Transcript Revision Lecture

DEPARTMENT OF PHYSICS AND ASTRONOMY
3677 Life in the Universe
Extra-solar planets: Revision
Dr. Matt Burleigh
www.star.le.ac.uk/mrb1/lectures.html
Dr. Matt Burleigh
3677: Life in the Universe
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
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
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
Dr. Matt Burleigh
Earth
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!)
• If the Kepler field of view is taken to be
representative of the local neighbourhood
then this 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”
Dr. Matt Burleigh
3677: Life in the Universe
Planet Hunting: The Radial Velocity Technique
(“Doppler Wobble”)
• Star + planet orbit common centre
of gravity
• As star moves towards observer,
wavelength of light shortens (blueshifted)
• Light red-shifted as star moves
away
873 planets detected by Doppler
Wobble inc. 142 multiple systems
Dr. Matt Burleigh
3677: Life in the Universe
Measuring Stellar Doppler shifts
• Method:
– Observe star’s spectrum through a cell of iodine gas
– Iodine superimposes many lines on star’s spectrum
– Measure wavelength (or velocity) of star’s lines relative to the
iodine
• Measure:
 Dl / le = (l0-le) / le = vr / c
Dr. Matt Burleigh
lo=observed wavelength, le=emitted wavelength
3677: Life in the Universe
M* from spectral type
Dr. Matt Burleigh
3677: Life in the Universe
Doppler Wobble Method
• Since measure K (= v* sin i), not v* directly, only know
mass in terms of the orbital inclination i
• Therefore only know the planet’s minimum mass, M sin i
– If i=90o (eclipsing or transiting) then know mass exactly
Orbital
plane
i=900
Orbital
plane
i0
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• Probability of transit (for random orbit)
Ptransit
R*
=
a
– For Earth (a=1AU), Ptransit=0.5%
– But for close, “hot” Jupiters, Ptransit=10%
– Of course, relative probability of detecting Earths is lower
since would have to observe continuously for up to 1 year
• (See Kepler mission)
Dr. Matt Burleigh
3677: Life in the Universe
Transits
• For an edge-on orbit, transit duration is given by:
æ PR* ö
Dt= ç
÷
è πa ø
• Where P=period, a=semi-major axis of orbit
• Example: HD209458b
–P=3.52475 days = 304538s
–R*=1.15RSun = 1.15x6.955x108m
–a=0.04747AU=7.1x109m
–Δt=10920s=3.03hours
–Note for Earth (a=1AU) Δt=46668s=12.96hours
Dr. Matt Burleigh
3677: Life in the Universe