Download Pluto, the dwarf planet

Pluto, the dwarf planet
By NASA.gov, adapted by Newsela staff on 11.10.16
Word Count 1,012
TOP: Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New
Horizons spacecraft, taken on July 13, 2015. MIDDLE: Pluto and Charon are shown in enhanced color in this image.
BOTTOM: In this image from NASA’s New Horizons spacecraft, great blocks of Pluto’s water-ice crust appear jammed
together in the informally named al-Idrisi mountains. Photo: Photo: NASA.
Pluto was discovered in 1930 by the astronomer Clyde Tombaugh. At the time,
astronomers believed that Pluto was the ninth planet in our solar system. However, it was
much smaller than the other planets, and scientists began to wonder if it should really be
considered a planet.
Pluto Was Given The Cold Shoulder
In 2006, astronomers got together at a meeting of the International Astronomical Union.
They put Pluto into a new category of space objects: dwarf planets. These small, spherical
bodies orbit the sun just like planets but are much smaller.
Pluto is also a member of a group of small, rocky objects that orbit the sun in a disc-like
region called the Kuiper Belt, which is located beyond the orbit of Neptune. Like the other
icy bodies in the Kuiper Belt, Pluto is an extremely cold world. Its average surface
temperature is 380 degrees below zero.
This article is available at 5 reading levels at https://newsela.com.
1
The diameter of Pluto is 1,475 miles, which is two-thirds the diameter of Earth's moon. It
most likely has a rocky core surrounded by a mantle of water ice. More unusual ices like
methane and nitrogen frost coat its surface.
Pluto is small and not very dense. The mass of Earth's moon is six times greater than
Pluto's, while Earth's mass is more than 454 times greater.
On An Elliptical
Pluto orbits the sun along an elliptical path, which is shaped like an oval. It takes 248 years
to complete one full orbit. Much of the time Pluto is located a great distance from the sun.
At the farthest point of its orbit, the dwarf planet is about 49 astronomical units (AU) from
the sun. One AU is the average distance between Earth and the sun, which is about 93
million miles. In 1989, Pluto came within 29.8 AU of the sun, close enough to the sun for
scientists to study it.
It's A Gas!
When Pluto is close to the sun, its surface ices change from a solid to a gas. As these
gases rise, they temporarily form a thin atmosphere. Pluto's low gravity — about six
percent of Earth's — allows the atmosphere to extend much higher than our planet's. Pluto
becomes much colder during the part of each orbit when it is traveling far away from the
sun. During this time, the bulk of the planet's atmosphere is thought to freeze and fall as
snow to the surface.
Seeing Double
Pluto has a very large moon that is almost half its size. Named Charon, this moon was
discovered in 1978. It is so big that Pluto and Charon are sometimes referred to as a
double dwarf planet system. The distance between them is 12,200 miles. Charon is grayer
than the reddish Pluto, indicating that they have different surface compositions and
structure.
Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day —
takes 6.4 Earth days. Charon neither rises nor sets. Instead, it hovers over the same spot
on Pluto's surface as the planet rotates, and the same side of Charon always faces Pluto.
In 2005, scientists using the Hubble Space Telescope discovered two tiny, previously
unknown, moons orbiting Pluto. Located two to three times farther away from Pluto than
Charon is, these were named Nix and Hydra. In 2011 and 2012, scientists spotted two
more moons, which were named Kerberos and Styx.
NASA's New Horizons space probe was launched in 2006 to conduct the first detailed
studies of Pluto. By the spring of 2015, it had approached close enough to begin recording
and sending back images of the planet's surface.
This article is available at 5 reading levels at https://newsela.com.
2
Results from New Horizons have left planetary scientists greatly puzzled about Pluto and
its moons. For example, pictures of the planet's surface reveal large mountains rising from
6,500 to 9,800 feet high. Scientists had not believed Pluto contained materials that could
hold up such mountains and maintain their shapes over millions of years. The New
Horizons science team has concluded the mountains are probably made of ice.
A Gap Here
Scientists were also puzzled to find fewer craters on Pluto than expected, though some
extremely large ones were spotted. Adding to the mystery, these large craters appear to
be partially filled in. Scientists had not thought Pluto had the kind of material that would
gradually fill in a crater, the way loose rock, dirt and vegetation do here on Earth.
Charon was also studied in detail. Its diameter came out slightly larger than expected, at
753 miles. Mountains seen on this moon suggest that it, like Pluto, has water ice that runs
deep in its structure, though there is no evidence that Charon has an atmosphere. Also
similar to Pluto, Charon exhibits both cratered and smooth plains.
The moons Nix are Hydra are not spherical, which separate them from most moons in the
solar system. There are variations in composition over the surface of Nix, but overall it
reflects more light than Charon, which suggests that its ice is cleaner than Charon's. Hydra
is also more reflective than Charon, leaving scientists to puzzle over how the ice on these
small moons could stay so bright over billions of years, given the processes known to
darken material over time.
How Pluto Got Its Name
Pluto is the only world named by an 11-year-old girl. In 1930, Venetia Burney of Oxford,
England, suggested to her grandfather that the new discovery be named for the Roman
god of the underworld. He forwarded the name to the Lowell Observatory, and it was
selected.
Pluto's moons are named for other mythological figures associated with the underworld.
Charon is named for the river Styx boatman who ferries souls in the underworld — as well
as honoring Sharon, the wife of discoverer James Christy. Nix is named for the mother of
Charon, who is also the goddess of darkness and night. Hydra is named for the nineheaded serpent that guards the underworld. Kerberos is named after the three-headed
dog of Greek mythology, and Styx is named for the mythological river that separates the
world of the living from the realm of the dead.
This article is available at 5 reading levels at https://newsela.com.
3
Quiz
1
2
Which statement BEST explains why the photo at the top of the article is included?
(A)
It is intended to create a sense of excitement about the topic of the article.
(B)
It provides a depiction of the unknown elements of scientific research.
(C)
It shows the surface of the planet, which is the central idea of the article.
(D)
It provides an overview image of the main topic of the article.
What is the MOST likely reason for including information about the re-categorization of Pluto?
(A)
to demonstrate the insignificance of Pluto in comparison to other planets
(B)
to provide information on how scientific understanding of the topic has
changed over time
(C)
to generate curiosity about how planets are categorized
(D)
to explain why scientists are interested in the surface of Pluto
3
Which paragraph from the section "Seeing Double" BEST explains the images in the section?
4
Read the paragraph from the article.
Pluto's moons are named for other mythological figures associated
with the underworld. Charon is named for the river Styx boatman who
ferries souls in the underworld — as well as honoring Sharon, the wife
of discoverer James Christy. Nix is named for the mother of Charon,
who is also the goddess of darkness and night. Hydra is named for the
nine-headed serpent that guards the underworld. Kerberos is named
after the three-headed dog of Greek mythology, and Styx is named for
the mythological river that separates the world of the living from the
realm of the dead.
What does this paragraph accomplish?
(A)
It explains the relationships between the mythological figures used to name
the moons of Pluto.
(B)
It shows that the moons of Pluto were also studied by the ancient Greeks.
(C)
It provides information on where the names of Pluto's moons come from.
(D)
It proves that scientists enjoy learning about other topics like literature and
history.
This article is available at 5 reading levels at https://newsela.com.
4
Answer Key
1
2
3
Which statement BEST explains why the photo at the top of the article is included?
(A)
It is intended to create a sense of excitement about the topic of the article.
(B)
It provides a depiction of the unknown elements of scientific research.
(C)
It shows the surface of the planet, which is the central idea of the article.
(D)
It provides an overview image of the main topic of the article.
What is the MOST likely reason for including information about the re-categorization of Pluto?
(A)
to demonstrate the insignificance of Pluto in comparison to other planets
(B)
to provide information on how scientific understanding of the topic has
changed over time
(C)
to generate curiosity about how planets are categorized
(D)
to explain why scientists are interested in the surface of Pluto
Which paragraph from the section "Seeing Double" BEST explains the images in the section?
Paragraph 8:
Pluto has a very large moon that is almost half its size. Named Charon, this
moon was discovered in 1978. It is so big that Pluto and Charon are sometimes
referred to as a double dwarf planet system. The distance between them is
12,200 miles. Charon is grayer than the reddish Pluto, indicating that they have
different surface compositions and structure.
This article is available at 5 reading levels at https://newsela.com.
5
4
Read the paragraph from the article.
Pluto's moons are named for other mythological figures associated
with the underworld. Charon is named for the river Styx boatman who
ferries souls in the underworld — as well as honoring Sharon, the wife
of discoverer James Christy. Nix is named for the mother of Charon,
who is also the goddess of darkness and night. Hydra is named for the
nine-headed serpent that guards the underworld. Kerberos is named
after the three-headed dog of Greek mythology, and Styx is named for
the mythological river that separates the world of the living from the
realm of the dead.
What does this paragraph accomplish?
(A)
It explains the relationships between the mythological figures used to name
the moons of Pluto.
(B)
It shows that the moons of Pluto were also studied by the ancient Greeks.
(C)
It provides information on where the names of Pluto's moons come
from.
(D)
It proves that scientists enjoy learning about other topics like literature and
history.
This article is available at 5 reading levels at https://newsela.com.
6