PH709
Extrasolar Planets
Professor Michael Smith
1
EXOPLANETS: Prof Michael SMITH
ph709-08-part1.doc
ph709-08-part2.doc
ph709-08-part3.doc
http://astro.kent.ac.uk/mds/Modules/modules.htm
TOPICS COVERED IN COURSE
Intro
1. Measurement and Theory: Dynamics, Binaries
2. Exoplanets
3. Detection
Techniques
Measurement
Derived Physical Information
4. Summary of detection
5. Populations, Distributions, Metallicity, Eccentricity
6. Star and Planet Formation
The accretion disk to the debris disc
Formation models
Evolution: Migration/eccentricity
Review of the Latest Status
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 312, 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.
PH709
Extrasolar Planets
Professor Michael Smith
2
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.
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 super-earth. 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 0.25 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 581 c and indicates that the third planet in the system, Gliese
581 d, 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.
PH709
Extrasolar Planets
Professor Michael Smith
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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")
Sources and sinks of atmospheric carbon dioxide. The photosynthesissustaining habitable zone (pHZ) is determined by the limits of biological
productivity on the planetary surface.
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.
PH709
Extrasolar Planets
Professor Michael Smith
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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.
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.
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 14 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 51 transiting planets are 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 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.
PH709
Extrasolar Planets
Professor Michael Smith
5
UPCOMING SPACE MISSIONS
The French/European COROT mission, launched in 2006 December, and
the American Kepler mission, set to launch in 2009 Nmarch 4 will revolutionize
the study of exoplanets. COROT will monitor 12,000 stars in each of five
different fields, each for 150 continuous days (Bordé et al. 2003).
COROT detected its first extrasolar planet, COROT-Exo-1b, in May 2007.
Planets as small as RE could be detectable around solar-type stars (Moutou et
al 2006). The mission lifetime is expected to be at least 2.5 yr.
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
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.
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.
PH709
Extrasolar Planets
Professor Michael Smith
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Resources. For observations, a good starting point is Berkeley extrasolar planets search
homepage
http://exoplanets.org/
Number: There are 228 confirmed planets listed.
Of the 312 candidates — there are 31 multiple planet systems, 178 in single
planet systems, 5 orbiting pulsars.
3 free floating?, plenty of candidates, retractions, cluster planets,……
Detection method: 293 planets have been detected by radial velocity or
doppler method, 51 by transit method, 8 by microlensing, 6 by direct
imaging, and 5 by timing method.
Mass: The planets are listed with indications of their approximate
masses as multiples of Jupiter 's mass (MJ = 1.898 × 1027 kg) or
multiples of Earth's mass (ME = 5.9737 × 1024 kg). Neptune = 17.1
ME, Mercury = 0.0553 ME.
Orbit/Distance: approximate distances in astronomical units (1) AU
= 1.496 108 km, distance between Earth and Sun) from their parent
stars.
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
There are currently (2007) 238 planets known in orbit around fusing stars.
There are currently 178 known planets in single-planet systems and 60 known planets
in 20 multiple-planet systems (14 with two planets, 4 with three and 2 with four).
"Single" here means that only one planet has been detected to date.
Detection methods are not sensitive to low-mass planets, these stars may have smaller
planets that are below the limits of detectability, or are so far from the star that they have
not yet been observed over an orbital period. Could ALL stars harbour planets?
Pulsars
There are currently four known planets orbiting two different pulsars. The planet of PSR
B1620−26 is in a circumbinary orbit around a pulsar and a white dwarf star.
PH709
Extrasolar Planets
Professor Michael Smith
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Brown dwarfs
There is currently one known planet orbiting a brown dwarf.
Free floating planets
There is currently one suspected free-floating planet, i.e. it doesn't appear to orbit a star.
PH709
Extrasolar Planets
Professor Michael Smith
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PH709
Extrasolar Planets
Professor Michael Smith
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PH709
Extrasolar Planets
Professor Michael Smith
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PH709
Extrasolar Planets
Professor Michael Smith
11
IN 2008 ……..(wikipedia)
2008, OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc
On February 14 the discovery of the, until now, most similar
Jupiter-Saturn planetary system constellation was announced,
with the ratios of mass, distance to their star and orbiting time
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Extrasolar Planets
Professor Michael Smith
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similar to that of Jupiter-Saturn. This can be important for
possible life in a solar system as Jupiter and Saturn have a
stabilizing effect to the habitable zone by sweeping away large
asteroids from the habitable zone.[50]
2008, HD 189733 b
On March 20 follow up studies to the first spectral analyses of
an extrasolar planet were published in the scientific journal
Nature, announcing evidence of an organic molecule found
on an extrasolar planet for the first time. In 2007 water vapor
was already detected in the spectrum of HD 189733 b, but
new analyses showed not only water vapor, but also methane
existing in the atmosphere of the giant gas planet. Although
conditions on HD 189733 b are too harsh to harbor life, it still
is the first time a key molecule for organic life was found on an
extrasolar planet.[51]
2008, HD 40307
On June 16, Michel Mayor announced a confirmed planetary
system with three super-Earths orbiting this K-type star. Their
masses are between 4 to 9 Earth masses and with periods
between 4 to 20 days. It is speculated that this may be the first
multi-planetary system without any known gas giants. All three
terrestrial planets were discovered by the HARPS
spectrograph in La Silla, Chile.
The discoveries represented a significant increase in the
numbers of known super-earths. Based on this, astronomers
now suggest that such low-mass planets may outnumber the
Jupiter-like planets by 3 to 1.