Published in The Patriot Ledger January 24, 2015
2015 will be a big year for dwarf planets. Two spaceships launched by NASA several years ago will soon be giving us our first close look at Ceres and at Pluto, giving us better photos and more scientific data on each of these dwarf planets than we have had before. But what are dwarf planets? When I was a boy, Pluto was one of the nine planets and Ceres was an asteroid.
In 2006, the International Astronomical Union finally got around to formally defining a planet, and Pluto didnít make the cut. The IAU decided that for an object in our solar system to be called a planet, it had to meet three criteria. First, it must orbit the sun (which rules out moons, which orbit planets). Second, it must be massive enough that gravity made it roughly spherical (which rules out most asteroids and comets). Third, there must be no other objects of comparable size in the neighborhood of its orbit (except its own moons).
The four inner planets, Mercury, Venus, Earth and Mars, and the four outer planets, Jupiter, Saturn, Uranus and Neptune, easily meet all three criteria. But if you pass the first two rules but fail the third rule, youíre now defined as a dwarf planet. Although Pluto is indeed small (about 1400 miles in diameter, smaller than the moon), it was not its size that demoted it to dwarf status. It was that there are many other massive objects in Plutoís neighborhood, a region out beyond Neptune called the Kuiper Belt. Pluto had enough gravity to form into a sphere in its early years, but not enough gravity to clear its neighborhood of other objects.
What about Ceres? Discovered way back in 1801, Ceres is the largest member of the asteroid belt, the region lying between the inner and outer planets, thus between Mars and Jupiter. There are many other asteroids in its neighborhood, so, like Pluto, it fails rule three. But unlike other asteroids, it is nearly spherical and satisfies rule two. Two out of three isnít bad, and by the definitions announced by the IAU in 2006, Ceres became defined as a dwarf planet. When Pluto was demoted, Ceres was promoted.
The spaceship Dawn, launched in 2007, will arrive at Ceres in March, thereby becoming, by a few months, the first mission to study a dwarf planet at close range. But Dawn has already been very busy, having spent much of 2011 and 2012 orbiting and collecting scientific data on Vesta, the second-largest asteroid. Asteroids are of special interest to scientists because they provide information on the early days of the solar system, when the sun and planets were still forming.
Iíll be pleased to see the first good pictures of Ceres in March, but Iím really looking forward to July, when NASAís New Horizons probe finally arrives at Pluto. This probe was launched in early 2006 to study what was then considered to be the ninth planet. A few months later, Pluto was officially removed from the list of planets. Must have been a blow to the New Horizons research team, whose nine-year mission to Pluto suddenly lost stature. And now, with Dawn arriving at Ceres a few months earlier, they wonít even be the first team to study a dwarf planet.
But Pluto remains special. From its discovery in 1930 to the IAU decision of 2006, it was the ninth planet. Itís way out there Ė nearly four billion miles from the sun. It has five known moons, and perhaps New Horizons will find more. It may be only a dwarf planet nowadays, but itís a lot bigger than Ceres, which is still only an asteroid to me.