January 16, 2019

On January 1, the United States National Aeronautics and Space Administration (NASA) probe New Horizons flew by a 21-mile- (34-kilometer-) long rock at the outer reaches of our solar system. The probe took the rock’s first “close-up” photos, revealing details of the snowman-shaped space object, called 2014 MU69 or “Ultima Thule.” The term Thule was used by ancient European cartographers to describe the farthest northern lands of Earth. Ultima Thule (farthermost Thule) once described areas beyond the known world. Ultima Thule, then, is a fitting name for the distant object: it is 4 billion miles (6.5 billion kilometers) from Earth.

The New Horizons spacecraft took this photo of Ultima Thule on Jan. 1, 2019, from a distance of some 18,000 miles (28,000 kilometers). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

New Horizons is the first spacecraft to study Pluto and the Kuiper belt, a band of icy bodies largely beyond the orbit of the planet Neptune. New Horizons flew past Pluto in July 2015, revealing a surprisingly varied and geologically active world. Despite its distance from the sun, Pluto has been molded and shaped by many of the same forces that created the eight planets of our solar system.

With the flyby of Ultima Thule, New Horizons became the first spacecraft to visit an object discovered after the spacecraft was launched. The craft blasted off from Cape Canaveral Air Force Station in Florida in 2006. Astronomers discovered Ultima Thule in 2014.

This NASA illustration shows New Horizons encountering 2014 MU69–“Ultima Thule.” Credit: NASA/JHUAPL/SwRI

What’s so special about Ultima Thule? It may look like just another space rock, but it is far different from the asteroids and comets found closer to the sun. The solar system’s planets and other inner objects have been transformed by the sun’s extreme heat, by catastrophic collisions, and by immense gravitational pressures. Ultima Thule and other Kuiper belt objects (KBO’s) are like frozen time capsules that show how things looked as the solar system was forming. The first grainy photographs of Ultima Thule show an object made up of two lobes that likely came together in a walking-speed “collision”—a rather serene birth in an otherwise violent universe.

New Horizons has completed its study of Ultima Thule, but NASA expects to learn more about the distant object over the next 20 months or so as information continues to trickle in. It takes a while for data to be transmitted across the immense distance to Earth. And this may not be the last distant flyby for New Horizons. NASA is searching for other space objects to visit before New Horizons runs out of power in the mid-2030’s.

This artist’s rendering shows a view of Planet Nine from a perspective facing towards the distant sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Credit: Caltech/R. Hurt (IPAC)

January 22, 2016

On Wednesday, January 20, astronomers Konstantin Batygin and Mike Brown from the California Institute of Technology (Caltech) in Pasadena released a study in The Astronomical Journal suggesting that there may be a ninth planet in the solar system after all. But it isn’t the dwarf planet Pluto, or Eris—or any other tiny, frozen ball of rock, for that matter.

Batygin and Brown analyzed the orbits of six objects with elongated orbits in the Kuiper belt. They noticed that all of these objects orbited the sun in a distinctive way, with their perihelia (the part of an orbit closest to the sun) clustered on one side of the solar system. Using computer simulations, the astronomers suggest that a ninth planet with about 10 times the mass of Earth is causing the orbits of the Kuiper belt objects to cluster in this way.

Predicting the existence of large objects on the edge of our solar system is not a new venture. After Uranus was discovered in 1781, astronomers noted that it was not orbiting exactly as it should, a hint that another large planet was altering its orbit. In 1846, French mathematician Urbain J. J. Le Verrier calculated the orbit of this outer planet and predicted its location in the night sky. Astronomers found the planet—now called Neptune—almost exactly where Le Verrier predicted.

Neptune’s discovery is the only successful prediction of this type to date. Around 1900, American businessman and amateur astronomer Percival Lowell became convinced that a ninth planet lurked beyond Neptune, based on more apparent inconsistencies in Uranus’s orbit. Clyde Tombaugh eventually discovered Pluto using the giant observatory in Flagstaff, Arizona, which Lowell built to look for Planet X. Pluto, however, was far too small to affect the orbit of Uranus. Much later, observations by space probes refined the values for Uranus’s size and mass. These values showed there was no problem with the planet’s orbit after all.

Although the evidence is strong, scientists will have to see Planet Nine to believe it. The planet will be extremely difficult to locate by telescope, despite its apparently large size. It is incredibly far away, from 600 to 1,200 times farther than is the sun from the Earth. Planet Nine completes an orbit every 15,000 years. For comparison, Pluto, the former ninth planet, is only about 40 times farther from the sun than Earth is and it completes an orbit every 248 years. It might take as long as five years of scientists combing the night sky with powerful telescopes to find Planet Nine. Until then, the solar system will have eight planets.

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)

July 12, 2012

Astronomers using the Hubble Space Telescope (HST) have discovered another moon orbiting Pluto, announces NASA. The new satellite is the fifth found circling the small body, which for decades was widely considered the ninth planet in the solar system. Pluto was “demoted” from planet to a new category of space object called “dwarf planet” after astronomers in the 1990′s found many objects similar to Pluto in the outer reaches of the solar system, in an area called the Kuiper Belt. Pluto was discovered by Clyde W. Tombaugh in 1930, based on predictions by astronomer Percival Lowell in 1905. Pluto is 1,460 miles (2,350 kilometers) in diameter.

Pluto is so far from Earth and is so small that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. (NASA)

Astronomers with the SETI Institute, a research organization that searches for life in the universe beyond Earth, discovered the new moon and gave it the temporary name P5. The astronomers are using the HST to study Pluto in preparation for a fly-by of the dwarf planet by the New Horizons space probe in July 2015. New Horizons was launched in 2006 to explore Pluto, its largest moon Charon, and other objects in the Kuiper Belt.

According to the researchers, P5 is a tiny, irregularly shaped body that measures from 6 to 15 miles (10 to 25 kilometers) across. The astronomers were unable to get a closer measurement because Pluto and its moons are about 2.8 billion miles (4.7 billion kilometers) away from Earth. Pluto’s moon Charon, which is about 750 miles (1,207 kilometers) in diameter, was discovered in 1978. Hydra and Nix, each up to 100 miles (160 kilometers) in diameter, were found in 2005. Pluto’s fourth moon, P4, discovered in 2011, is about 8 to 21 miles (13 to 34 kilometers) in diameter.

The scientists were a bit disconcerted by the discovery of an unknown moon as New Horizons is nearing Pluto. “All of this stuff poses a navigation hazard for New Horizons,” according to Ray Villard, news director of the Space Telescope Science Institute. However, tiny P5 has also produced a sense of wonder at the complexity of ex-planet Pluto’s system of satellites.

Additional World Book articles:

• Space exploration
• Exploring the Suburban Solar System (a special report)
• Astronomy 1930 (Back in Time article)
• Astronomy 1978 (Back in Time article)
• Astronomy 2006 (Back in Time article)
• Space exploration 2006 (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