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 – 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
Dr. Matt Burleigh 3677: Life in the Universe 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
Dr. Matt Burleigh 3677: Life in the Universe 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
Dr. Matt Burleigh 3677: Life in the Universe 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
Dr. Matt Burleigh 3677: Life in the Universe 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
Dr. Matt Burleigh 3677: Life in the Universe Useful numbers
8 • RSun = 6.995x10 m 7 • Rjup = 6.9961x10 m ~ 0.1RSun 7 • Rnep = 2.4622x10 m ~ 4Rearth 6 • Rearth = 6.371x10 m ~ 0.1Rjup ~ 0.01RSun
30 • MSun= 1.989x10 kg 27 • Mjup= 1.898x10 kg ~ 0.001MSun = 317.8Mearth 26 -5 • Mnep= 1.02x10 kg ~ 5x10 MSun ~ 0.05Mjup = 17.15Mearth 24 -6 -3 • Mearth= 5.97x10 kg = 3x10 MSun = 3.14x10 Mjup
• 1AU = 1.496x1011m • 1 day = 86400s Dr. Matt Burleigh 3677: Life in the Universe 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!
Dr. Matt Burleigh 3677: Life in the Universe 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!
Dr. Matt Burleigh 3677: Life in the Universe 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 Barnardʼs 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 Dr. Matt Burleigh 3677: Life in the Universe 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 • >1700 exoplanets confirmed to date by all methods – Kepler has several thousand more candidates
Dr. Matt Burleigh 3677: Life in the Universe Blue: radial velocity, Green: transiting, Red: microlensing, Orange: direct imaging, Yellow: pulsar timing
Dr. Matt Burleigh 3677: Life in the Universe Dr. Matt Burleigh 3677: Life in the Universe Dr. Matt Burleigh 3677: Life in the Universe 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 Dr. Matt Burleigh 3677: Life in the Universe 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!
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 (blue-shifted) • Light red-shifted as star moves away
517 planets detected by Doppler Wobble and many more transiting planets confirmed by this method
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:
‒ - observed wavelength, =emitted wavelength Δλ / λe = (λ0 λe) / λe = vr / c λο= λe
Dr. Matt Burleigh 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 Example: 51 Peg
• P= 4.15days = 4.15x86400s = 3.5856x105s
• G5V star, M*=1.11Msun = 1.11x1.989x1030kg = 2.21x1030kg • Find r = 0.052AU, 5 -1 vpl=1.37x10 ms Mpl sin i = 0.45Mjup Dr. Matt Burleigh 3677: Life in the Universe Above: eccentric orbit (e=0.93)
Top right: 55 Cancri multiple-planet system (4, maybe 5 planets)
Bottom right: 3 planet HD37124 system
Dr. Matt Burleigh 3677: Life in the Universe 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 Italian Galileo telescope on la Palma 2012
Dr. Matt Burleigh 3677: Life in the Universe 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 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