Chapter 14-15 Solar System Debris & Exoplanets
14.2 Comets
Comets that come close enough to the sun to be detectable from Earth have very eccentric orbits
Comet Structure
a very small nucleus
a coma of gas and dust, the most visible part and can be very large
a hydrogen envelope, invisible
a dust tail
an ion tail
The comet’s tail always points away from the Sun, due to the solar wind. The ion tail is straighter than the dust tail.
Halley’s comet is one of the most famous; it has a period of 76 years and has been observed since antiquity. Its most recent visit, in 1986, was not spectacular.
Typical cometary mass: 1012 to 1016 kg
14.3 Beyond Neptune
The Kuiper belt is outside the orbit of Pluto and has many icy chunks
Current theory is that Pluto is the nearest, and largest, of these objects
14.4 Meteoroids
On an average dark night, you can see a few meteors every hour. The flash is caused by heating; most meteors do not survive to reach the ground
Meteoroids are defined as being less than 100m in diameter. Most of the smaller ones are remnants of comets that have broken up
If the Earth’s orbit intersects the comet’s, meteor showers will occur every year on the same date, until the meteoroids have burned out
15.1 Modeling Planet Formation
Solar system is evidently not a random assemblage, bit 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 by the model
15.5 Solar System Regularities and Irregularities
Mercury’s large metallic core may be the result of collision between two planetesimals, where much of mantle was lost
Two large bodies may have merged to form Venus
Late collision may have caused Mar’s north— south asymmetry and stripped most of its atmosphere
Uranus’s tilted axis may be the result of a glancing collision
15.6 Planets Beyond the Solar System
Most extrasolar planets have been discovered indirectly, through their gravitational or optical effects, and cannot be seen directly due to the glare of their star.
Planets around other stars can be detected if they are large enough to cause the star to “wobble” as the planet and star orbit around their common center of mass
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
More than 200 extrasolar planets have been discovered so far:
Most have masses comparable to Jupiter’s
Orbits are general much smaller, and in some cases very much smaller, than the orbit of Jupiter
Orbits have high eccentricity
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
15.7 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
About 5% of stars have been measured have planets around them of the sort that can now be detected
A method of detecting Earth-like planets is much desired but will not be available for some time.
The most promising detection method involves looking for changes in star’s brightness as a planet transits across it,
Until we can observe such planets, we will not be able to draw conclusions about the uniqueness of our own system