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Extrasolar Planets - 1
Professor Michael Smith
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EXOPLANETS: Prof Michael SMITH
An extrasolar planet, or exoplanet, is a planet outside the Solar System
Module page:
http://astro.kent.ac.uk/mds/Modules/modules.htm
latest on discovery methods:
http://www.youtube.com/watch?v=uicxfBcQIog
latest on discoveries:
http://adsabs.harvard.edu/abs/2013arXiv1307.2944A
http://exoplanetarchive.ipac.caltech.edu
TOPICS COVERED
1.
2.
3.
4.
5.
6.
Introduction: Review & Status
Measurement : Dynamics, Binaries2-component systems
Definitions, planets, disks; Detection methods
Populations
Theory of formation
Theory of evolution, Migration/eccentricity
Review
We are still in the early days of a revolution in the field of planetary
sciences that was triggered by the discovery of planets around other
stars.
http://exoplanet.eu/catalog.php .
Showing 661 planetary systems / 838 planets / 125 multiple planet
systems as of 22/09/2012
Showing 750 planetary systems / 986 planets / 168 multiple planet
systems as of 28/09/2013
Comparative planetology, which once included only our Solar
System's planets and moons, now includes sub-Neptune to superJupiter-mass planets in other solar systems.
Overview of units: mass, distance and constitution
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Mass:
Sun
Jupiter 's mass
Earth's mass
Sun
2
1.989 10 30 kg
MJ = 1.898 1027 kg
ME = 5.974 1024 kg
= 300,000
Jupiter =
Neptune =
Mercury =
Professor Michael Smith
ME
300
ME
17.1
ME
0.0553 ME.
Orbit/Distance: 1 astronomical units (AU) = 1.496 108 km, distance
between Earth and Sun
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Distance from Sun in AU
0.39
0.72
1.0
1.5
5.2
9.5
19.2
30.1
39.5
Constitution
Sun + MVEM
gas
rock
+ asteroids (Ceres) + JSUN
rock
gas
2
+
P
rock/ice
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Extrasolar Planets - 1
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SUN: 99.85% of SS mass, 92% H, 8% He
Density kg/m3
1
Earth
5515
2
Mercury
5427
3
Venus
5243
4
Mars
3933
5
Moon
3350
6
Pluto
1750
7
Neptune
1638
8
Sun
1408
9
Jupiter
1326
10
Uranus
1270
11
Saturn
687
+ moons, comets, asteroids, Kuiper belt, Oort cloud,
debris, dust
Back to Review
HOT JUPITERS
Began earnestly in 1995 by definite discovering hot jupiters.
SUPER-EARTHS
Thanks to remarkable progress, radial velocity surveys are now able to detect
terrestrial planets at habitable distance from low-mass stars.
The unexpected diversity of exoplanets includes a growing number of superEarth planets, i.e., exoplanets with masses smaller than 10 Earth masses.
Unlike the larger exoplanets previously found, these smaller planets are more
likely to have similar chemical and mineralogical composition to the Earth.
EARTHS-TWINS
And since 2011 we discuss earth twins …….
In April 2013, using observations by NASA's Kepler Mission, a team led
by William Borucki, of the agency's Ames Research Center, found five
planets orbiting in the habitable zone of a Sun-like star, Kepler-62, 1,200
light years from Earth. These new super-Earths have radii of 1.3, 1.4, 1.6,
and 1.9 times that of Earth. Theoretical modelling of two of these superEarths, Kepler-62e and Kepler-62f, suggests both could be solid, either
rocky or rocky with frozen water.
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On June 25, 2013 Three “super Earth” planets have been found orbiting a
nearby star at a distance where life in theory could exist, according to a
record-breaking tally announced on Tuesday by the European Southern
Observatory. They are part of a cluster of as many as seven planets that
circle Gliese 667C, one of three stars located a relatively close 22 light
years from Earth in the constellation of Scorpio, it said. The planets orbit
Gliese 667C in the so-called Goldilocks Zone — a distance from the star
at which the temperature is just right for water to exist in liquid form rather
than being stripped away by stellar radiation or locked permanently in ice.
DOPPLER
METHOD
SPECTROSCOPY:
RADIAL
VELOCITY
Gliese 581d & Goldilocks.
In April 2007, a team of 11 European scientists announced the discovery of a
planet outside our solar system that is potentially habitable, with Earth-like
temperatures.
The planet was discovered by the European Southern Observatory's telescope
in La Silla, Chile, which has a special instrument that splits light to find wobbles
in different wave lengths, HARPS. Those wobbles can reveal the existence of
other worlds.
Gliese 581c. What they revealed is a planet circling the red dwarf star,
Gliese 581. The discovery of the new planet, named Gliese 581c, is sure to fuel
studies of planets circling similar dim stars.
About 80 percent of the stars near Earth are red dwarfs.
Super-earth.
The new planet is about five times heavier than Earth, classifying it as a superearth.
Its discoverers aren't certain if it is rocky, like Earth, or if it is a frozen ice ball
with liquid water on the surface. If it is rocky like Earth, which is what the
prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our
planet. If it is an iceball, it would be even bigger.
Gliese 581: M star: 3480K, mass: 0.31 solar masses; 0.29 solar radii
Luminosity: 0.013 solar
Hot Neptune Gl 581b 15.7 ME
0.041 AU
Super-earth Gl 581c 5.06ME
0.073 AU
Super-earth Gl 581d 8.3 ME
0.22 AU
Gl 581c:
a pleasant 20C (albedo = 0.5 assumed) Greenhouse?
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Tidal locking?
Extremophiles?
However, further research on the potential effects of the planetary
atmosphere casts doubt upon the (extremophile life form) habitability
of Gliese 581c and indicates that the third planet in the system, Gliese
581d, is a better candidate for habitability.
An extremophile is an organism that thrives in and may even require physically
or geochemically extreme conditions that are detrimental to the majority of life
on Earth.
Most known extremophiles are microbes.
Currently, Gliese 581d, the third planet of the red dwarf star Gliese 581
(approximately 6.12 parsecs from Earth), appears to be the best example yet
discovered of a possible terrestrial exoplanet which orbits close to the
habitable zone of space surrounding its star.
Going by strict terms, it appears to reside outside the "Goldilocks Zone", but
the greenhouse effect may raise the planet's surface temperature to that
which would support liquid water.
HZ – Habitable Zone: life zone", "Comfort Zone", "Green Belt" or
"Goldilocks Zone" (because it's neither too hot nor too cold, but "just right")
Planet “c” receives 30% more energy from its star than Venus from the Sun,
with an increased radiative forcing caused by the spectral energy distribution of
Gl 581.
This planet is thus unlikely to host liquid water, although its habitability
cannot be positively ruled out by theoretical models due to uncertainties
affecting cloud properties and cloud cover.
Highly reflective clouds covering at least 75% of the day side of the planet
could indeed prevent the water reservoir from being entirely vaporized.
Planet “d”. Irradiation conditions of planet “d” are comparable to those of
early Mars, which is known to have hosted surface liquid water. Thanks to the
greenhouse effect of CO2-ice clouds, also invoked to explain the early Martian
climate, planet “d” might be a better candidate for the first exoplanet known to
be potentially habitable.
Sources and sinks of atmospheric carbon dioxide. The photosynthesissustaining habitable zone (pHZ) is determined by the limits of biological
productivity on the planetary surface.
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Although Gliese 581 d orbits outside the theoretical habitable zone of its star,
scientists surmise that conditions on the planet may be conducive to supporting
life. Scientists originally believed that Gliese 581 d would be too cold for liquid
water to exist, and therefore could not support life in forms as existing on
Earth.
However, since Earth's temperature would be about -19°C without any
greenhouse gases, and due to a theorized greenhouse effect of Gliese 581
d, research now suggests that atmospheric conditions on the planet could
create temperatures at which liquid water can exist, and therefore the planet
may be capable of supporting life.
Now: f and g have been discovered but NOT confirmed. Gliese 581
g has attracted attention because it is near the middle of the
habitable zone of its parent star. That means it could sustain liquid
water on its surface and could potentially host life similar to that on
Earth.
Top view of the GJ 581 system. For reference, the orbits of Earth,
Venus, and Mercury are overlaid as dashed blue, green, and red
lines, respectively.
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DOPPLER METHOD
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Radial velocity method permits a minimum planet mass to be
determined.
HARPS is a vacuum spectrograph designed to measure precise
radial velocities, with the specific goal of searching for exoplanets
in the Southern hemisphere. This high-resolution Echelle spectrograph
(R=115000) is fiber-fed by the ESO 3.6-meter telescope at La Silla
Observatory.
R is the spectral Resolution: R = c / v.
So R=15,000 is not itself very good.
TRANSITS
Currently the most important class of exoplanets are those that
transit the disk of their parent stars, allowing for a determination
of planetary radii.
SELECTION: Of course, while planets close to their parent stars will
preferentially be found, due to their shorter orbital periods and
greater likelihood to transit, planetary transits will be detected at
all orbital separations.
CONFIRMATION: In general, the detection of three successive
transits will be necessary for a confirmed detection, which will limit
confirmed planetary-radius objects to about 1.5 AU.
DENSITIES: The first confirmed transiting planets observed were all
more massive than Saturn, have orbital periods of only a few days,
and orbit stars bright enough such that radial velocities can be
determined, allowing for a calculation of planetary masses and bulk
densities. A planetary mass and radius allows us a window into
planetary composition (Guillot 2005).
The first transiting planets were mainly gas giants although one
planet, HD 149026b, appears to be 2/3 heavy elements by mass
(Sato et al. 2005; Fortney et al. 2006; Ikoma et al. 2006).
Understanding how the transiting planet mass-radius relations
change as a function of orbital distance, stellar mass, stellar
metallicity, or UV flux, will provide insight into the fundamentals of
planetary formation, migration, and evolution.
The transit method of planet detection is biased toward finding
planets that orbit relatively close to their parent stars. This means
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that radial velocity follow-up will be possible for some planets as the
stellar "wobble" signal is larger for shorter period orbits.
However, for transiting planets that are low mass, or that orbit very
distant stars, stellar radial velocity measurements may not be
possible. For planets at larger orbital distances, radial velocity
observations may take years. Therefore, for the foreseeable future a
measurement of planetary radii will be our only window into the
structure of these planets.
Orbital distances may give some clues as to a likely composition,
but our experience over the past decade with Pegasi planets (or
"hot Jupiters") has shown us the danger of assuming certain types
of planets cannot exist at unexpected orbital distances.
SPACE MISSIONS
The French/European COROT mission, launched in 2006 December, and the
American Kepler mission, launched 2009 March 6 have revolutionized the
study of exoplanets. COROT monitored 12,000 stars in each of five different
fields, each for 150 continuous days.
COROT detected its first extrasolar planet, COROT-Exo-1b, in May 2007.
Planets as small as RE could be detectable around solar-type stars. The
mission lifetime is expected to be at least 2.5 yr (extended to 2010
COROT-7b (previously named COROT-Exo-7b)[4][5] is a reported
exoplanet orbiting around the star COROT-7. It was detected by the
French-led COROT mission in 2009. It is the smallest exoplanet to
have its diameter measured, at 1.7 times that of the Earth (which
would give it a volume 4.9 times Earth's).
The mass of COROT-7b is about 4.8 Earth masses,[2] so its density
is similar to Earth's. It is possible from this to exclude that the planet
is made purely of iron, but other compositions, including a
predominantly rocky one, are possible.[1] It orbits very close to its
star with an orbital period of 20 hours. The star, in the constellation
Monoceros, is 150 parsecs (490 ly) away and is slightly smaller than
the Sun.
The Kepler mission (Transit Method) continuously monitors one patch of sky
in the Cygnus region, monitoring over 100,000 main-sequence stars. The
expected mission lifetime is 3-5 years. Detection of sub-Earth size planets is
the mission's goal, with detection of planets with radii as small at 1 Mercury
radius is possible around M stars.
http://kepler.nasa.gov/
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Until the announcement of Kepler-10b in January 2011, it was the
smallest exoplanet to have its diameter measured, at 1.58 times that
of the Earth (which would give it a volume 3.95 times Earth's), and
the first potential extrasolar terrestrial planet to be found. The planet
is also notable for its very short orbital period, revolving around its
host star in less than one day.
Kepler-10b is the first confirmed terrestrial planet to have been
discovered outside the Solar System.[3] Discovered after several
months of data collection during the course of the NASA-directed
Kepler Mission, which aims to discover Earth-like planets crossing in
front of their host stars, the planet's discovery was announced on
January 10, 2011. Kepler-10b has a mass between 3.3 and 5.7
Earth masses and a radius of 1.4 Earth radii. However, it lies
extremely close to its star, Kepler-10, and as a result is too hot to
support life.
As of December 2011, there are a total of 2,326
candidates.[9][10] Of these, 207 are similar in size to Earth,
680 are super-Earth-size, 1,181 are Neptune-size, 203 are
Jupiter-size and 55 are larger than Jupiter. Moreover, 48
planet candidates were found in the habitable zones of
surveyed stars. The Kepler team estimated that 5.4% of all
stars host Earth-size planet candidates, and that 17% of all
stars have multiple planets.
Multiple-Planet Systems: The latest: HD10180
http://www.eso.org/public/archives/releases/sciencepapers/eso1035/eso1035.pdf
HD 10180 is a solar-type star that is thought to harbour seven planets, possibly
9!
The system contains five planets with minimum masses from 12 to 25 times
Earth's (comparable to the mass of the ice giant planets Uranus and
Neptune in our Solar System) at orbital radii of 0.06, 0.13, 0.27, 0.49 and
1.42 AU.
There is also an Earth-sized planet located at 0.02 AU (minimum mass 1.4
times Earth's; and a orbital period of 1.17 days.
A Saturn-sized giant planet at 3.4 AU (minimum mass 65 times Earth's.
Orbital radii ranging from about one seventeenth that of Mercury . The
outermost planet revolves at a distance from HD 10180 comparable to the
distance of the outer part of the main asteroid belt from our Sun.
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The planetary system contains no planets in mean motion resonances,
although it has a number of near resonances.[8] The approximate ratios of
periods of adjacent orbits are (proceeding outward): 1:5, 1:3, 1:3, 2:5, 1:5,
3:11.
Very massive systems are all found around metal-rich stars more massive than
the Sun, while low-mass
systems are only found around metal-deficient stars less massive than the Sun.
It thus appears that both quantities independently impact the mass of formed
planets. When both effects of stellar mass and metallicity are combined, we
obtain an even stronger correlation between total planetary system mass and
total metal content in the star.
Exoplamets are associated with: Fusing stars, Pulsars, Brown
dwarfs There is currently at least one known planet orbiting a
brown dwarf.
Direct detection: must be large and distant from star
RESOURCES
http://exoplanet.eu/
http://en.wikipedia.org/wiki/Extrasolar_planet
Book: Chapter 23 of Carroll & Ostlie, Modern Astronomy, second edition
http://exoplanets.org/
732 planets
3469 unconfirmed Kepler candidates
Candidates detected by microlensing
13 planets
12 planetary systems
1 multiple planet systems
Candidates detected by imaging
25 planets
22 planetary systems
1 multiple planet systems
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Candidates detected by timing
14 planets
9 planetary systems
4 multiple planet systems
+ some cluster and free floating?, plenty of candidates, retractions, ……
Names: According to astronomical naming conventions, the official
designation for a body orbiting a star is the star's catalogue number
followed by a letter. The star itself is designated with the letter 'a',
and orbiting bodies by 'b', 'c', etc
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Log scale:
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