1. Chapter 15

2. Chapter 15 Exoplanets
Astronomers routinely observe other young star systems, hoping to gain insight into the origins of our own solar system. This is actually a composite image, taken in the optical domain by two telescopes: The Hubble Space Telescope imaged the central parts and Japan’s Subaru Telescope extended the field of view around the edges. It shows the region called S106, a nebula about 3300 light-years away in the constellation Cygnus. Amid its chaotic gas and dust spanning a few light-years (hence thousands of times larger than a typical planetary system), many young stars—and probably planets—are now forming. (NASA; NAOJ)

3. 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?

4. 15.1 Modeling Planet Formation
We now have many planetary systems other than our own to test formation theories – 900 confirmed exoplanets and 2500 candidates as of mid-2013
Figure Extrasolar Planet Most known extrasolar planets are too faint to be detectable against the glare of their parent stars. However, in this system, called 2M1207, the parent itself (centered) is very faint—a so-called brown dwarf (see Chapter 19)—allowing the planet (lower left) to be detected in the infrared. This planet has a mass about 5 times that of Jupiter and orbits 55 AU from the star, which is 230 light-years away. (ESO)

5. 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
Figure Solar System Origin The condensation theory of planet formation is artistically illustrated by these half-dozen changes, from infalling interstellar cloud at the top to newly emerged planetary system at the bottom. Consult the text on the opposite page for descriptions of each of the frames of this figure. The condensation theory was devised to explain the observed properties of our own solar system. Now astronomers have the opportunity to test it against observations of planetary systems elsewhere in the universe.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.
Figure Detecting Extrasolar Planets As a planet orbits its parent star, it causes the star to “wobble” back and forth. The greater the mass of the planet, the larger is the wobble. The center of mass of the planet–star system stays fixed. If the wobble occurs along our line of sight to the star, as shown by the yellow arrow, we can detect it by the Doppler effect.

11. 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.
Figure Planets Revealed (a) Measurements of the Doppler shift of the star 51 Pegasi reveal a clear periodic signal indicating the presence of a planetary companion of mass at least half the mass of Jupiter. (b) Radial-velocity data for Upsilon Andromedae are much more complex, but are well fit (solid black, wobbly line) by a three-planet system orbiting the star. (c) A sketch of the inferred orbits of three planets from the Upsilon Andromedae system (in orange), with the orbits of the terrestrial planets superimposed for comparison (in white).

12. 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.
Figure An Extrasolar Transit (a) If an extrasolar planet happens to pass between us and its parent star, the light from the star dims in a characteristic way. (b) Artist’s conception of the planet orbiting a Sun-like star known as HD The planet is 200,000 km across and transits every 3.5 days, blocking about 2 percent of the star’s light each time it does so.

13. 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

14. 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.
Figure 15.6 Extrasolar Planetary Parameters (a) Masses and orbital semimajor axes of hundreds of known extrasolar planets. Each point represents one planetary orbit. Planets are classified by familiar solar system names, depending on mass, and as hot or cold, depending on distance from their parent star. (b) Radii and semimajor axes of thousands of extrasolar Kepler candidates.

15. 15.4 Exoplanet Properties
Orbits of many of the known extrasolar planets. Note that some of them are very close to their star:
Figure Extrasolar Orbits The orbits of many extrasolar planets residing more than 0.15 AU from their parent star, superimposed on a single plot, with Earth’s orbit shown for comparison (in white). All these extrasolar planets are comparable in mass to Jupiter. A plot of all known extrasolar planets would be very cluttered, but the message would be much the same: These planetary systems don’t look much like ours!

16. 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.

17. 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).
Discovery 15-1 Figures (D. DeMartin/ESO; L. Calcada, N. Risinger/ESO)

18. 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.
Figure 15.8 Earth-like Comparison Two transiting super-Earths whose masses and radii are accurately known are depicted at the top alongside Earth and Neptune. Based on their average densities, these two new worlds seem to be very different from one another—one is rocky, somewhat like Earth or Neptune’s core, but the other may well be composed predominantly of water and ice. Depicted below are the nine official or candidate “habitable” exo-Earths shown in Figure All objects in this figure are drawn to scale and have a color scheme of brown (rocky), blue (icy), yellow (gassy), gray (unknown).

19. 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.

20. 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.
Figure Jupiter-like Planet? (a) Velocity “wobbles” in the star HD reveal the presence of an extrasolar planet with one of the most “Jupiter-like” orbits yet discovered. The parent star is almost identical to the Sun, and the 0.95-Jupiter-mass planet orbits at a distance of 4.2 AU with an orbital eccentricity of 0.04.

21. 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
Figure Sinking Planet Friction between a giant planet and the nebular disk in which it formed tends to make the planet spiral inward. The process continues until the disk is dispersed by the wind from the central star, possibly leaving the planet in a “hot-Jupiter” orbit.

22. 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.

23. 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.
Figure Habitable Zones Every star is surrounded by a habitable zone, within which an Earth-like planet could have liquid water on its surface. Marked in or near their habitable zones are the eight planets of our solar system, 12 extrasolar super-Earths, and three exo-Earths. About 30 more Kepler candidates, as yet unconfirmed, are marked with smaller dots.

24. 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.