Chapter 14. Discovering Planets

14.1 Introduction

Uranus and Neptune: Two planets not observable with the unaided eye.

The goals of this module: After completing this exercise, you should be able to:

1. Explain how astronomers distinguish between observations of planets and stars.
2. Explain how astronomers used the basic laws of physics to calculate the existence of Neptune.

In this module you will explore:

1. How a planet appears to move through the sky relative to the stars.
2. How astronomers used motion to detect the existence of undiscovered planets in our solar system.
3. How astronomers use a series of observations, and not just one, to determine the existence and orbital parameters of a planet.

Why you are doing it: Only five planets can be observed without a telescope (the first six planets, minus Earth). Objects further out in our solar system were discovered with the use of a telescope. Given the size of the entire sky and the small fraction of it observed through a telescope, it is amazing that astronomers have been able to detect as many objects as they do. The detection of the seventh and eighth planets in our solar system not only changed our view of the solar system, but it also tripled the scale of it.

14.2 Background

Even before the invention of telescopes, ancient astronomers looked up into the sky and noted what they saw. They observed thousands of stars in the sky, and plotted their positions. Some of the names of the patterns of stars they recognized, like Orion, Cygnus, and Sagittarius, we still use today. They knew where each star belonged. Before telescopes, astronomers also knew that there existed a number of planets as well (5 to be exact). Looking up in the sky without a telescope, though, it's difficult to tell the difference between a star and a planet, so how did they know?

The above animation shows a view of the night time sky, with what appears to be a field of stars. Rapid click on the arrows at the bottom of the animation to advance the observation forwards or backwards in time. Each time you click about two weeks of time have passed. Do all of the objects appear stationary?

14.3 A Planet Named George

William Herschel

Before the invention of the telescope, ancient astronomers knew of the existence of five planets in the sky: Mercury, Venus, Mars, Jupiter and Saturn. These planets are visible to the unaided eye, and astronomers noted that they moved relative to the background stars. Up until the 1700's, these were the only planets that astronomers were aware of. In 1781, though, that all changed. On March 13, William Herschel, a musician turned mathematician and astronomer, was systematically observing the sky with a telescope when he happened to observe what appeared to be a fuzzy blue-green object. At first Herschel believed that this object was a comet, since comets appear fuzzy when viewed through a telescope. Over the course of the year, though, as Herschel continued to observe this object, he determined that its orbit was not one that would be expected of a comet. Comets move on highly elliptical orbits, which bring them close to the Sun. This object's orbit was nearly circular, and it resided beyond Saturn. In fact, Herschel had discovered the seventh planet of our solar system. Herschel wanted to name the new planet Georgium Sidus (George's Star) after England's King George III, but many in the astronomical community (especially those from other countries) refused to call the planet by this name, and eventually it came to be known as Uranus (pronounced YOOR-uh-nus).

14.4 Determining Uranus' Orbit

After Herschel's discovery of Uranus, astronomers realized that he was not the first to observe the planet. Since 1690, Uranus had been observed at least 20 times, but without noticing its motion relative to background stars. It appeared simply as yet another dim star in a field of stars. Based on its distance away from the Sun, Uranus appears to move through the sky only 4° per year.

14.5 The Power of Physics

After Herschel's discovery of Uranus in 1781, astronomers continued to track its motion. Over time, it became apparent that Uranus was not following the path it was supposed to take, according to Kepler's laws. By 1830, Uranus was 2 arcminutes away from its predicted position.

Some astronomers began to worry that Newton's law of gravitation was not accurate at large distances. Others, though, provided another alternative. Two scientists, Le Verrier in France and Adams in England (working independently), were investigating the possibility that there existed an object with enough mass to cause slight changes (perturbations) in Uranus' orbit, pulling it away from its predicted orbit. Calculations suggested that Uranus had sped up in its orbit as it approached this massive object, and then slowed down after passing by. Using their calculations and the latest star maps, another astronomer, Galle in Germany, searched for and found the eighth planet in our solar system, Neptune.

14.6 Quick Check Quiz

Indepth Activity: Discovering Planets