91av

WHEN the Sun and planets formed from an interstellar cloud of gas and dust
some 4.5 billion years ago, most of the original material was lost. Solar
heating and geological processes drastically changed all of the planets, and it
was only on the fringes of the young Solar System that temperatures were low
enough—and collisions rare enough —for primordial material to
survive. It is this rubble, left over from the building of the planets, that
holds clues about the nebula from which the Solar System formed.

Comets are occasional visitors from these distant, icy regions, bearing
information for astronomers about the early Solar System. Unfortunately, by the
time such comets are discovered, they are usually approaching the Sun, so their
outer coating of ice is disappearing fast. Now, as powerful computers and
sensitive new instruments push back the frontiers, astronomers have discovered a
whole new class of celestial icy bodies, objects that could finally reveal the
secrets of the Solar System’s birth.

The Solar System is thought to have formed from one of the many clouds of gas
and dust that pervade interstellar space. Theory has it that
something—what exactly is not yet fully understood—caused part of
the cloud (the pre-solar nebula) to collapse. As the collapse progressed, the
increasing gravitational pull at the centre of the collapsing core dragged more
and more material inwards. At the centre, temperatures and pressures rose high
enough for nuclear reactions to start and a star was born. But some material
stayed behind in a flattened disc, orbiting the new star close to its
equator.

Within this disc, the grains of dust and ice began to stick together. Soon
the Solar System was awash with objects a few metres in diameter which clumped
together to form larger planetesimals. These eventually swept one another up to
form the planets. Close to the Sun, temperatures were high enough to evaporate
frozen substances such as ice from the grains and drive them outward. Because of
this, rocky planets formed in the inner regions of the disc, while farther out,
above the “snow line” of the solar nebula, the gas giant planets and their icy
moons were born.

Solar secrets

The terrestrial planets and the main asteroid belt between Mars and Jupiter
are made up of solar nebula material with a high melting point, while the gas
giants mostly comprise hydrogen and helium. Although the giants also contain
simple molecules such as water, ammonia and methane, their atmospheres have been
chemically changed by the heat generated when they formed so, apart from the
ratio of hydrogen to helium, they tell us little about the original molecular
content of the solar nebula.

Where then can we look for remnants of the solar nebula? In the late 1940s
and early 1950s, K. Edgeworth and Gerard Kuiper both argued that there should be
a cloud of frozen planetesimals at the edge of the Solar System which formed at
the same time as the planets but which were so far from the Sun and from each
other that they never coalesced. Objects in this region would not be large
enough to have significant internal heating so they should preserve a record of
the oldest material in the Solar System. The limited technologies of the 1950s,
however, meant that there was no hope of detecting these faint objects and
interest in “Kuiper’s disc” waned.

Then, in the late 1980s, Martin Duncan of the Lick Observatory in California
working with Thomas Quinn and Scott Tremaine from the University of Toronto
showed that there were too many comets trapped in the inner regions of the Solar
System to be explained by the prevailing notion that the comets were all
arriving at random from a spherical cloud way beyond the planets. They concluded
that although such a cloud could generate some of the comets, others must be
arriving from a disc close to the plane of the Solar System from which they
could be more easily captured.

At about the same time, astronomer Dave Jewitt from the University of Hawaii
began searching for something beyond Pluto. Because the project was considered
“too speculative”, he was denied time on national telescopes in the US. Instead,
he turned to the University of Hawaii’s own 88-inch telescope on Mauna Kea, and
in 1992 he and collaborator Jane Luu struck gold. They discovered a distant
faint object, which they called 1992QB1, Q because each fortnight of the year is
given a letter of the alphabet, and the object was found in the seventeenth
fortnight of 1992, and B1 because each new object found every month is given a
different letter, and August was so busy that astronomers had already used up
the alphabet once, and were now on to A1 and B1 (“The planet that came in from
the cold”, 91av, 14 November 1992). Since the object moved
slowly, working out its orbit was not easy, but when Jewitt and Luu discovered a
second object, designated 1993FW, six months later, most astronomers were
convinced that the Kuiper disc had been found.

Since 1993, more than two dozen such objects have been spotted, some by
Jewitt and company from Hawaii, others by new groups who have joined the fray
using telescopes in La Palma, Australia and Chile. All of these objects are
relatively large—more than 100 kilometres in diameter—and
researchers estimate that there are at least 35 000 such objects throughout the
Kuiper disc, several hundred times more than are found in the main asteroid
belt. Moreover, in 1994 Anita Cochran from McDonald University in Texas and
various collaborators used statistical analysis on images from the Hubble Space
Telescope to estimate that the Kuiper disc could hold up to 100 million objects
with a diameter of around 10 kilometres—the typical size for the nucleus
of a comet. As Jewitt puts it, “discovering the Kuiper belt is like waking up
and finding that your house is ten times bigger than you thought it was”.

Seeing red

Having discovered the objects, the next step is to try to work out what they
are made of, and hence what the original solar nebula was really like. The
comet-sized objects are too small to study from the Earth, but researchers have
managed to obtain spectral information from some of the larger objects.
Surprisingly, there are tantalising clues that these objects are very different
from one another. For example, 1992QB1 is fairly red in colour, but Jewitt and
Luu reported that 1993FW seemed to be less red and might have a different kind
of surface coating. In 1994, I confirmed this by showing that 1993FW was much
harder to detect than 1992QB1 in the infrared. Later that year, Luu and Jewitt
reported that another new object, 1993SC, seemed to be of intermediate
redness.

The problem is that the objects are just too faint to be really sure what
they are made of and whether they really are different from one another, even
using the biggest and best telescopes: 1992QB1 is over six million times fainter
than the faintest star visible to the naked eye. Fortunately, nature has given
astronomers a sneak preview of what the Kuiper disc objects look like. While
some of the objects appear to lie beyond Neptune and Pluto in fairly circular
orbits that are quite stable over the expected lifetime of the Solar System,
others are in orbits which wander across the orbit of Neptune and could be
disturbed out of their present orbits by Neptune’s massive gravity.

Many of these objects turn out to be stabilised by a particular orbital
“resonance” with Neptune—making exactly two trips around the Sun in the
time it takes Neptune to make three orbits. This means that although they cross
Neptune’s orbit, they never approach the planet itself and so do not risk having
their orbits drastically modified by Neptune’s gravity. However, some of the
Neptune-crossing orbits do not fit this neat pattern, and could “leak” into the
inner Solar System.

Eccentric asteroid

One such icy interloper was discovered back in 1977, when Charles Kowal found
Minor Planet 2060 in an eccentric orbit between Saturn and Uranus. Astronomers
quickly realised that the object could not have formed in its present
location—the orbit is highly unstable and would be disrupted by the
gravity of the giant planets within a few million years. There were no similar
objects anywhere near the new minor planet—it looked like an asteroid but
it was hundreds of millions of kilometres from the main asteroid belt, and its
orbit looked more like that of a comet. It was an anomaly, and its indeterminate
nature was reflected in the name it was given, Chiron, a member of the mythical
group of half-man, half-horse creatures called Centaurs.

Chiron remained a lonely aberration until 1991 when the Spacewatch automatic
asteroid search telescope in Arizona discovered a new distant asteroid, soon to
be named 5145 Pholus after another mythical centaur. By this time, the idea of
the Kuiper disc had been resurrected, Chiron and Pholus both had orbits that
were close to the plane of the Solar System, and Luu and Jewitt were scanning
the skies for Kuiper disc objects. Everything fell into place, and astronomers
realised that Pholus and Chiron must both have come from the Kuiper disc.

Pholus was in a planet-crossing orbit rather similar to Chiron’s, but there
the similarity ended. The surface of Chiron is neutral in colour—it
reflects all wavelengths equally—but when three different groups of
astronomers observed Pholus within weeks of its discovery, they all discovered
that it was astonishingly red—the reddest object ever seen in the Solar
System. Four other Centaurs have since been discovered, but the others are very
faint and not much is known about them. The best observed of these, 1993HA2, was
discovered in 1993 by the Spacewatch team. Using the United Kingdom Infrared
Telescope on Mauna Kea in Hawaii in 1993 and 1994, I discovered that 1993HA2 is
very red, although it is not quite as red as Pholus.

The differences between the Centaurs seem to confirm the early results that
the colours of objects that lie in the Kuiper disc are different. But does this
mean they are made from different things? One possibility is that they could
have been different from birth. If they formed in different parts of the outer
Solar System, they could have started life with different chemical compositions.
Similar differences are seen within the main asteroid belt where objects closer
to the Sun tend to be more reflective than those further out. These differences
are generally attributed to temperature differences in the original solar
nebula—the closer to the snow line the asteroids were when they formed,
the more likely they are to contain volatile, “original” material which reacted
to form complex organic compounds, darkening and reddening their surfaces.

Kuiper-disc objects, however, are thought to have formed way out in the
Uranus-Neptune region, far from the snow line, where temperatures would have
varied much less with distance from the Sun—all of which makes this
scenario improbable.

Another possibility is that they all began the same, but that some have since
had their surfaces changed. This would mean that only the objects with pristine
surfaces could provide direct clues about the original condition of the solar
nebula. But how could the surfaces be changed? Astrochemists have long suggested
that ice bombarded by cosmic rays will form new, reddish compounds when simple
molecules such as carbon dioxide, methane and ammonia combine to form more
complex organic material called tholins. These complicated compounds are like
the red-brown residue found in dirty organic chemistry labs and, as anyone whose
has tried to clean up knows, they are extremely stable. A coating of reddish
tholins could explain the colour of Pholus, and at last year’s American
Astronomical Society Planetary Science meeting in Hawaii, Dale Cruikshank of
NASA’s Ames Research Center presented models containing a mixture of organic
materials, water and dust that matched the spectrum of Pholus.

Cutting the crust

But if this idea is right, why should Pholus be so much redder than Chiron?
Earlier this year, Luu and Jewitt suggested that the reddish-brown crust might
have been disrupted when some of the objects may have collided with smaller
objects within the disc, causing craters which splashed out cleaner material
from beneath the crust and diluted the overall redness. Despite the huge volume
of the disc, which probably extends from 6 to 10 billion kilometres from the
Sun, Alan Stern of the Southwest Research Institute in San Antonio, Texas
believes that collisions could happen often enough to make this explanation
viable. Stern estimates that a QB1-sized body will be struck by a smaller body
within the disc about once every 1 to 10 million years, leaving time for plenty
of collisions over the lifetime of the Solar System. His figures suggest that
these collisions might even produce dust clouds, or trails, which though
invisible from the ground could be detected using far-infrared detectors on
satellites such as the recently launched ISO mission.

Resurfacing is also possible if, like Neptune’s moon, Triton, some sort of
geyser of cleaner material sometimes squirts out from below the crust and falls
back onto the surface. The sublimation of frozen carbon monoxide trapped in
pockets beneath the surface might generate enough pressure to do the trick. This
idea is supported by the fact that, since it was first spotted, Chiron has
developed a comet-like cloud of gas and dust, probably caused when gases trapped
below the surface blew a hole in an insulating crust. Similar outbursts are seen
on the distant comet Schwassmann-Wachmann-1 about once a year.

However, if Pholus and Chiron do have the same underlying structure, it is
not clear why Pholus should now be inactive and red, especially since, like
Chiron, Pholus recently made its closest approach to the Sun (see Diagram).
Perhaps the cometary activity of the Centaurs does not just depend on distance
from the Sun. After all, measurements of Chiron when it reached its closest
distance to the Sun earlier this year revealed that it seems to have “turned
itself off” (91av, Science, 9 March), and in 1993 Bobby Bus
from the Massachusetts Institute of Technology re-examined old photographs and
discovered that Chiron was active when it was still very distant from the
Sun.

In any case, these resurfacing ideas fail to explain why none of the Kuiper
disc objects seen so far are anything like as red as Pholus. If resurfacing is
the explanation, it seems odd that the reddest object, and presumably the one
with the oldest crust, should be in the relatively warm and crowded regions of
the Solar System rather than on its colder fringes.

If we are to discover the true nature of the original solar nebula, we will
need much more information about the Centaurs and their icy cousins back home in
the Kuiper disc. Unfortunately, this requires large telescopes, which are
heavily in demand. In fact, some of the new Kuiper disc objects have actually
been “lost”—the errors in the calculated orbits built up so much that by
the time a telescope was available to look for them again, they had moved too
far to be found.

Heroic efforts by Jewitt and Luu recently relocated several objects that
looked as if they were headed for obscurity, and another was recovered by
amateur astronomer Warren Offutt using a 0.6 metre telescope in his home-made
observatory near Cloudcroft, New Mexico. Still, the Kuiper objects are in no
special hurry. They have guarded their secrets for more than 4 billion years so
they can afford to wait a few more.