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Final Answers
© 2000-2005 Gérard P. Michon, Ph.D.

The Solar System

Man's most valuable trait is a judicious sense of what not to believe.
 Euripides (c.480-406 BC)

Related articles on this site:

  • Kepler's Third Law: The relation between orbital period and orbit size.
  • Nemesis: A distant companion to the Sun could explain extinction periodicity.
  • Cosmology 101: Basic concepts for studying the physical Universe as a whole.

Related Links (Outside this Site)

Views of the Solar System  by Calvin J. Hamilton
The Sun (Sol)  at  nineplanets.org  by Bill Arnett

The Solar System

Mark Neumeyer  (2004-11-18; email)   Solar Radiation and Solar Mass
Since the Sun gives off energy, wouldn't its mass decrease?

The Sun's mass does decrease, not only because it gives off energy, but also because it gives off some matter particles (Solar wind) at an initial speed that's sometimes sufficient to let them escape the Solar System.  As a result, the planets are slowly drifting outward.  Let's quantify this:

First, let's dispose of the  Solar wind  issue...  The escape velocity near the surface of the Sun can be computed to be about 600 km/s.  Well, the so-called "fast" Solar wind emanates from the polar regions of the Sun faster than that (at about 800 km/s) and is thus eventually lost to interstellar space.  On the other hand, what's called "slow" Solar wind emanates from the equatorial region at a speed (around 300 km/s) which doesn't allow it to escape the Solar system  (the Sun's gravitation eventually pulls the stuff back in, after a fairly long journey).  All told, the mass of the Solar wind that does escape has been estimated to be at most a few million tons per year.  The Sun loses this much through light and other electromagnetic radiation in just a couple of seconds  (there are over 30 million seconds in a year).  In other words the Sun loses about 10 million times less mass through Solar wind than it does via regular radiation, as discussed next...

The total bolometric power of the Sun is about  3.826 1026 W.  In terms of lost mass, this translates into about  4.257 109 kg/s.  (over 4 million metric ton(ne)s per second).  In one year (31557600 seconds), that's about  1.3434 1017 kg,  which is still minuscule compared to the entire mass of the Sun itself (about 1.989 1030 kg).  It takes about 15 million years for the Sun to lose one millionth of its mass in the form of radiation.  Assuming that the power output of the Sun has been constant ever since its formation 4550 000 000 years ago (which is not quite so) the Sun has thus lost to radiation about 0.03% of its original mass.

The fusion of hydrogen into helium converts about 0.7% of mass into energy.  For a star like the Sun, an opaque layer exists which slows down radiation emanating from the core.  The regime in which such a star settles imposes a "nuclear time scale" allowing only 10% of its hydrogen to be consumed over the star's lifetime  (hydrogen makes up roughly 75% of the initial mass).  The above thus indicates that the Sun has already burned about half of what its current regime allows.

This and other effects concerning the so-called decay of planetary orbits have been incorporated into our long-term mathematical models of the Solar system.

There are other more dramatic evolutions of astronomical motions:  For example, we have biological evidence that the lunar month was only 9 days long 420 million years ago (instead of about 29.5 days today).  Each of these ancient days was itself about 12% shorter than a modern day.

(2004-11-04)   The Titius-Bode Law
An empirical formula for the Solar distribution of planetary distances.

d(n)  =  0.4 + 0.3´2n     for  n = , 0, 1, 2, (3), 4, 5, 6 ...

This formula happens to give a good approximation of distances to the Sun (expressed in asronomical units) for the successive planets:  The Earth is, by definition, at a unit distance: d = 1 (n = 1).  The main asteroid belt is at the approximate location of a "missing" planet (n = 3) between Mars (n = 2) and Jupiter (n = 4)...  All told, the approximation is surprisingly good as far as Uranus (n = 6) but it's about 29% too large for Neptune (n = 7) and fails by almost a factor of 2 for Pluto (n = 8).

This empirical relationship is most commonly known as Bode's Law.  It was named after Johann Elert Bode (1747-1826), who published it in 1768.  Bode was to become director of the Observatory of Berlin, and he collaborated with Johann Heinrich Lambert on the first ephemeris ever published in German.

The first calculations concerning the distribution of planetary distances are due to Christian Freiherr von Wolf  (1679-1754).

Wolf's calculations were first made popular in 1766 by Johann Daniel Dietz (1729-1796), a professor of physics at the University of Wittenberg (Germany) who is best known as Titius [of Wittenberg].  The thing is thus also known as the Titius-Bode law...

Of course, it's not a "law" at all, it's just an approximative relationship between the rank of a planet in the Solar system and the size of its orbit.  Yet, the pattern is sufficiently simple and sufficiently precise that it does beg for an explanation of some kind.  The Solar system's major planets came from the condensation of a rotating cloud of dust and gas.  Most of this was hydrogen which aggregated at the center to form a ball (the Sun) hot enough to ignite an ongoing nuclear reaction as it was compressed by its own gravity...  The rest aggregated in a small number of planets around the Sun, at distances which are fairly well described by the Titius-Bode law.  The details of the condensation of this primal cloud are not understood well enough to allow any kind of "derivation" of the Titius-Bode relation, at least for now.

(2005-08-30)   The Inner Solar System
Four rocky planets: Mercury, Venus, Earth and Mars.

 Come back later, we're
 still working on this one...

(2005-08-30)   The Asteroid Belt
Between Mars and Jupiter.

 Come back later, we're
 still working on this one...

Discovery of Ceres

(2005-08-30)   The gaseous planets
Four gaseous planets: Jupiter, Saturn, Uranus and Neptune.

 Come back later, we're
 still working on this one...

(2005-08-27)   Pluto, Plutinos and other planetoids in the Kuiper belt.
A Plutonian year is 1.5 times as long as a Neptunian year.

Pluto was discovered in 1930 by the American astronomer Clyde William Tombaugh (1906-1997).  It is about 2360 km in diameter  (roughly 2/3 the diameter of the Moon). 

In proportion to the planet, Pluto has the largest satellite of the Solar System:  It is 1250 km in diameter and was discovered in 1978, by American astronomer James Christy, who named it Charon  (after the mythical boatman of the Styx)  because the first syllable was the nickname of his wife Charlene...  Pluto and Charon are about 19000 km apart.  They present the same face to each other as they revolve around their center of gravity, in about 6.38 days.

In 1988, Pluto was found to have a very thin atmosphere of nitrogen, with traces of methane and carbon monoxide.  The atmospheric pressure at the surface of Pluto is roughly 1 Pa  (about 100 000 times less than on the surface of the Earth).

Pluto revolves around the Sun in 247.7 years.  This is  50 %  more than the giant planet Neptune, because of a gravitational synchronization dominated by Neptune.  Planetoids which are in this same 3 to 2 resonance with Neptune are called  plutinos.  Pluto and such planetesimals are located in the  Kuiper Belt.

Since the recent discovery in the Kuiper Belt of several planetoids (i.e. nearly spherical solar objects) whose sizes approach or exceed Pluto's,  the status of Pluto as the Solar System's ninth planet has been questioned.  It has been reaffirmed by the International Astronomical Union in 1998, but such a status is already best viewed as cultural and/or historical, rather than astronomical or physical...  Pluto has just too much competition in its own neighborhood to qualify, on merits alone, for the same status as the other 8 planets.

Some Large Kuiper Belt Objects  (KBO)
DiscoveryNameIDDiameter Period
of Orbit
2003-10-21  2003UB3132800 km56044°  
1930-02-18 Pluto 2360 km247.717.2° 6.38
2005-03-31  2005FY92000 km30729°  
2004-02-17 Orcus2004DW1500 km247.520.7°  
2002-06-02 Quaoar2002LM601280 km2887.983°  
2003-03-09  2003EL611200 km285.3328.194°  
2000-11-28 Varuna2000VR106  900 km28217.2° 0.13 ?
2001-02-22 Ixion2001KX76  407 km24919.6°  
1998-11-19 Chaos1998WH24  372 km30912.1°  
1992  1992QB1  283 km29112.1°  

Such discoveries are often announced many months after being first observed, for healthy scientific reasons which occasionally yield to a combination of peer pressure and media greed:  For example, three of the above  (2003UB313, 2005FY9, 2003EL61)  were announced on July 27 and 28, 2005...

Kuiper Belt Objects may be arbitrarily divided into 3 categories: 

  • The inner belt, consisting mostly of Plutinos.
  • "Classical" Kuiper Belt Objects, with a period of 400 years or less.
  • Scattered disk objects (SDO) beyond that point.

The Kuiper Belt was so named shortly after the discovery of 1992QB1, the first object  (besides Pluto and Charon)  found in it, by David C. Jewitt (University of Hawaii) and Jane X. Luu (Berkeley).

Gerard Peter Kuiper (1905-1973) was a Dutch-born American astronomer who served as chief scientist of NASA's "Ranger" lunar probe program in the 1960's.

The existence of the Kuiper Belt was formally proposed in the 1980s as the origin of short-period comets.

Transneptunian Objects   |   Objects Transneptuniens
Largest Bodies of the Solar System by Haluk Akcam

(2005-09-03)   Planetoids with Distant Aphelions
Scattered disk objects (SDO) and wanderers, beyond the Kuiper Belt.

Some Transneptunian Object beyond the Kuiper Belt
DiscoveryNameIDDiameter Period
of Orbit
2003-11-14 Sedna2003VB121500 km1148711.9° 0.4
2000  2000CR105 342022.735°  
2000  2000OO67 1330020.083°  
1996-08-09  1996PW10 km400029.787°  

  With a semimajor axis of 480 AU, Sedna is so distant that it has been considered part of the Oort Cloud.

 Come back later, we're
 still working on this one...

(2005-08-30)   Heliosphere and Heliopause
The region aftected by solar wind, and its boundary.

 Come back later, we're
 still working on this one...

(2005-08-27)   The Oort Cloud
The outermost spherical shell at the fringe of the Solar System.

The Dutch astronomer Jan Hendrik Oort (1900-1992) helped establish the rotation of our  Milky Way  galaxy in the 1920's.  In 1950, he proposed that the outermost part of the Solar System was a spherical reservoir of comets...

As passerby stars come within a few light-years from the Sun, they could disturb the orbits of some distant solar objects and turn them into "long-period" comets bound for the inner Solar System.

This view is now universally accepted, although Oort's explanation for the formation of the "cloud" is not...  Oort envisioned it as a remnant from the explosion of a planet between Mars and Jupiter.

The fringe of the Solar System is now called the  Oort Cloud.  It is bounded by a huge sphere centered on the Sun, whose radius is estimated to be about one light-year  (1730 astronomical units) corresponding to orbital periods of 70000 years or so.

visits since Nov. 20, 2004
 (c) Copyright 2000-2005, Gerard P. Michon, Ph.D.