Astronomy 10: Lecture 10

Lecture 10

Wednesday, June 12th 2002. Reading: Cosmic Perspective Chapters 1, 8, & 12

Pluto and Charon

Planet dSun
P (yr)
P (days)
moons symbol
Pluto 39.5 249 0.2 0.0025 2.1 6.4 120° 1

Planet #9, 1 moon

Discovered in 1930 (C. Tombaugh), and its moon, Charon, was not discovered until 1978 (J. Christy)

Sometimes Pluto is closer to the Sun than Neptune because of its highly elliptical orbit (e = 0.25). It was closer 1979-1999, it's currently back to being the furthest from the Sun.

The orbit is also inclined by 17° with respect to the ecliptic plane. Very large.

Despite its being an outer planet, Pluto is completely unlike the jovian worlds. It's actually very similar in composition to Triton (Neptune's largest moon).

Charon is has a radius = 0.1R = 0.5R. It's really like a "double planet"! The orbital period is 6 days (tidally locked).

Every 250 years there is a 5-year period where Pluto and Charon eclipse eachother as seen from Earth. This allowed astronomers on Earth to determine the physical sizes and densities of the system.

The rotation axis is "on it's side" like Uranus.

Pluto has a thin, methane atmosphere. Able to keep it because it is so cold there (40 K).

2 thoughts about Pluto:
(1) A former moon of Neptune that was stripped away
(2) The largest of the Kuiper belt objects

Kuiper Belt

The Kuiper Belt is a recently discovered debris field that lies just beyond the orbit of Neptune. So far there have been over 60 small icy worlds discovered here and it is estimated that there may be as many as over 30,000 such icy worldlets in a flattened belt extending out to as far as 1000 AU. This region is very much like the inner asteroid belt, but because of its distance from the Sun these big rocks are able to keep water ice frozen on their surface.

There has been a controversy in the Astronomical community on whether or not to classify Pluto as a planet or big Kuiper belt object.


Often referred to as "minor planets". The majority of these giant rocks occupy a belt between the orbits of Mars and Jupiter (~2 AU) where there could be a planet.

The first one discovered was in 1801, named Ceres. Ceres is about 1000 km in diameter and one of the largest asteroids. There are now thousands known and there are estimated to be about 100,000 out there in interplanetary space.

There are only about 6 that are larger than 300 km across; most are smaller (< 10 km). Most do not have enough mass to be spherical.

3 types: rocks, iron/nickel, carbon rich

Tidal forces exerted by Jupiter likely kept these bodies from being able to form into a planet in the early Solar System.


Seen in the sky as diffuse, luminous patches, often with long tails. They are basically "dirty snowballs" that evaporate as they approach the Sun.

The tail is dust and gas pushed away by the Sun's radiation pressure and solar wind. The tail therefore always points away from the Sun (not always trailing behind the comet!).

Comets are like asteroids that are covered with ice. They have typical masses = 10-11 of Earth. They are made of primitive material and provide clues to the early composition of the original "Solar Nebula".

Periodic Comets: these comets are bound to the Sun and orbit in very elliptical orbits. They spend most of their existence very far from the Sun and only a short time near it (when the tail develops).

Most comets are not periodic. They originate from either the Kuiper Belt or the Oort Cloud: sphere of ~ 1012 comets, ~ 50,000 AU from the Sun. Sometimes the cloud is perturbed by a passing star and sends some comets heading toward the inner Solar System.


Meteors and Impacts

Meteors: are the name given any piece of space debris that enters the Earth's atmosphere.

Meteorites: are debris which actually touch down on the Earth's surface

Meteroids: chunks floating through interplanetary space, not in the asteroid belt. Most are small (< 10 m). Most probably come from the asteroid belt; produced by collisions.

When they enter Earth's atmosphere (< 100 km), they burn due to friction. They are coming in with very high velocity and hence have much kinetic energy which is converted into heat and light during the burn. Shooting Star

When they are found on the surface they can be studied to give clues as to the composition of the early Solar System and its age.

Age = 4.6 Billion years

Some meteorites seem to come from the Moon, and Mars. Some meteorites have carbon-rich molecules, like amino acids: life precursors.

Meteor Showers occur when Earth passes through the debris field that a comet leaves behind after passing through the inner Solar System. Most re-occur each year since the debris field is in orbit around the Sun as well.

Some meteors are bigger and when they enter the atmosphere they burn very brightly and leave smoke trails behind.

Others are bigger still (~ 0.5 - 50 m) and blow-up upon entering the atmosphere.

Then there are even larger objects that collide with Earth:
Some groups of Asteroids (mainly "Apollo") have orbits that cross Earth's orbit. Eventually most of them can collide with Earth. Plus there are many comets and random meteroid that cross Earth's orbit and can collide.


~ 200 Earth-crossing asteroids known, with typical diameter = 1 km. There are an estimated 2,000 - 4,000 of them out there. We expect a collision to occur once every 300,000 years; 105 - 106 megatons. The smaller blasts (Tunguska) every ~ 300 years.
Statistically, averaged over tens of millions of years, you are as likely to die from a cosmic collision as you are from an airplane crash, flood, or tornado.

An asteroid with d = 10 km hits the Earth roughly every 30 million years. Such a collision in our time would likely be the end of human civilization. 108 - 109 megaton explosion devastates surrounding area (r = 1000 km); rest of Earth like an oven set to "broil". This is followed by impact "winter" produced by dust in the atmosphere; sunlight is blocked for many years.

Hypothesis: The cause of the Cretacious - Tertiary extinction 65 million years ago, when the dinosaurs along with 2/3 of all other life on the planet went extinct, was caused by such a collision.

Luis Alvarez (UC Berkeley) 1979 - excellent evidence: thin iridium layer found in rock strata at t = -65 million years. Also the site of an impact crater in the Yucatan peninsula has been dated at about 65 million years old.

A Real Threat: in July 1994 comet Shoemaker-Levy 9 collided with Jupiter and all the worlds telescope were there to watch. The comet had been tidally broken up into 20 smaller pieces by a previous encounter with Jupiter. The chunks crashed into Jupiter with energies released of about 6 million megatons each!

Formation of the Solar System

To understand the observed characteristics of the Solar System we need a Scientific Model that explains how it formed. Many models have been proposed over the last few centuries (as should always be the case). The one that has come to the forefront and is the best so far is called the Nebular Theory.

The Nebular Theory has these key features:

Protoplanetary disks such as those predicted by the theory have been observed in regions of active star formation.

Other Planetary Systems

If our ideas about how our own Solar System formed are correct then it is reasonable to conclude that many, many other stars would have formed in the same manner and would have planetary systems of their own.

Planets around other stars are hard to find. We cannot see their reflected light (too dim and drowned out by star's light).

Until recent years this had only been a hypothesis with no data. But now we have very strong evidence that most stars, in fact, have planets around them.

Several teams of astronomers across the world have been using a similar technique to find them. One team lead by UC Berkeley's Geoff Marcy has been very successful. They have found these new worlds by their gravitational influence on their parent star.

They look for a periodic "wobble" in the motion of the star due to the gravitational tug and motion of the planet around it. The "wobble" is measured as a Doppler Shift in the light from the Star.

Can calculate the mass of the planet using Kepler's 3rd law.

There are now over 50 Extra-solar planets known. Most have masses similar to Saturn and Jupiter (and greater) and are very close to their parent star. This is likley a selection effect. The Doppler Shift Method is most sensitive to massive planets with short periods and eccentric orbits.

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