Lecture Outlines Astronomy Today 8th Edition Chaisson/McMillan © 2014 Pearson Education, Inc. Chapter 15

© 2014 Pearson Education, Inc. Chapter 15 Exoplanets

© 2014 Pearson Education, Inc. 15.1Modeling Planet Formation 15.2Solar System Regularities and Irregularities 15.3Searching for Extrasolar Planets 15.4 Exoplanet Properties Discovery 15-1 The Closest Exoplanet 15.5Is Our Solar System Unusual? Units of Chapter 15

© 2014 Pearson Education, Inc 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.

© 2014 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 Searching for Extrasolar Planets

© 2014 Pearson Education, Inc 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.

© 2014 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 15.4 Exoplanet Properties

© 2014 Pearson Education, Inc 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.

© 2014 Pearson Education, Inc. Orbits of many of the known extrasolar planets. Note that some of them are very close to their star: 15.4 Exoplanet Properties

© 2014 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 Exoplanet Properties

© 2014 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). Discovery 15-1 The Closest Exoplanet

© 2014 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 Exoplanet Properties

© 2014 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 Is Our Solar System Unusual?

© 2014 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 Is Our Solar System Unusual?

© 2014 Pearson Education, Inc. 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?

© 2014 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 Is Our Solar System Unusual?

© 2014 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 Is Our Solar System Unusual?

© 2014 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 Chapter 15