1. 3677 Life in the Universe: Extra-solar planets
Dr. Matt Burleigh

2. 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

3. 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

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 = x107m ~ 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 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

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 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

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
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