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Final Answers
© 2000-2018   Gérard P. Michon, Ph.D.

The Solar System

Daybreak is one of the greatest disadvantages
of living under the solar system
 Ogden Nash (1902-1971)
 Tycho Brahe 
 1546-1601   Galileo Galilei 
 1564-1642   Johannes Kepler 
 1571-1630   Isaac Newton 
 1643-1727   William Herschel 

É, si muove !  (Nevertheless, it moves.)
Galileo Galilei  (1564-1642)

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Related articles:

Related Links (Outside this Site)

The Sun's Sizzling Corona  at  FirstScience.com
Views of the Solar System  by Calvin J. Hamilton
The Sun (Sol)  at  nineplanets.org  by Bill Arnett
Transneptunian Objects  by John Baez  (Week #222).
Center for Astrophysics  (CfA)

Video  New Dimension Media #24 :   Our Sun and the Planets  (75.7 MB)
Carl Sagan's Pale Blue Dot (famous picture taken in 1990 by Voyager 1)
Nova: Origins of the Solar system  (Nickel-60 from a supernova explosion)
National Geographic Special: Secrets of the Sun.  Coronal Mass Ejection (CME).
Why is the Solar System Flat?  by  Henry Reich  (MinutePhysics, 2014).

 International Year 
 of Astronomy (2009)

The Solar System

 Carl Friedrich Gauss 
 (1777-1855)  Jean-Dominique Cassini 
 (1625-1712) (2007-10-10)   The  astronomical unitfixed in 2012.
A unit of distance now  defined  as  149597870700 m.

The  astronomical unit  (au) was first defined in 1672 by Jean-Dominique Cassini (1625-1712) as the mean distance from Earth to Sun.  It was later redefined more precisely as the semimajor radius of the orbit of a small mass which would take one sidereal year to go around one solar mass.  The two definitions differ by about one part in a million.  So it was, at the beginning.

In 1672 the perihelion of Mars was simultaneously observed by Cassini from Paris and by Jean Richer from Cayenne  (French Guiana).  The parallax allowed them to derive a good approximation of the newly minted astronomical unit of length.  They underestimated the correct value  Johannes Kepler 
 (1571-1630)  Tycho Brahe 
 (1546-1601) by less than  7%.  (Previous guesses by Tycho Brahe and Johannes Kepler were more than 18 and 6 times too small, respectively.)

In 1938,  the  International Astronomical Union  modified the definition of the  astronomical unit  to eliminate any reference to the actual sidereal year  which decays over time  (it was equal to  365.256363004 D  at  J2000.0).  Instead, they used a fixed duration equal to the  Gaussian year  of about  365.2568983 D  that  Gauss  worked out in 1809  (as his best estimate of the  sidereal real  which he thought to be constant).

By definition (1938) the  Gaussian year  is the following exact duration:

2p / (0.01720209895)   =   (365.25689832632816455955142419...) days

It is used to define the unit of length,  not  the unit of time.  More precisely, in 1976, an  astronomical system of units  was established by the IAU, with the  astronomical unit  as the unit of length  (A)  and the  solar mass  (S)  as the unit of mass.  The basic unit of time is the day  (D)  of 86400 s  (thus defined as an exact multiple of the  "atomic"  SI second).

The secundary units of time are  exact  multiples of that astronomical or atomic day  (same thing).  The  year  is  365.25 D  (31557600 s)  and the  [Julian]  century  is  36525 D  (exactly  3.15576 109 s).

Until 2012  (when the astronomical unit of length itself was redefined as an exact number of meters)  there were no exact conversion factors between SI units and astronomical units, except for units of time.  The value of the  astronomical unit  in meters and the mass of the Sun expressed in kilograms were supposed to be derived from measurements  (both vary  extremely  slowly with time as the Sun  loses mass  and gravitational pull).

Newton's gravitational constant  (G)  had  an exact value under the old definition of the  astronomical system of units  outlined above.  Namely:

G   =   k2   =   (0.01720209895)2  A3 S-1 D-2
=   0.0002959122082855911025  A3 S-1 D-2

k = 0.01720209895 rad/D   is the  Gaussian gravitational constant which was a  defining  constant, with that exact value, before the 2012 reform.

The product  G . S  is the  heliocentric gravitational constant  which is known with excellent precision in SI units, although neither of its factors is:

G . S   =   1.32712440042 (8)  1020 m3/s2

The following relation  used to  link the value  A  of the  astronomical unit  in meters and that  heliocentric gravitational constant  G . S  (the relative uncertainty on the former is thus  a third  of the uncertainty on the latter).

(0.01720209895)2 A3   =   (86400 s)2  (1.32712440042(8) 1020 m3/s2 )

Solving for  A,  we obtain:   A   =   149597870700 (3)  m

On August 31, 2012, the IAU decided that this was good enough to make the metric value of the  astronomical unit  a defining constant of the astronomical system of units  (instead of the Gaussian "constant"  k  which need not be mentioned anymore).  They argued that there was no longer any precision to be gained by evaluating ratios of astronomical distances rather than expressing them directly in meters, or in any convenient fixed multiple thereof, which is what the astronomical unit of length thus becomes:

Astronomical unit   (IAU, August 2012)
A   =   1 au   =   149597870700  m       ( exactly )

In a vacuum, a photon travels one astronomical unit in  499.004783836... s.
Checksum :   both 299792458 and 149597870700 are divisible by 73.
1 au   =   149597870700 m   =   22 . 3 . 52 . 73 . 877 . 7789   m
c   =   299792458 m/s   =   2 . 7 . 73 . 293339   m/s

Here's what I've gleaned from the bygone era when the  heliocentric gravitational constant  (G.S)  and the  astronomical unit of length  (A)  where firmly tied to each other by a  de jure  value of the ratio  G.S/A3.

Heralded values when  G.S / A3  was a defining constant  (1938-2012).
Data SetHeliocentric Constant  (G . S)Astronomical Unit  (A)
IAU, 1976 1.32712438 (2)  1020 m3/s2 149597870000(750) m
Ephemerides, 1992 1.32712439935 (54)  1020 m3/s2 149597870660 (20)  m
IAU, 1994 1.32712440042 (80)  1020 m3/s2 149597870700 (30)  m
IERS 2003 1.32712440018 (16)  1020 m3/s2 149597870691 (6)   m
before IAU, 2012 1.32712440042 (8)   1020 m3/s2 149597870700 (3)   m

Curiously enough, some lists quoted values of the two constants that were grossly incompatible at the stated accuracy.  Also, the strict 3:1 ratio of the relative uncertainties  (excluding rounding)  translates into a clean 8:3 ratio when uncertainties are expressed in units of the least significant digit...

Resolutions of the IAU, since 1922   |   Value of the astronomical unit (2012) by NASA.

(2012-11-06)   The  mean  distance between the Earth and the Sun.
It's  slightly more  than one astronomical unit.

The  mean distance  (d)  between two bodies of masses  M  and  m  orbiting each other with a period  T  is given by  Kepler's third law :

4 p 2  d 3   =   G  ( M + m )  T 2

If the period  (T)  of the Earth around the Sun was exactly one  Gaussian year,  the definition of the  astronomical unit  that was valid before 2012 would turn Kepler's law into the following equation for the Earth-Sun distance  (d)  expressed in astronomical units :

d 3   =   1  +  m / M       or,  very nearly :     d   =   1  +  m / 3M  - m2 / 9M2

As the Earth-Moon system is  3.040432685(9)  10-6  solar masses, this gives the mean Earth-Sun distance as  1.000001013476535(4)  astronomical units.

To be valid at the claimed precision of  4 10-15  (about  0.6 mm)  the so-called "mean distance" must be ultimately  defined  in terms of orbital energies,  not  on raw distances averaged over time  (or else the averaging time would have to be unrealistically long).
Indeed, in the ideal Keplerian motion of perfect spheres  (classically equivalent to two orbiting point-masses)  the time-average of the distance happens to be exactly equal to the semimajor radius of the orbit, which depends only on the orbital energy.

If the value of  T  is not exactly equal to  1  (one Gaussian year)  then the above result has to be multiplied by  T2/3.

( 1 + x ) 2/3   =   1  +  2x/3  - x2/9  +  ...

For example, when the sidereal year is given as  365.256363004 D  then:

T   =   (365.256363004) (0.01720209895) / 2p
T2/3   =   0.99999902293

Multiplying this into the above, we obtain  d  =  1.00000003641 au

Using the metric equivalent of the  au  (149597870700 m)  we obtain the  mean  Earth-Sun distance to a precision of  3 m :  149597876146 m.

(2012-11-06)   The  parsec  (pc).  The basic unit of interstellar distance.
Obtaining interstellar distances by triangulation.

By definition, a  parsec  is the radius of a circle in which an angle of  one arcsecond  subtends an arc of one astronomical unit  (A = 1 au).

Thus, the  parsec  (pc)  is an exact multiple of the  astronomical unit,  since a circle with a radius of  1 pc  has a circumference  of  1296000 au :

p pc   =     648000  au
1 pc   =   ( 206264.80624709635515647335733...)  au

As the astronomical unit has an exact metric equivalent  (149597870700 m)  since 2012,  so does the parsec:

1 pc   =   3.0856775814913672789139...   1016  m

The  parsec  is a fairly large unit which is adapted to the description of  interstellar  distances.  (The closest star to the Sun is  1.3 pc  away).

Light-year :

The  light-year  is a commensurable unit with an exact metric value.  It is equal to the distance traveled by light in a vacuum over a period of one year  (recall that the only year recognized as a standard unit of time in astronomy is worth exactly 365.25 days, or 31557600 s).

(31557600 s) (999792458 m/s)   =   9460730472580800 m

There are  3.261563777167433562138639707...  light-years  in a  parsec.

attoparsec (apc)   |   Parsec and Parallax  by  Michael Merrifield   ("Sixty Symbols" Video  by  Brady Haran)

(2006-11-28)   The Interior of the Sun
Temperature and pressure are high enough to allow nuclear fusion.

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

(2006-11-28)   The Chromosphere is the Surface of the Sun
After a long journey, core photons shine through the chromosphere.

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

 Solar eclipse 
 of August 11, 1999
 (Courtesy of Jonathan Kern) (2004-11-11)   The Corona
The Corona is a very hot region of rarefied gas which surrounds the Sun, beyond its chromosphere.

It's normally visible only during a total solar eclipse.  At right is the eclipse of August 11, 1999  (which I witnessed from my late grandparents' backyard).

The light spectrum of the corona features a weak green emission line which was first observed during the total solar eclipse of August 7, 1869.  This defied all explanations until 1939, when Grotrian and Edlen attributed this to the presence in the Corona of highly ionised iron:  Fe XIV  ("iron 14").

An atom of iron would lose 14 of its 26 electrons only under incredibly high temperature:  more than  1000 000 K,  as pointed out in 1942 by the Swedish astronomer Bengt Edlén (1906-1993).  This scorching temperature is still not fully explained.

(2018-04-06)   The  Carrington event  started on August 28th, 1859.
It was observed optically by  Richard Carrington  and  Richard Hodgson.

Carrington and Hodgson were both amateur astronomers.  Their independent reports were published side-by-side in the  Monthly Notices of the Royal Astronomical Society  (November 1859).

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

Solar storm of 1859   |   Richard C. Carrington (1826-1875)   |   Richard Hodgson (1804-1872)
The Carrington Event of 1859 (5:26)  by  John Michael Godier   (2017-03-12).

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  from the surface of the Sun is about 617 km/s.  The so-called  fast  solar wind emanates from the polar regions of the Sun at a speed of 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.  Actually, at an outward velocity  v = 300 km/s = ½ v,  a particle would only travel a distance  (d)  of    15%  of the solar radius  (R)  before falling back:

1 + d/R   =   1 / (1 - v2/vo2 )½

All told, the mass that does escape via solar winds 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  (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 isn't quite so, but close enough)  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  decays  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.

Is the Sun losing enough mass to affect planetary orbits?   by  Sten Odenwald

(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 astronomical 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)  [ formerly considered a planet ].

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...

 The planet Earth, 
 as seen from Appollo 17. 
 December 7, 1972 (2007-10-11)   Earth
This  is home :

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

Ceres  is, by far, the moss massive body in the  asteroid belt  between the orbits of Mars and Jupiter.  It contains about 32% of the total mass in it.  The  current classification  makes Ceres the only  dwarf planet  in the asteroid belt. 

Ceres  was discovered on the first day of the  19th  century  (January 1, 1801)  by Giuseppe Piazzi (1746-1826).  In his  History of MathematicsFlorian Cajori points out that the discovery of Ceres occurred just after the philosopher G.W.F. Hegel had published a "proof"  a priori  that such a thing was not possible!

For nearly half a century, Ceres was known as the  eighth planet  of the Solar system (Uranus, the seventh planet, had been discovered in 1781 and Neptune would only be observed in 1846).

Discovery of Ceres

 William Herschel 
 1738-1822 (2005-08-30)   Four giant gaseous planets :
Jupiter, Saturn, Uranus and Neptune.

On March 13, 1781, using a 6.2" (16 cm) telescope, William Herschel (1738-1822)  discovered the  seventh planet  (now called Uranus). 

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

(2015-06-12)   Chiron and the other  centaurs
Asteroids with decaying orbits, in the midst of the giant planets.

On October 18,  1977,  using images taken two weeks earlier at  Palomar ObservatoryCharles T. Kowal (1940-2011)  discovered a minor planet near aphelion just outside the perihelion of Uranus.  At the time, no other minor planet besides Pluto had been observed this far out.  It was named  Chiron  in 1979 and the rule was proposed that the names of other centaurs be reserved for other bodies with similar charasteristcs.

2060 Chiron  had previously been photographed as early as 1895 and those records helped determine its eccentric orbit with great precision.  It was found that a close encounter with Saturn in  AD 720  had brought Chiron's previous semimajor axis of  14.4 au  down to its current value of  13.7 au.

Chiron is at least 130 km un diameter.  It originates from the  Kuiper belt  and will become a short-period comet in the next million years.

Likewise, other centaurs have relatively unstable decaying orbits.

(2008-09-01)   The Discovery of Neptune   (September 1846)
Urbain Le Verrier (1811-1877; X1831)  scooped  John Couch Adams.

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

Urbain Le Verrier (1811-1877; X1831)   |   John Couch Adams (1819-1892)   |   Discovery of Neptune

(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 of its size, Pluto has the largest satellite of the Solar System  (unless 2003EL61 turns out to consist of two similar bodies in a very tight orbit).  This moon 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.

After 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 appeared mostly cultural 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.  Although the status of Pluto as a planet was reaffirmed by the International Astronomical Union in 1998, Pluto fell from grace on 2006-08-24, when a new definition of a planet was adopted which rules it out...

Some Large Kuiper Belt Objects  (KBO)
DiscoveryNameIDDiameter Period
of Orbit
2003-10-21 Eris 2003UB3132800 km55744°  
1930-02-18 Pluto 2360 km248.0917.2° 6.38725
2005-03-31 Makemake 2005FY92000 km30729°  
2004-02-17 Orcus2004DW800 km247.520.7°  
2007-07-17 2250882007OR10 1300 km552.5230.70°  
2002-06-02 Quaoar2002LM601280 km2887.983°  
2003-03-09 Haumea 2003EL611200 km285.3328.194° 0.16315
2000-11-28 Varuna2000WR106  900 km28217.2° 0.26434
2001-02-22 Ixion2001KX76  407 km24919.6°  
1998-11-19 Chaos1998WH24  372 km30912.1°  
1992 157601992QB1  283 km29112.1°  

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

Eris is the largest known  dwarf planet; it's bigger than Pluto  (which ceased to be an official planet on 2006-08-24).  Prior to the official adoption of its name (on 2006-09-13), Eris had been known as Xena  (or "the tenth planet")  to its discoverer, Mike Brown, and many others.  Dysnomia  (the satellite of Eris discovered on 2005-09-10)  was previously dubbed  Gabrielle  (after Xena's sidekick in the eponymous TV series).

Eris  is the name of the Greek goddess of strife, whose Latin name is  Discordia,  as opposed to  Harmonia  (Greek)  and  Concordia  (Latin).

Although a definite conclusion has yet to be reached,  an animated picture of 2003EL61 is consistent with the interpretation that it consists of two kernels in tight orbit (4-hour period) with at least 2 distant moons around them.

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.  Centaurs  being the transitional state.

Kuiper belt   |   Updated list of transneptunian objects   |   Objets Transneptuniens (2001)
ONE picture of all known bodies in the Solar system larger than 320 km   by  Alan Taylor

(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 Sedna2003VB121000 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 inner  Oort Cloud.

90377 Sedna

(2006-10-06)   Latest definition of a "Planet".
Back to 8 planets  (down from 9, 12 or more).

2006-08-16:   Press release from the IAU presents a proposal for 12 planets.
2006-08-24:   Pluto's fall from grace.  Back to 8 planets, after 76 years.

On 2006-08-24, Resolution 5A (amended by resolution 5B for the locution "classical planet") introduced the distinction between the  8  so-called  classical planets :  (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune)  and a new concept of  dwarf planet  which includes Pluto and Ceres (the largest asteroid).

Both types of planets are bodies sufficiently large for gravitation to overcome rigid body forces, so that a nearly spherical shape is obtained.  Neither type can be a satellite of a larger planet.  However, a new condition was imposed that a [classical] planet must dominate its orbital neighborhood and it should have cleared it from other bodies  (by collision or by capturing them as  satellites).  That last requirement does not apply to  dwarf planets.

By that new definition,  Pluto  is now a dwarf planet, which is merely taken as the prototype of trans-Neptunian dwarf planets, for which the companion resolutions 6A and 6B introduce the denomination of  plutonian objects.  An IAU process was instated to decide between the status of  dwarf planet  or "Small Solar System Body" in borderline cases.

Eris (2800 km) is the largest dwarf planet.  It's the fact that Eris is larger than Pluto which prompted the new classification.  The largest Kuiper belt objects (KBO) tabulated above should also be classified as  dwarf planets.

1 Ceres (950 km) is by far the largest asteroid in the main belt and it's definitely considered a  dwarf planet.  The asteroids  4 Vesta  (525 km)  and  2 Pallas  (544 km)  could eventually be assigned the same status.

Wikipedia:   Dwarf Planets

(2010-04-24)   In 7837, Neptune and Pluto come "close" to each other...
Can Neptune eventually capture Pluto?

I played around with a  nice clockwork model  of the solar system, until I found a "near miss" of Neptune and Pluto in the year 7837  (when their planar locations actually seem to coincide).  Other people made the same remark.

Of course, this is mostly an optical illusion, since the two orbits do not really intersect  (their closest points are 2 billion kilometers apart).

Nevertheless, as each such close encounter can only reduce the distance between the two orbits, we may wonder if the dynamics of the situation is such that Neptune will eventually capture Pluto.  The answer is  no  because the close encounters of Neptune and Pluto are not powerful enough to destroy at once the  stable  3 to 2 ratio in their orbital periods, which prevents a slow drift to the 1 to 1 ratio that would be needed before Pluto could become a satellite of Neptune.

(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 between 50 000  and  100 000 au  (with ludicrous precision, one light-year is  63241.07708426628... au)  The diameter of that spherical cloud is thus often quoted as being  three light-years.

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