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DEPARTMENT OF PHYSICS AND ASTRONOMY
3677 Life in the Universe:
Extra-solar planets
Dr. Matt Burleigh
www.star.le.ac.uk/mrb1/lectures.html
Course outline
• Lecture 1
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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
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 5
– The quest for an Earth-like planet
– Results from the Kepler mission
– Habitable zones
– Biomarkers
– Future telescopes and space missions
Dr. Matt Burleigh
3677: Life in the Universe
Useful numbers
•
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RSun = 6.995x108m
Rjup = 6.9961x107m ~ 0.1RSun
Rnep = 2.4622x107m ~ 4Rearth
Rearth = 6.371x106m ~ 0.1Rjup ~ 0.01RSun
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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
Dr. Matt Burleigh
3677: Life in the Universe
Towards other Earths
Telescope
Method
Date
Corot (Fr)
Transits
2007
Kepler (NASA)
Transits
2009
GAIA (ESA)
Astrometry
2013
SIM (NASA)
Astrometry
2015 (?)
CHEOPS
(ESA)
TESS (NASA)
Transits
2017
Transits
2017
Plato (ESA)
Transits
2024
Darwin (ESA)
Imaging
2030+ (?)
39m E-ELT
Imaging
2021
JWST
Imaging
2018
Dr. Matt Burleigh
3677: Life in the Universe
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
3677: Life in the Universe
Kepler
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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
Dr. Matt Burleigh
Earth
3677: Life in the Universe
Kepler results
By Nov 2014 Kepler had identified 4,173
candidate planets (most unconfirmed)
Dr. Matt Burleigh
3677: Life in the Universe
Kepler results
Petigura et al. 2013:
• 11±4% of sun-like stars have an Earthsized 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
Dr. Matt Burleigh
3677: Life in the Universe
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: ”SuperEarths”
Dr. Matt Burleigh
3677: Life in the Universe
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”?
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
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
Dr. Matt Burleigh
3677: Life in the Universe
• 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
Dr. Matt Burleigh
3677: Life in the Universe
Tatooine-like planet Kepler-16-b
Dr. Matt Burleigh
3677: Life in the Universe
Kepler-11
• Multiple system of 6 planets
• Masses measured through transit timing variations
– Effect of gravity on time of centre of successive transits
Dr. Matt Burleigh
3677: Life in the Universe
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?
Dr. Matt Burleigh
3677: Life in the Universe
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
3677: Life in the Universe