International Astronomical Union (IAU) members vote on a new planet definition during a meeting in Prague Aug. 24, 2006. The vote redefined Pluto as a dwarf planet.

MICHAL CIZEK/AFP/GETTY IMAGES DEFINITION: PLANET 11,000 scientists around the world have the final say on what’s a planet — or not

By Mary Helen Berg a planet is a planet or just an orbiting ice of planetary scientists, academics and ball. Right now, the worlds that make historians support research, confirm HE NUMBER OF PLANETS IN OUR the cut are Mercury, Venus, Earth, Mars, celestial discoveries, document and solar system is … well, it depends Jupiter, Saturn, Uranus and Neptune. preserve data and even track potentially on who you ask. Pluto fans, sporting The IAU is “responsible for managing the dangerous asteroids. T-shirts that read “Never Forget,” astronomical world,” said Gareth Williams, Tstill say there are nine. Meanwhile, some associate director of the NASA-funded IAU PLUTO OUT astronomers say the tally should be 13. But Minor Planet Center (MPC). “They define Most of these working groups fly under the arbiter on all things astronomical, the everything that astronomers need to talk the radar. But in 2006, one committee International Astronomical Union (IAU), about objects in a consistent way. So, if I’m found itself under global scrutiny when, for recognizes only eight planets. talking about an object at a certain point the first time, the astronomy community As the world’s largest professional in the sky, some other astronomer knows demanded an official definition of “planet.” organization for astronomers, the IAU exactly what I am talking about.” The seven-member Planet Definition represents 11,000 scientists from 95 In other words, the IAU controls cosmic countries and has the final say on whether chaos on Earth. Volunteer committees CONTINUED IN THE SHADOWS FITS IN UTAH Orbital analysis of 2012 VP113, a tiny planetoid beyond Pluto, and other dwarf planets like Sedna indicates an undetected planet, a giant Super Earth, Astronomers estimate the diameter of 2012 VP113 at lurking on the outer edge of our solar system. Here’s how 2012 VP113 and Super Earth compare with some of their neighbors: 280 miles, about the width of Utah.

SIZES Utah (Diameters in miles)

2012 VP113 280 miles EARTH MOON PLUTO SEDNA 2012 VP113 1 1 7,918 2,159 1,430 800 TO 1,100 279 SUPER EARTH 16,000 TO 80,0001

DISTANCE FROM SUN Positions at perihelion, closest position to sun 270 miles (In miles)

Mars Saturn Neptune, Pluto 2012 VP113 128.4 million 839.7 million 2.8 billion 7.4 billion

Note: Sizes not to scale

Earth Jupiter Uranus Sedna Super Earth 91.4 million 460.2 million 1.7 billion 7.1 billion 19 billion

1 – estimates. SOURCES: SPACE.COM; NASA; GEORGE PETRAS AND TRACI WATSON, USA TODAY KARL GELLES/USA TODAY

not clear objects out of contains thousands of objects. Even more certain to influence Pluto’s planet status, PICK A NUMBER HOW PLANETS ARE NAMED its path. In a controversial exoplanets exist outside the solar system. said Jack Burns, a vice president of the So how many planets are there really? Among “The Pluto case is the only one I’m WHAT’S A PLANET? The International Astronomical Union (IAU) has been responsible other things, vote, IAU members at- In 2003, one Kuiper Belt object (KBO) American Astronomical Society (AAS). “Ask 100 scientists and you’ll get 100 tending the 2006 General appeared so similar in size to Pluto that “If Eris had been known at the same answers,” said MIT’s Binzel. In 2006, the International Astro- for approving the names of heavenly bodies since 1919. The IAU lists proposed aware of in which public emotional Assembly approved the astronomers thought it had potential as the time that Pluto was named a planet (in David Aguilar’s answer is in the title of nomical Union (AU) decided that for a more than 300 sources for planetary names, from mythologies, names should be: reaction (call it interest) has forced new definition. 10th planet. 1930), it probably would have had a big his middle-school book 13 Planets: The Lat- celestial body to be defined as a planet it legends and folktales to famous women and scientists. “There was no real Intense interest over the discovery of the impact on the whole original discussion of est View of the Solar System. He combines needed to meet three criteria: The 15-member Committee for Small Body Nomenclature (CSBN), v One word the IAU to issue a definition. The need for a definition of KBO, Eris, in 2003 prompted the need for a what are planets and Pluto’s designation as the five dwarf and the eight major planets 1 It must orbit the sun a global group of astronomers, votes to approve proposed names. v No more IAU decides to give a definition planet before this,” said definition of planet and for a new category a planet,” Burns said. in a baker’s dozen. 2 Its mass is great enough that its They become official when published in a journal of the Minor Planets than 16 PDC member Richard of dwarf planets, objects that share some, The number of planets has been more “A planet is a planet is a planet” characters of an object if there is a scientific own gravity pulls it into a round or Center (MPC). Binzel, a Pluto specialist but not all, of the criteria for a planet, said fluid than one might think. The IAU regardless of nitpicky definitions, said nearly round shape For example, planetary scientist Mike Brown nicknamed his 2003 v Not at the Massachusetts Thierry Montmerle, IAU general secretary. debated for two years before establishing Aguilar, director of science information necessity to do so.” 3 It has cleared debris from the discovery Xena. The MPC assigned the object a provisional designa- offensive Institute of Technology. “It “The Pluto case is the only one I’m aware eight major planets and re-assigning Pluto at the Harvard-Smithsonian Center for neighborhood of its orbit tion, 2003 UB313. The number includes the year the object was v Different — Thierry Montmerle, general secretary, was more like, ‘You know of in which public emotional reaction (call and Eris to the dwarf category, along with Astrophysics. discovered (2003), which half of the month it was discovered in (U) than existing International Astronomical Union a dog when you see one.’ it interest) has forced the IAU to issue a the asteroid Ceres, and KBOs Makemake Far from the debate, planetary scientist That same year, the IAU approved the and the order in which it was found during the month (B313). When planet names You know a planet when definition,” Montmerle wrote in an e-mail. and Haumea. But an earlier resolution Nicolle Zellner of Albion College in Michi- first definition for dwarf planet. It must the object’s orbit was confirmed, the IAU assigned it a permanent v Not the you see one.” Usually, “the IAU decides to give a would have named 12 planets — the gan, whose lunar research is supported also orbit the sun and achieve a roundish number (136,199), and Brown proposed a permanent name: Eris. name of a Technology has definition of an object if there is a scientific current eight, plus Pluto, Eris, the asteroid by NASA, among others, suggests that the shape, cannot be a satellite and does not recent military Now, the world can join the name game by participating in the first Committee (PDC), an international group changed that. Increasingly powerful land necessity to do so (like for a catalog), and Ceres and Charon, Pluto’s satellite. Ceres astronomy community use “a worldwide need to clear its neighborhood. or political consisting of astronomers, educators, telescopes and space telescopes such as the if it is possible or useful,” he said. “For was considered a planet for decades after online poll” of interested scientists to seek IAU-sponsored NameExoWorlds contest. After nonprofit organiza- Likewise, the IAU deemed that any leader or historians and novelist Dava Sobel, dared one used in NASA’s Kepler mission have instance, there is no IAU definition of a its discovery in 1801. more input the next time it debates an other object that orbits the sun, except tions and astronomy clubs suggest names for more than 300 event to go where no man had gone before. They revealed a bonanza of celestial objects, star or of a galaxy. A definition has to be In fact, early 19th-century astronomers important topic like planet status. planetary satellites, should be called exoplanets, the public can vote online for their favorites. New names v Not a pet set criteria that an object must meet to be causing astronomers to question what they quantitative or refer to a specific physical also thought about extending the planets And to think that ancient astronomers will be announced at the IAU General Assembly in August 2015. animal name considered a planet — criteria that had thought needed no definition. law (like gravity for planets vs. dwarf to a dozen, believing that asteroids Ceres, could simply look up to the inky sky, “Small Solar System Bodies.” — Mary Helen Berg excluded Pluto because while it orbits the Galactic traffic beyond Neptune’s orbit planets).” Pallas, Vesta and Juno were all planets, marvel at a moving light and call it a planet. — Mary Helen Berg sun like other planets, it is smaller and can- in the icy region called the Kuiper Belt Sooner or later, the discovery of Eris was Williams said. Ah, the good old days. ●