January 21, 2015

The photos are fuzzy, but astronomers eagerly studying the first images of the dwarf planet Ceres taken by NASA’s Dawn spacecraft think they may be seeing craters. Astronomers wouldn’t be surprised to learn that Ceres, like many planets and moons in the solar system, has been whacked by space rocks during its billion-year history. Still, the images are the clearest yet of Ceres–three times better than the previous best, which were taken by the Hubble Space Telescope. The new images were taken when Dawn was about 238,000 miles (383,000 kilometers) away from Ceres, about the distance between Earth and the moon. Over the next six weeks, as Dawn speeds closer and closer, astronomers will be able to determine if the surface of Ceres really is dotted by craters. And if all goes well, that finding will be only the first of many discoveries about this largely mysterious object. Astronomers expect to learn much more, especially after Dawn enters orbit around Ceres on March 6. Launched in 2007, Dawn was designed to study both Ceres and Vesta, the third-largest asteroid in the solar system.

The dwarf planet Ceres appears in both visible and infrared light in some of the first images of Ceres taken by the Dawn spacecraft. The craft was 238,000 miles (383,000 kilometers) away at the time. In the infrared image, which is really a temperature map of Ceres, white is warmer and red is colder. (NASA/JPL-Caltech/UCLA/ASI/INAF)

Ceres, which is shaped like a slightly squashed sphere, is about 590 miles (950 kilometers) in diameter. It is so large that it accounts for more than one-fourth of the total mass (amount of matter) of all of the millions of asteroids in the Main Belt, a region between the orbits of Mars and Jupiter. In fact, Ceres has enough mass to be considered a dwarf planet.

On entering obit around Ceres, Dawn will become the first spacecraft to circle two deep-space objects. The craft reached Vesta for a one-year study in 2011. While on this mission, Dawn revealed that Vesta has three layers–an inner core, middle layer called the mantle, and an outer crust. Scientists think both Ceres and Vesta probably formed early in the solar system’s history from many smaller bodies that collided and stuck together, a process called accretion. However, the gravitational pull of Jupiter likely prevented the two from combining with other objects and growing to the size of a planet.

Additional World Book articles:

• Space exploration
• Exploring the Suburban Solar System (a special report)

March 28, 2014

A probable dwarf planet that orbits much farther from the sun than any other known object has been discovered by American astronomers Chad Trujillo and Scott Sheppard. The little planet is so far away that its orbit takes it 15 times as far from the sun as the orbit of Neptune, the most distant major planet in the solar system. The astronomers first sighted the object, designated 2012 VP113, or VP for short, using the Dark Energy Camera mounted on the 4-meter Blanco telescope in Chile. However, to confirm the shape of the object’s orbit around the sun, the astronomers needed to watch the object for many months. The would-be dwarf planet VP is the second such object found orbiting the sun at such a vast distance. The first, named Sedna, was discovered in 2003 by a team that included Trujillo. VP appears to be about 280 miles (450 kilometers) across, about half the size of Sedna and the most commonly known dwarf planet, Pluto. Trujillo and Sheppard nicknamed the new dwarf planet Biden, for United States Vice President Joe Biden.

The distance from Earth to the sun is, on average, about 93 million miles (150 million kilometers). This distance is called an astronomical unit or AU. Neptune is about 30 AU from the sun. The orbit of Sedna is a stretched out oval that carries it as far as 1,000 AU from the sun. However, Sedna also comes to within 76 AU of the sun at its closest approach. The orbit of VP is more circular. For this reason, VP doesn’t stray as far as Sedna, only about 450 AU. On the other hand, VP never gets closer than 80 AU to the sun at its closest approach, farther than any other solar system body known.

Dwarf planets Ceres, Eris, Haumea, Makemake, and Pluto and their moons, compared with Earth and its moon. All of these dwarf planets are smaller than the moon. (©UCAR/University of Michigan)

The area to which Pluto and several other dwarf planets belong is called the Kuiper belt. It is the region beyond Neptune and includes millions of icy bodies. Far beyond the Kuiper belt is a region called the Oort cloud, an area thought to be filled with millions of comets. Both Sedna and VP orbit between the Kuiper belt and the Oort cloud. Many astronomers believed this area was empty. The discovery of Sedna and now VP puts that idea into question.

The discovery offers new information about how the solar system was formed. How did such large objects assume orbits so far from the sun? How many are there yet to be found? One interesting conclusion suggested by astronomers studying the new data is that the orbits of Sedna and VP may be influenced by a large, yet undiscovered planet.

Additional World Book articles:

• Ceres
• Eris
• Planet (the dwarf planets)
• Exploring the Suburban Solar System (a Special Report)
• Astronomy (2004) (a Back in Time article)
• Astronomy (2006) (a Back in Time article)

Nov. 2, 2011

The dwarf planet Eris, whose discovery in 2005 contributed to Pluto’s demotion from planet to dwarf planet, is actually a bit smaller than Pluto, new observations suggest. Discovered in 1930, Pluto was widely considered a full-fledged planet­, until scientists found Eris. At first, it seemed that Eris, which appeared to be slightly larger than Pluto, might become the 10^th planet. Instead, the possibility that astronomers might find many more objects about the size of Pluto and Eris led to a scientific debate about the definition of a planet. In 2006, the International Astronomical Union, the body in charge of naming objects in space, created a formal definition for a planet as well as a new category of objects called dwarf planet for Eris and Pluto.

Eris, Pluto, and two other dwarf planets, called Makemake and Haumea, are found in the Kuiper belt, the area of the solar system immediately beyond the orbit of Neptune. A fifth dwarf planet called Ceres is the largest member of the Main Belt, a region of asteroids between the orbits of Mars and Jupiter. The new observations indicate that Eris is less than 1,454 miles (2,340 kilometers) in diameter. Pluto is about 1,455 miles (2,342 kilometers) wide. All the dwarf planets are smaller than Earth’s moon. Some scientists think there may be thousands of dwarf planets in the solar system.

This illustration shows the dwarf planets Ceres, Eris, Haumea, Makemake, and Pluto and their moons in comparative size to Earth and the moon. UCAR/University of Michigan.

Despite their similar size, Eris and Pluto are remarkably different. Eris is much denser and shinier than Pluto. In fact, Eris is one of the brightest objects in the solar system. Scientists think Eris may be so shiny because of methane ice on its surface.

Additional World Book articles:

• Back in Time (Astronomy,  1930)
• Back in Time (Astronomy, 2005)
• Exploring the Suburban Solar System (special report)
• Tombaugh, Clyde William