3 lectures on:

Extrasolar planetsA/Prof. Quentin A Parker

PHYS178 - other worlds: planets and planetary systems 1

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 2

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Based on slides produced by Simon O’Toole (AAO), Fraknoi’s book Voyages and various web resources

Quentin A ParkerMacquarie/AAO

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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 4

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.PHYS178 - other worlds: planets and planetary systems 6

But is there another Earth-like planet out there? Does is harbour life….?Intelligent life?

We now know there are other planets in the universe outside of our solar system

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PHYS178 - other worlds: planets and planetary systems 8

1995

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 Brunoin De L'infinito

Universo E Mondi

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..

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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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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 usedby amateurs

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Light gathering Power The ability of a telescope to collect light

Resolving power The ability of a telescope to reveal fine detail

Magnifying power (least important) 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 mirrorcollects more light and has a brighter image thana smaller telescope of the same focal length.

LGP is proportional to the areaof the telescope objective and not the diameter.

The area of a circular lens/mirrorof 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 mirrordiameter 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

No AO correction

Recent advances in wave-front sensing allows active compensation of the wavefront and permits sub-arcsecond imaging and dramatically sharper images.

AO corrected

PHYS178 - other worlds: planets and planetary systems 22

And other planets were “discovered.”

Uranus

Pluto

Neptune

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The year 1781The first planet “discovered.” William and Caroline Herschel

The year 1846 First observed by Galle and d'Arrest (based on calculations by Adams and Le Verrier).

The year 1930 Discovered by Clyde Tombaugh

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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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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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 Earth-masses and one lunar mass object

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• 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!

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 toDiscover a significant fraction of the planets that have been found around other stars so far.

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PHYS178 - other worlds: planets and planetary systems 31