Jupiter – Key Facts Largest and Most Massive Planet in the Solar System: ~ 11 Earth Radii and 318 Earth Masses

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 composion: 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 gravitaonal contracon is radiated into space

Internal heat and rapid rotaon 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 altudes 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 composion: 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 gravitaonal contracon is radiated into space

Internal heat and rapid rotaon 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 altudes 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 conducng

On top of this lies an layer of molecular hydrogen (+ helium and molecules such as ammonia).

The rapid rotaon of Jupiter combined with the electrically conducng metallic hydrogen give rise to a powerful magnec field – 20 mes stronger at the surface of Jupiter than the Earth’s field is at the surface of the Earth. Jupiter has a renue of 67 confirmed moons: the 4 galilean satellites and a collecon of much smaller objects (called irregular satellites) many of which appear to be captured asteroids/comets in inclined, eccentric and oen retrograde orbits Io – the closest of the galilean satellites. Volcanically acve due to internal heang caused by dal flexing by Jupiter’s gravitaonal 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 des 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 magnec ﬁeld which requires the presence of an electrically conducng 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 suggesng that a semi-liquid mantle may exist under the icy crust

Callisto – the most distantly orbing 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 erupons 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 composion to Jupiter, but signiﬁcant deﬁciency of He in atmosphere - suggesng that He is “raining out” of the atmosphere. This also leads to Saturn radiang more heat than it receives from the Sun.

The spin period is ~ 10 hrs. Rapid rotaon combined with internal heat generates turbulence and jets. Bands and zones are present but less disnct than on Jupiter because of hazes high in Saturn’s atmosphere.

Hosts an impressive ring system and a large collecon of satellites (62 conﬁrmed in total): 1 large moon, 6 moderate sized moons, and a large collecon 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

Hosts an impressive ring system and a large collecon of satellites (62 conﬁrmed in total): 1 large moon, 6 moderate sized moons, and a large collecon 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 conducng liquid metallic hydrogen, on top of which sits an envelope of molecular hydrogen.

The existence of a metallic hydrogen layer combined with the rapid rotaon leads to generaon of a magnec ﬁeld whose strength at Saturn’s surface is about 3% of Jupiter’s.

Saturn’s rings are composed of small icy parcles 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. Observaon of the lower density D, C, F, G and E rings require either larger telescopes or in situ space cra such as Voyager or Cassini. Saturn’s rings are highly structured due to the gravitaonal inﬂuence of embedded moons and more distantly orbing 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 conjuncon with Mimas every two orbits, leading to regular gravitaonal perturbaons that remove parcles from locaons 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 gravitaonal perturbaons due to Daphnis. These perturbaons create and maintain the gap through a process of angular momentum and energy exchange between the parcles in the ring and the satellite. Parcles orbing 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 parcles that orbit outside Daphnis, and they move onto orbits slightly further from Saturn, creang the gap. Collisions between ring parcles have the eﬀect of trying to close the gap, so a balance is maintained between satellite perturbaons on the ring material and collisional spreading of the ring parcles.

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 dal 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 acvity. Geysers of ice parcles and water vapour are acve in the regions covered by blue stripes. The ice parcles act as the source of the E-ring.

The presence of these geysers indicates a heat source – most likely dal damping by Saturn of Enceladus’ orbital eccentricity that is maintained by its gravitaonal interacon 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 spacecra.

Temperatures near Titan’s surface (~95 K) allow methane and ethane to exist in vapour, liquid or solid phases. Exploraon 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 rotaon axis is lted by ~98o relave to its orbital angular momentum vector – this is assumed to arise because Uranus experienced a giant impact with a smaller planetary body shortly aer formaon.

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.

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 aer mathemacal predicon (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 acve than Uranus, displaying surface features such as the great dark spot.

Neptune is the outermost planet in the solar system. It was the ﬁrst planet to be discovered aer mathemacal predicon (Le Verrier, Couch Adams, Galle 1846)

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 acve 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 quanes of methane lies at the surface. Interior models that match the observaons 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 absorpon of sunlight at red wavelengths by methane. Extrasolar Planets

The ﬁrst planet discovered orbing a main sequence star outside of the solar J system was 51 Peg b in 1995 (Mayor & Queloz). The method of S detecon was the radial velocity method (see later).

There are currently 863 conﬁrmed extrasolar planets (678 planetary E systems and 129 mulple planet systems). NASA’s Kepler mission has announced the detecon 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 posion is indicated by the red ‘E’, Jupiter’s by the ‘J’ and Saturn’s by the ‘S’ Detecon methods

Radial velocity method – the orbit of a planet around a star actually involves both planet and star orbing their common centre of mass. This technique detects the line-of-sight moon of the star by measuring the Doppler shi 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 gravitaonal field of the intervening star will cause the background star to increase in brightness for a short me (few days). This effect results from Einstein’s theory of general relavity. If the intervening star hosts a planetary system the paern of increasing brightness is modified in a detectable fashion.

Thermal emission – a closely orbing 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 orbing 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 orbing 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-shied. As it recedes the starlight is red- shied.

Absopon lines in the star’s spectrum are seen to move back and forth in phase with the star’s radial moon relave 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 moon 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 funcon of the planet’s posion on its orbit (orbital phase)

Radial velocity measurements tell us: • Orbital period

• Planet mass - m p 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 informaon about the planet’s bulk composion. Note that for a transing planet the inclinaon 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 relaon for planets with diﬀerent composions. When a planet transits in front of its star some of the starlight passes through the atmosphere, imprinng spectral absorpon features that can be detected. This technique has led to the detecon of Na, H2O, CO2 and CH4 in exoplanet atmospheres

When the planet goes behind the star (the “secondary eclipse”) the IR radiaon from the planet is blocked out. The reducon in IR radiaon can be used to determine the temperature of a close-orbing 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 direcon of Orion Kepler mission:

Photometric monitoring of more than 150,000 stars in constellaon Cygnus looking for transit signals

More than 2700 planetary candidates announced so far HD 209458 b – a transing ‘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 fracon of starlight passes through planet atmosphere during transit.

Absorpon features due to sodium observed in the spectrum - in agreement with theorecal predicons 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).

Observaons 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 distribuon of velocies leads to signiﬁcant escape from surface As HD 209458b goes behind the star, the infrared radiaon emied by the planet is blocked out.

Using this method it can be determine how much infrared radiaon is emied - and how hot the planet is. Spitzer space telescope measurements suggest T ~ 1300 K. GJ436 b – a transing ‘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 hoer 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 orbing at 14.5, 24, 38 and and 68 AU. Planet masses are esmated to be between 7-10 Jupiter masses. The top image was obtained using adapve opcs 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 hosng a giant planet because it has an edge-on debris disc that is warped – the warp was believed to be generated by the gravitaonal inﬂuence of a planet on an inclined orbit. The existence of a ~ 9 Jupiter mass planet orbng 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 connuously measures the brightness of 150,000 stars.

2780 planet candidates have been detected – 105 are conﬁrmed.

Several mulplanet systems discovered where all planets transit their host star – e.g. Kepler 11 (see later).

Planets have been discovered orbing both stars in close binary systems (e.g. Kepler 16)

Mass esmates are obtained either by radial velocity measurements (only possible for the brightest and nearest stars) or by measuring transit ming variaons. This laer technique relies on the fact that mutual gravitaonal interacons within a mulplanet system induce predictable orbital changes that can be used to constrain planet masses.

Without an esmate of the planet masses the planetary nature of the transit signal cannot be conﬁrmed. False posives can be generated through other sources of photometric variaon: star spots; eclipsing binaries; blends – a foreground star occurs in the same pixel as a background eclipsing binary system creang a signal very similar to a transing planet. Analysis of the exisng 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 underesmates of the true number.

The lower diagram shows the results of a recent aempt to esmate the fracon of stars in our Galaxy that host planets based on an extrapolaon 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 interesng 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 spacecra CCD chips are not constant but dri over me.

The lower ﬁgure shows the calibrated data, and each small black dot corresponds to a measured brightness of the Kepler 11 star, demonstrang that there is a complex paern of transits. Careful analysis shows that this paern is caused by 6 transing 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 conﬁguraon of the planets, compared to the solar system. The panel below shows the posion of the Kepler 11 planets on a mass-radius diagram, where the size of the ellipse indicates the uncertainty.