Here Be Dragons

HERE BE DRAGONS
believed to be the building blocks of planets in the late stages of disk
evolution. Rather than blast them with photon torpedoes, Wetherill
simply lets them collide with each other under the influence of gravity.
In this way, he makes terrestrial planets with ease and, by adding a
Jupiter in the right place, he prevents any planet from forming in the
vicinity of the asteroid belt, just as seems to have happened in our own
solar system.
In the outer part of the model solar system, however, planet formation
proceeds very reluctantly. Beyond Neptune's orbit, the average
time between collisions becomes as long as the present age of the
solar system. So Wetherill is left with a population of cold icy planetesimals
that still occasionally collide but that never completely accumulate
into a single planet.
These objects do in fact still exist in the outer reaches of our own
solar system. They're called "Kuiper-belt objects," after Gerard Kuiper,
who first postulated a family of comets orbiting in this region.
Examples of such icy bodies have recently been detected by a number
of researchers, especially by Jane Luu of Harvard and Dave Jewitt of
the University of Hawaii. 10 The objects bear a striking resemblance to
Pluto and its moon Charon, as well as to Neptune's captured moon
Triton. Of importance to us is the fact that the objects form in the
outer region of the developing solar system where interstellar organic
compounds can survive the accretion shock.
When the solar system was young, according to Wetherill, there
should have been many more rocky and icy planetesimals than the
current inventory of asteroids and Kuiper-belt objects. Some of the
missing objects, his model indicates, were slung out of the solar system
after near encounters with planets or with each other. Judging by
the cratering record on the oldest known planetary surfaces, however,
an astonishing number were not ejected from the solar system but
wended their way to a fiery collision with one or another planet or
moon, including Earth. The recent collision of comet "Shoemaker-
Levy 9" with Jupiter serves to remind us that such events continue
even today. 11
Could infalling comets and icy planetesimals, packed full of organic
molecules, have contributed substantially to the origin of life on earth
We already visited this subject in Chapter i. According to Chris Chyba
and the late Carl Sagan, the inflow of organics from space could have
been as high as the local (Miller-style) production rate in a weakly reducing
atmosphere. Chyba also points out that the inflow could have
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