Hot Jupiters modify our solar system theories
If hot Jupiters did not form where they are seen today, it is possible their
orbits shifted?
Density wave braking
Gravitational effects from the planetary disk. This would work on
planets that formed early, when the proto-planetary disk was still
thick, and had not yet been dispelled by the stellar wind.
Jovian-jovian gravitational interactions
Encounters between planets could expel one, and send the other into
an elliptical, near-star orbit.
Could terrestrial planets survive the inward migration of Jovian planets?
It might be the case that planetary systems with hot Jupiters cannot have
terrestrial planets in the habitable zone.
Modern thoughts on our solar system is that the planets were not always
in their current locations. Solar systems change over time!
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Highly elliptical orbits
In our solar system, orbits are very nearly circular:
Mercury
Venus
Earth
Mars
0.206
0.007
0.017
0.093
Jupiter 0.048
Saturn 0.056
Uranus 0.046
Neptune 0.010
We have also discovered that many exoplanets have very elliptical
orbits (~50% have e > 0.2, ~17% have e > 0.5).
In some cases (~35%) these could result from additional, undetected
planets confounding our interpretations of the data. Others (~40%)
might be due to simple misinterpretations of the data.
Or…our planetary system is something of an oddity.
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Rare Earth hypothesis
“Life in the Universe” authors certainly seem to lean towards the notion
of a universe filled with life. But what of the counter-hypothesis?
Rare Earth Hypothesis
“Life, at least in an advanced multicellular form, is exceedingly rare in
the Universe. The Earth may even be unique in this respect.”
Let us consider five factors that might make life rare.
1. The galactic habitability zone is small
Frequent supernovae set the inner limit of habitability.
Bennett & Shostak argue that such pulses of radiation might not be
so bad; they may be shielded by the atmosphere, and might even
encourage mutations that enhance evolution.
The rarity of elements more massive than helium sets the outer limit
of habitability.
Bennett & Shostak argue that the difference (0.1% vs. 2%) does not
necessarily prohibit the formation of rocky planets.
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Rare Earth hypothesis
2. A Jupiter is necessary
Possibly, Jupiter was critical in expelling inner solar system comets to purgatory in
the Oort cloud. Without this cleanup service, comets would continue to pelt the
terrestrial planets, repeatedly sterilizing them.
(However, Jupiters and super-Jupiters have already been discovered in abundance,
and so are not likely to be rare.)
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Rare Earth hypothesis
3. Having a large satellite
The terrestrial planets are constantly being tugged and jostled
gravitationally by the other planets. The tidal forces from our
Moon overwhelms these other tugs, and keep our axial tilt more or
less stable at 23.5º.
On the other hand, Mercury, Venus, and Mars do not have such a
large moon. This could contribute to very large climate variations
on an otherwise habitable world.
Obtaining a massive moon may be both critical for life, and highly
unlikely!
Maybe large moons are not highly unlikely, some of the Kuiper
Belt Objects have them.
And is climate stability really important? Recall that the Cambrian
explosion of life diversity may have resulted from a massive
climate transition from a snowball Earth phase to a hothouse Earth
phase.
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Rare Earth hypothesis
4. Having plate tectonics
The CO2 cycle is a stabilizing influence for our climate. This
requires active plate tectonics.
It might be the case that having plate tectonics is rare. For
example, we do not see it well developed in Mercury, Venus, or
Mars.
Since Mercury and Mars are both small, we should not be
surprised at the lack of plate tectonics—but what about Venus?
Venus’ enormous greenhouse effect may be to blame for the lack of
plate tectonics—the water was cooked out of the crust.
There is no reason to insist that an Earth-sized planet in a habitable
zone must have a runaway greenhouse effect (the Earth is proof of
this).
If runaway greenhouses were the norm, why did the Earth dodge
this bullet?
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Rare Earth hypothesis
5. Having an ocean, but not too much
You can argue that, as a technological civilization, we are the
results of an amphibious pattern of evolution:
– Life on Earth may have developed around undersea
hydrothermal vents.
– In order to develop our necessary technological skills such as
mastery of fire, our aquatic predecessors had to evolve into
land-based life forms.
Therefore, in order to develop an advanced, technologically adept
civilization, a planet must have adequate water, but not so much
that continents do not form. This might be a delicate and
improbable balance.
Opponents to this argue that aquatic civilizations may very well
exist, and that the argument is based in a prejudiced perspective.
Furthermore, the details of the land-ocean balance may not be very
critical.
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