All of the recent Space Exploration glory has been taken by The European Space Agency (ESA) with it’s comet-chasing probe Rosetta. However, from today (2015/03/06) NASA can hold it’s head high when, after a 7½ year journey, it’s Dawn probe has finally reached it’s target Ceres & gone into orbit around it.
Ceres is a dwarf planet that lies between the orbit of Mars and Jupiter.
Wrong question. We need to start with “What is a body of planetary mass?” (which is given the horrible name ‘planemo’) and can then ask “What is the difference between a planet and a dwarf planet?”
This was defined by the International Astronomical Union (IAU) in 2006. Before that date, everyone accepted that our Sun had 9 planets. After that date, Pluto was downgraded into a dwarf planet & thus the Sun had only 8 planets:
A Planet needs to have 3 properties to be true to be deemed a Planet:
Hydrostatic equilibrium: to try to keep this one simple, it means that the body is big enough to have become ellipsoidal (ovoid).
This is purely a function of size & composition. If composed of liquid/gas it will more easily become close to a sphere than if mostly composed of rock. However, it would then still need to be at a minimum size so that gravity will prevent the material from evaporating into space. Celestial bodies with a rocky/metallic composition also need a large size/gravity to be able to compress & deform their ultimate shape.
All the planets are spinning, and that spin prevents their shape from being perfectly spherical. The most normal shape – as is true with the Earth – is oblate spheroid, with the equator being larger than expected (if a perfect sphere).
Clearing the Neighbourhood: this is also a question of size, and means that the planet will not share it’s immediate orbit with any other bodies than it’s own satellites (moons).
A Dwarf Planet has the first 2 properties true, but not the 3rd:
Pluto shares it’s orbit with the Kuiper belt objects such as the plutinos (pluto-like objects named after Pluto itself). Both Pluto & the plutinos have highly elliptical orbits, rising high out of the normal elliptic plane shared by the other planets. Ceres lies within the asteroid belt, and thus shares it’s orbit with a very large number of other bodies.
5 dwarf planets are currently accepted by the IAU. It is suspected that another 100 of the currently known objects will fit the definition, and estimated that another 200 will be found within the Kuiper belt and perhaps 10,000 beyond that belt.
Many of the satellites (moons) are also in Hydrostatic equilibrium (but not in independent orbit around the sun) (our Moon is a good example of this):
The Dawn probe has been taking shots of Ceres as it has got closer, and this has provoked some excitement due to one crater in the Northern hemisphere that shows 2 very bright spots:
No-one currently has any certainty over what could be causing these bright shapes.
The probe now has about a year and 6 months to conduct the first close research into a dwarf planet. It will end it’s days as a satellite of Ceres, for all time (at least a hundred years).
--------- Alex Kemp