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DEPARTMENT OF PHYSICS AND ASTRONOMY Extra-solar planets Dr. Matt Burleigh A brief history of extra-solar planets • In the 16th century the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round them • 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+12 – Variations in arrival times of pulses suggests presence of three or more planets – Planets probably formed from debris left after supernova explosion • 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” method – Most successful detection method by far, but other methods like transits are now very successful – 405 exoplanets found to date Dr. Matt Burleigh Planet Hunting: The Radial Velocity Technique (“Doppler Wobble”) • Star + planet orbit common centre of gravity 377 planets detected by Doppler Wobble inc. 38 multiple systems Dr. Matt Burleigh • As star moves towards observer, wavelength of light shortens (is blueshifted) • Light red-shifted as star moves away Planet Hunting 2: Transits Dr. Matt Burleigh Transits • The depth of the transit is simply the ratio of the planetary and stellar disk areas: – pRp2 / pR*2 = (Rp / R*)2 – so we can measure the planet’s radius • But the probability of a transit is very low: only 1 in >10,000 stars has a transiting planet Dr. Matt Burleigh Super WASP • Wide Angle Search for Planets (by transit method) • First telescope located in La Palma, second in South Africa • Operations started May ‘04 • Data stored and processed at Leicester • >20 new planets detected • www.superwasp.org • www.wasp.le.ac.uk Dr. Matt Burleigh Super WASP • SuperWASP monitors about 1/4 of the sky from each site • That means millions of stars, every night! Dr. Matt Burleigh “Hot Jupiter” planets • Doppler Wobble and transit surveys find many gas giants in orbits of 2-4 days – cf Mercury’s orbit is 80 days • Surveys are biased towards finding them – Larger Doppler Wobble signal – Greater probability of transit • These planets are heated to >1000oF on “day” side – And are “tidally locked” like the Moon – Causes extreme weather conditions • Must have formed much farther away from star and migrated inwards – Environment too hot for dust and ice to form Dr. Matt Burleigh Planet Hunting 3: Direct Imaging • Last year 3 planets imaged around the star HR8799 • 130 light years away (40pc) • Three planets at 24, 38 and 68AU separation – In comparison, Jupiter is at 5AU and Neptune at 30AU • Masses of 7Mjup, 10Mjup and 10Mjup Dr. Matt Burleigh Fomalhaut (alpha Piscis Austrini) • One of the brightest stars in the sky (southern hemisphere) • There is a dust disk around Fomalhaut • Sub-mm image taken from James Clarke Maxwell telescope on Hawaii • Disk has a hole in centre like a doughnut • Part of disk appears to be perturbed • Is there a giant planet orbiting Fomalhaut? Dr. Matt Burleigh Fomalhaut (alpha Piscis Austrini) • 2Mjup planet imaged inside disk • 200Myr old • Like early solar system Dr. Matt Burleigh Dr. Matt Burleigh What we know about extra-solar planets • 405 planets now found • 38 multiple systems • 62 transiting planets • Unexpected population with periods of 2-4 days: “hot Jupiters” • Planet with orbits like Jupiter discovered (eg 55 Cancri d) • Smallest planet: CoRoT-7b - 1.7Rearth Extra-solar planet masses • Mass distribution peaks at 1-2 x mass of Jupiter • Lowest mass planet so far: 5.5xMEarth Dr. Matt Burleigh Results of the Planet Hunting surveys • Of 2000 stars surveyed – 5% have gas giants between 0.02AU and 5AU – 10% may have gas giants in wider orbits – <1% have Hot Jupiters • How many have Earths…..? Dr. Matt Burleigh Hunting for Earth-like planets Dr. Matt Burleigh Towards other Earths Telescope Method Date Corot (Fr) Transits 2007 Kepler (NASA) Transits 2008 GAIA (ESA) Astrometry 2012 SIM (NASA) Astrometry 2015 (?) Plato (ESA) Transits 2017 Darwin (ESA) Imaging 2025+ (?) 42m E-ELT Imaging 2018 Dr. Matt Burleigh Kepler • • • • • Searching for Earths by transit method Launched last year by NASA Aims to find an Earth around a Sun-like star in a one year orbit Need three transits to confirm So mission lasts at least three years… Dr. Matt Burleigh Towards Other Earths: Habitable Zones • Habitable zone defined as where liquid water exists • Changes in extent and distance from star according to star’s spectral type (ie temperature) Dr. Matt Burleigh Towards Other Earths: Biomarkers • So we find a planet with the same mass as Earth, and in the habitable zone: – How can we tell it harbours life? • Search for biomarkers – Water – Ozone – Albedo Dr. Matt Burleigh Recommended reading • • • • • • Wikipedia Extra-Solar Planets NASA JPL PlanetQuest University of California Planet Search Extra-Solar Planets Encyclopaedia SuperWASP Kepler Dr. Matt Burleigh