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Extrasolar Planets - 1
Professor Michael Smith
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EXOPLANETS: Prof Michael SMITH
http://astro.kent.ac.uk/mds/Modules/modules.htm
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.
Candidate exoplanets now number 490, with masses as small as 5–7 ME
http://exoplanet.eu/catalog.php .
Comparative planetology, which once included only our solar system's
planets and moons, now includes sub-Neptune to super-Jupiter-mass planets in
other solar systems.
Overview: mass, distance and constitution
Mass:
Sun
Jupiter 's mass
Earth's mass
Sun
1.989 10 30 kg
MJ = 1.898 1027 kg
ME = 5.974 1024 kg
= 300,000
Jupiter =
Neptune =
Mercury =
ME
300
ME
17.1
ME
0.0553 ME.
Orbit/Distance: 1 astronomical units (AU) = 1.496 108 km, distance
between Earth and Sun
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Extrasolar Planets - 1
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Professor Michael Smith
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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
+
P
rock/ice
Density kg/m3
1
2
3
4
5
6
7
8
9
10
11
Earth
Mercury
Venus
Mars
Moon
Pluto
Neptune
Sun
Jupiter
Uranus
Saturn
5515
5427
5243
3933
3350
1750
1638
1408
1326
1270
687
Back to Review
HOT JUPITERS
We began in 1995 by 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 in 2010 we discuss earth twins …….
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Extrasolar Planets - 1
Professor Michael Smith
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DOPPLER SPECTROSCOPY: RADIAL VELOCITY METHOD
Gliese 581. 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.
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.
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;
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: 20C (albedo = 0.5 assumed) Greenhouse? Tidal locking?
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.
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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
2009: The HARPS search for southern extra-solar planets XVIII. An Earthmass planet in the GJ 581 planetary system (Mayor et al, A&A, 507, 487)
The GJ 581 planetary system was already known to harbour three planets,
including two super-Earths planets which straddle its habitable zone. We report
here the detection of an additional planet -- GJ 581e -- with a minimum mass of
1.9 Mearth. With a period of 3.15 days, it is the innermost planet of the system
and has a ~5% transit probability. We also correct our previous confusion of the
orbital period of GJ 581d (the outermost planet) with a one-year alias, thanks to
an extended time span and many more measurements. The revised period is
66.8 days, and locates the semi-major axis inside the habitable zone of the low
mass star.
The dynamical stability of the 4-planet system imposes an upper bound on
the orbital plane inclination. The planets cannot be more massive than
approximately 1.6 times their minimum mass.
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
.
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.
The confirmed transiting planets observed to date are 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 (see Charbonneau et al. 2007a). A
planetary mass and radius allows us a window into planetary composition
(Guillot 2005).
The 101 transiting planets are mainly gas giants although one planet, HD
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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 massradius 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 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.
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.
SPACE MISSIONS
The French/European COROT mission, launched in 2006 December, and
the American Kepler mission, launched 2009 March 6 will revolutionize the
study of exoplanets. COROT will monitor 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).
The Kepler mission (Transit Method) will continuously monitor one patch of
sky in the Cygnus region, monitoring over 100,000 main-sequence stars (Basri
et al. 2005). 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.
With these missions, perhaps hundreds of planets will be discovered
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with masses ranging from sub-Mercury to many times that of Jupiter.
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.
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.
INTERFEROMETRY (mid-IR ? )
There are several potential advantages to the use of interferometry for direct
detection of extrasolar planetary emission. Destructive interference can be
used to strongly suppress emission from the much brighter primary star. High
angular resolution, which can be significantly better that the diffraction limit of
the individual telescopes, will help to separate the emission from an
extrasolar planet and its primary star as well as from sources of background
emission.
RESOURCES
http://exoplanet.eu/
http://en.wikipedia.org/wiki/Extrasolar_planet
Book: Chapter 23 of Carroll & Ostlie, Modern Astronomy, second edition
Rapidly developing subject - first extrasolar planet around an ordinary star only
discovered in 1995 by Mayor & Queloz.
Resources. For observations, a good starting point is Berkeley extrasolar
planets search homepage
http://exoplanets.org/
490 planets
Candidates detected by radial velocity or astrometry
459 planets
388 planetary systems
45 multiple planet systems
Transiting planets
101 planets
100 planetary systems
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7 multiple planet systems
Candidates detected by microlensing
10 planets
9 planetary systems
1 multiple planet systems
Candidates detected by imaging
13 planets
11 planetary systems
1 multiple planet systems
Candidates detected by timing
8 planets
5 planetary systems
2 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
Fusing stars
Pulsars
Brown dwarfs
There is currently at least one known planet orbiting a brown dwarf.
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Professor Michael Smith
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Log scale:
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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
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.
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.
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