The term dwarf planet was introduced in 2006, the same year I started teaching astronomy. It has been a source of confusion for students ever since. The confusion lies not only with the re-designation of Pluto, but also that an asteroid is considered a dwarf planet while the other four objects so designated lie in the Kuiper Belt outside the orbit of Neptune. The term dwarf planet was something of a compromise for those reluctant to modify Pluto’s status as a planet. As the living memory of Pluto as a planet fades, I suspect the term dwarf planet will fade as well. So how should we categorize the objects that lie in our Solar System?
Rather than looking at the shape and size of these objects, I prefer to examine how they were created in the solar nebula that formed the Solar System. When we divide the solar nebula into concentric circles, each section formed a distinct type of object. The solar nebula concept was first hypothesized by Immanuel Kant in The Universal Natural History and Theories of the Heavens published in 1755. As often is the case in astronomy, it took awhile for the ability to verify the theory to emerge. In this case, the process spanned some three centuries.
Part of the holdup was determining if the Sun, planets, and asteroids are all the same age as they must be if formed together in the solar nebula. A model of stellar evolution had to be created and that required Einstein’s relativity theory to explain nuclear fusion. This model puts the Sun’s age at 4.6 billion years. The age of the Earth also had to be determined and given the amount of erosion that takes place on the surface, finding rocks from Earth’s early days is difficult. Nonetheless, radiocarbon dating has put the age of zircon found in Australia at 4.4 billion years. This result also matches up with the age of the oldest Moon rocks brought back from the Apollo program. Also during the 1970’s, Russian physicist Victor Safronov formulated a modern solar nebular theory to compare the evidence against.
The Solar System began when a rotating interstellar cloud containing gas and dust grains began to collapse under its gravity. The rotation of the cloud caused this collapse to create a disk. The bulk of the matter was still in the center of the solar nebula and this is where the Sun materialized. Today, the Sun contains 99.8% of the Solar System’s mass. There is no difficulty categorizing the Sun as a star as nuclear fusion occurs in its core. The difficulty comes in the layers of the solar nebula outside the Sun.
The first concentric ring around the Sun is where the inner, rocky planets are located. These would include Mercury, Venus, Earth, and Mars. In this zone, as the Solar System was forming, the Sun kept temperatures warm enough to keep hydrogen and helium from condensing. As these two elements comprised 98% of the solar nebula, only trace amounts of heavier elements were left to construct planets. This explains the smaller size of the rocky planets. However, they were still large enough to become spherical in shape. The gravity of the planets pulled equally inward from all sides, overcoming the internal mechanical strength of its constituent material forming a sphere.
At the outer edge of this zone beyond the orbit of Mars lies the asteroid belt. During the epoch of the solar nebula, there was enough material here to form a planet. However, the presence of Jupiter’s gravitational influence caused enough disruption to keep this material from coalescing into a single planet. Today, there is not enough matter here to form a body the mass of the Moon. Nonetheless, one asteroid, Ceres, was large enough to become spherical in shape. As such, it was designated as a dwarf planet in 2006. When discovered in 1801, is was classified as a planet and remained so until the mid-1800’s. Here you can see the ephemeral nature of this categorizing. Ceres was in fact the first object discovered in a belt consisting of over 1 million asteroids. Once it was understood Ceres was simply the most visible of a large number of asteroids, its classification was changed. In this, Ceres is very similar to Pluto but their point or origin makes their physical makeup very different.
Between the orbits of Mars and Jupiter lies what is called the frost line. Beyond the frost line, both heavy elements and hydrogen compounds such as water, methane, and ammonia condensate (convert directly from gas to solid). As the hydrogen compound ice particles and rocky material began to stick against each other, they eventually grew large enough to gravitationally attract the surrounding hydrogen and helium gas. In this region, the hydrogen and helium gases’ temperature was colder than inside the frost line. Colder gases move with slower velocity than hot gases and this enabled planets outside the frost line to trap huge amounts of hydrogen and helium. Consequently, being outside the frost line allowed the outer planets to grow significantly larger than the inner planets. Thus, the gas giant planets Jupiter, Saturn, Uranus, and Neptune bear little resemblance to Mercury, Venus, Earth, and Mars as you can see below.
The third concentric ring in the Solar System beyond the orbit of Neptune is the Kuiper Belt, of which Pluto is a member. Also, short period comets such as Halley’s are thought to originate from the Kuiper Belt. These objects differ from the asteroid belt in that they are more icy than rocky in nature. This makes sense as the Kuiper Belt lies beyond the frost line where hydrogen compounds can condensate. Pluto was the first Kuiper Belt object to be discovered in 1930. It stood alone until 1992 when the second Kuiper Belt object was found. While Pluto is highly reflective, most Kuiper Belt objects are dark, in fact, darker than coal. That, along with their small size makes them difficult to detect. To date, more that 1,000 Kuiper Belt objects have been discovered giving the Solar System a third zone of objects orbiting the Sun.
Looking at the above image, it is tempting to think that the Kuiper Belt objects were formed beyond the orbit of Neptune. However, the origins of the Kuiper Belt are still a matter of debate among astronomers. As these are icy bodies, they originated beyond the frost line, but precisely where is uncertain. One theory, called the Nice model, postulates these objects are left over remnants from where the gas giant planets formed and pushed outward by the migration of Neptune’s orbit beyond Uranus. This model explains Kuiper Belt objects that have highly elliptical orbits but not those with circular orbits. As it is estimated some 200,000 objects exist in the Kuiper Belt, there is quite a bit of discovery and mapping to perform to pin down the origins of these objects.
Beyond the Kuiper Belt is the Oort Cloud where long period (orbits that last thousands of years) comets are thought to originate. The Oort Cloud consists of trillions of icy bodies ranging from 1-20 km and extends about 1 light year from the Sun. To date, astronomers have not directly detected the Oort Cloud but we have observed long period comets traveling through the Solar System at different angles indicating an origination point from a spherical cloud. Like the Kuiper Belt, models have determined these icy objects formed beyond the frost line near the gas giant planets and were ejected by the gravity of these planets to their current location.
If the solar nebula existed 4.6 billion years ago, how can we prove this theory is correct? We cannot observe the formation of our own Solar System, but we can observe, thanks to the Hubble and the next generation ALMA radio telescope, stellar and planetary systems forming around other stars. Below is perhaps the most iconic image taken by the Hubble, the Pillars of Creation in the Eagle Nebula located 7,000 light years from Earth. This is a large (the column on the left is four light years long) interstellar gas cloud acting as a nursery for new stars. In fact, ultraviolet radiation from newly born stars eats away at the dust cloud which gives it its shape.
Below is an image of a spinning protoplanetary disk in the Orion Nebula 1,500 light years from Earth. The spinning motion has flattened the dust cloud to a disk shape. The disk contains dust grains that will clump together to form planets, asteroids, and small icy bodies such as Kuiper Belt objects.
The next image is planet creation in process around HL Tauri 450 light years from Earth. Taken by the ALMA radio array in Chile, you can see the gaps in a protoplanetary disk cleared of dust by planets forming in the rings. This is direct evidence of stars and planets creation matching up with the solar nebula theory of how our Solar System formed.
Getting back to the first point, when thinking how to categorize Solar System objects, it is best to consider how these objects formed. The term dwarf planet covers objects that originated inside and outside the frost line in the solar nebula and is of little use here. And as I mentioned before, most likely will be discarded as the collective memory of Pluto as a planet fades. It is a transition term much like Ceres was referred to as a minor planet between its time as a planet and asteroid. As such, I do not consider it a good point of emphasis when learning about the Solar System. Instead, I would summarize as follows:
Objects formed inside the frost line: Rocky planets with thin atmospheres, rocky asteroids, some of these asteroids are large enough to become spherical in shape, but most are not such as Eros below.
Objects formed outside the frost line: Gas giant planets with small icy-rocky cores and large atmospheres, small icy bodies such as the Kuiper Belt and Oort Cloud objects. Some, like Pluto, are large enough to form a spherical shape but most are not.
The Solar System is not static: After these objects are created and the solar nebula was dissipated by the young Sun’s solar wind, gravitational perturbations caused migration of these objects. In our Solar System, the orbital resonance of Jupiter and Saturn caused Neptune to migrate outward and took the Kuiper Belt objects with it. However, around other stars, Jupiter sized gas giants have migrated inwards to occupy orbits closer to their host star than Mercury is to the Sun. Over the course of 4.6 billion years, the Solar System has been a dynamic place.
Our lifetimes are very small compared to the cosmic time scale and thus, we tend the think of the Solar System as a static system. Nonetheless, we do see migrations of objects whenever a comet pays us a visit from the outer reaches of the Solar System. By classifying objects in the Solar System by their composition, it allows you to understand how the Solar System formed and what path those objects took to reach their final destination. And that is more important than worrying if a celestial body is a planet or a dwarf planet.
*Image atop of post is sunset over the mountains of Pluto taken 15 minutes after New Horizons closest approach. Credits: NASA/JHUAPL/SwRI.
