Uncovering the Secrets of PlutoSpecial Feature

By NASA’s Amazing Space reporters June 2015

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IMAGE: NASA, ESA, and M. Showalter (SETI Institute)

A P U B L I C A T I O N O F N A S A ’ S “A M A Z I N G S P A C E ” E D U C A T I O N P R O G R A M

WITNESSSTARTHE

National Aeronautics and Space Administration

L ONG BEFORE THE ICY dwarf planet Pluto was discovered on the outskirts

of our solar system, amateur astronomer Percival Lowell began his search for an object he called “Planet X.” It was 1905, and, at the time, our solar system had eight known planets.

Astronomers had found Neptune, the

eighth planet, in 1846 after noticing

that Uranus, the planet’s next-door

neighbor, had a slightly irregular

motion around its orbit. Researchers

calculated that the gravity of Neptune

was tugging on Uranus and creating

its inconsistent motion.

Lowell, along with some astronomers,

extended this idea, and predicted

there was at least one other planet

orbiting far away in our solar system.

The hunt is on

Objects far away in our solar system

are small pinpoints of light and are

This Hubble Space Telescope image shows five moons orbiting the distant, icy dwarf planet Pluto. Pluto and its bright inner moon, Charon, were observed with a short exposure, while the fainter, outer moons required a longer exposure time.

Pluto

Hydra

Nix

StyxKerberos

Charon

hard to see with the best telescopes.

Lowell, however, was ready to give

the search a try. He conducted a

photographic search for Planet X

using a telescope at his observatory

in Arizona. Lowell hunted for the

planet for 11 years before giving up

in 1916. It would later be revealed

that Pluto showed up in two of his

images. The object was overlooked

because it was much dimmer than

anyone expected.

More than 10 years later, a young

amateur astronomer named Clyde

Tombaugh took up the search

at Lowell’s Arizona observatory.

Beginning in 1929, he spent 10

months photographing the sky,

looking for any slight movement

of objects over that time. While

comparing two images taken in

January 1930, Tombaugh found an

object that changed position in the

expected manner. The discovery

of a new planet was officially

announced on March 13, 1930.

Interestingly, Pluto’s size and orbit

did not match the expectations for

Planet X. In fact, Lowell’s predictions

had been incorrect, and it is

Tombaugh’s careful observing that

really deserves credit for

the discovery.

Pluto has a family

Astronomers would have to wait

another 48 years before finding the

first of Pluto’s five known moons.

Charon, the largest and innermost

moon, was discovered in 1978

with ground-based telescopes. The

sharp vision of the Hubble Space

Telescope spotted the other four,

smaller, outer moons. Nix and Hydra

were found in 2005; Kerberos in

2011; and Styx in 2012.

Charon, Pluto’s largest moon, is

746 miles (1,200 kilometers) across.

Nix and Hydra range from 20 to

70 miles (32 to 113 kilometers) in

diameter. Kerberos has an estimated

diameter of 8 to 21 miles (13 to 34

kilometers), and Styx is estimated to be 6 to 15 miles (10 to 24

kilometers) across.

Borrowing names from Greek mythology

The names for Pluto and its

family of moons come from Greek

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mythology. Discovered in 1930,

Pluto was named for the Greek

god of the underworld, who was

perhaps better known as Hades. The

moon, Charon, discovered in 1978,

was named for the ferryman who

carried souls to the underworld.

Nix, one of two moons discovered

in 2006 with Hubble, is named for

Nyx, the goddess of the night and

Charon’s mother. Hydra, the other

moon spied in 2006, is named for

the many-headed, serpent-like

beast of ancient times. Kerberos,

discovered in 2011, was named for

the many-headed dog that guarded

the entrance to the underworld.

Styx, spotted in 2012, is the name

of the goddess who ruled over the

underworld river.

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Pluto is demoted

During the flurry of Hubble

observations of Pluto, the faraway

object lost its planetary status. The

International Astronomical Union

(IAU) voted in 2006 to demote Pluto

from planet to dwarf planet. The

reason for the change is because

Pluto is now known to be part of

a vast collection of objects called

the Kuiper Belt. The Kuiper Belt

is a region beyond Neptune that

contains small, icy objects much

like the asteroid belt contains small,

rocky objects. Pluto is the second-

largest of five currently recognized

dwarf planets. Eris, the largest dwarf

planet, is only slightly bigger

than Pluto.

The discovery of Charon

IMAGE: USNO

In 1978, the U.S. Naval Observatory was making measurements to refine Pluto’s orbit. Astronomer James Christy noticed that a number of the Pluto images appeared elongated. These Pluto images show a normal, roughly circular image on the right, and an elongated one on the left. Upon further study, Christy concluded that the only explanation was a previously unknown moon, now called Charon, with an orbital period of just over six days.

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Pluto’s dismissal from the planetary

ranks is not unusual. During the

1820s, astronomers counted 11

planets. Years later, several of them

were dropped from the planetary

list, including Ceres, now considered

a dwarf planet. Ceres is the largest

object in the asteroid belt.

The dwarf planet is full of surprises

Despite Pluto’s demotion,

astronomers are still interested in

the dwarf planet and its moons.

In fact, recent Hubble observations

of the frigid system have revealed

another surprising discovery. Two

of the moons, Nix and Hydra, are

rotating chaotically as they orbit

Pluto. If you lived on Nix or Hydra,

you would have a hard time setting

your alarm clock. You would not

know when the Sun would rise or

even in which direction it would

rise. You might even have a hard

time figuring out when to celebrate

your birthday.

In preparation for the New Horizons

flyby, Hubble has been making

detailed observations of Pluto and

its neighborhood. Launched in 2006,

the New Horizons spacecraft has

been traveling for nine years to the

far-flung dwarf planet. The satellite

will arrive at the Pluto system on

July 14, 2015, and will take the first

detailed close-up views of the dwarf

planet and its moons. Following that

encounter, the spacecraft may study

other objects in the Kuiper Belt.

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The dwarf planet, Ceres

IMAGE: NASA, ESA, and J. Parker (Southwest Research Inst.)

The dwarf planet Ceres is the largest object in the asteroid belt, and has a diameter about 40 percent that of Pluto. Ceres has a rocky core with an icy outer layer and a dusty surface. This Hubble image is low resolution compared to the high-resolution ones coming from the Dawn mission, which began to orbit Ceres in early 2015. Dawn will map its features and measure its characteristics for about a year.

The New Horizons mission encounters Pluto in July 2015

IMAGE: JHUAPL/SwRI

As depicted in this artist’s drawing, New Horizons will be the first spacecraft to visit the Pluto system. It will be able to study the dwarf planet and its moons in greater detail than ever before. Following that encounter, the spacecraft may be re-directed in order to study another object in the Kuiper Belt.

SEE MORE Hubble images and read more

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