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PH507
Astrophysics
Prof M Smith
1
EXOPLANETS: Prof Michael SMITH
TOPICS COVERED
1.
2.
3.
4.
5.
Introduction: Review & Status
Definitions, planets, disks.
Discovery and detection methods
Populations
Properties: metallicity, eccentricity
RESOURCES
MOODLE
+
Module page:
http://astro.kent.ac.uk/mds/Modules/modules.htm
THE EXOPLANET NEWSLETTER:
Past editions of the Newsletter http://exoplanet.open.ac.uk
TABLES AND DATA
http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nphexotbls?dataset=planets
see Table Plotter
and
http://exoplanets.org
http://exoplanet.eu/
http://en.wikipedia.org/wiki/Extrasolar_planet
Watch latest on discovery methods:
http://www.youtube.com/watch?v=uicxfBcQIog
See latest on discoveries:
http://adsabs.harvard.edu/abs/2013arXiv1307.2944A
http://exoplanetarchive.ipac.caltech.edu
DESCRIPTION
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Astrophysics
Prof M Smith
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Definition of a Planet:
• Discovery of large (bigger than Pluto) trans-Neptunian Objects
(e.g. Eris, which has a mass that is over 25% bigger than
Pluto) led to a problem – were these planets? Was Pluto not a
planet?
• International Astronomical Union ruled (in 2006) that a planet:
Is in orbit around the Sun.
Has sufficient mass to be in hydrostatic equilibrium and hence
roughly spherical.
Has “cleared the neighbourhood” of its orbit.
• If only the first two criteria are fulfilled, the object is defined as
a Dwarf Planet – e.g. Ceres (orbits Sun, is spherical, but lies in
asteroid belt) and Pluto (orbits Sun, is spherical, but has not
cleared orbital region).
• What do you think?
An extrasolar planet, or exoplanet, is a planet outside the Solar
System.
1. Outside our solar system
2. Nuclear fusion not significant
3. Less than 13/1000 of our Suns mass
= Less than 13 Jupiter masses
Definition of a star/brown dwarf/planet
• Stars: burn hydrogen (M > 0.075 Msun)
• Brown dwarfs: burn deuterium
• Planets: do not burn deuterium (M < 0.013 Msun)
Deuterium burning limit occurs at around 13 Jupiter masses (1 MJ =
1.9 x 1027 kg ~ 0.001 Msun
objects, there is no large change in properties at the deuterium
burning limit. ALL young stars / brown dwarfs / planets liberate
gravitational potential energy as they contract
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Astrophysics
Prof M Smith
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STATUS: Since the first exoplanet discoveries in the 1990s
(Wolszczan & Frail 1992; Mayor & Queloz 1995)
1992: 1 discovered
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
Showing 1125 planetary systems / 1810 planets / 466
multiple planet systems as of 20/07/14
Showing 1160 planetary systems / 1849 planets / 471
multiple planet systems as of 01/11/14
Showing 1331 planetary systems / 2085 planets / 509
multiple planet systems as of 20/2/16
DETECTION: Although few of the planets have been directly
imaged, the effects of the gravity tugging at the stars, as well as the
way that gravitation affects can affect material close to the stars, has
been clearly seen.
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Astrophysics
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Disc of material around the star Beta Pictoris – the image of the bright
central star has been artificially blocked out by astronomers using a
‘Coronograph’
Types of planet
Giant planets (gas giants, `massive’ planets)
• Solar System prototypes: Jupiter, Saturn, Uranus...
• Substantial gaseous envelopes
• Masses of the order of Jupiter mass
• In the Solar System, NOT same composition as Sun
• Presence of gas implies formation while gas was still
prevalent
Cores: Gas giants may have a rocky or metallic core—in fact,
such a core is thought to be required for a gas giant to form.
H and He: The majority of its mass is in the form of the
gaseous hydrogen and helium, with traces of water, methane,
ammonia, and other hydrogen compounds.
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Astrophysics
Prof M Smith
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Terrestrial planets
Mercury: 75% iron/nickel :
• Prototypes: Earth, Venus, Mars
• Primarily composed of silicate rocks (carbon/diamond planets?)
• In the Solar System (ONLY) orbital radii less than giant
planets
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Astrophysics
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Much more massive terrestrial planets could exist (>10 Earth
masses), though none are present in the Solar System. The Solar
system also has asteroids, comets, planetary satellites and rings we won’t discuss those in this course.
Core: A central metallic core, mostly iron with a surrounding silicate
mantle. The Moon is similar, but lacks an iron core.
Terrestrial planets have canyons, craters, mountains, and
volcanoes.
Types of EXOplanet
Hot Jupiters
– 300 earths
Neptune-Twins – 17 earths
Super-Earths
Earth-Twins
(since 2011)
Mercury?
Small planets between the sizes of Earth and Neptune substantially
outnumber Jupiter-sized planets.
NAMING CONVENTION
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', et
Currently the most important class of exoplanets are those that
transit the disk of their parent stars, allowing for a determination
of planetary radii.
TRANSITS: KEPLER MISSION SURVEY
Kepler was launched in 2009 and obtained precise photometric
measurements with nearly continuous coverage for four years.
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The Kepler mission (Transit Method) continuously monitored one
patch of sky in the Cygnus region, monitoring 160,000 mainsequence stars. Detection of sub-Earth size planets was the
mission's goal, with detection of planets with radii as small at 1
Mercury radius possible around M stars.
http://kepler.nasa.gov/
Search of Kepler stellar light curves for the characteristic dips in flux
indicative of a planet
Kepler-10b is the first confirmed terrestrial planet to have been
discovered outside the Solar System
The Kepler mission has recently discovered a number of
exoplanetary systems, such as Kepler-11 and Kepler-32, in which
ensembles of several planets are found in very closely packed orbits
(often within a few per cent of an AU of one another).
The Kepler-11 planetary system contains six transiting planets
ranging in size from 1.8 to 4.2 times the radius of Earth. Five of
these planets orbit in a tightly packed configuration with periods
between 10 and 47 days.
CIRCUMBINARY & CIRCUMPRIMARY PLANETS
Kepler-16: We report the detection of a planet whose orbit surrounds a
pair of low-mass stars.
http://adsabs.harvard.edu/abs/2011Sci...333.1602D
PH507
Astrophysics
Prof M Smith
Intensity of starlight from Kepler 11
Kepler 11 b c d e f g
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TOP: Geometry of the Kepler 16b stellar system. Two stars move
about their center of mass, while Kepler 16b orbits both stars.
LOWER: Kepler 16b is Saturn-like, but the view from its cloud tops
could be similar to the view imagined from the planet Tatooine in
the movie Star Wars.
Atmosphere of Planet
Gas giants possess primary atmospheres — atmospheres
captured directly from the originalsolar nebula.
Terrestrial planets possess secondary atmospheres —
atmospheres generated through internal vulcanism and comet
impacts.
Temperature of Planet
Estimate the temperature of an exoplanet based on the intensity of
the light it receives from its parent star.
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Astrophysics
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We calculate the equilibrium blackbody temperature of a
planet. We assume that thermal equilibirium (i.e., constant
temperature) applies, and consequently that the power ( =
energy/time) emitted by the planet is the power absorbed from its
parent star:
Pabsorbed = Pemitted
The left hand side is found from geometry, corrected by a coefficient
that takes into account reflected light; the right hand side is given by
the Stefan-Boltzmann law:
Lstar (1 - A) ( Rp/4  dp)2 = 4 
p
2
 Tp4
Lstar = luminosity (power) of the parent star
A = planet's albedo = (light reflected)/(light incident)
Rp = planet's radius
Tp = planet's temperature
dp = distance of planet from parent star
= Stefan-Boltzmann constant
Solving for Tp gives
Tp4 = Lstar(1 - A)/(16
p
2
)
Albedo? Notice that the equilibrium temperature depends on the
"guessed" albedo of the planet;
This calculation doesn't take into account the thermal energy
released from the planet's interior, tidal energy released via a starplanet interaction, the greenhouse effect in the atmosphere, etc.
DISTRIBUTIONS: MASS, log(MASS), log(number)-log(MASS)
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Astrophysics
ECCENTRICITY
Prof M Smith
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Astrophysics
METALLICITY
Prof M Smith
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