3 lectures on: Extrasolar planets A/Prof. Quentin A Parker PHYS178 - other worlds: planets and planetary systems1

 Voyages, Chapter by Fraknoi, Morrison & Wolff  Astronomy & Geophysics, Vol 47, June 2006, The first cool rocky/icy exoplanet by Dominik, Horne & Bode  PHYS178 - other worlds: planets and planetary systems2

3 Based on slides produced by Simon O’Toole (AAO), Fraknoi’s book Voyages and various web resources Quentin A Parker Macquarie/AAO

2 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 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 systems6

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 PHYS178 - other worlds: planets and planetary systems7

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

 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..

PHYS178 - other worlds: planets and planetary systems10 The telescope was improved further and further… Galileo and his Refractive Telescope, 1609Herschel’s Reflecting Telescope, 1789 The Hooker Telescope - Mount Wilson, ca 1920

11  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

12 Different possible configurations for reflecting telescopes Mostly used by amateurs

13  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

14 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

15  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.

16 Examples of modern reflecting telescope mirror diameters 4m 8-10m

17  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

18 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

19  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 = 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 arcseconds

20  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.

21 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 systems22 And other planets were “discovered.” Uranus Pluto Neptune 5 The year 1781 The 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 PHYS178 - other worlds: planets and planetary systems23

 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 PHYS178 - other worlds: planets and planetary systems24

 In 1991, astronomers at Jodrell Bank claimed to have found a planet around the pulsar PSR  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 PHYS178 - other worlds: planets and planetary systems25

 In 1992, astronomers used Arecibo to find a planetary system around the pulsar PSR  Three planets: two about 4 Earth- masses and one lunar mass object PHYS178 - other worlds: planets and planetary systems26 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 to Discover a significant fraction of the planets that have been found around other stars so far.

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