When at New Year’s eve of 1801 - the first night of the new century - Guiseppe Piazzi watched through his telescope and saw a tiny point of light that no star map had registered, he could hardly gave anticipated how, by this discovery, he revolutionised our understanding of the structure of our solar system.
Up to then, only the sun, seven planets (the planet Uranus had been discovered by F. W. Herschel as late as 1781), their moons and a few comets had been known. When Ceres was discovered, it was first considered a comet, then it was classified as a planet. It was supposed that Ceres was that very world that had been predicted by astronomers for long to fill the gap between Mars and Jupiter. Within a short time, Pallas, Juno and Vesta were also discovered, that orbit in the same gap between Mars and Jupiter. Thus our planetary system grew to 11 planets. 38 years went by, however, until Astraea was found in 1845 (also between Mars and Jupiter), followed by Neptune in the year after. Now we reckoned even 13 planets.
When in 1847, the first detection of Hebe initiated a deluge of new discoveries, the true nature of these objects between Mars and Jupiter was finally acknowledged, and scientists coined the terms asteroids, planetoids or minor planets for them, took them out of the list of planets and numbered them by order of discovery, beginning with (1) Ceres. Now there were only eight major planets left, including Uranus and Neptune, while the number of minor planets steadily grew. When Clyde Tombaugh discovered Pluto in the vastness beyond Neptune in 1930, it was listed as a ninth planet.
Only sixty years after, in the nineties of the last century, evidence was found that the asteroid belt between Mars and Jupiter was paired with another belt of minor solar system bodies beyond Neptune, now called the Kuiper Belt. The objects there consist mainly of ice, mixed with silicates and organic compounds. Some of them are wider than 1000 km, meaning they are larger than Ceres. And only then it became obvious that Pluto is nothing else than another of these bodies, now called TNOs (Trans-Neptunian Objects) or KBOs (Kuiper Belt Objects).
In summer 2006, the International Astronomical Union, or IAU, during its general assembly in Prague decided for a new definition of the term planet. Since then, a planet of the sun has to meet two criteria: 1.) It must be in hydrostatic equilibrium on account of its gravity, i. e. be spherical. 2.) It must be the dominating object in its sphere of influence and have its orbit cleared of other objects.
Though Pluto is perfectly spherical, as the images of the New Horizons space probe have impressively shown us, it fails on the other criterion, because it shares its orbit, intersecting that of Neptune, with about a third of the newly discovered TNOs. Therefore Pluto was no more a planet. Not to degrade it too lowly, however, a new category was designed for it: the dwarf planets. Though they must fulfil the first criterion, to be spherical, they may share similar orbits with a variety of objects. Within the scope of this new definition, spherical Ceres, reckoned as a minor planet for 150 years, has suddenly become a dwarf planet.
Ceres is the largest body is in the asteroid belt between Mars and Jupiter. With a diameter of 1000 km, it is about twice as wide as the next largest asteroids, Pallas and Vesta. About one third of the total mass of the asteroid belt is included in the mass of Ceres.
Once it had been believed that the asteroids were fragments of a larger planet that had orbited between Mars and Jupiter and burst in a cosmic disaster. This hypothetical planet, however, could not have been very big: If you could assemble all the minor bodies of the asteroid belt on Ceres, its diameter would grow only by about half its current size. The asteroid belt simply does not contain enough material to provide for a larger planet. Its total mass amounts only to about half a thousandth that of Earth or about 4 % of the mass of our Moon. In the early stage of the solar system, the gravity of the rapidly growing proto-Jupiter had sucked too much material away from this area so that no proper planet could have developed there. Ceres is only a protoplanet that had not been given a chance to accrete into a true planet.
To understand these processes better, that is why the Dawn mission had been launched. (The name was chosen as a reference to the dawn of the solar system because the mission was intended to provide us insight into the origin of planets.) The camera on board of Dawn has been built in Germany, by the same team that had been in charge of the camera on the comet probe Rosetta. It is a masterpiece of German engineering that should make us proud and the product of long-standing experience with cameras on probes like Giotto or Mars Express. This should be underlined in an era of receding space budgets and fading interest in natural sciences. The camera has not only produced marvellous images of Ceres but, in the end, even saved the continuation of the Dawn mission because, when the US administration had stopped the Dawn project because it was surpassing the budgetary limit, it was just this international cooperation that yet kept the project alive.
Join a fascinating trip to Ceres in this book by Andreas Möhn and Metka Klemenčič with its impressive images: to a body that has not fully managed to become a true planet and has been caught in the protoplanetary stage yet given us thereby important knowledge about the dawn of our solar system and the origin of our own planet.
Dr Rainer Riemann
Heidelberg, in March 2017
The English translation is online and available for a sample read:
https://www.academia.edu/32503897/Leseprobe_Ceres_GB.pdf
The complete printed book is available here: https://www.amazon.com/dp/1545462933/ref=sr_1_1…
and the eBook is here: https://www.amazon.com/dp/B071CVXXKH/ref=sr_1_1…
Our solar system does not have only eight major planets. Unfortunately, this discussion presents only one side of the debate on planet definition. The IAU decision was adopted by just four percent of its members, most of whom are not planetary scientists but other types of astronomers, and was rejected by an equal number of professional planetary scientists immediately in a formal petition. These scientists instead adhere to the geophysical planet definition, which rejects the notion that an object must clear its orbit to be a planet, focusing instead on the object's instrinsic properties rather than its location. According to the geophysical definition, a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. If an object is not a star itself and is large enough and massive enough to be squeezed into a round or nearly round shape by its own gravity, it is a planet.
AntwortenLöschenAccording to this definition, Ceres IS a planet, and its 19th century demotion was in error. That is understandable since 19th century telescopes could not resolve Ceres into a disk, so no one knew it is spherical and rounded by its gravity. Today, we know this, so we know it meets the geophysical definition of planet.
Dwarf planets are planets too, in spite of the controversial IAU decision. Alan Stern coined the term "dwarf planet" in 1991 to designate a third class of planets in addition to terrestrials and jovians. The four percent of the IAU who voted in 2006 misused his term by stating dwarf planets are not planets at all. The Dawn and New Horizons missions have revealed Ceres, Pluto, and even Vesta to be complex geological worlds with the same processes as the larger planets. The only difference is they are smaller. A scientist on the Dawn mission even stated, "perhaps Vesta should be seen as the solar system's smallest terrestrial planet." At minimum, it is a protoplanet, a borderline case that is almost but not quite in hydrostatic equilibrium.
It is therefore misleading to say Pluto--or Ceres--are planets no more. That statement is true only if one accepts the IAU position and embraces the dynamical planet definition.
Kuiper Belt Objects that are in hydrostatic equilibrium are both small planets and KBOs, at least according to the geophysical definition. The two are not mutually exclusive. The first tells us where they are while the second tells us what they are.