Transcript Quentin Parker Lecture 1b - PowerPoint file.

3 lectures on:
Extrasolar planets
A/Prof. Quentin A Parker
PHYS178 - other worlds: planets and
planetary systems
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Voyages, Chapter 13.4-13.6 by Fraknoi,
Morrison & Wolff
Astronomy & Geophysics, Vol 47, June 2006,
The first cool rocky/icy exoplanet by Dominik,
Horne & Bode
http://www.exoplanet.eu/
PHYS178 - other worlds: planets and
planetary systems
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Quentin A Parker
Macquarie/AAO
Based on slides produced by Simon O’Toole (AAO),
Fraknoi’s book Voyages and various web resources
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There are 200 billion stars in our galaxy…
…one of them is our Sun.
PHYS178 - other worlds: planets and
planetary systems
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There are 200 billion stars in our galaxy . . . one of them is our Sun.
Our galaxy is so large that if you could travel at the speed of light, it
would take you 100,000 years to go from one side to the other. And
this giant "city of stars" known as the Milky Way Galaxy is only one of
billions of other galaxies beyond the Milky Way. There are more stars
in the universe than there are grains of sand on all the beaches of
Earth.…..
The center of our Milky Way Galaxy is located in the constellation of Sagittarius.
In visible light the lion's share of stars are hidden behind thick clouds of dust.
This obscuring dust becomes increasingly transparent at infrared wavelengths.
This 2MASS image, covering a field roughly 10 X 8 degrees (about the area of
your fist held out at arm's length) reveals multitudes of otherwise hidden stars,
penetrating all the way to the central star cluster of the Galaxy.
On a dark starry night, it seems as though we can see countless stars. In reality,
however, we can only see about 2000. If we made a model of our whole Milky
Way Galaxy that was 10 feet across, almost all the stars we could see with our
naked eyes on a clear, dark night would exist within a little bubble a few inches
(about 5 centimeters) across centered on our solar system. All the other stars in
our galaxy lie beyond.
The sun has eight planets…
…we know of one that has life.
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We now know there are other planets in
the universe outside of our solar system
But is there another Earth-like planet out there?
Does is harbour life….?
Intelligent life?
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The year 1584
"There are countless suns and countless
earths all rotating around their suns in
exactly the same way as the seven
planets of our system... The countless
worlds in the universe are no worse
and no less inhabited than our Earth”
Giordano Bruno
in De L'infinito
Universo E Mondi
1995
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planetary systems
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Philosophical conjecture and Religious dogma
over the centuries was GRADUALLY replaced
by more scientific rationale
This was made possible through the advent of
technology
There was one invention in particular which
has transformed our understanding of our
Universe..
The telescope was improved further
and further…
Galileo and his Refractive Telescope, 1609
Herschel’s Reflecting Telescope, 1789
The Hooker Telescope Mount Wilson, ca 1920
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planetary systems
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Uses a concave mirror as the primary (objective)
Once ground to the correct shape the special glass (such as cervit
or Zerodur) is coated with a reflective surface to create a mirror
which reflects light (Q: desirable properties for the glass?)
The primary mirror brings light to a focus at the so called `prime
focus’. Until the advent of modern CCD detectors astronomers
would often `ride’ in the prime focus cage when performing
astronomical photographic imaging
A small secondary mirror is often placed at prime or in the
converging beam to redirect light to a more conveniently located
focus.
Most modern professional large telescopes use Cassegrain or
prime focus
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Different possible configurations for reflecting telescopes
Mostly used
by amateurs
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Light gathering Power
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Resolving power
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The ability of a telescope to collect light
The ability of a telescope to reveal fine detail
Magnifying power (least important)
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the ability to make a resolved image bigger
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Telescope Light Gathering Power
The mirror as a photon bucket!
A larger diameter mirror
collects more light and
has a brighter image than
a smaller telescope of the
same focal length.
LGP is proportional to the area
of the telescope objective and
not the diameter.
The area of a circular lens/mirror
of diameter `D’ is: π (D/2)**2
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Consider the LGP of two telescopes A & B of diameters
DA and DB
Calculate the ratio of the areas of their objectives
(which reduces to the ratio of their diameters D
squared).
 LGPA/LGPB = (DA/DB) **2
 So if DA = 4m & DB = 1m then telescope A collects
16 times as much light as telescope B
 Hence a small increase in mirror diameter produces
a large increase in light-gathering power allowing
astronomers to probe to much fainter limits.
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Examples of modern reflecting telescope mirror diameters
4m
8-10m
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Light behaves as a wave and thus produces a small
diffraction fringe around every point of light in the
image
We cannot see any detail finer than the fringe size
Such fringes cannot be eliminated but the larger the
telescope diameter the smaller the fringes
Hence the large the telescope objective the higher the
resolving power and the ability to reveal fine detail
For optical telescopes we estimate resolving power by
calculating the minimum angular separation between
two stars which can just be distinguished as being two
objects
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Diffraction rings and telescope resolving power
Stars are so far away that their images are point sources.
However the wave nature of light surrounds each star image
with diffraction fringes. Closely separated stars can have
overlapping diffraction patterns that limits resolution at a
separation that depends purely on the telescope mirror
diameter in the absence of an atmosphere
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The resolving power α, in seconds of arc is
given by 11.6 divided by the telescope primary
mirror diameter in cm.
i.e. α = 11.6/D
e.g. if D = 100cm then α = 11.6/100 = 0.116
arcseconds – this is the diffraction limit of this
telescope
In practice for earth bound telescopes
turbulence in the earths atmosphere rarely
permits imaging better than 0.2-0.5arcseconds
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Apart from optical imperfections which can limit imaging
performance (but which these days is not usually an issue) the
most significant factor in determining the resolution of any
earthbound telescope with D>1m is the atmosphere.
When we image through a telescope we are looking through
tens of Km of the earth’s atmosphere. Turbulence here makes
any image dance and blur in an effect we refer to as `seeing’
Rarely if ever is the inherent diffraction limit of a large
telescope reached regardless of atmospheric stability though
obviously the higher the observatory is situated the less
atmosphere there is to contend with.
Note also that the final resolution in an astronomical image is
determined by the resolution of the detector used to record the
image and the accuracy with which measurements can be
made.
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Adaptive optics
Recent advances in wavefront sensing allows active
compensation of the
wavefront and permits
sub-arcsecond imaging and
dramatically sharper images.
No AO correction
AO corrected
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And other planets were “discovered.”
Uranus
The year 1781
The first planet “discovered.”
William and Caroline Herschel
Neptune The year 1846
First observed by Galle and d'Arrest
(based on calculations by Adams
and Le Verrier).
Pluto
The year 1930
Discovered by Clyde Tombaugh
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Putative evidence for the existence of planets
outside our solar system has been presented
before..
They proved to be a false dawn until quite
recently
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planetary systems
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In 1963, Peter Van de Kamp
claimed to have found a planet
around Barnard’s Star using
astrometry
It orbited at 4.4 AU and was
1.6 Jupiter masses
Sadly, it was shown in 1973 to
be a systematic measurement
error, and not a planet
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planetary systems
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In 1991, astronomers at Jodrell Bank claimed to have found
a planet around the pulsar PSR 1829-10
By measuring the arrival times of the object’s pulses, they
determined a 10 Earth-mass planet orbited every 6 months
The following year they realised they had not accounted for
the eccentricity of Earth’s orbit, and the planet was
retracted
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In 1992, astronomers used Arecibo
to find a planetary system around
the pulsar PSR 1257+12
Three planets: two about 4 Earthmasses and one lunar mass object
• In 1995, two Swiss astronomers found the
first extra-solar planet around a Sun-like
star
• The planet around 51 Pegasi orbits in only
4 days!
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Michel Mayor and Didier Queloz of the Geneva Observatory
were the first to discover a giant planet around a sun-like star.
They are continuing their work using telescopes in Europe and
in Chile and have found several other planets since 1995.
Giant Planet Close to a Sun-Like Star
This artist’s concept shows what the giant planet
discovered orbiting the star 51 Pegasi might look like
close up.
This planet was the first of over a dozen jovian planets
found around other stars whose orbits turned out
smaller than the orbit of Mercury in our own system.
The planet around 51 Pegasi is at a distance of ~7million
Km from its star, taking a mere 4.2 days to complete
its orbit.
The artist has shown prominences and sunspots on
51 Pegasi, evidence of an active atmosphere that might
extend a significant way to the giant planet.
The planet is shown with bands like Jupiter, although our
measurements can only allow us to estimate the mass of
the planet, not its density, and thus we have no idea
what sorts of materials the planet is made of.
(Painting by Lynette Cook)
Paul Butler and Geoff Marcy were both at San Francisco State University when they
confirmed the discovery of the planet around 51 Pegasi and went on to
Discover a significant fraction of the planets that have been found around other stars so
far.
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planetary systems
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