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NOVA scienceNOW: 10th Planet

Viewing Ideas

Before Watching

1. Discuss the following questions as a class:

   • Why is it important to have a clear definition of a planet?

     A clear definition would help astronomers know how to classify new objects they discover. For example, if size were the only criteria, some asteroids could be considered to be planets.

   • Why is it important, in science, to classify objects and use precise vocabulary?

     Grouping and classifying objects helps us compare, contrast, and draw connections. Precise vocabulary aids our ability to accurately describe objects and understand the way they relate to one another.

   • How do advancing technologies make it increasingly necessary to have a clear definition?

     New technologies, such as telescopes, computers, and spacecraft, have aided scientists in discovering new bodies that orbit our sun as well as those that orbit distant stars. These bodies need names, categories, and definitions so people can discuss them unambiguously.

2. Some scientists do not believe Pluto is a planet. They say it is smaller than Earth's moon, its mass is far less than that of the other planets, and its orbit is unlike any other (Its orbit is tipped 17 degrees compared to Earth's. This orbital tilt is considerably different from the other eight planets.) On the board, write the mass and diameter of the nine planets and have students calculate how Pluto compares. Ask them if the data suggest that size should be an important part of the definition of a planet.

   	Mercury	Venus	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto
   Mass (x1024 kg)	0.330	4.87	5.97	0.642	1899	568	86.8	102	0.0125
   Diameter (km)	4879	12,104	12,756	3475	142,984	120,536	51,118	49,528	2390

3. Have student pairs match astronomy words to their definitions. You can read out a definition and see if students can name the term. Alternatively, say the term and ask for a definition.

   Astronomy Term	Definition
   Universe	The space that contains all existing matter and energy
   Solar System	A sun and the celestial objects bound to it by gravity
   Inner Planets	Planets and their moons that formed closest to a sun. (For example: Mercury, Venus, Earth, and Mars)
   Terrestrial Planets	Another name for the four inner planets, because they are dense and rocky
   Outer Planets	Planets beyond Mars, sometimes called the Jovian planets or gas giants due to their composition and size. These include Jupiter, Saturn, Uranus, and Neptune. Pluto is sometimes included with the outer planets, but it is small, solid, and its composition is more like an asteroid than a gas giant.
   Satellite	An object that orbits another object
   Asteroids	Small (i.e. boulder size to a few kilometers in length) solid objects that orbit the sun, mostly in a belt between Mars and Jupiter
   Comets	Frozen gas-and-ice bodies orbiting the sun in large, elliptical orbits that extend beyond Pluto
   Kuiper Belt	Disc-shaped region of icy debris orbiting the sun at a distance of 12-15 billion kilometers from the sun

After Watching

1. Ask students what technologies astronomers use to observe the heavens. (Telescopes, satellites, charge-coupled device (CCD) cameras, computers, and conventional cameras) Ask them to speculate about how advancing technology has affected our understanding of the solar system and universe.

   Before 1600, people observed the sky with the naked eye. They thought the universe had eight bodies: the sun, Earth, Earth's moon, Mercury, Venus, Mars, Jupiter, and Saturn. They considered Earth to be the center, with the other bodies revolving around it. After the advent of the telescope in the 17^th century, people realized that the sun occupied the center of the solar system, with the planets in elliptical orbits around it. Today, astronomers use spacecraft, ground-based telescopes, computers, and CCD and conventional cameras to study space. Many thousands of objects have been discovered, and our conception of the universe continues to evolve.

2. Tell students that the speed of light is about 300,000 km per second. Using the distances in the table below, have pairs of students use calculators to find how long it takes light to travel from the sun to each planet. Discuss that sunlight reflects off planets, and that, when we see starlight, we are actually looking back in time to when the light left the star long ago.

   	Mercury	Venus	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto
   Distance from sun (x106 km)	57.9	108.2	149.6	227.9	778.6	1433.5	2872.5	4495.1	5870.0
   Time (minutes)	3.2	6.0	8.3	12.7	43.3	79.6	159.6	249.7	326.1

   Extension: Have students calculate how long it would take to fly from the sun to each planet if they were in a plane averaging 900 km per hour.

3. Scientist Neil deGrasse Tyson discusses how students' knowledge of planets is often limited to naming and ordering them based on their approximate distance from the sun. To help students learn more about the formation and composition of the terrestrial planets and Earth's moon, make 10 groups of students. Assign each one a planet or the moon. Have them research their celestial object and make a poster that includes: its size, distance from the sun, composition, surface temperature (average, high, low), and any other interesting facts they find. Have students determine what a "typical day" is on their planet. Does it have seasons or precipitation? What might astronauts need to survive this object? Ask groups to choose a term to describe their object's "personality," such as a flighty comet, a sleepy moon, or an angry or mysterious planet. Have them share their posters and hang their work on a wall at relative distances from the sun (see scale in After Viewing question 2). As an assessment, ask students to name some characteristics of the inner planets and of the outer gas giants.

Web Sites

Hands-on Universe
www.handsonuniverse.org/
Allow students to examine the planets and objects in the universe.

Welcome to the Planets
pds.jpl.nasa.gov/planets/welcome.htm
Includes planet profiles, photographs, and information about the space-exploration missions.

Windows to the Universe
www.windows.ucar.edu/
Consists of leveled sections for students, teacher resources, and information about space missions, our solar system, and other bodies in the universe.

Books

DK Guide to Space by Peter Bond. Dorling Kindersley, 1999.
Includes many NASA photographs and explores the solar system and beyond.

Skywatching by David H. Levy and John O'Byrne (editor). Time-Life Books, 2000.
Describes the planets, the sun, comets, and eclipses, and includes photographs and sky-viewing charts.