3677 Life in the Universe: Extra-solar planets

1. 3677 Life in the Universe:Extra-solar planets Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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!

10. 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!

11. 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

12. 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

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

16. 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

17. 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!

18. 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

19. 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

20. M* from spectral type

21. 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

22. 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

23. Above: eccentric orbit (e=0.93) Top right: 55 Cancri multiple-planet system (4, maybe 5 planets) Bottom right: 3 planet HD37124 system

24. 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

25. 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

26. 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?