206
See also: Halley’s comet 74–77 ■ The Kuiper belt 184 ■
The composition of comets 207 ■ Exploring beyond Neptune 286–87I
n 1950, reviving a theory
proposed by Estonian
astrophysicist Ernst Öpik, a
Dutch astronomer named Jan Oort
argued that there is a reservoir
of comets at the edge of the solar
system. At the time, it was known
that two main classes of comets
visit the inner solar system—
the region comprising the four
rocky planets. Short-period comets
visit at intervals of less than 200
years, and orbit in the plane in
which the planets lie. Long-period
comets visit at intervals that are
longer than 200 years and have
orbits inclined at all directions
and angles to the plane of the solar
system. The origin of either class
was subject to speculation.Long-period comets
Oort’s idea provided a solution to
the origin of long-period comets.
A comet periodically visiting the
inner solar system will eventually
collide with the sun or a planet,
or will be ejected from the solar
system after its orbit is disturbed by
passing near a planet. This meansthat comets cannot simply have
been going around in their orbits
since the solar system formed. Oort
suggested that long-period comets
passing into the inner solar system
are just a small subset of all the
comets orbiting the sun. The
comets seen from Earth have been
nudged out of the distant comet
reservoir, perhaps by a passing star,
and have plummeted toward the
sun, taking up long, elliptical orbits.Spherical cloud
Examining the orbits of numerous
long-period comets and the farthest
distance from the sun they reach,
Oort reasoned that the reservoir
for the long-period comets is a shell-
like, spherical region ranging up
to a maximum of about 4.5–19
trillion miles (7.5–30 trillion
kilometers) from the sun. This
region, envisaged to contain billions
or trillions of comets, is now known
as the Oort cloud. However, it has
since been established that short-
period comets probably originate
from a disklike region much closer
to the sun, the Kuiper belt. ■A VAST CLOUD
SURROUNDS THE
SOLAR SYSTEM
THE OORT CLOUD
IN CONTEXT
KEY ASTRONOMER
Jan Oort (1900 –1992)BEFORE
1705 Edmond Halley correctly
predicts the return of a comet.1932 Estonian astronomer
Ernst Öpik proposes that
long-period comets originate
in an orbiting cloud at the
edge of the solar system.1943 Kenneth Edgeworth
suggests that the solar system
beyond the orbit of Neptune is
occupied by many small bodies,
some of which become comets.1950 Fred Whipple proposes
that comets are a conglomerate
of ices and rocky material.AFTER
1992 David Jewitt and Jane
Luu discover the first Kuiper
belt object other than Pluto.2014 The Philae lander of
the Rosetta spacecraft
lands on the comet 67P/
Churyumov–Gerasimenko.