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Uranus
Neptune & Pluto Questions
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Q1. How were Uranus, Neptune, and Pluto discovered? Answer
Q2. What observation tells us that Uranus and Neptune are composed mostly of water? Answer
Q3. Why are Uranus and Neptune blue instead of the reds, browns, and yellows that are typical of Jupiter and Saturn? Answer
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Q1. Describe how seasons are different on Uranus (with its sideways rotation) compared to Earth. Answer
Q2. How are the rings of Uranus different in appearance from those at Saturn? What is thought to cause this difference in appearance? Answer
Q3. What component of Uranus’ atmosphere gives it its blue color? Why? Answer
Q4. How were the rings of Uranus first discovered? Answer
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Q1. How was Neptune discovered? Answer
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Q1. What is the significance of the discovery of Pluto’s moon? Answer
Q2. Why was the mass and size of Pluto unknown for many years? How were they finally determined? Answer
Q3. What observations finally allowed astronomers to determine the mass of Pluto? Why did the result surprise them? Answer
Uranus Neptune & Pluto Answers
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A1. Uranus was discovered completely by accident as William Herschel mapped the heavens.
After Uranus was discovered, astronomers watched its orbit very carefully. Over time they noticed that it was not following the path predicted by the known forces of gravity from the sun and other planets. Predictions were made that another , unknown planet must be causing the deviations from the predicted path. These predictions led to the discover of Neptune.
The same process of watching and predicting was followed for Neptune. While none of the predicted planets have ever been found, they did motivate astronomers to search the heavens for additional planets. One of these systematic searches led to the discovery of Pluto.
A2. While the average density of the planet gives some hint that they are composed mostly of water, more detailed evidence comes from comparing their radius and mass with models composed of different substances.
A3. The blue color of Uranus and Neptune is caused by methane
in their atmospheres which absorbs red and yellow, but reflects blue.
Uranus Answers
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A1. When the pole points toward the sun that part of the planet will be in perpetual sunlight (for many years) and will experience an intense summer. At the same time the opposite pole will be in darkness and winter. When the axis of rotation is more nearly sideways to the sun, the equatorial regions will experience regular day/night cycles and a moderate summer.
A2. The rings of Uranus are widely separated from each other and narrow in width. The rings of Saturn cover a broad expanse of radius, but are divided into thousands of finely divided ringlets. Saturn also has one narrowly defined ring outside the main belt of rings. A narrow ring is the result of tiny moons on either side of the ring that shepherd the ring particles to keep them from spreading out.
A3. Uranus appears blue to us because of the methane in its outer atmosphere. Methane absorbs red light, leaving the blue to be reflected by the atmosphere.
A4. The rings of Uranus were discovered just before a star was scheduled to pass behind Uranus, an even called an occultation. Before the occultation caused by the planet, the light of the star was briefly interrupted several times by an unknown series of objects. When the same sequence of disappearance was observed after the primary occultation, it was realized that rings must account for the temporary blockages of light from the star.
Neptune Answers
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A1. After observing the motion of Uranus for approximately
50 years, astronomers concluded that its motion could not be explained by the effects of
gravity of the known objects in the solar system. The motion could only be explained if
the gravity of another planet was included in the calculation. These calculations allowed
astronomers to predict the precise location of this unseen planet, and Neptune was quickly
discovered exactly where it had been predicted.
Pluto Answers
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A1. Once the moon was discovered and its orbit determined, we were able to determine the mass of Pluto and able to estimate the size of Pluto more accurately than before. With this information, we found that Pluto is an icy body that is much smaller than many had thought – much too small to affect the orbits of the other planets.
A2. Pluto is so tiny and distant that its size cannot be directly observed with ground-based telescopes. The mass of a planet can only be determined by its gravitational influence on another body, and no nearby body was known for Pluto until its moon was discovered in 1978. Once its moon was discovered, the orbital motion could be used to determine its mass. Eclipses of the moon by Pluto also allow astronomers to determine its size.
A3. Pluto’s mass was finally measured after the motion of its newly discovered moon was analyzed. It turns out the Pluto is much lighter than astronomers had previously thought. It is much too small to cause any significant deviations in the motions of Neptune.