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Beyond Pluto
We are only beginning to discover how vast and strange our solar system truly is
By Kathy A. Svitil
Illustrations by Don Foley
DISCOVER Vol. 25 No. 11 | November 2004 | Astronomy & Physics
As a child, Mike Brown had all the trappings of an astronomer-in-the-making, with space books, rocket drawings, and a poster of the
planets on his bedroom wall. On it, Pluto was depicted as ―this crazy and very eccentric planet,‖ he says. ―It was everyone‘s favorite
crazy planet.‖ Brown still recalls the mnemonic he learned for the names of the planets: Martha visits every Monday and—a for
asteroids—just stays until noon, period. ―The ‗period,‘ for Pluto, was always suspicious,‖ Brown says with a laugh. ―It didn‘t seem to
fit. So maybe that was when I first got the idea that Pluto didn‘t belong.‖
Nowadays Brown, a planetary astronomer at Caltech, has no doubt about Pluto‘s place in the solar system: ―Pluto is not a planet.
There is no logical reason to call Pluto a planet.‖ Like a growing number of his colleagues, Brown believes Pluto is best understood as
the largest known member of the Kuiper belt, a band of rocky, icy miniplanets that orbit the sun in a swath stretching from beyond
Neptune to a distance of nearly 5 billion miles. ―I don‘t think it denigrates Pluto at all to say that it is not a planet. I think Pluto is a
fascinating and interesting world, and being the largest Kuiper belt object is an honorable thing to be.‖
No longer is Pluto a lonely outpost in an otherwise empty frontier. A string of discoveries has revealed that it is merely the entry point
to a vast and still mysterious wilderness that teems with uncountable numbers of unusual objects. They come in a variety of shapes,
colors, and sizes, many with their own moons, some in peculiar orbits that have been pushed by Neptune or pulled by passing stars.
Stranger objects are likely to be found. Astronomers are only on the edge of discovering this vast new world.
In the 1940s and 1950s, astronomers Kenneth Edgeworth and Gerard Kuiper independently predicted that a reservoir of icy rocks lay
beyond the orbit of Neptune. Many became short-period comets, with orbits of 200 years or less, that blasted in toward the sun,
crossing the orbits of most planets. Excluding Pluto (discovered in 1930), the first official Kuiper belt object was not found until 1992,
by astronomers Jane Luu and David Jewitt. Since then, in excess of 800 have been detected in the 2-billion-mile-wide EdgeworthKuiper belt (commonly truncated to Kuiper belt), including a few huge objects that are as much as three-quarters the size of Pluto
(which is 1,430 miles wide). The two biggest have been found by Brown and his colleagues. At least 100,000 objects 30 miles wide
may occupy the belt. Some of them, say researchers, are almost certainly the size of Pluto—if not larger.
But our solar system doesn‘t end there. Far beyond the Kuiper belt lies the mysterious Oort cloud, a spherical shell that stretches to the
boundaries of interstellar space and blasts its own dark ice balls toward the sun. Trillions more bodies may lurk there. A few may be as
big as Mercury or Mars. Imprinted in those far-flung worlds, scientists say, is the history of the solar system before planets came to be.
Every Kuiper belt object and Oort cloud entity is a geologic fossil, preserved at low temperatures, largely unaltered by time, and made
up of the material from which the solar system formed 4.5 billion years ago. Understanding their compositions—and why they are
where they are today—will help scientists reconstruct the nascent moments of our planetary neighborhood and our sun‘s younger
days, when it was just one of a cluster of stars.
Each of those objects has a tale to tell that is as lively as those that the nine—call it eight—planets have told so well. Let their stories
begin.
NAME THAT CELESTIAL OBJECT
Scientific nomenclature can barely keep up with the range of odd objects that astronomers are discovering in the solar system these days.
Future discoveries will no doubt suggest new categories and further blur the boundaries of old ones. In the meantime, here’s a field guide to
the known residents.
PLANETS lack a standard definition. If a body orbits a sun and was made spherical by its own gravity, astronomers tend to call it a planet. Yet
that definition would include at least four asteroids and dozens of Kuiper belt objects.
ASTEROIDS (a.k.a. minor planets) are rocky, metallic, or carbonaceous bodies whose solar orbit takes them beyond Mars, into the so-called
asteroid belt. They typically lack sufficient gravity to retain an atmosphere. The latest count of asteroids in the inner solar system, inside
Jupiter, is about 700,000.
COMETS are icy bodies that follow elliptical orbits. Those that have orbital periods shorter than 200 years originate in the Kuiper belt. Longerperiod comets originate much farther out in the Oort cloud. The known comets number in the thousands.
CLASSICAL KUIPER BELT OBJECTS orbit the sun at 3.9 billion to 4.5 billion miles out. They are sometimes called cubewanos, after QB1, the
first Kuiper belt object discovered (1992).
RESONANT KUIPER BELT OBJECTS orbit in synchrony with Neptune. Pluto is the prototype: It orbits twice around the sun for every three solar
orbits made by Neptune. About 20 percent of known RKBOs are thought to orbit with a similar 2:3 resonance and thus are called plutinos. Six
other resonant orbits have been detected.
SCATTERED KUIPER BELT OBJECTS have very eccentric and tilted orbits that carry them from around 3.3 billion miles from the sun to almost
100 billion miles out. They may have been kicked into their far-flung orbits by Neptune’s gravity.
A New Miniplanet Rises
Detected last November by Brown and his colleagues, using the 48-inch Samuel Oschin Telescope, Sedna is the largest, farthest,
coldest, and arguably weirdest world yet discovered in the distant solar system. It may be as large as 1,100 miles in diameter, and it
has a mysterious deep red surface—neither particularly dark, like a typical rocky object, nor bright and icy like Pluto and other Kuiper
belt objects. Its orbit is even more peculiar. It passes no closer to the sun than 7 billion miles, nearly twice as far away as Pluto ever
gets, and it sails beyond the outer edge of the Kuiper belt, possibly into the Oort cloud. Is it a stray Kuiper belt object, a comet from
the inner Oort cloud, or something different?
Sedna‘s eccentric orbit indicates that it must have formed at the edge of the same disk of gas and dust from which Earth and the other
planets coalesced 4.5 billion years ago. Then it was somehow bumped far out into space and into an offbeat orbit. Although the details
are debated, the most likely explanation involves both Neptune and a nearby star.
Sedna may have begun its days in the Kuiper belt. A chance close encounter with Neptune could have knocked it off track, pushing
the most distant point of its orbit farther and farther away from the sun. Then a second event—the gravitational tug of a passing
star?—could have yanked Sedna away from Neptune‘s influence and even farther out. Mathematically, Brown says, the ideal
arrangement would have involved a star passing sufficiently close to ours that it would have shone ―brighter than the full moon for
something like 20,000 years.‖
Most stars are born in the company of others, as in the stellar nurseries of the Orion nebula. But until Sedna there had been no direct
evidence that our sun had a social childhood; the nearest stars today are light-years away. ―Sedna‘s orbit seems to provide the first
evidence that our sun formed in a dense cluster,‖ says Oort cloud expert Luke Dones of the Southwest Research Institute in Boulder,
Colorado. If other objects with similar orbits are found, they might enable scientists to retrospectively piece together a group portrait
of the sun‘s nursery.
―We basically have a fossil record of this early cluster of stars,‖ Brown says. ―A year ago I would have never guessed that the record
even existed. Now I‘m convinced that it can tell us exactly how many stars there were, where they came from, and where they have all
gone.‖
An Old Planet Sets
Neither the Kuiper belt nor the Oort cloud had been predicted in February 1930, when astronomer Clyde W. Tombaugh spotted the
faint movement of Pluto during a painstaking search of thousands of photographic plates he had snapped of the night sky. Tombaugh
had been commissioned by Percival Lowell, founder of the Lowell Observatory in Flagstaff, Arizona, to find Planet X, an object
thought to be of prodigious size. Little Pluto was not what anyone expected, but it nevertheless gained worldwide fame as the ninth
planet.
―If we
found
Pluto
Discovered: February 18, 1930
Distance from the sun: 3.7 billion miles; ranges from 2.8 billion
today, we
to 4.6 billion miles
probably
Orbital period: 249 years
wouldn‘t
Diameter: 1,430 miles
Features: Pluto has a moon,
call it a
Charon, only slightly smaller
planet,‖
than itself. Both objects rotate
says
at precisely the same rate
astronome
(6.387 Earth days)
r Chad
Trujillo of the Gemini North Observatory on Mauna Kea in Hawaii, ―but there is a big historical component to astronomy. Maybe 70
years from now, when everyone who knew Pluto as a planet is gone, they‘ll change its status, but I don‘t see a big push to do it now.‖
The International Astronomical Union, the organization responsible for naming planets, moons, asteroids, and other cosmic
discoveries, has no intention of demoting Pluto.
PLUTO
Planetary scientist Alan Stern of the Southwest Research Institute says there are reasons for preserving the status quo. Stern, the
principal scientist for NASA‘s New Horizons mission to Pluto and the Kuiper belt, scheduled to launch in January 2006, argues that
any object tugged into a round shape by the force of its own gravity is a planet. ―We can argue about adjectives,‖ he says. ―I don‘t
mind if you want to call Pluto a dwarf planet, which it is, but it is a planet‖—as are, by his definition, thousands of other objects in the
solar system.
―His definition makes me mad,‖ says Caltech‘s Mike Brown, who discovered the two largest Kuiper belt objects aside from Pluto. ―It
is a definition of convenience. Just by coincidence, the size at which things become round just happens to be a little bit smaller than
Pluto. If Alan Stern is right, then I‘ve discovered more planets than anyone in human history.‖
The problem is precisely one of history, Brown says. Originally, any large nearby object orbiting the sun was deemed a planet. ―When
the very first asteroid, Ceres, was discovered in 1801, it was called a planet. So was the next asteroid, and the next.‖ Eventually
astronomers concluded it was unwise to call them all planets, in part because they comprise a whole population of objects that orbit
beyond Mars; they were called minor planets, then asteroids, and their neighborhood is known as the asteroid belt. Similar
classification should also apply to the objects in the Kuiper belt, says Brown. Anything that‘s part of a group of similar things and not
a distinct and unique body just isn‘t a planet.
―If you don‘t understand that, you get a misguided impression of the architecture of the solar system and how things formed and
where things are,‖ he says. ―Pluto will not be the largest object in the outer solar system for long, and then we will really have to deal
with this issue.‖
Understanding Your Solar System 2.0
It‘s not about planets anymore. In the past decade, astronomers have spotted hundreds of objects in the Kuiper belt, some nearly the
size of Pluto; countless more await discovery. Even the distant and far-reaching Oort cloud is coming into scientific focus. Preserved
in these nether realms are trillions of rocky, icebound insights into the early days of the sun and the formation of the solar system. In
spatial scale and information potential, they are the solar system—past, present, and future.
Inner Solar System
0–5.2 astronomical units, or AU
One astronomical unit (AU) equals the distance from
the sun to the Earth, or about 93 million miles.
The inner solar system spans everything from the sun out to (but not including) Jupiter, 460 million miles away. The solar system as a
whole formed roughly 4.5 billion years ago from a collapsing disk of dust and gas. Swarms of boulder-size objects called
planetesimals slowly accreted to form the rocky planets—Mercury, Venus, Earth, and Mars—and their various and sundry satellites.
The leftovers linger today in the asteroid belt. Farther out, well away from the sun‘s warmth, gases like hydrogen and helium failed to
vaporize and instead accumulated to form the gaseous outer planets: Jupiter, Saturn, Neptune, and Uranus.
Asteroid Belt The majority of asteroids occupy an orbit about 210 million miles from the sun, between the orbits of Mars and
Jupiter. A smaller group orbits at about 370 million miles; a third group occupies the same orbit (though not the same spot) as Jupiter.
At least a hundred asteroids are a mile or more in diameter, and a dozen or so are 200 miles wide. The largest, Ceres, is 600 miles
wide.
Kuiper Belt
30–50 AU
The Kuiper belt is a vast reservoir of icy rocks that extends from about 3 billion miles from the sun, beyond Neptune‘s orbit, to about
5 billion miles out. Beyond that, the number of detected objects drops precipitously until one reaches the even more distant Oort
cloud. There may be billions of Kuiper belt objects in all, including at least 35,000 that are more than 60 miles wide. Pluto, at 1,430
miles wide, is the largest known member and the only one known to have an atmosphere. Its possession of a moon, Charon, was long
thought to set Pluto apart from other denizens of the frozen region, but a dozen Kuiper belt objects are now known to have partners.
Bodies in the Kuiper belt are believed to have formed roughly where they now reside, and many have stayed largely undisturbed in
temperatures averaging –375°F. They come in a variety of colors, from gray to dark red, indicating their compositions, but all are
thought to be half ice and half rock. On occasion, a gravitational bump from Neptune tosses a Kuiper belt object toward the sun. Then
the heated ices sublime, producing a bright halo of glowing gas that trails behind the rock: the coma and the tail of a comet, which
fade again as the object retreats past Neptune.
Oort Cloud
10,000–100,000 AU
The entire solar system is enveloped by the Oort cloud, an enormous sphere of nascent comets extending more than 9 trillion miles
into space. The cloud is thought to contain trillions of loose lumps of dust and volatile gases, near-perfect samples of the stuff from
which the planets were made.
Dutch astronomer Jan Hendrik Oort first proposed the existence of the cloud in 1950 to explain the funny paths of comets that take
more than 200 years to orbit the sun. This model suggests that the cloud‘s contents started out closer by, near Jupiter and the other gas
giants, until gravitational encounters with those planets knocked the primordial bits into the hinterlands. Barely 5 percent of that
original material remains in the Oort cloud; the rest was ejected entirely from the solar system.
Over time, the gravitational yanks of passing stars jostled the orbits of the objects, leaving no hint of their formerly neat, flat paths
around the sun. (However, the orbits of objects in the loosely defined inner Oort cloud, located a few thousand astronomical units
from the sun, do retain some memory of their origins, so this part of the cloud is thought to be flatter.) The outer, round cloud would
have taken about a billion years to form, making it the youngest structure in the solar system.
New Horizons
NASA is gearing up for an historic first visit to the frigid world of Pluto and its moon, Charon. The designated explorer, a 1,000pound, $550-million spacecraft called New
Horizons, is scheduled to launch in about 14 months, swing around Jupiter in February 2007 for a gravity boost, then arrive in July
2015. It will snap the first close- up images of Pluto and Charon, map their surface features with visible-wavelength cameras, study
their compositions in the near-infrared spectra, and monitor Pluto‘s thin atmosphere with ultraviolet spectrometers and radio waves.
The closest approach will bring the probe within 6,000 miles of Pluto‘s surface. Among their many aspirations, NASA scientists hope
New Horizons will clarify Pluto‘s relationship to Triton, a moon of Neptune with a similar size, mass, and composition. Are they
sister worlds or historically distinct entities? After 2015, the spacecraft will venture further out into the Kuiper belt, where it will
spend a decade studying at least one of the smaller (30- to 60-mile-wide) objects.
―This is our first foray into the deep, frozen, ancient outer solar system,‖ says planetary scientist Alan Stern. ―This is what humans do,
what great civilizations do: They explore and they make history.‖