The Discovery of the Asteroid Belt

The euphoria over the discovery of Ceres didn't last.

Quickly, more objects were discovered.  Note in this table their semi-major axes:

asteroid number/name
discovery date
semi-major axis
1 Ceres
1801, Jan 1
925 km
2.768 AU
1/10,000 Mearth
2 Pallas
1802, Mar 28
583 km
2.773 AU
3 Juno
1804, Sept 1
249 km
2.671 AU
4 Vesta
1807, Mar 29
555 km
2.362 AU
5 Astraea
1845, Dec 8
(3 more)
(1 more)
(1 more) 
(47 more)
(52 more)
(102 more)

Now, we know of more than 9000 named asteroids (with well known orbits) and tens of thousands more with less well known orbits. Of these, the biggest are those four listed above and #10 with d = 443 km and #65  with d = 311 km.  Most have diameters smaller than 150 km.  Almost all have orbits between Jupiter and Mars.

By 1802, with the discovery of Pallas, the astronomy community was calling these objects "minor planets," not planets.  Soon they would be renamed "asteroids" and the region where they are located would be called the asteroid belt.  Thus, within a year of the "missing planet" being discovered, it became clear that there was no missing planet to be found.  There is no planet in the asteroid belt.

Or is there?  Do any of the asteroids meet our criteria for planet status?  They all have primary orbits around the sun.  They all are too small to be stars or brown dwarfs.  But are they round? Images of Gaspra, Ida, Dactyl, Phobos, Deimos, Eros, Toutatis, and Mathilde reveal that these asteroids are not round.  But these are small objects, the biggest being about 60 km across its longest axis.  [click here for a rotation movie of Eros; ; click here for information on NEAR landing on Eros].  Many moons are also odd-shaped, including Saturn's moons Pandora (114 x 84 x 62 km), Epimetheus (144 x 108 x 98 km), Janus (196 x 192 x 150 km), Phoebe  (d~220km). Hyperion (410 x 260 x 220 km) is clearly more spherical but yet is clearly not spherical. Neptune's moon Nereid (d=340 km) is most certainly not spherical.

How big must an object be to be round? Jupiter's moons are all large spherical objects, the smallest one being Europa with a diameter of 3138 km.  These are quite big compared to asteroids.  What smaller objects do we have in the solar system? Neptune's moon Triton is a bit smaller (d = 2700 km) but is clearly spherical. Working our way downwards through the moons of Saturn and Uranus, we find all of these are spherical:

Titania (d=1578 km)(Uranus)
Rhea (d=1530 km) (Saturn)
Oberon (1523 km) (Uranus)
Iapetus (d=1470 km) (Saturn)
Umbriel (d=1170 km) (Uranus)
Ariel (d=1158 km) (Uranus)
Dione (d=1120 km) (Saturn)
Tethys (d=1060 km) (Saturn)
Enceladus (d=498 km) (Saturn)
Miranda (d=472 km) (Uranus) is pretty round, but has some rough edges;
and Mimas  (d=398 km) (Saturn) is spherical, albeit with one enormous crater and lots of topographic relief.
So we might conclude, fairly safely, that 400 km is a good boundary, above which an object is spherical, below which objects become less so.

What about our asteroids? At least three of them are more than large enough "to be round." Therefore, Ceres, Pallas and Vesta all meet our criteria for being planets, with a few other asteroids being above or close to our size limit.  Yet we don't consider them planets!

Before leaving behind the asteroids and the asteroid belt, for the time being, we should note one other aspect of the discovery of these objects: which ones were discovered first? The biggest ones.  In fact, the very biggest was discovered first.  This isn't surprising because the biggest would reflect the most sunlight and therefore would appear brightest in our telescopes, thereby being easiest to discover. Similarly, the biggest moons of the giant planets were all discovered long before the smaller ones.

... the discovery of Neptune ...