Pluto is a difficult planet
to find, but with a good-sized backyard telescope, it is possible. The challenge
highlights the difficulty professional astronomers faced many decades ago when
trying to find the elusive world in the first place.
This year marks the 75th
anniversary of the discovery of Pluto. Clyde Tombaugh
discovered the ninth planet in the solar system on the afternoon of Feb. 18,
1930 while he meticulously examined a pair of photographic plates he had exposed
in January at Lowell Observatory. The official announcement was made on March
13.
Tombaugh exposed the photographs
on the nights of Jan. 23 and 29 using the Observatory's 13-inch Abbott Lawrence
Lowell Telescope. Then, as part of the carefully planned and executed planet
search, Tombaugh "blinked" the two exposures using a machine called
a comparator, looking for motion of objects captured on film.
"One need only visit Lowell Observatory and view copies of the discovery
images through the same eyepiece used by Clyde Tombaugh to appreciate what a
remarkable discovery this was," notes Lowell's Director, Bob Millis. "The
images are extremely faint and testify to the skill, concentration, and dedication
that Clyde Tombaugh brought to his work."
This most renegade of all
the planets has been an odd, mysterious world ever since. It will remain so
for at least another decade, until the yet to be launched New Horizons Pluto-Kuiper
Belt Mission arrives at Pluto, the last planet in our solar system to be visited
by a spacecraft.
New Horizons is scheduled
to launch in January 2006, and is due to swing past Jupiter for a gravity boost
and scientific studies in February 2007, before finally reaching Pluto and its
moon, Charon, in July 2015.
There is little comfort
in knowing, however, that close-up images of Pluto might already have been in
hand, had one of NASA's original "Grand Tours" of the outer planets
had been carried out back in the 1970s. Nevertheless, astronomers have been
inexorably closing in on the secrets of what to most of us is still little more
than a star like speck in the sky - piecing together hints gathered from the
other remote worlds by the Voyager probes.
Pluto is being observed
by growing numbers of nonprofessional astronomers as well.
From the backyard
Up until about 25 or 30
years ago, any amateur who had the equipment to spot this speck against the
starry background enjoyed a sort of special status among their peers. But today,
thanks to the "Dobsonian revolution" - those large aperture telescopes
that first appeared on the market back in the early 1980s - Pluto is being glimpsed
by increasing numbers of people.
We are actually quite fortunate
to be living around the time when Pluto is near the perihelion point in its
orbit, or the point closest to the Sun. For Pluto, this occurs once every 2½
centuries. It reached this point back in September 1989, but the planet's current
brightness of magnitude 14.0 is still significantly above average, and it's
destined to rise slightly to 13.8 when it arrives at opposition to the Sun on
June 14. On this astronomers' scale, higher numbers represent dimmer objects,
with magnitude 6.5 being about the dimmest visible to the naked eye under perfect
conditions.
This is very nearly as bright
as Pluto ever gets. Those who have access to at least an eight-inch or (preferably)
larger telescope with high magnification and are blessed with dark, transparent
skies and steady seeing have a chance of catching a glimpse of it.
Currently, Pluto is located
within the constellation of Serpens Cauda. The map near the top-right of this
page serves as a guide to those who choose to join the hunt.
Odd orbit
What really makes Pluto
most renegade is its eccentric orbit.
From 1979 to 1999, this
path brought Pluto even closer to the Sun than Neptune. Pluto's orbit is also
tilted more than 17º to the plane of Earth's orbit. The two-dimensional orbit
diagrams usually published in most astronomy books give a false impression that
Pluto intersects Neptune's orbit. But Pluto is presently well "above"
Neptune's orbit, and the paths of these two planets don't really intersect at
all.
What we usually see are
curtate orbits - true orbits that are projected onto the plane
of the solar system. The orbital inclinations of the other eight planets are
small enough to show little distortion between the actual orbits and the curate
versions. Not so, however, with Pluto.
To get a really good three-dimensional
conception of Pluto's orbit, as well as of the other planets and comets and
asteroids, I highly recommend obtaining a sophisticated computer program like
Starry Night Pro [a product of
SPACE.com's parent company, Imaginova] that will not only shows such information
graphically, but allows you to view motion and changing perspective.
The old days
Lowell Observatory's search
for a ninth planet was begun by founder Percival Lowell in 1905. While Dr. Lowell
did not live to see the discovery of Pluto, the Observatory made the official
announcement of the discovery on Percival Lowell's birth date, March 13, 1930.
The privilege of naming the newly discovered object presumably belonged to Lowell
Observatory (located in Flagstaff, Arizona).
Clyde Tombaugh, the person
who first spied Pluto on photographic plates urged the director of the observatory,
V.M. Slipher to suggest a name before someone else did. Soon suggestions indeed
poured in from all quarters: Cronus, Odin, Persephone, Erebos, Atlas, Prometheus
. . . the list seemed endless. One young couple even wrote to Tombaugh asking
that the planet be named after their newborn child!
But it was an 11-year old
English girl by the name of Venetia Burney who first suggested Pluto to her
grandfather, who was a professor of astronomy at Oxford. The name of Pluto was
highly favored by Slipher who made the official announcement on May 1, 1930
giving full credit to little Venetia. In addition, Slipher suggested interlocking
the letters P and L as the official symbol for Pluto. Not only do they stand
for the first two letters of the planet, but they're also the initials of Percival
Lowell.
Several Lowell Observatory
astronomers continue to study Pluto, particularly Marc Buie and Will Grundy
who have studied the cold, dark outer regions of the solar system - with a
special interest in Pluto - since the early 1980s. Their observations include
a long-term project to monitor Pluto's brightness changes on a decade, or longer,
time scales. Another project is aimed at producing a new generation of Pluto
maps based on images taken with the Hubble Space Telescope.
Way out there
Pluto is currently located
at a distance of roughly 2.87 billion miles from both the Earth and the Sun.
It appears to be rotating on its axis once every 6.39 days and has a mass estimated
to be just 0.0022 that of the Earth. In 1988, an extremely thin atmosphere was
detected around Pluto. We know that its surface pressure is about 100,000 times
smaller than that on Earth but still large enough for us to expect weather,
winds, haze, chemistry, and an ionosphere.
At the same time, Pluto's
weak gravity does not hold the atmosphere very tightly, and its upper portions
may resemble the atmosphere of a comet
Finally, a rather interesting
astronomical issue has surfaced in recent years as to whether Pluto should legitimately
be included in the roster of nine major planets. It's decidedly small size (1,413
miles, or 2,274 kilometers across) makes it considerably smaller than any other
planet, even smaller than several planetary satellites (including our own Moon).
In addition, it has a moon (Charon) that is more than half its size (728 miles,
or 1,172 kilometers), as well as a most unplanetary, asteroid like orbit. For
some time people have suggested that Pluto is really an escaped satellite of
Neptune, one that closely resembles its largest satellite, Triton.
Other's have proposed that
Pluto should not be considered a planet, but rather as the "King of the
Kuiper Belt Objects."
The Kuiper Belt is a ring
of icy objects strung like a dirty diamond necklace in the vastness of space
beyond Neptune. Kuiper Belt Objects are very faint, and extremely hard to study
from the Earth. Most of the detected "KBOs" are small, with diameters
typically around 62 miles (100 kilometers). The first KBOs were discovered only
in 1992, but already the Kuiper Belt is changing the way we think about the
outer solar system and the formation of the outer planets and comets. Pluto
might be the largest of these strange bodies: some sort of condensed clumping
of icy cometary nuclei.
In fact, after visiting
this "neither fish nor fowl" planet, the New Horizons Pluto-Kuiper
Belt spacecraft is scheduled to head deep into the Kuiper Belt to spend several
years studying one or more of the icy mini-worlds in that vast region. All this
begins happening a decade from now. Until then, Pluto remains an enigma.
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Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium.
He writes about astronomy for The New York Times and other publications, and
he is also an on-camera meteorologist for News
12 Westchester, New York.