When I was in high school we knew about nine planets in our solar system, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto.
Since that time it was discovered that Pluto had a moon, Charon, about half the size of Pluto (1170km verses Pluto 2270km) and really could be considered a double planet system. Two more moons of Pluto were discovered in Hubble images, but they are tiny, between 60-200km in diameter.
Ceres which orbits between Mars and Jupiter, discovered in 1801, was considered to be an asteroid, but with a diameter of 950km, a spherical form, and differentiated core and mantle, it really shares characteristics that normally would be attributed to a planet.
Vesta, about 538km in diameter, also orbits between Mars and Jupiter, and has a basaltic surface indicating that it once had lava flows and a molten core. It has many features resembling our moon and the terrestrial planets.
Then there is Eris, originally 2003 UB313, slightly larger than Pluto at about 2500km in diameter, and on an orbit that is more elliptical. At it’s closest approach (280 years from now), it’s orbit takes it just inside of Pluto’s orbit, but presently it is about 2.6 times further out. The spectrum of light reflected off of Eris is nearly identical to that of Pluto, suggesting a surface largely made up of frozen methane, but Eris is more reflective. Eris has a moon, Dysnomia, estimated to be about 150km in diameter. I preferred Xena and Gabrielle myself, but the IAU had to ruin that.
Quaoar is about 1250km in diameter and has a similar orbit but with an inclination almost opposite that of Plutos relative to the orbits of the major planets. Part of Quaoar’s orbit brings it inside the orbit of Pluto and part of the time it is outside of Pluto’s orbit. The orbit of Pluto is in a 3:2 resonance with Neptune, but this is not the case with Quaoar. While Pluto and Eris have relatively high albedos (reflectivity) of .68 and .86 respectively, Quaoar is very dark with an albedo of only .1 suggesting less surface ice.
Orcas has a diameter of about 1600km, and has an orbit almost identical to Pluto. Like Pluto, it has a 3:2 resonance with Neptunes orbit, and nearly the same degree of eccentricity as Pluto. The diameter of Orcas isn’t known precisely yet because it’s albedo has not been precisely determined.
Varuna, orbiting at a distance of approximately 43 AU, is believed to have a diameter of around 900km. It is not very reflective with an albedo of only .07 indicating very little surface frost.
Ixion has an orbit very similar to Plutos, but where Plutos perihelion (the point in it’s orbit closest the sun) is above the ecliptic, Ixion’s is below. Like Pluto and Orcus, Ixion is in a 3:2 resonance with Neptune. It’s diameter is estimated at 800km, and albedo at .15. It has a reddish color.
2003 EL61 is undoubtedly one of the strangest objects out there, it is cigar or egg shaped even though it’s length is nearly 2000km, it has not collapsed to a spherical state like other similarly sized and even much smaller bodies have. 2003 EL61 also has two moons, one approximately 300km in diameter and one approximately 150km in diameter.
In addition to the large asteroids between Mars and Jupiter, there are numerous objects between the orbits of the gas giants, most of them have diameters of 200km or less, and then many objects out in the vicinity of Pluto in the Kuiper belt, the major ones noted above, there is now believed to be hundreds, thousands, and perhaps even tens of thousands of minor planets in the Oort cloud, a spherical region of space extending about a light year from the Sun. Based on statistical analysis it is believed that there may be as many as a dozen frozen Earth sized objects in this region and perhaps even some gas giants. There is some speculation that there may even be a brown dwarf solar companion out in this vast region.
I find the whole situation very fascinating. We, humanity, if Bush doesn’t go and invade Iran and start WW III, are very close to harnessing hydrogen fusion. With such an abundant source of energy at our disposal, we might be able to thaw some of these distant worlds and make them livable.
Prior to knowing these worlds existed, I had dreams of traveling to minor planets in our solar system that were in the process of being terraformed. In my dreams they were at a state where there was an atmosphere that was breathable but not ideal, where the temperature was survivable but less than ideal, and where it was possible to grow food crops but other than cultivated vegetation the land was largely barren.
Now with the 7th (and last before a commercial level power reactor for naval propulsion) Bussard polywell reactor is built and in the processes of being tested, with the discovery of all of these minor planets, this no longer seems like an impossibility.
The density of these minor planets often lies between that of ice and dirt suggesting a mixture of the two. I wonder how long it would take to defrost these small planets sufficiently to grow something. There is a hint that some of these may already contain liquid interior oceans due to tidal forces or the decay of radioactive elements.
If there are tens of thousands of these objects and they extend a quarter of the way to the nearest star, then there is a lot of real estate available to advanced civilizations. There is some evidence that advanced civilizations may have arisen on Earth in the past and I can’t help but wonder if some of these distant minor planets might already be colonized by our ancestors.