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The Origin of
the Solar System
In the beginning in a cold gas cloud near where the solar system lies today,
there was a competition between a physical process, gravitational collapse, and
a chemical process, the formation of dust and rocks. When stars formed the strong
UV light emitted by the stars evaporated the gas and stopped the formation of
the dust, rocks and ultimately the planets. Such a competition is taking place
today in the Trifid nebula (left) as imaged by the Hubble Space Telescope.
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Trifid
nebula |
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"A cocoon nebula, perhaps the primordial solar nebula"
by William K. Hartmann |
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Inside the early solar nebula, the sun shone dimly in visible
light as the dust and protoplanets formed as illustrated in a painting
by W. Hartmann (right). In the region now called the asteroid belt,
this process stopped when Jupiter formed by a gravitational instability.
Again physical and chemical processes were in competition. Jupiter
orbited the sun at a different rate than the protoplanets, closer
to the sun, and its periodic gravitational pull stirred up the
bodies in the "asteroid belt" shutting off the planetary formation
process leaving baby planets a fraction of the size of Earth but
very much indicative of the protoplanets that led to the formation
of the terrestrial planets.
The figure below shows Vesta and Ceres as seen by the Hubble Space
Telescope and the Near-Earth asteroid Eros that was recently explored
by the NEAR mission. These two bodies are much more massive than
any body yet visited in this region of space and are truly small
planets.
CERES
Giuseppe Piazzi pointing
at Ceres
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Ceres, the largest asteroid and the first to be discovered, is named
after the Roman goddess of agriculture. It was discovered by Giuseppe
Piazzi of the Palermo Observatory on Jan. 1, 1801. Additional observations
by Piazzi were cut short due to illness. Carl Friedrich Gauss, at
the age of 24, was able to solve a system of 17 linear equations
to determine Ceres' orbit and to allow it to be rediscovered, a remarkable
feat for this time. As a result within one year of its initial discovery,
both Heinrich Olbers and Franz von Zach were able to relocate Ceres.
It revolves around the Sun in 4.6 terrestrial years and has a diameter
estimated at about 960 km (600 miles).
VESTA
Vesta, the brightest asteroid, is named for the ancient Roman goddess of the
hearth and is the only asteroid ever visible with the naked eye. Found on March
29, 1807, by Heinrich Olbers, it was the fourth minor planet to be discovered.
It is the second most massive and the third largest asteroid. It revolves around
the Sun in 3.6 terrestrial years and has an average diameter of about 520 km
(320 miles). Its surface composition is basaltic.
WHY IS THERE SUCH A LARGE GAP BETWEEN THE ORBITS
OF MARS AND JUPITER?
The large gap between Mars and Jupiter is associated with the large mass of
Jupiter but there has been an evolution in our understanding of the nature
of this effect. Isaac Newton, for example, regarded this gap as part of the
divine plan for the stable and clockwork universe: the massive planets, Jupiter
and Saturn, had been located by Providence at the outside of the planetary
system, well clear of the smaller planets whose orbits their gravitational
force would otherwise disrupt. Later, Johann Heinrich Lambert, who in general
is as committed to an eternal, unchanging clockwork universe as was Newton,
proposed that change had been brought about by the attractive power of Jupiter: "And who knows whether already planets
are missing which have departed from the vast space between Mars and Jupiter?
Does it then hold of celestial bodies as well as of the Earth, that the stronger
chafe the weaker, and are Jupiter and Saturn destined to plunder forever?" This
view is much closer to our modern view that the formation of Jupiter brought
an end to the formation of planetary bodies in the gap between Mars and Jupiter
and then caused the small bodies that had already formed in this region to
collide with one another, leading to destruction of many of these bodies.
WHERE SHOULD THE PLANETS BE? THE
LAW OF PROPORTIONALITIES
We know today that Jupiter, the most massive body in the solar system has acted
to disrupt the region we know as the asteroid belt. What if Jupiter was small?
We would expect a planet to form in this region. Beginning with the work of Copernicus
in the 16th century, followed by Kepler who determined the laws that govern planetary
motion, the precise locations of the planets came to be understood. Early in
the 18th centery David Gregory, in his widely-read The Elements of Astronomy puts
the planetary distances into proportional numbers: "...supposing the distance
of the Earth from the Sun to be divided into ten equal Parts, of these the distance
of Mercury will be about four, of Venus seven, of Mars fifteen, of Jupiter fifty
two, and that of Saturn ninety five." The same numbers - indeed, a paraphrase
of the same sentence - in a work published in 1724 by Christian Wolff. In 1764,
the French natural philosopher Charles Bonnet published his Contemplation de
la Nature, a successful work that was quickly translated into other European
languages. The German translation was undertaken by Johann Daniel Titius of
Wittenberg. Translators took a greater initiative than is now thought proper;
Titius, probably because he was by nature self-effacing, not only left his
additions unsigned but actually incorporated them in the text itself, with
no hint that they were not the original work of the author. He chose to make
such an addition to the paragraph where Bonnet remarks that ``We know seventeen
planets that enter into the composition of our solar system [that is, major
planets and their satellites]; but we are not sure that there are no more'',
going on to anticipate more discoveries as telescopes improve. Titius then
inserts what we now called Bode's Law:
Take notice of the distances of the planets from one another, and
recognize that almost all are separated from one another in a
proportion which matches their bodily magnitudes. Divide the
distance from the Sun to Saturn into 100 parts; then Mercury
is separated by four such parts from the Sun, Venus by 4+3=7
such parts, the Earth by 4+6=10, Mars by 4+12=16. But notice
that from Mars to Jupiter there comes a deviation from this so
exact progression. From Mars there follows a space of 4+24=28
such parts, but so far no planet was sighted there. But should
the Lord Architect have left that space empty? Not at all. Let
us therefore assume that this space without doubt belongs to
the still undiscovered satellites of Mars, let us also add that
perhaps Jupiter still has around itself some smaller ones which
have not been sighted yet by any telescope. Next to this for
us still unexplored space there rises Jupiter's sphere of influence
at 4+48=52 parts; and that of Saturn at 4+96=100 parts. What
a wonderful relation!
As it happened, Titius published a second edition of his translation
- with the law now properly located in a footnote - just as the
promising young astronomer Johann Elert Bode was putting the finishing
touches to the second edition of his introduction to astronomy,
Anleitung zur Kenntniss des gestirnten Himmels, which he had published
in 1768 when he was only nineteen. Bode came across the relationship
proposed by Titius, was convinced by it, and inserted it as a footnote
in his text:
This latter point seems in particular to follow from the astonishing
relation which the known six planets observe in their distances
from the Sun. Let the distance from the Sun to Saturn be taken
as 100, then Mercury is separated by 4 such parts from the Sun.
Venus is 4+3=7. The Earth 4+6=10. Mars 4+12=16. Now comes a gap
in this so orderly progression. After Mars there follows a space
of 4+24=28 parts, in which no planet has yet been seen. Can one
believe that the Founder of the universe had left this space
empty? Certainly not. From here we come to the distance of Jupiter
by 4+48=52 parts, and finally to that of Saturn by 4+96=100 parts.
It is clear from the wording that Bode is following Titius, although
he of course realized that the suggestion that the missing planet
was a moon of Mars was preposterous, a fact he emphasized in the
third edition of his book. But he makes no acknowledgement to Titius;
indeed, it is only in later editions that Bode identifies his source
(possibly because Titius had pressed him to do so). In the hands
of Bode the relationship assumed a new importance, for Bode was
a professional astronomer soon to take on international stature,
and he was well-placed to act as apostle of the new law.
THE SEARCH FOR A PLANET BETWEEN
MARS AND JUPITER: THE FIRST INTERNATIONAL SCIENTIFIC PROGRAM
The discovery of Uranus in 1781, close to the location predicted by the Titius-Bode
law of proportionality, persuaded Baron Franz Xaver von Zach, the court astronomer
at Gotha to search for the proposed missing planet between Mars and Jupiter.
Not unreasonably, he limited his investigation to the Zodiac, and believing that
only a methodical search offered hope of success, he produced for himself a catalogue
of zodiacal stars arranged by right ascension; but without success. In the autumn
of 1799 the idea of a cooperative attack on the problem emerged:
It was the opinion of these men of discernment, that to get onto the trail
of this so-long-hidden planet, it cannot be a matter for one or two astronomers
to scrutinise the entire Zodiac down to the telescopic stars.
It was on 21 September the following year that the cooperative attack - probably
without precedent in the history of science - became a reality. On that day
six astronomers met in Lilienthal: von Zach himself; J.H. Schröter, the chief
magistrate of Lilienthal, whose world-famous collection of instruments included
a Herschel reflector of 27ft focal length; H.W.M. Olbers, physician from nearby
Bremen and longtime collaborator with Schröter; C.L. Harding, who was employed
by Schröter and who was himself to discover the third asteroid in 1804; F.A.
Freiherr von Ende; and Johann Gildemeister. They decided that even six observers
were too few for the task ahead, and nominated instead a group of twenty-four
practicing astronomers chosen from throughout Europe. Schröter was to be president
and von Zach, secretary. The entire Zodiac was divided up into twenty-four
zones each of 15 degrees in longitude, and extending some 7 or 8 degrees north
and south of the ecliptic in latitude. The zones were allocated to the members
by lot. Each member was to draw up a star chart for his zone, extending to
the smallest telescopic stars. Accordingly von Zach sent out the invitations
to join this society of celestial cops. Shortly thereafter on January 1, 1801
Guiseppe Piazzi of Palermo discovered the first asteroid, 1 Ceres, but, ironically,
despite the methodical approach of von Zach and his colleagues, Piazzi was
not an original member of the celestial cops. They had been scooped by a lone
observer.
Note: Some of this information was excerpted from Bode's
Law and the Discovery of Ceres by Michael Hoskin, Churchill College,
Cambridge.
+ Vesta:
A Brief Review of Current Knowledge by K. Tschann-Grimm
OTHER SITES OF INTEREST
+ Solar Views - Vesta
+ Asteroids
2001: From Piazzi to the 3rd Millennium
+ Meteorite: International
Quarterly of Meteorites and Meteorite Science
+ The Minor Planet Observer |
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accountability report