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 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 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 .