3/25/2013 Extrasolar planets Astronomy 9601 1 Topics to be covered • 12.1 Physics and sizes • 12.2 Detecting extrasolar planets • 12.3 Observations of exoplanets • 12.4 Exoplanet statistics • 12.5 Planets and Life 2 What is a planet? What is a star? • The composition of Jupiter closely resembles that of the Sun: who’s to say that Jupiter is not simply a “failed star” rather than a planet? • The discovery of low-mass binary stars would be interesting, but (perhaps) not as exciting as discovering new “true” planets. • Is there a natural boundary between planets and stars? 3 1 3/25/2013 Planets and brown dwarfs • A star of mass less than 8% Luminosity “bump” due to short- of the Sun (80x Jupiter’s lived deuterium burning mass) will never grow hot Steady luminosity due to H burning enough in its core to fuse hydrogen • This is used as the boundary between true stars and very large gas planets • Object s b el ow thi s mass are called brown dwarfs • The boundary between BD and planet is more controversial – some argue it should be based on formation – other choose 0.013 solar masses=13 Mj as the boundary, as objects below this mass will never reach even deuterium fusion 4 Nelson et al., 1986, AJ, 311, 226 5 Pulsar planets • In 1992, Wolszczan and Frail announced the discovery of a multi‐ Artist’s conception of the planet planet planetary system around the orbiting pulsar PSR B1257+12 millisecond pulsar PSR 1257+12 (an earlier announcement had been retracted). • These were the first two extrasolar planets confirmed to be discovered, and thus the first multi‐planet extrasolar planetary system discovered, and the first pulsar planets discovered • However, these objects are not in planetary systems as we usually think of them 6 2 3/25/2013 Worlds Beyond Our Sun • In 1995 a team of Swiss astronomers discovered the first planet (in a non- pulsar system) outside our solar system, orbiting asuna sun-like star called 51 Pegasi. • Further discoveries bring the grand total of known extrasolar planets to 861 (as of March 2013) and counting. Artist's rendition of the star 51 Pegasi and its planetary companion 51 Pegasi B. 7 Unseen Companions • Curiously enough, most extrasolar planets remain unseen • They are usually detected by indirect means, though their effects on their parent This artist's concept shows the Neptune-sized extrasolar planet star. circling the star Gliese 436. 8 Obstacles to Direct Detection • Direct detection is the only way to tell what these planets are made of and whether there's water or oxygen in their atmospheres. • But most known exoplanets are impossible to see with current technology • Two reasons why: – known exoplanets are too dim • Jupiter, for example, is more than a billion times fainter than the Sun. However it could easily be seen at large distances except for… – known exoplanets orbit too close to their parent stars • most known exoplanets have orbits smaller than that of Mercury "It's like trying to see a firefly next to a searchlight from across town." 9 3 3/25/2013 The “first confirmed” image of an exoplanet: GQ Lupi & Planetary Companion 21 Mj, 100 AU orbit. Imaged by ESO’s VLT, then HST and Subaru confirmed (early Apr 2005) 10 Detection methods: Astrometry • oldest method, used since 1943 • the wobble induced in the plane-of-sky motion of the star by planets is measured by accurately observing its position over time • 1 detection 11 Astrometry • STEPS (Stellar Planet Survey) detected periodic proper motion of VB 10, a nearby brown dwarf. • VB 10b is approximately 6 Jupiter masses, with a period of 9 months. • No sign of planet when examined with other techniques: busted! 12 4 3/25/2013 Astrometry:Difficulties •Example: The Sun wobbles by about its diameter, mostly due to Jupiter. •At 30 light-years, this woldprodceanould produce an apparent motion of less than 1 milliarcsecond. • Typical good ground- based observing conditions produce positions with accuracies below but around 1 arc- Apparent motion of Sun from 30 ly second. 13 Detection methods: Pulsar planets • Pulsar planets are planets that are found orbiting pulsars – Pulsars are rapidly rotating neutron stars. • Pulsar planets are discovered through radio pulsar timing measurements, to detect anomalies in the pulsation period. Any bodies orbiting the pulsar will cause regulhlar changes iitltiSilin its pulsation. Since pulsars normally rotate at near-constant speed, any changes can easily be detected with the help of precise timing measurements. • The first ever planets discovered around another star, were discovered around a pulsar in 1992 by Wolszczan and Frail around PSR 1257+12. Some uncertainty initially surrounded this due to an earlier retraction of a planet detection around PSR 1829-10 14 PSR 1257+12 • Pulsar located 2630 light years away • These were the first extrasolar planets ever discovered • Pulsar mass 0.3 Msun, rotational period 0.0062 seconds Mass (ME)a (AU) Period (days) e Firs t pltlanet 0. 020 0190.19 25. 26 000.0 Second planet 4.3 0.36 66.54 0.02 Third planet 3.9 0.46 98.21 0.025 – possible small fourth object has an upper mass limit of 0.2 MPluto and an upper size of R < 1000km. 15 5 3/25/2013 5 of the 12 known pulsar planet systems Pulsar planet Mass Orbit distance Orbit period PSR B1620-26 c 2.5 Jupiters 23 AU 100yr V391 Peg b 3.2 Jupiters 1.7 AU 1170 days PSR 1257+12 a 0.02 Earths 0.19 AU 25 days b 4.3 Earths 0.36 AU 66 days c 3.9 Earths 0.46 AU 98 days d 0.0004 Earths 2.7 AU 3.5 years QS Vir b 6.4 Jupiters 4.2 AU 7.9 years HW Vir b 19.2 Jupiters 16 years c 8.5 Jupiters 332 days •Since neutron stars are formed after the violent death of massive stars (supernovae), it was not expected that planets could survive in such a system. •Its now thought that the planets are either the remnant cores of giant planets that were able to weather the supernova, or later accretion products of supernova debris. 16 Detection methods: Transits • Planets observed at inclinations (measured with respect to the plane of the sky) near 90o will pass in front of (“transit”) their host stars, dimming the light of the star. This may be detectable by high-precision photometry. •Note that the planet is invisible, being unresolved, only the brightness variation in the star is seen. 17 The Observational Challenge The fraction of stars expected to have transits is: f = fs fMS fCEGP pt fs = fraction of stars that are single = 0.5 fMS = fraction of those on the main sequence = 0.5 fCEGP = fraction of those that have a close-in planet = 0.01 pt = fraction of those with an inclination to transit = 0.1 • Need to look at 4000 stars to find 1 that transits. • Need to sample often compared to transit duration. • Need 1% accuracy for a 3s detection of a 2 hour transit. • Need to look on sky for at least 1 orbital period. Requires 1,000,000 15-minute samples with 1% accuracy to detect one transit. 18 6 3/25/2013 Transits • Assuming – The whole planet passes in front of the star – And ignoring limb darkening of the star as negligible • Then the dep th o f the ec lipse is s imp ly the ra tio of the planetary and stellar disk areas: 2 2 Δf πR ⎛ R ⎞ f = light flux = p = ⎜ p ⎟ 2 ⎜ ⎟ f* πR* ⎝ R* ⎠ • We measure the change in brightness, and estimate the stellar radius from the spectral type 19 Transits • Advantages – Easy. Can be done with small, cheap telescopes • WASP, STARE, numerous others – Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, launched Mar 2009) • Disadvantages – Probability of seeing a transit is low • Need to observe many stars simultaneously – Easy to confuse with binary/triple systems – Needs radial velocity measurements for confirmation, masses • Has found 294 exoplanets in 238 systems so far (March 2013) 20 • OGLE-TR-10: Konacki et al. 2004 • 0.57Mj, 1.24Rj, P=3.1days 21 7 3/25/2013 Kepler (transits) With a total of 95 mega-pixels of CCDs Kepler is capable of observing over 100,000 stars all at once and measuring their brightness to an accuracy of better than 1 part in 100,000. 22 Kepler Orrery 23 Detection methods: microlensing • If the geometry is correct, a planet can actually produce a brightening (rather than a dimming) of a background star (not the parent star) through gravitational microlensing. 24 8 3/25/2013 First detection: OGLE 2003 BLG-235 Analysis of the light curve reveals second object in lens with .4% of mass of the other • 17,000 light years away, in the constellation Sagittarius. • The planet, orbiting a red dwarf parent star, is most likely one-and-a-half times bigger than Jupiter. • The planet and star are three times farther apart than Earth and the Sun. • Together, they magnify a farther, background star some 24,000 light years away, near the Milky Way center. 25 Microlensing • Microlensing has some disadvantages – model-dependent – only see the planet once • However, it is the “best” technique for finding smaller planets, farther from their star – ie. more Earth-like planets than RV technique (next) OGLE 2005-BLG-390 (Artist’s • 18 detections so far impression): Five Earth mass (Mar/2013) planet on a 10 yr orbit around a red dwarf star.