Lecture Outlines Chapter 15 Astronomy Today 8th Edition Chaisson/McMillan
Chapter 15 Exoplanets
Units of Chapter 15 15.1 Modeling Planet Formation 15.2 Solar System Regularities and Irregularities 15.3 Searching for Extrasolar Planets 15.4 Exoplanet Properties Discovery 15-1 The Closest Exoplanet 15.5 Is Our Solar System Unusual?
15.1 Modeling Planet We now have many planetary systems other than our own to test formation theories 900 confirmed exoplanets and 2500 candidates as of mid- 2013 Formation
15.1 Modeling Planet Formation 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
15.2 Solar System Regularities and Irregularities Condensation theory covers the 10 points mentioned at the beginning. What about the exceptions? 1. Two large bodies may have merged to form Venus. 2. Earth Moon system may have formed after a collision.
15.2 Solar System Regularities and Irregularities (cont.) 3. Late collision may have caused Mars s north south asymmetry and stripped most of its atmosphere. 4. Uranus s tilted axis may be the result of a glancing collision. 5. Miranda may have been almost destroyed in a collision. 6. Interactions between jovian protoplanets and planetesimals could be responsible for irregular moons. 7. Binary Kuiper belt objects (including the Pluto-Charon system) could have formed through collisions before ejection by interactions with the jovian planets.
15.2 Solar System Regularities and Irregularities (cont.) 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.
15.3 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.
15.3 Searching for Extrasolar Planets 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.
15.3 Searching for Extrasolar Planets 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.
15.3 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.
15.4 Exoplanet Properties 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
15.4 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.
15.4 Exoplanet Properties Orbits of many of the known extrasolar planets. Note that some of them are very close to their star:
15.4 Exoplanet Properties 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.
Discovery 15-1 The Closest Exoplanet 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).
15.4 Exoplanet Properties This figure shows two transiting super-earths plus the nine habitable exo-earths (as of mid-2013), compared to Earth and Neptune.
15.5 Is Our Solar System Unusual? 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.
15.5 Is Our Solar System Unusual? 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.
15.5 Is Our Solar System Unusual? Current theories include the possibility that Jupiter-like planets could migrate inward, through friction with the solar nebula
15.5 Is Our Solar System Unusual? 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.
15.5 Is Our Solar System Unusual? 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.
Summary of Chapter 15 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.