Astronomy Update. Planets, Dwarf Planets, Pluto, Eris & Ceres Unit 3 Cycle 2 Activities 11, 12 & 13

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1 Astronomy Update Planets, Dwarf Planets, Pluto, Eris & Ceres Unit 3 Cycle 2 Activities 11, 12 & 13 Purpose Scientists are constantly revising and updating their theories and definitions as they gain new evidence and information. This update has to do with the International Astronomical Union s (IAU) new definition of a planet, along with the category of dwarf planet, which resulted in the demotion of Pluto from a planet to a dwarf planet. There are three sections to this update. The first section reviews the historical usage of the word planet. The second section reviews the IAU s new definitions. You should use the material in these two sections in class discussions about the meaning of planet. The third section suggests changes to Activities 11 through 13 and Scientists Ideas: Earth and Space Science Part 1 that reflect the new IAU definition of planet and the addition of dwarf planet as a type of Solar System object. Two handouts for Activities 11 and 12 have also been prepared for students and are available at the on-line Teacher Resource. Historical Background For millennia, the word planet was used to designate objects that wandered across the sky. That is, they did not stay in a fixed place on the celestial sphere like stars appeared to do. Thus, planet not only included the wandering stars Mercury, Venus, Mars, Jupiter and Saturn, but also included the Sun and the Moon. Only after people firmly accepted the fact that Earth revolved around the Sun along with the five wanderers did planet come to mean a body that orbited the Sun. Now Earth was counted as a planet, but not the Sun or the Moon. The number of planets rose from six to seven when Uranus was discovered by William Herschel in Astronomers counted eight planets after Ceres was discovered in 1801; nine, when Pallas was found in 1802; ten, with the discovery of Juno in 1804; and eleven with Vesta s discovery in All four of the planets discovered between 1801 and 1807 were small bodies that orbited between Mars and Jupiter in similar orbits. Although some astronomers (notably Herschel) suggested that these small bodies be re-classified as asteroids (Greek for star-like ), most astronomers resisted demoting them until after 1845, when the first asteroid since Vesta was found and several more soon followed. In 1851, astronomers began designating asteroids as minor planets and gave them numeric designations such as 1 Ceres, 2 Pallas, etc. Ceres and its companions were effectively demoted as planets. Not until the mid 1860s were Ceres, Pallas, Juno and Vesta dropped from most lists of planets InterActions Project 1

2 Neptune, discovered in 1846, now became the eighth planet. The Solar System remained a system of eight planets until Clyde Tombaugh discovered Pluto in 1930 out beyond the orbit of Neptune. Originally believed to be a cold, icy planet around the size and mass of Mars, in 1978 Pluto was found to have a large moon, Charon, orbiting close to Pluto. That allowed scientists to get a firm grasp on Pluto s mass because Charon s orbital period is a function of the Pluto-Charon combined mass. They discovered that Pluto s mass was only about 20% of the Moon s mass. Though it continued to be classified as a planet, Pluto was now ranked as the smallest planet by far. Then in 1992, history began to repeat itself. A new icy body smaller than Pluto, provisionally designated 1992 QB1 (it still has not been named), was found beyond Neptune s orbit. This snowball s discovery was followed rapidly by the discovery of hundreds of other icy bodies with similar orbits, including scores of plutinos with orbits similar to Pluto s own (see the last couple pages of Activity 13). Soon astronomers realized that Pluto was just part of a larger group of objects, a belt of icy bodies beyond Neptune s orbit that was roughly analogous to the Asteroid Belt. They named this belt the Kuiper Belt, after a scientist who predicted its existance back in As the years passed and the number of KBOs and similar trans-neptunian objects (TNOs) discovered increased, astronomers found more and more bodies that approached Pluto in size. Many began to question the continued inclusion of Pluto as a planet. If Pluto was a planet, it seemed inevitable that a number of TNOs would eventually be counted as planets as well, and the Solar System could increase in size to dozens of planets. Distant Sedna, discovered in 2002 far beyond the Kuiper Belt and which is probably not much smaller than Pluto, was hailed by some in the press as the tenth planet. When Eris (originally designated 2003 UB313 and nicknamed Xena) was discovered in 2005, even NASA hailed the new discovery as the tenth planet because it was estimated to be larger than Pluto. Two other bodies, discovered at the same time, were also nearly Pluto s size. Rather than immediately accepting Eris as a new planet in 2005, the IAU decided to consider a new definition for planet. One year later, after much debate, the IAU accepted a new definition for planets in the Solar System that excluded both Pluto and Eris. The IAU simultaneously defined a new category of dwarf planet that included Pluto and Eris. Another former planet, Ceres, the largest asteroid, was also counted as a dwarf planet effectively getting promoted while Pluto got demoted. Another twelve bodies (asteroids and KBOs/TNOs alike) were candidates for dwarf planet status. The Solar System is back to eight planets and an unknown number of dwarf planets. While some astronomers were exploring the icy frontier of the Solar System, other astronomers were using new techniques to search beyond the Solar System for planets around other stars. Since 1992, astronomers have found over 200 such planets, and they are discovering new ones at the rate of each year. Most of these planets have been 2

3 the size of Jupiter or larger. Indeed, some have been so massive that they fell just short of having enough mass (about 13 Jupiter masses) to become stars. These discoveries prompted the International Astronomical Union to issue guidelines in 2003 on what differentiated planets from stars and related objects (most notably brown dwarfs ). See the Teacher Resource for on-line references on this topic. Defined by the IAU: Planets and Dwarf Planets On August 24, 2006, the International Astronomical Union adopted the following resolution that defined the terms planet, dwarf planet, and small Solar System objects for bodies in the Solar System: The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". Criterion (b) in the definitions of both planet and dwarf planet, the rounding of a body due to its self-gravity, is the topic of How Gravity Determines Shapes of Planets and Stars in Activity 13 (page 278 in the Student Edition). In Criterion (c), the phrase cleared the neighborhood does not mean that no other bodies share a planet s orbit or cross its orbital path. It does mean is that a planet has cleared the neighborhood of any competing masses (see the discussion of planet formation in Activity 13, pages ), and that it largely controls the orbital dynamics of masses that remain. For example, Neptune s gravity determines the orbits of its moons. Its gravity also keeps Pluto, which crosses its orbit, and hundreds of other plutinos (see Activity 13, pages ) in orbits around the Sun that are resonant with Neptune s own. Neptune is also much more massive than the combined mass of its moons, the plutinos, and all other objects close to its orbit. In 2003, the IAU had also released an official statement that differentiated planets from stars and other massive objects. This is the part of the statement that pertains to planets: 3

4 Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System. Note that deuterium is a heavy isotope of hydrogen, and a stellar remnant are bodies like black holes, neutron stars and white dwarfs that are left over after a star dies. Changes to Activities The IAU s definitions have been incorporated into two new Science Words for InterActions in Physical Science, planet and dwarf planet, as well as an added Scientists Idea. In addition, to complete the set of astronomy Science Words, definitions have been added for star, solar system, and galaxy. These changes and other changes are indicated below. Changes for Activity 11: 4 Copy and distribute Astronomy Handout 1, which is available on this web site. It is designed to be printed on both sides of a single sheet of paper. The information on this handout is always more current than information in the text. New Science Words: star, solar system, planet, dwarf planet. star: a celestial body consisting of hot gases that shines due to the energy produced by nuclear fusion reactions in its core; the Sun is a small star solar system: one star (or more), all the bodies that orbit the star, and their satellites (moons); our Solar System is one of hundreds that we know of planet: a body smaller than a star that has been made round by gravity, orbits a star such as the Sun, and dominates its orbital neighborhood dwarf planet: a small body made round by gravity that orbits the Sun and does not dominate its orbital neighborhood; dwarf planets include Pluto, Eris and Ceres Planets are now objects that orbit the Sun or another star, are round due to selfgravity and dominate their orbital neighborhoods. That means that planets are much more massive than the combined mass of other bodies in their orbital neighborhood. Because it does not dominate its orbital neighborhood, Pluto is no longer considered a planet. That means the Solar System has only eight planets. Dwarf planets are a new category of Solar System objects. They orbit the Sun and are round due to self-gravity, but do not dominate their orbital neighborhoods like planets do. The first dwarf planets identified by the IAU are:

5 o Ceres, the largest asteroid and from or so the eighth planet o Pluto, a large KBO and from the ninth planet o Eris (nicknamed Xena ), a KBO a bit larger than Pluto, which upon discovery in 2005 was billed as the tenth planet Slide Show: There is a new slide, after the gas giants slide, featuring the first three dwarf planets. See the slide show posted at the Teacher s Edition web site. Also, the KBO formerly designated 2004 DW now has a name: Orcus. Question 2: Omit Question 2f. The answer was Pluto, but since this text was written two more small moons have been discovered around Pluto. Visiting Our Neighbors: Although no spacecraft have yet visited the distant Solar System, it is worth noting that the New Horizons spacecraft, launched in January 2006, is on its way and will reach Pluto in July Key Question 1: Add dwarf planets to the list of objects found in the Solar System. Changes for Activity 12: Copy and distribute Astronomy Handout 2, which is available on this web site. It is designed to be printed on both sides of a single sheet of paper. The information on this handout is always more current than information in the text. New Science Word: galaxy. Introduce during Distances between Stars and Galaxies. galaxy: a giant cluster of stars bound by gravity; galaxies can contain between hundreds of millions and hundreds of billions of stars Elliptical orbits: The most elliptical orbit among the planets now belongs to Mercury, whose distance from the Sun varies between 0.31 AU and 0.47 AU. That means that Mercury is 50% further away from the Sun at its furthest than it is at its closest. Slide Show: The slides for the Solar System proper have been modified. Terrestrial planets now have green orbits, gas giants have blue orbits, and the orbits of dwarf planets (including Sedna, a dwarf planet candidate in late 2006) are brown. The orbits for Ceres and Eris are new. Changes for Activity 13: Formation of Stars and Solar Systems: Toward the end of this section (ideally, right after the first paragraph following Question 2), note the following: 5

6 Unlike planets, asteroids and KBOs, including dwarf planets like Ceres, Pluto and Eris, are too small and lack the gravity to sweep up the rock and gas in their orbits, and therefore they do not dominate their orbital neighborhoods like planets do. How Gravity Determines the Shapes of Planets and Stars: Before Question 4, note the following: For the Solar System, the International Astronomical Union considered the rounding of a body by its own gravity to be important enough that they included it in their definitions for planet and dwarf planet. Thus, any body in the Solar System that became round by its own gravity and orbits the Sun is either one of the eight planets or a dwarf planet. Little Plutos and Wandering Planets: Again, the plutino formerly designated 2004 DW is now called Orcus. You might note the number of planets found orbiting other stars as of late 2006 exceeded 200, with a new planet being discovered approximately once every 12 days. These planets range in mass from several times the mass of Earth to several times the mass of Jupiter. Scientists Ideas: Earth and Space Sciences Part 1 Dwarf planets should be added to the list of Solar System objects in Idea 1. Specifically, amend Idea 1a to say: 1. The Solar System consists of : a) the Sun, eight planets, and several dwarf planets Add the following idea: 14. Both planets and dwarf planets are bodies that orbit the Sun and have been made round due to gravity. They differ in one important respect: a) Planets dominate their orbital neighborhoods, which means that they are much more massive than combined mass of all other objects in their orbits, and have much stronger gravity. b) Dwarf planets do not dominate their orbital neighborhoods and usually belong to other groups that have similar orbits. For example, Ceres is an asteroid, and both Pluto and Eris are KBOs. 6