Download ph709-09

PH709
Extrasolar Planets
Professor Michael Smith
1
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 374, 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.
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.
DOPPLER SPECTROSCOPY: RADIAL VELOCITY METHOD
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
PH709
Extrasolar Planets
Professor Michael Smith
2
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 0.25 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.
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.
PH709
Extrasolar Planets
Professor Michael Smith
3
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
2009: The HARPS search for southern extra-solar planets XVIII. An Earthmass planet in the GJ 581 planetary system (Mayor et al, A&A, in press)
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
PH709
Extrasolar Planets
Professor Michael Smith
4
1.9 M_earth. 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 Mayor et al. (2003). 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 62 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
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
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.
PH709
Extrasolar Planets
Professor Michael Smith
6
RESOUCES
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/
374 planets
Candidates detected by radial velocity or astrometry
347 planets
295 planetary systems
35 multiple planet systems
Transiting planets
62 planets
62 planetary systems
2 multiple planet systems
Candidates detected by microlensing
9 planets
8 planetary systems
1 multiple planet systems
Candidates detected by imaging
11 planets
9 planetary systems
1 multiple planet systems
3 free floating?, plenty of candidates, retractions, cluster planets,……
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.
PH709
Extrasolar Planets
Professor Michael Smith
7
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.
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
8
PH709
Extrasolar Planets
Professor Michael Smith
9
PH709
Extrasolar Planets
Professor Michael Smith
10
PH709
Extrasolar Planets
Professor Michael Smith
11
PH709
Extrasolar Planets
Professor Michael Smith
12
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
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
PH709
Extrasolar Planets
Professor Michael Smith
13
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.