Download Jupiter – key facts Largest and most massive planet in the Solar

Jupiter – key facts Largest and most massive planet in the Solar System: ~ 11 Earth radii and 318 Earth masses Composed primarily of H and He, but enriched in heavy elements compared to the Sun. Bulk composiFon: 71% H; 24% He; 5% heavier elements Rotates in just less than 10 hrs. Emits twice as much energy as it receives from the Sun – thermal energy generated by slow gravitaFonal contracFon is radiated into space Internal heat and rapid rotaFon combine to generate a turbulent atmospheric flow that stretches into eastward and westward directed winds called jets (c.f. the jet stream on Earth) Zones and belts represent different alFtudes in atmosphere (IR image on right) Jupiter – key facts Largest and most massive planet in the Solar System: ~ 11 Earth radii and 318 Earth masses Composed primarily of H and He, but enriched in heavy elements compared to the Sun. Bulk composiFon: 71% H; 24% He; 5% heavier elements Rotates in just less than 10 hrs. Emits twice as much energy as it receives from the Sun – thermal energy generated by slow gravitaFonal contracFon is radiated into space Internal heat and rapid rotaFon combine to generate a turbulent atmospheric flow that stretches into eastward and westward directed winds called jets (c.f. the jet stream on Earth) Zones and belts represent different alFtudes in atmosphere. Jupiter’s internal structure appears to consist of a rock+ice core lying at its centre with mass ~ 10 Earth masses. Central temperatures are ~ 20,000 K On top of this lies a mantle composed mainly of “metallic hydrogen” – a state of hydrogen that exists under extremely high pressures (> 106 atmospheres) in which it behaves as a liquid and the electrons are stripped off atoms making it electrically conducFng On top of this lies an layer of molecular hydrogen (+ helium and molecules such as ammonia). The rapid rotaFon of Jupiter combined with the electrically conducFng metallic hydrogen give rise to a powerful magneFc field – 20 Fmes stronger at the surface of Jupiter than the Earth’s field is at the surface of the Earth. Jupiter has a reFnue of 67 confirmed moons: the 4 galilean satellites and a collecFon of much smaller objects (called irregular satellites) many of which appear to be captured asteroids/comets in inclined, eccentric and oden retrograde orbits Io – the closest of the galilean satellites. Volcanically acFve due to internal heaFng caused by Fdal flexing by Jupiter’s gravitaFonal field because of Io’s eccentric orbit. Mean density suggests it is largely made of rock. Europa – in a 2:1 orbital resonance with Io caused by Jupiter’s Fdes expanding the orbits of the galilean satellites. Mean density suggests Europa composed largely of rocky material. Images and spectroscopy indicate a smooth surface made of water ice. Lack of craters suggests regular resurfacing by a liquid ocean lying ~100 km below the ice crust. Further evidence for subsurface ocean is provided by Europa’s magneFc field which requires the presence of an electrically conducFng fluid. Ganymede – the largest and most massive satellite in the solar system. In a 2:1 resonance with Europa. Mean density suggests this satellite is largely icy, but with an iron and rocky core. Surface shows different ages suggesFng that a semi-­‐liquid mantle may exist under the icy crust Callisto – the most distantly orbiFng of the galilean satellites. Mean density suggests that Callisto is largely icy. Its highly cratered surface indicates that it is an ancient surface, not subject to renewal by erupFons from an interior mantle. The galilean satellites are believed to have formed in a disc of gas and rocks that surrounded the young Jupiter, just like the planets formed around the Sun. Saturn – key facts The second largest and most massive planet in the solar system: approximately 100 Earth masses or ~1/3 of Jupiter’s mass Similar bulk composiFon to Jupiter, but significant deficiency of He in atmosphere -­‐ suggesFng that He is “raining out” of the atmosphere. This also leads to Saturn radiaFng more heat than it receives from the Sun. The spin period is ~ 10 hrs. Rapid rotaFon combined with internal heat generates turbulence and jets. Bands and zones are present but less disFnct than on Jupiter because of hazes high in Saturn’s atmosphere. Hosts an impressive ring system and a large collecFon of satellites (62 confirmed in total): 1 large moon, 6 moderate sized moons, and a large collecFon of small irregular satellites. Titan is only moon in solar system with significant atmosphere. Saturn – key facts The second largest and most massive planet in the solar system: approximately 100 Earth masses or ~1/3 of Jupiter’s mass Similar bulk composiFon to Jupiter, but significant deficiency of He in atmosphere -­‐ suggesFng that He is “raining out” of the atmosphere. This also leads to Saturn radiaFng more heat than it receives from the Sun. The spin period is ~ 10 hrs. Rapid rotaFon combined with internal heat generates turbulence and jets. Bands and zones are present but less disFnct than on Jupiter because of hazes high in Saturn’s atmosphere. Hosts an impressive ring system and a large collecFon of satellites (62 confirmed in total): 1 large moon, 6 moderate sized moons, and a large collecFon of small irregular satellites. Titan is only moon in solar system with significant atmosphere. Saturn’s interior is similar to that of Jupiter: a rock+ice core surrounded by a mantle of electrically conducFng liquid metallic hydrogen, on top of which sits an envelope of molecular hydrogen. The existence of a metallic hydrogen layer combined with the rapid rotaFon leads to generaFon of a magneFc field whose strength at Saturn’s surface is about 3% of Jupiter’s. Saturn’s rings are composed of small icy parFcles ranging in size from 1 cm to 5 m, with the most abundant being ~ 10 cm. A small telescope from Earth allows the more massive A and B rings to be observed, along with the Cassini division. ObservaFon of the lower density D, C, F, G and E rings require either larger telescopes or in situ space crad such as Voyager or Cassini. Saturn’s rings are highly structured due to the gravitaFonal influence of embedded moons and more distantly orbiFng satellites. The Cassini division is a gap between the B and A rings that coincides with a 2:1 orbital resonance with the satellite Mimas. The gap is created because material located there has a conjuncFon with Mimas every two orbits, leading to regular gravitaFonal perturbaFons that remove parFcles from locaFons in the vicinity of the 2:1 resonance. The Keeler gap is formed and maintained by a small embedded satellite Daphnis. Wave-­‐like features are observed at the edges of the gap induced by the gravitaFonal perturbaFons due to Daphnis. These perturbaFons create and maintain the gap through a process of angular momentum and energy exchange between the parFcles in the ring and the satellite. ParFcles orbiFng interior to Daphnis orbit faster and are tugged back by the satellite’s gravity as they overtake it. They lose angular momentum and move onto orbits slightly closer to Saturn. The opposite happens to parFcles that orbit outside Daphnis, and they move onto orbits slightly further from Saturn, creaFng the gap. Collisions between ring parFcles have the effect of trying to close the gap, so a balance is maintained between satellite perturbaFons on the ring material and collisional spreading of the ring parFcles. A pair of satellites can also act to shepherd and maintain a narrow ring. Saturn’s F-­‐ring is shepherded by a pair of satellites that orbit interior (Prometheus) and exterior (Pandora) to the ring. Saturn has 6 medium sized regular satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus) and one large satellite (Titan – the 2nd largest satellite in the solar system). All of these regular satellites are located outside of the so-­‐called Roche zone where the Fdal forces of Saturn would disrupt a body held together by its own gravity. The rings lie inside of the Roche zone. Enceladus is an icy moon with clearly defined linear surface features that appear to be the source of cryo-­‐ volcanic acFvity. Geysers of ice parFcles and water vapour are acFve in the regions covered by blue stripes. The ice parFcles act as the source of the E-­‐ring. The presence of these geysers indicates a heat source – most likely Fdal damping by Saturn of Enceladus’ orbital eccentricity that is maintained by its gravitaFonal interacFon with Dione with which it shares a 2:1 resonance, similar to the Jovian satellites. Titan is the only moon in the solar system with a dense atmosphere. The atmosphere extends ~ 200 km from Titan’s surface, and is composed primarily of nitrogen with a mixture of organic compounds such as methane (CH4) and ethane (C2H6). The image to the right is a composite of visible and IR images taken by the Cassini spacecrad. Temperatures near Titan’s surface (~95 K) allow methane and ethane to exist in vapour, liquid or solid phases. ExploraFon of Titan by the Huygens probe in 2005 demonstrated the existence of river channels caused by liquid ethane and methane on the surface which falls as rain from the atmosphere. The surface was imaged by the probe showing the landing site to be strewn with ice boulders. Uranus – key facts Uranus is the inner-­‐most of the ice-­‐giant planets and has a featureless surface. The most unusual fact about Uranus is that its rotaFon axis is Flted by ~98o relaFve to its orbital angular momentum vector – this is assumed to arise because Uranus experienced a giant impact with a smaller planetary body shortly ader formaFon. Despite being closer to the Sun than Neptune, Uranus is colder. Unlike the gas giants it does not have an internal heat source. This explains why Uranus is devoid of surface features such as bands, zones and spots. Uranus has a system of 13 dark rings, and 27 moons, 5 of which are considered to be regular satellites. They are composed of ~50 % ice and rock. Uranus – key facts Uranus is the inner-­‐most of the ice-­‐giant planets and has a featureless surface. The most unusual fact about Uranus is that its rotaFon axis is Flted by ~98o relaFve to its orbital angular momentum vector – this is assumed to arise because Uranus experienced a giant impact with a smaller planetary body shortly ader formaFon. Despite being closer to the Sun than Neptune, Uranus is colder. Unlike the gas giants it does not have an internal heat source. This explains why Uranus is devoid of surface features such as bands, zones and spots. Uranus has a system of 13 dark rings, and 27 moons, 5 of which are considered to be regular satellites. They are composed of ~50 % ice and rock. Neptune – key facts Neptune is the outermost planet in the solar system. It was the first planet to be discovered ader mathemaFcal predicFon (Le Verrier, Couch Adams, Galle 1846) It has a smaller radius and larger mass than Uranus, and radiates ~ 2 x more heat than it receives from the Sun. The surface appears to be more acFve than Uranus, displaying surface features such as the great dark spot. Neptune has a system of ~ 3 dark rings, and 13 known moons. Triton is the largest of these, and the most massive irregular satellite in the solar system. Being slightly more massive than Pluto, and in a retrograde orbit, Triton appears to be a captured Kuiper belt object. It displays evidence of acFve cryovolcanism. Neptune – key facts Neptune is the outermost planet in the solar system. It was the first planet to be discovered ader mathemaFcal predicFon (Le Verrier, Couch Adams, Galle 1846) It has a smaller radius and larger mass than Uranus, and radiates ~ 2 x more heat than it receives from the Sun. The surface appears to be more acFve than Uranus, displaying surface features such as the great dark spot. Neptune has a system of ~ 3 dark rings, and 13 known moons. Triton is the largest of these, and the most massive irregular satellite in the solar system. Being slightly more massive than Pluto, and in a retrograde orbit, Triton appears to be a captured Kuiper belt object. It displays evidence of acFve cryovolcanism. Uranus has a mass ~ 15 Earth masses and Neptune ~ 17 Earth masses. Unlike the gas giants Jupiter and Saturn, these “ice-­‐giant” planets are composed primarily of water + ammonia “ice” that forms a highly compressed liquid mantle. Interior to this lies a rock+metal core. A H-­‐He rich gaseous envelope containing trace quanFFes of methane lies at the surface. Interior models that match the observaFons suggest that the core contains 1-­‐2 Earth masses, the liquid mantle ~ 10 – 14 Earth masses, and the envelope 1-­‐2 Earth masses. The blue-­‐ish colour of both planets arises because of absorpFon of sunlight at red wavelengths by methane. Extrasolar Planets The first planet discovered orbiFng a main sequence star outside of the solar system was 51 Peg b in 1995 (Mayor & Queloz). The method of detecFon was the radial velocity method (see later). There are currently 863 confirmed extrasolar planets (678 planetary systems and 129 mulFple planet systems). NASA’s Kepler mission has announced the detecFon of 2740 planet candidates. The upper diagram on the right plots the planet mass (in units of Jupiter’s mass) versus the orbital period (in days) for all known exoplanets. For reference the Earth’s posiFon is indicated by the red ‘E’, Jupiter’s by the ‘J’ and Saturn’s by the ‘S’ J S E DetecFon methods Radial velocity method – the orbit of a planet around a star actually involves both planet and star orbiFng their common centre of mass. This technique detects the line-­‐of-­‐sight moFon of the star by measuring the Doppler shid of spectral lines Transit method – planets whose orbit planes are edge-­‐on as seen from Earth will pass in front of their host star once per orbit, causing the star’s measured luminosity to dim periodically. Microlensing – If a distant background star is observed from Earth, and another star passes directly across the line of sight, the gravitaFonal field of the intervening star will cause the background star to increase in brightness for a short Fme (few days). This effect results from Einstein’s theory of general relaFvity. If the intervening star hosts a planetary system the pauern of increasing brightness is modified in a detectable fashion. Thermal emission – a closely orbiFng planet will be heated by its star, and its thermal IR emission may be detected by an IR telescope (i.e. Spitzer space telescope) Direct imaging – young, hot planets orbiFng at large distances from their host stars may be directly imaged. Radial velocity technique When a planet orbits a star it causes the star to wobble back and forth since both planet and star are orbiFng their common centre of mass Looking down on planet and star Looking at planet and star edge-­‐on As star moves towards observer, starlight is blue-­‐shided. As it recedes the starlight is red-­‐
shided. AbsopFon lines in the star’s spectrum are seen to move back and forth in phase with the star’s radial moFon relaFve to the observer. This technique relies on the planetary orbit not being exactly face-­‐on to the observer so that a component of the orbital moFon is along the line of sight. Lack of knowledge about the orbital plane of the exoplanet usually leads to uncertainty in its measured mass. See on-­‐line supplementary lecture notes. The diagrams on the right are radial velocity diagrams showing the velocity of the star in the 51 Peg and Gliese 876 systems as a funcFon of the planet’s posiFon on its orbit (orbital phase) Radial velocity measurements tell us: •  Orbital period •  Planet mass -­‐ mp sin (i) •  Eccentricity -­‐ from the shape of the curve (a circular orbit gives a pure sinusoidal curve, an eccentric orbit gives an asymmetric curve because the velocity of the planet varies on its orbit). •  Number of planets in system -­‐ a composite radial velocity diagram can be decomposed into a number of superposed keplerian orbits Transit method For ‘edge-­‐on’ planet systems the transit method detects the dimming of the starlight as the planet passes between star and observer during each orbit. From transit can determine radius from decrease in stellar flux. Combining the measured radius with mass from radial velocity measurements gives the mean density, and informaFon about the planet’s bulk composiFon. Note that for a transiFng planet the inclinaFon of the planets orbits is known, so the actual mass of the planet is known. See on-­‐line supplementary lecture notes. The lower diagram shows the mass-­‐radius relaFon for planets with different composiFons. When a planet transits in front of its star some of the starlight passes through the atmosphere, imprinFng spectral absorpFon features that can be detected. This technique has led to the detecFon of Na, H2O, CO2 and CH4 in exoplanet atmospheres When the planet goes behind the star (the “secondary eclipse”) the IR radiaFon from the planet is blocked out. The reducFon in IR radiaFon can be used to determine the temperature of a close-­‐orbiFng planet that is heated by its star. The ground-­‐based SuperWASP project looks for transits by staring at large patch of sky looking for periodic dimming of the stars within the field of view. The above image shows the large number of stars being monitored in the direcFon of Orion Kepler mission: Photometric monitoring of more than 150,000 stars in constellaFon Cygnus looking for transit signals More than 2700 planetary candidates announced so far HD 209458 b – a transiFng ‘hot Jupiter’ •  Originally discovered using radial velocity technique •  Found to transit in front of its star in 1999 •  Combining transit data and radial velocity measurements gives the planet mass and radius: Mass=0.69 Jupiter masses Radius=1.347 Jupiter radii (see below) ⇒ Gas giant planet with mean density of about 1 g /cm3 Diagram on right shows Hubble Space Telescope measurement of transit light curve Transmission spectroscopy: Small fracFon of starlight passes through planet atmosphere during transit. AbsorpFon features due to sodium observed in the spectrum -­‐ in agreement with theoreFcal predicFons based on chemical modelling of atmosphere. Using a similar technique water, carbon dioxide and methane has been detected (and in atmosphere of the “hot Jupiter” HD 189733b). ObservaFons using HST suggest that the atmosphere of HD 209568b is boiling off -­‐ producing a long ‘cometary tail’. The upper atmosphere is predicted to have T≈10000 K, so Maxwell-­‐Boltzmann distribuFon of velociFes leads to significant escape from surface As HD 209458b goes behind the star, the infrared radiaFon emiued by the planet is blocked out. Using this method it can be determine how much infrared radiaFon is emiued -­‐ and how hot the planet is. Spitzer space telescope measurements suggest T ~ 1300 K. GJ436 b – a transiFng ‘hot Neptune’ Discovered using radial velocity method (see right diagram) but also found to transit across host star (see figure below) → mass, radius & mean density Computer models that provide a good fit to the data on the mass and radius of the planet indicate that planet has internal structure very similar to Neptune and Uranus -­‐ but much houer surface layers (800 K) Planets observed by direct imaging HR8799 is a young (~ 30 Myr old) main sequence star with mass ~ 1.5 solar masses. It hosts 4 planets orbiFng at 14.5, 24, 38 and and 68 AU. Planet masses are esFmated to be between 7-­‐10 Jupiter masses. The top image was obtained using adapFve opFcs techniques in the IR using the Keck telescopes. The lower image shows that HR8799 is surrounded by a “debris disc” – a disc of warm dust thought to be generated through collisions between planetesimals. The star Beta Pictoris was long suspected of hosFng a giant planet because it has an edge-­‐on debris disc that is warped – the warp was believed to be generated by the gravitaFonal influence of a planet on an inclined orbit. The existence of a ~ 9 Jupiter mass planet orbFng at a distance of ~ 10 AU was confirmed in 2009 (Lagrange et al 2010) from images taken with the VLT in Chile – archive images going back to 2003 were found to contain the planet The Kepler mission The Kepler satellite conFnuously measures the brightness of 150,000 stars. 2780 planet candidates have been detected – 105 are confirmed. Several mulFplanet systems discovered where all planets transit their host star – e.g. Kepler 11 (see later). Planets have been discovered orbiFng both stars in close binary systems (e.g. Kepler 16) Mass esFmates are obtained either by radial velocity measurements (only possible for the brightest and nearest stars) or by measuring transit Fming variaFons. This lauer technique relies on the fact that mutual gravitaFonal interacFons within a mulFplanet system induce predictable orbital changes that can be used to constrain planet masses. Without an esFmate of the planet masses the planetary nature of the transit signal cannot be confirmed. False posiFves can be generated through other sources of photometric variaFon: star spots; eclipsing binaries; blends – a foreground star occurs in the same pixel as a background eclipsing binary system creaFng a signal very similar to a transiFng planet. Analysis of the exisFng data indicate that Super-­‐Earth and Neptune-­‐like planets are very common, as indicated by the histogram on the right. Earth-­‐like planets are more difficult to detect because they produce a smaller dip in the lightcurve, so the numbers for these planets may be underesFmates of the true number. The lower diagram shows the results of a recent auempt to esFmate the fracFon of stars in our Galaxy that host planets based on an extrapolaFon of the Kepler data. It seems likely that planetary systems containing an Earth-­‐like planet in the habitable zone where liquid water can exist are rather common… Kepler 11 Perhaps the most interesFng of the Kepler discoveries is the Kepler 11 system that hosts 6 planets that all transit the central star. The figure below shows the raw photometric data in the upper panel for this system (the broken black line). This shows that the response of the spacecrad CCD chips are not constant but drid over Fme. The lower figure shows the calibrated data, and each small black dot corresponds to a measured brightness of the Kepler 11 star, demonstraFng that there is a complex pauern of transits. Careful analysis shows that this pauern is caused by 6 transiFng planets The top diagram shows the measured sizes of the Kepler 11 planets compared to other planets discovered by Kepler, and Jupiter. The lower right panel shows the orbital configuraFon of the planets, compared to the solar system. The panel below shows the posiFon of the Kepler 11 planets on a mass-­‐radius diagram, where the size of the ellipse indicates the uncertainty.