PY 124: Solar System Astronomy: Homework 3: SOLUTIONS

1) Define an Astronomical Unit.

An astronomical unit is the average distance from the Earth to the sun. It is approximately equal to 1.5 x 1011 meters. (150,000,000,000 meters, or 93 million miles)

The world's air speed record is held by a U.S. Air Force SR-71 "Blackbird" reconnaissance plane. It is near 2000 miles/hour. At this speed, it would take 5.3 YEARS for the world's fastest aircraft to get to the sun from Earth. At the speed of the typical passenger aircraft (about 600 miles/hour), it would take 17.7 YEARS to get to the sun. An Astronomical Unit is a LONG distance!

2) What attributes do the Jovian planets have in common?

They are all:

very large,

very massive,

have a ring or rings,

low density (near 1g/cc)

No solid surface ("gas balls")

3) Which planets were discovered in "modern" times? (After AD 1600.)

In order: First was Uranus (1781), then Neptune (1846) , and then Pluto (1930), then the planets orbiting other stars: PSR 1257+12 (1992), 51 Pegasi (1995), 70 Virginis (1995), 47 Ursae Majoris (1995), LaLande 21185 (1996).... There are currently (November 2006) more than 150 planets known to be orbiting other stars.

4) Why don't the objects in Saturn's or Jupiter's rings condense to make one or two large moons?

The rings of all the Jovian planets are within their respective Roche limit. The Roche limit is the closest that a large object can come to a body such as a planet before it would be torn apart by the different gravitational pulls on its near side and its far side. Since all of the objects in the rings are within the Roche limit, they cannot form one or two large moons, since these would be torn apart, and you'd end up with rings again.

5) Why do comet's tails always point away from the sun?

Comets' tails always point away from the sun because the gas and dust melted off of the comet is pushed back by the solar wind, a stream of particles that flows away from the sun. Since the solar wind always flows away from the sun, it always pushes the stuff melted from the comet away from the sun, forming the tail.

6) List the Jovian planets in order of size, biggest to smallest.

Jupiter, Saturn, Uranus, Neptune. (Uranus and Neptune are almost the same size.)

7) What's the difference between a meteoroid, a meteorite, and a meteor?

When a rock that will hit the Earth is in space it is called a meteoroid, when its in the atmosphere of the Earth its a meteor, and once its hits the ground, its a meteorite. Same object, three different names, depending on where it is located.

8) The fact that some of Jupiter's moons have highly inclined orbits is an indication of what concerning these moons?

This indicates that the moons were captured, rather than forming along with the planet. If they formed with the planet, we would expect them to be in the equatorial plane of the planet.

9) What are the Lagrangian points in a planet's orbit? Where are they located?

The Lagrangian points in a planet's orbit are points where a object could orbit in synchrony with the planet there are 5 such points, but only two, the L4 and L5 points, are stable (meaning that an object at the L4 or L5 point could remain there for some length of time.) The L4 and L5 points are located 60 degrees ahead of and 60 degrees behind the planet, along the path of the planet's orbit.

10) Where are the Kuiper Belt, the Asteroid Belt, and the Oort Cloud, and what is located in each?

The Kuiper Belt is located about 40 - 200 AU, The Asteroid belt is located at about 2.8 AU, and the Oort Cloud is located at about 20,000 - 50,000 AU. The Asteroid belt has (not surprisingly) asteroids, while both the Kuiper belt and Oort Cloud have comets, the Kuiper belt being the source for short period comets and the Oort Cloud being the source for long period comets.

11) Draw the layers of the sun.

The sun has a core in the center which is where thermonuclear fusion takes place, surrounded by the Radiative Zone in which the principle method of energy transport is radiation, which is itself surrounded by the Convection Zone, where the principle means of energy transport is convection. At the top of the Convection Zone there is the Photosphere, the layer of the sun in which most of the visable light is produced. Surrounding the Photosphere is the Chromosphere, followed by the solar corona.

12) Describe the astrometric method of searching for extrasolar planets (planets orbiting other stars.)

Any two objects orbit their common center of mass, or barycenter. This is the point in space where the objects would balance if they were stuck on a very long stick. In the case of a planet and a star, this point is within the star, but NOT at the center of the star. If we measure the position of a star with a planet very carefully over a period of time, we would find that the star "wobbles" a little as the planet orbits. The bigger the planet is compared to the star, the more the star wobbles. The science of very precisely measuring the positions of stars is know as astrometry, so searching for the wobble is a star's position is known as the astrometric method. Most stars that wobble do so because of other companion stars, but by analyzing the size and period of the wobble we can tell if the wobble is caused by another star or a planet.

13) What is the source of the sun's energy? (What powers the sun?)

The fusion of Hydrogen into Helium.

14) What gas is the largest component of the Jovian planets? What gas is the largest component of the sun. Why would we expect / not expect that the giant planets and the sun have the same major components?

Hydrogen, for both the sun and the Jovian planets. The second largest component is Helium. Since Hydrogen and Helium are the two simplest elements, made of just 1 and 2 protons each respectively, it is not surprising that we find that most of the mass of the solar system (in fact most of

the universe) is made up of these two gasses.

15) Which Jovian planets, if any, have retrograde rotation?

Only one Jovian planet has retrograde rotation. Uranus. Uranus also essentially lies on its side.

Answers to Sample Test 3 on the Web:

1-A, 2-A, 3-A, 4-A, 5-A, 6-C, 7-C, 8-B, 9-C, 10-B, 11-B, 12-A, 13-D, 14-C, 15-B, 16-B, 17-A, 18-B, 19-A, 20-C, 21-B, 22-C, 23-D, 24-A, 25-A