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

Course outline Lecture 1 Lecture 2 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

Course outline Lecture 3 Lecture 4 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

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

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

Towards other Earths Telescope Method Date Corot (Fr) Transits 2007 Kepler (NASA) 2009 GAIA (ESA) Astrometry 2013 SIM (NASA) 2015 (?) CHEOPS (ESA) 2017 TESS (NASA) Plato (ESA) 2024 Darwin (ESA) Imaging 2030+ (?) 39m E-ELT 2021 JWST 2018

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)

Kepler Jupiter Earth 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

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

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

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”

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”?

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

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

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

Tatooine-like planet Kepler-16-b

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

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?

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