© 2011 Pearson Education, Inc. Planetary Systems

© 2011 Pearson Education, Inc. 1Modeling Planet Formation 2Terrestrial and Jovian Planets 3Interplanetary Debris 4Solar System Regularities and Irregularities 5Searching for Extrasolar Planets 6 Properties of Exoplanets 7Is Our Solar System Unusual? Units

© 2011 Pearson Education, Inc. Any model must explain 1. Planets are relatively isolated in space 2. Planetary orbits are nearly circular 3. Planetary orbits all lie in (nearly) the same plane 4. Direction of orbital motion is the same as direction of Sun’s rotation 5. Direction of most planets’ rotation is also the same as the Sun’s 1: Modeling Planet Formation

© 2011 Pearson Education, Inc. 6. Most moons’ orbits are also in the same sense 7. Solar system is highly differentiated 8. Asteroids are very old, and not like either inner or outer planets 9. Kuiper belt, asteroid-sized icy bodies beyond the orbit of Neptune 10. Oort cloud is similar to Kuiper belt in composition, but farther out and with random orbits 1: Modeling Planet Formation (cont.)

© 2011 Pearson Education, Inc. Solar system is evidently not a random assemblage, but has a single origin. Planetary condensation theory, first discussed in Chapter 6, seems to work well. Lots of room for variation; there are also irregularities (Uranus’s axial tilt, Venus’s retrograde rotation, etc.) that must be allowed for by the model. 1: Modeling Planet Formation

© 2011 Pearson Education, Inc. Review of condensation theory: Large interstellar cloud of gas and dust starts to contract, heating as it does so Sun forms in center; dust provides condensation nuclei, around which planets form As planets grow, they sweep up smaller debris near them 1: Modeling Planet Formation

© 2011 Pearson Education, Inc. Terrestrial (rocky) planets formed near Sun, due to high temperature—nothing else could condense there. 2: Terrestrial and Jovian Planets

© 2011 Pearson Education, Inc. 2: Terrestrial and Jovian Planets T Tauri stars are in a highly active phase of their evolution and have strong solar winds. These winds sweep away the gas disk, leaving the planetesimals and gas giants.

© 2011 Pearson Education, Inc. Jovian planets: Once they were large enough, may have captured gas from the contracting nebula Or may not have formed from accretion at all, but directly from instabilities in the outer, cool regions of the nebula 2: Terrestrial and Jovian Planets

© 2011 Pearson Education, Inc. Detailed information about the cores of jovian planets should help us distinguish between the two possibilities. Also possible: The jovian planets may have formed farther from the Sun and “migrated” inward. 2: Terrestrial and Jovian Planets

© 2011 Pearson Education, Inc. Asteroid belt: Orbits mostly between Mars and Jupiter Jupiter’s gravity kept them from condensing into a planet, or accreting onto an existing one Fragments left over from the initial formation of the solar system 3: Interplanetary Debris

© 2011 Pearson Education, Inc. General timeline of solar system formation 3: Interplanetary Debris

© 2011 Pearson Education, Inc. Icy planetesimals far from the Sun were ejected into distant orbits by gravitational interaction with the jovian planets, into the Kuiper belt and the Oort cloud. Some were left with extremely eccentric orbits and appear in the inner solar system as comets. 3: Interplanetary Debris

© 2011 Pearson Education, Inc. Kuiper belt objects have been detected from Earth; a few are as large as, or larger than, Pluto, and their composition appears similar. About 1/3 of all Kuiper belt objects (including Pluto) have orbits that are in a 3:2 resonance with Neptune; such objects are called “plutinos.” 3: Interplanetary Debris

© 2011 Pearson Education, Inc. Condensation theory covers the 10 points mentioned at the beginning. What about the exceptions? 1. Mercury’s large metallic core may be the result of a collision between two planetesimals, where much of the mantle was lost. 2. Two large bodies may have merged to form Venus. 3. Earth–Moon system may have formed after a collision. 4: Solar System Regularities and Irregularities

© 2011 Pearson Education, Inc. 4. Late collision may have caused Mars’s north–south asymmetry and stripped most of its atmosphere. 5. Uranus’s tilted axis may be the result of a glancing collision. 6. Miranda may have been almost destroyed in a collision. 7. Interactions between jovian protoplanets and planetesimals could be responsible for irregular moons. 8. Binary Kuiper belt objects (including the Pluto-Charon system) could have formed through collisions before ejection by interactions with the jovian planets. 4: Solar System Regularities and Irregularities (cont.)

© 2011 Pearson Education, Inc. Many of these explanations have one thing in common—a catastrophic, or near-catastrophic, collision at a critical time during formation. Normally, one does not like to explain things by calling on one-time events, but it is clear that the early solar system involved almost constant collisions. Some must have been exceptionally large. 4: Solar System Regularities and Irregularities (cont.)

© 2011 Pearson Education, Inc. 5: Searching for Extrasolar Planets Most extrasolar planets have been discovered indirectly, through their gravitational or optical effects, and they cannot be seen directly due to the glare of their star. However, a few dozen exoplanets have indeed been detected this way.

© 2011 Pearson Education, Inc. Many planets around other stars have been detected because they are large enough to cause the star to “wobble” as the planet and star orbit around their common center of mass. 5: Searching for Extrasolar Planets

© 2011 Pearson Education, Inc. If the “wobble” is transverse to our line of sight, it can also be detected through the Doppler shift as the star's motion changes. 5: Searching for Extrasolar Planets

© 2011 Pearson Education, Inc. 5: Searching for Extrasolar Planets An extrasolar planet may also be detected if its orbit lies in the plane of the line of sight to us. The planet will then eclipse the star, and if the planet is large enough, some decrease in luminosity may be observed. Transit method

© 2011 Pearson Education, Inc. More than 900 extrasolar planets have been discovered so far, with about 2700 more candidates waiting to be confirmed: Most are in the “cold Jupiter” or “cold Neptune” category due to size and distance from parent star Orbits are generally somewhat smaller than the orbit of Jupiter Most orbits have high eccentricity 6: Exoplanet Properties

© 2011 Pearson Education, Inc. 6: Exoplanet Properties The upper plot shows masses and orbital semimajor axes for hundreds of knows extrasolar planets, with Jupiter, Neptune, and Earth for comparison. The lower shows planetary radii and orbital semimajor axes for thousands of exoplanet candidates.

© 2011 Pearson Education, Inc. Orbits of many of the known extrasolar planets. Note that some of them are very close to their star: 6: Exoplanet Properties

© 2011 Pearson Education, Inc. Planets orbiting within 0.15 AU of their stars are called “hot Jupiters”; they are not included in the previous figure but are numerous. Stars with composition like our Sun are much more likely to have planets, showing that the “dusty disk” theory is plausible. Some of these “planets” may actually be brown dwarfs, but probably not many. 6: Exoplanet Properties

© 2011 Pearson Education, Inc. Types of Exoplanets: Gas Giants Gas Giants Gas giants are planets similar to Jupiter and Saturn. Their mass is mostly composed of hydrogen and helium with possibly a dense rocky or metallic core. Exoplanets with a mass of more than 10 times the Earth are classified as gas giants. Due to their size the majority of planets detected outside our solar system are gas giants.

© 2011 Pearson Education, Inc. Types of Exoplanets: Hot Jupiters Hot Jupiters are gas giant planets similar in mass to Jupiter but which orbit very close to their parent star. As a result of the close proximity to their star their surface temperatures exceed 700C (1300F). Apart from gas giants Hot Jupiters are the most common type of exoplanet detected.

© 2011 Pearson Education, Inc. Types of Exoplanets: Super Earth/Mini Neptune Super Earths are ‘potentially’ rocky planets with up to 10 times the mass of Earth. The term ‘Super Earth’ simply refers to the fact that these planets are larger than earth in mass (the largest rocky planet), but smaller than uranus (the smallest gas giant). The first two exoplanets to be detected were Super Earths orbiting around the pulsar PSR B

© 2011 Pearson Education, Inc. Types of Exoplanets: Free Floating Planets Free floating planets or orphan planets do not orbit around any star. It is believed that these isolated worlds were somehow ejected from developing systems and now free-float around the galaxy. Although very few have been detected they are believed to very common in our galaxy.

© 2011 Pearson Education, Inc. Types of Exoplanets: Pulsar Planets Pulsar planets orbit around Pulsars or Neutron Stars. These super dense, rapidly spinning stars are the core remains of a large star after a supernova explosion. It is highly unlikely that any orbiting planet could survive the blast from a supernova so Pulsar Planets probably formed after the event and now orbit around the dead star.

© 2011 Pearson Education, Inc. Types of Exoplanets: Water Worlds Water Worlds are planets whose surfaces are entirely covered in water. Evidence suggests that these planets originally formed as objects made of ice and rock far from their parent star. As they drifted towards the star they heated up melting the ice and so became covered in oceans. Under the surface the water is so dense its consistency is more like ice.

© 2011 Pearson Education, Inc. Types of Exoplanets: Chthonian Planets Chthonian Planets were once gas giants but have migrated far too close to their parent star. As a result their atmospheres have been roasted away, leaving only a rocky or metallic core. It is possible their surface may be covered in molten lava. Due to their similarity to terrestrial planets some Super Earths may actually be Chthonian Planets.

© 2011 Pearson Education, Inc. Types of Exoplanets: Exo Earths These are planets that have a similar mass, radius and atmosphere to Earth and orbit in the habitable zone of its star. This is an area where the temperature would allow water to flow on the planet’s surface and possibly allow life to flourish. To date there have been no discoveries of any Exo Earths.

© 2011 Pearson Education, Inc. Recently, a planet with a mass close to the mass of the Earth has been discovered orbiting our closest neighbor star system, Alpha Centauri. But it is about 25 times closer to its parent stars (Alpha Centauri A and B) than the Earth is to our Sun. The figures below show the Alpha Centauri system (left) and an artist’s conception of this planet (right). The Closest Exoplanet

© 2011 Pearson Education, Inc. Significant Exoplanets Planet Name Exoplanet Type Distance From Earth Detection Method Alpha Centauri βbSuper-Earth4 Light YearsRadial Velocity GJ 1214bWaterworld42 Light YearsTransit Kepler 16bGas Giant200 Light YearsTransit Corot 7bChthonian 489 Light Years Transit HAT-P-7bHot Jupiter1044 Light YearsTransit PSR B bPulsar Planet12,400 light-yearsPulsar Timing CFBDSIR Rogue Planet130 light years Direct imaging (infrared)

© 2011 Pearson Education, Inc. This figure shows two transiting super-Earths plus the nine “habitable” exo-Earths (as of mid-2013), compared to Earth and Neptune. 6: Exoplanet Properties

© 2011 Pearson Education, Inc. The other planetary systems discovered so far appear to be very different from our own. Selection effect biases sample toward massive planets orbiting close to parent star; lower-mass planets cannot be detected this way. 7: Is Our Solar System Unusual?

© 2011 Pearson Education, Inc. This is an example of a “cold Jupiter” in another system. Its orbit is very similar to that of Jupiter’s (blue). Also included is an artist’s conception of such a planet. 7: Is Our Solar System Unusual?

© 2011 Pearson Education, Inc. Current theories include the possibility that Jupiter-like planets could migrate inward, through friction with the solar nebula 7: Is Our Solar System Unusual?

© 2011 Pearson Education, Inc. A number of Earthlike planets have now been observed, although due to detection difficulties most exoplanets still fall into the “hot Jupiter” category, making other planetary systems look quite different from our own. Until we are able to observe much smaller planets at much larger distances from their parent stars, we will not know just how unusual our own system is – or if it is unusual at all. 7: Is Our Solar System Unusual?

© 2011 Pearson Education, Inc. This figure shows the size of the habitable zone – where there is a possibility of liquid water being present – as a function of the mass of the parent star. Note that the presence or absence of a greenhouse effect (runaway or otherwise) can affect the surface temperature of a planet considerably. 7: Is Our Solar System Unusual?

© 2011 Pearson Education, Inc. Condensation theory leads us to expect that other systems will be coplanar with planets orbiting in the same sense. Random collisions will lead to irregular properties. Most extrasolar planets have been discovered through wobbling of parent stars, or through transits. There are 900 planets and 2500 candidates so far. 20% of systems, and 33% of candidates, have multiple planets per system. 20 Earths and super-Earths are in habitable zones. We don’t yet have enough information to tell how unusual our own system is. Summary of this Unit