Meet your new neighbor: it treks around the sun, and it's large than Pluto. Find out if a newly spotted object could be the 10th planet.

DID YOU KNOW?

* Generally, the farther a planet is from the sun, the cooler the planet's temperature. Exception: Venus is surrounded by a thick atmosphere, which acts like a greenhouse to hold in solar heat. Temperatures on Venus reach 462[degrees]C (864[degrees]F). Astronomers estimate that the surface temperature of distant 2003 UB313 is a frosty -243[degrees]C (-405[degrees]F).

* Neptune takes 164.8 Earth years to orbit the sun. That means that in 2011, this planet will have completed one orbit since its discovery in 1846.

CRITICAL THINKING:

* The International Astronomical Union (IAU) convened a panel of experts to come up with a definition for the word "planet." Their decision will determine the fate of Pluto and 2003 UB313. Debate: Are the two cosmic bodies planets?

CROSS-CURRICULAR CONNECTIONS:

HISTORY: When Galileo first scanned the skies with a telescope in the early 17th century, his observations altered human understandings of the planets. Have students research to create a time line on how advancement in telescope designs lead to discoveries about the solar system.

RESOURCES

* To learn more about 2003 UB313, read this interview with Brian Marsden of the IAU: www.earthsky.com/shows/ astrophysics_interviews.php?id=44587

* For fun facts about the solar system, including an activity for finding your weight on different planets, check out: www.planetary.org/learn/solarsystem/index.html

"My-Very-Educated-Mother-Just-Served-Us-Nine-Pizzas." The various phrases that have helped you memorize the order of the solar system's planets may need revisions. That's because a team of astronomers has discovered what could be the 10th planet.

Currently dubbed 2003 UB313, the newly spotted chunk of ice and rock seems to show planetlike characteristics: It has a spherical shape. It orbits, or revolves around, a star--in this case, the sun. Plus, with an estimated diameter of 3,450 kilometers (2,144 miles), it is larger than Pluto. "This is the first time anyone has found something in the solar system that is bigger than Pluto since 1846, when they discovered Neptune," says David Rabinowitz, a planetary scientist at Yale University in Connecticut.

The cosmic find could do more than complicate your memorization of the solar system's planets. The idea of what defines a planet has been called into question.

That's because the organization responsible for classifying space objects, the International Astronomical Union (IAU), has never decided on criteria that need to be met for an object in space to gain planetary status. Now, astronomers are scrambling to piece together a strict definition. The wording will dictate size limits, and other factors. Due to its small size and wacky orbit, the new find may not qualify as a planet. And if not, what does this mean for the status of Pluto--a rocky object whose planethood has been debated since its discovery in 1930?

ON THE MOVE

Since 2001, scientists have been scanning the skies with giant telescopes connected to cameras in search of new planets the solar system. To spot a sun-orbiting object, the camera snaps three photos of a swath of the sky over a 90-minute period. Since planets make a steady trek around the sun, astronomers examine the pictures to see if an object has changed its location relative to the starry background.

Last January, Michael Brown, an astronomer at the California Institute of Technology, spied this motion in photos of the distant rock.

He noticed that one bright speck had changed its position between sequential images. "When he saw [2003 UB313], he noticed it was moving slowly. That means it's very far away," says Rabinowitz, the space object's codiscoverer. In fact, 2003 UB313 resides in the celestial neighborhood that rings Neptune's orbit. Called the Kuiper (KIE-puhr) belt, this region comprises ice and other space debris left over from the formation of the solar system (see Nuts & Bolts, below).

Another surprise: In September, Brown spotted a moon orbiting the distant object. He estimates that the satellite is eight times smaller than Earth's moon.

ODDBALL

Why didn't planet hunters spy the orbiting rock and its moon sooner? "We're using a new instrument that can look over the whole sky," says Rabinowitz. This telescope, at Palomar Observatory in California, can view larger regions of the sky, and much fainter objects, compared with instruments used decades ago.

Another reason for 2003 UB313's elusiveness: This miniworld has an unusual orbit (see Planet Trek). The eight planets in the solar system, excluding Pluto, circle the sun on a relatively flat plane called the ecliptic. For instance, Venus's path around the sun inches just three degrees above this plane. But 2003 UB313 swoops above the plane at a super-tilted 45 degrees. Until recently, planet seekers were scanning the skies within the ecliptic. So 2003 UB313 had been out of view.

PLANET PUZZLE

Does the object deserve planetary status? Depending on who you ask, you'd likely get a different answer. "There's a debate going on about what the [new object] should be called," says Alan Boss, a planet-formation scientist at the Carnegie Institution of Washington.

Some scientists say that both Pluto and 2003 UB313 are too small to be considered planets. How did Pluto get its planet tag? In 1930, when it was discovered, astronomers estimated that Pluto was Earth-size and declared it the ninth planet. In the 1960s, further study shrank that estimate to one fifth of Earth's diameter. "By 1980, I said that maybe we shouldn't be calling Pluto a planet anymore," says Brian Marsden, director of the IAU's Minor Planet Center. And although the new object may be one-and-a-half times the size of Pluto, it's a mere one-fortieth that of Jupiter.

Besides not sizing up, the new discovery, like Pluto, is one of many objects residing in the Kuiper belt. In recent years, astronomers have spotted more than 1,000 objects out there--fewer than a dozen of which come close to Pluto's size. Since there are several Plutolike objects, Boss says that Pluto and 2003 UB313 shouldn't be called planets. Still, Rabinowitz argues that if Pluto is a planet, then an object larger than Pluto should qualify as a planet too.

NAME TAG

To help solve the cosmic conundrum, the IAU has created a panel of experts, including Marsden and Boss, to craft a precise definition for "planet." The result could decide the status of Pluto and 2003 UB313.

If the distant discovery advances into the planetary lineup, the IAU will determine its name. The discoverers are keeping their name suggestions secret, though for now they have nicknamed it Xena. But Rabinowitz did reveal a clue: "If it were a planet, it should be given a name similar to the other planets." That narrows it down to the Roman gods. Any ideas?

PLANET TREK

The solar system's nine planets follow orbits around the sun that are shaped like ellipses, or stretched-out circles. The new object has an elongated orbit that takes 560 years to complete. Currently, the space object is at a distance from the sun of 97 astronomical units (AU), or the average distance from Earth to the sun.

Relative Sizes: The newly discovered object is roughly one fourth the size of Earth, and a bit larger than Pluto.

Nuts & Bolts

The solar system can be divided into the inner and the outer solar system. Scientists theorize that nearly 5 billion years ago, a spinning cloud of gas and dust began to form the planets. As the particles collided, they grew into larger and larger globules. The inner planets--Mercury, Venus, Earth, and Mars--formed closest to the sun.

There, temperatures were so hot that the gases "boiled off" and escaped into space. So these planets are made of dense, rocky materials. The outer planets, called gas giants, formed farthest from the sun, where gases could remain stable. Pluto--an object in the Kuiper belt--is different from the inner and the outer planets.

CHECK FOR UNDERSTANDING

DIRECTIONS: On a separate sheet of paper, use details from the article to help you write the following:

1. You're the president of your school's astronomy club. Create a leaflet to help the local community learn the difference between the inner planets and the gas giants.

2. You're a member of the International Astronomical Union. Write an argument on why you believe 2003 UB313 should or should not be classified as a planet.

ANSWERS

Answers will vary but should contain the following points:

1. Scientists theorize that more than 5 billion years ago, a spinning cloud of gas and dust began to form the planets As the particles collided, they grew into larger and larger globules The inner planets--Mercury, Venus Earth, and Mars--formed closest to the sun There, temperatures were so hot that the gases "boiled off" and escaped into space So these planets are made of dense, rocky materials The outer planets, called "gas giants," formed farthest away from the sun, where gases could remain stable Pluto--an object in the Kuiper belt--is different from the inner and the outer planets

2. The space object 2003 UB313 is a chunk of ice and rock and seems to have planetlike characteristics: It has a spherical shape; it orbits the sun: and it is planet-size--larger than Pluto But some scientists say that both Pluto and 2003 UB313 are too small to be considered planets Pluto got its planet tag because in 1930, when it was discovered, astronomers estimated that Pluto was Earth size, and immediately declared it the ninth planet in the 1960s, further study shrink that estimate to one fifth of Earth's diameter The new object is only approximately 15 times the size of Pluto. Also, the newly found object, along with Pluto, may be one of many objects residing in the Kuiper belt. In recent years, astronomers have spotted more than 1,000 Kuiper belt objects--only four of which come close to the size of Pluto. Since there are several Plutolike objects orbiting the sun. some scientists say that Pluto and 2003 UB313 shouldn't be called planets.