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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

3677 Life in the Universe:Extra-solar planets

Dr. Matt Burleigh


Course outline
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: secondary eclipses & transmission spectroscopy

    • Transit timing variations

Course outline1
Course outline

  • Lecture 3

    • Microlensing

    • Direct Imaging

    • Other methods: astrometry, eclipse timing

    • Planets around evolved stars

  • Lecture 4

    • Statistics: mass and orbital distributions, incidence of solar systems, etc.

    • Hot Jupiters

    • Super-Earths

    • Planetary formation

    • Planetary atmospheres

    • The host stars

Course outline2
Course outline

  • Lecture 5

    • The quest for an Earth-like planet

    • Habitable zones

    • Results from the Kepler mission

      • How common are rocky planets?

      • Amazing solar systems

    • Biomarkers

    • Future telescopes and space missions

Useful web sites
Useful web sites

  • Extra-solar planets encyclopaedia: exoplanets.eu

  • Exoplanet Data Explorer (California Planet Survey): exoplanets.org

  • NASA exoplanet archive: exoplanetarchive.ipac.caltech.edu

  • Planet hunters (Zooniverse): www.planethunters.org

  • Kepler mission: kepler.nasa.gov

  • Next Generation Transit Survey: www.ngtransits.org

Useful books
Useful books

  • Extrasolar planets & Astrobiology: Caleb A. Scharf

  • Extrasolar planets: the search for new worlds: Stuart Clark

  • Transiting Exoplanets: Carole A. Haswell

  • The Exoplanet Handbook: Michael Perryman

  • An Introduction to Astrobiology: Iain Gilmore & Mark Sephton

  • Life in the Universe: Bennett & Shostak

Useful numbers
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

What is a planet
What is a planet?

  • International Astronomical Union definition –

    • An object orbiting a star

    • Too small for dueterium fusion to occur

      • Less than 13 times the mass of Jupiter

    • Formation mechanism?

      • Forms from a circumstellar disk of dust and gas around a young star

    • Lower mass limit – IAU decided that Pluto should be downgraded!

What is a planet1
What is a planet?

Above, left to right: limb of Sun, late M (red) dwarf, L brown dwarf, T brown dwarf, Jupiter.

The coolest stars, old brown dwarfs and gas giant planets have the same radii!

A brief history of extra solar planets
A brief history of extra-solar planets

  • 16th century: the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round them

  • 19th Century: astronomers believed orbital anomalies in the binary star 70 Oph could be explained by an unseen planet, but later disproved

  • 1950s & 60s: Peter van de Kamp concluded that irregularities in the high proper motion of nearby Barnard’s Star were caused by a planet. Sadly, this too turned out to be erroneous.

  • late 1980s: Canadian Gordon Walker found tentative evidence for exoplanets using radial velocity method: but not confirmed until 2000s!

  • 1991: Andrew Lyne & Setnam Shemar at Jodrell Bank claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. They withdrew the claim later that year due to an error in their calculations.

New York Times 16th April 1963

A brief history of extra solar planets1
A brief history of extra-solar planets

  • 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+12

    • Variations in arrival times of pulses suggests presence of three or more planets

    • Planets probably formed from debris left after supernova explosion

  • 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” method

    • Most successful detection method by far, but other methods like transits are now very successful

  • >1000 exoplanets confirmed to date by all methods

    • >100 found since I gave this lecture last year

    • Kepler has several thousand more candidates

Blue: radial velocity, Green: transiting, Red: microlensing, Orange: direct imaging, Yellow: pulsar timing

Pulsar planets
Pulsar planets

  • Pulsars are neutron stars that emit radio pulses every ~second as they spin

    • More stable and accurate than an atomic clock

  • If a planet accompanies the pulsar, then the pulsar will orbit the centre of mass of the system

    • The pulses will then arrive earlier or later than expected

  • Radio observations have found a dozen or so such “pulsar planets”

    • Wolszczan & Frail’s discovery of PSR1257+12’s planets in 1991 at Arecibo in Puerto Rico were the first confirmed exoplanets

    • PSR1257+12’s planets are all ~Earth mass or smaller

Pulsar planets1
Pulsar planets

  • Pulsars are created when a massive star (>8Msun) explodes as a supernova

    • Their original planetary systems will not survive

    • Radio-detected planets thought to have formed from supernova debris

    • Planets will be bathed in high energy radiation from pulsar – no chance of life!

Planet hunting the radial velocity technique doppler wobble
Planet Hunting: The Radial Velocity Technique(“Doppler Wobble”)

  • Star + planet orbit common centre of gravity

  • As star moves towards observer, wavelength of light shortens (blue-shifted)

  • Light red-shifted as star moves away

873 planets detected by Doppler Wobble inc. 142 multiple systems

Measuring stellar doppler shifts
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 lo=observed wavelength, le=emitted wavelength

M* from spectral type

Doppler wobble method
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


Orbital plane


Example 51 peg
Example: 51 Peg

  • P= 4.15days = 4.15x86400s = 3.5856x105s

  • G5V star, M*=1.11Msun = 1.11x1.989x1030kg = 2.21x1030kg

  • Find r = 0.052AU, vpl=1.37x105ms-1Mplsin i = 0.45Mjup

Above: eccentric orbit (e=0.93)

Top right: 55 Cancri multiple-planet system (4, maybe 5 planets)

Bottom right: 3 planet HD37124 system

Harps radial velocity spectrograph
HARPS radial velocity spectrograph

  • Built by Geneva Observatory

  • First installed on ESO 3.6m at La Silla, Chile in 2002/3

  • Has found over 130 planets

  • Precision 30cm/s – 1m/s

  • Simultaneously observes star and a reference Thorium lamp through two separate fibres

  • Highly stable optical bench, housed in sealed, thermally stable room

  • Second HARPS installed on Italain Galileo telescope on la Palma 2012

Doppler wobble method1
Doppler Wobble Method

  • Precision of current surveys routinely <1m/s

    • Jupiter causes Sun’s velocity to vary by 12.5m/s

    • All nearby, bright Sun-like stars are good targets

      • Lots of lines in spectra, relatively inactive

    • Smallest planet found by this method is ~1Mearth: Alpha Cen Bb – nearest star system to us!

    • Most are Neptune size and larger

  • Length of surveys limits distances planets have been found from stars

    • Earliest surveys started 1988

    • Jupiter (5AU from Sun) takes 12 yrs to orbit Sun

    • Saturn takes 30 years

      • Would be strongly hinted at but not yet completed one orbit since surveys began

    • Do not see planet directly

Alpha cen bb
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?