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3677 Life in the Universe: Extra-solar planets

3677 Life in the Universe: Extra-solar planets. Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html. Course outline. Lecture 1 Definition of a planet A little history Pulsar planets Doppler “ wobble ” (radial velocity) technique Lecture 2 Transiting planets Transit search projects

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3677 Life in the Universe: Extra-solar planets

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  1. 3677 Life in the Universe:Extra-solar planets Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html

  2. Course outline • Lecture 1 • Definition of a planet • A little history • Pulsar planets • Doppler “wobble” (radial velocity) technique • Lecture 2 • Transiting planets • Transit search projects • Detecting the atmospheres of transiting planets

  3. Course outline • Lecture 3 • Microlensing • Direct Imaging • Planets around evolved stars • Lecture 4 • Statistics: mass and orbital distributions, incidence of solar systems, etc. • Hot Jupiters • Super-Earths • Planetary formation • The host stars

  4. Course outline • Lecture 5 • The quest for an Earth-like planet • Results from the Kepler mission • Habitable zones • Biomarkers • Future telescopes and space missions

  5. Useful numbers • RSun = 6.995x108m • Rjup= 6.9961x107m ~ 0.1RSun • Rnep= 2.4622x107m ~ 4Rearth • Rearth= 6.371x106m ~ 0.1Rjup ~ 0.01RSun • MSun= 1.989x1030kg • Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth • Mnep= 1.02x1026kg ~ 5x10-5MSun~ 0.05Mjup = 17.15Mearth • Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup • 1AU = 1.496x1011m • 1 day = 86400s

  6. Towards other Earths

  7. 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)

  8. Kepler • Searching for Earths by transit method • Launched 2009 by NASA • 0.95m schmidt telescope, FOV ~ 105 square deg. • 150,000 MS stars, with V=10 - >14 mag • Aim: find an Earth around a Sun-like star in a one year orbit • Need three transits • So mission lasts at least three years… Jupiter Earth

  9. Kepler results By Nov 2014 Kepler had identified 4,173 candidate planets (most unconfirmed)

  10. Kepler results • Petigura et al. 2013: • • 11±4% of sun-like stars have an Earth-sized planet receiving 1 to 4 times the stellar intensity that Earth receives • define “Earth-sized” as 1-2REarth • • 5.7% (+1.7% - 2.2%) of Sun-like stars have an Earth-sized planet with orbital periods between 200 & 400 days • • 22% of Sun-like stars have Earth-sized planets orbiting within their habitable zones • (generous definition of HZ includes Venus & Mars!) • Implies >10 billion Earths in milky way galaxy! • • suggests that the nearest such planet could be within 12 light years

  11. Kepler’s limitations • Kepler observes probable transit events • “Validates” these • with 3 transits • tries to rule out background binaries/triple star systems • Statistical likelihood planet is “real” • But transit only gives radius • To confirm it’s a planet, need mass, and hence density • Confirm structure: gas giant or terrestrial • Although we don’t know where boundary lies: ”Super-Earths”

  12. Kepler’s limitations • Mass comes from two sources • Radial velocity observations • But most Kepler stars are too faint for Doppler Wobble measurements with current telescopes • Transit timing variations • Kepler & media confuse planet status • “Earth-sized”? • “Earth-like”? • “Earth-mass”?

  13. Earth-like planet detected! • “Astronomers Find First Habitable Earth-Like Planet” • Science Daily, 24 April 2007 • “Gliese 581g: the most Earth like planet yet discovered” • Daily Telegraph, 29 Sep 2010 • “Super-earth exoplanet found that could support life” • The Guardian, 13 Sep 2011

  14. Kepler-22b • Kepler has observed 3 transits • Lies within its star’s habitable zone • 290 day orbit • The radius is 2.4x Earth • NO mass measurement • Upper limit of 38x Earth • Could be rocky, or gaseous • Host star too faint (>11th) for RV

  15. Ranked by “Earth Similarity Index” where Earth = 1.00 • See http://phl.upr.edu/projects/earth-similarity-index-esi • Define “Earth-like” as ESI>0.9: none meet this criterion

  16. Tatooine-like planet Kepler-16-b

  17. Kepler-11 • Multiple system of 6 planets • Masses measured through transit timing variations • Effect of gravity on time of centre of successive transits

  18. Alpha Cen Bb • Alpha Cen system is the nearest star system to us • Alpha Cen B has been monitored by radial velocity method • Very recent discovery of a rocky planet: • Minimum mass 1.1xEarth • Period 3.2 days • Dumusque et al. 2012, Nature • Thought: if Alpha Cen B has a rocky planet, do most stars have rocky planets?

  19. 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

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