Dwarf Planet Pluto
For Inzilbeth, who wanted to know more about it. ;)
One of the most common questions we hear on our lectures on our observatory is: why is Pluto no longer called a planet? So here is the answer...
Pluto was discovered in 1930 by American astronomer Clyde Tombaugh. But soon after it was discovered, it became clear that it had many unusual characteristics. One of them is eccentricity of its orbit. While all other planets, with the only exception being Mercury, have the orbits that are very close to circular, Pluto's orbit is quite elongated. In scale in which distance Earth-Sun is 1 meter, Pluto's minimum distance from the Sun would be 29,6 m, and maximum 49,3 m. Deviation of those extremes from the average distance is almost 25%; for comparison, Earth's extremes are only about 1,67% from the average distance.
Another peculiarity of Pluto's orbit is high inclination. Most planets' orbital planes are very close to our plane (called the ecliptic): inclination varies from 0.8 to 3.3, and only Mercury is slightly more inclined - about 7 degrees to the ecliptic. But Pluto's inclination is 17 degrees! That is a lot, when compared to the rest.
Pluto's composition is also "strange" and unlike that of the planets. The first four are mutually similar and are called Terrestrial planets: they are mostly made of metals and sillicate rocks and have high mean density. The next four are gas-giants, they are mutually similar and are called Jovian planets: they are mostly made of hydrogen and helium and have low mean density (I have to add here that they don't have that low density that they are really gaseous; because of their gravity, density and pressure are high enough that their gasses are liquid; those planets are spheres of dense liquid with a solid rocky core). But Pluto is made of rock and different types of ice (water ice and frozen gases); no planet of Solar system is made of ices! Pluto resembled small satellites of big gaseous giants.
And speaking of "small"... yes, Pluto is small. Not that we've always known it, though. When it was discovered, and some years afterwards, it was thought to be the size of Mars. That means, about twice smaller than the Earth (just like the size of Mars, which is on the picture):
But as the years and decades passed, our data became much more accurate, and today we know its true size. Look at the Earth, Moon and Pluto in scale:
Yes, it is smaller than our Moon! While it still had the status of the planet, it used to be told on lectures, in articles and similar occasions that seven different planets' satellites were bigger than planet Pluto (Jupiter's Io, Europa, Ganymede and Callisto, Saturn's Titan, our Moon and Neptune's Triton).
(Another digression: today, Mercury is the smallest planet of our system, and Jupiter's Ganymede and Saturn's Titan are bigger than it!)
Very elongated and very inclined orbit; strange composition; very small diameter... most unusual planet, isn't it?
-x-x-x-
Long ago, in the mid-20th century, astronomers came to conclusion that Solar system did not end with Pluto (which at the time had the title of the most distant planet), but that there must be many many (many, *really* many) small bodies in those very distant, outer parts of the system. Those objects are remnants from the time when Solar system was made: gases and dust that were not "spent" for Sun and the planets. They are small, icy bodies, made of rock, dust and different ices (again, water ice and frozen gases). Btw, if you wonder how gases can be frozen, then I have to remind you that the temperatures in those parts are about -240°C (-400F), so yes, many gases that we know as gases (here on our temperatures and pressure) are frozen when it is so cold. Astronomers calculate that there are billions and billions of such objects; of course, most are far too small, and above all, far too distant, to be observed. Ever.
But not all!
For a long time, it was impossible to see any of them. But in time, our telescopes, cameras, and other astronomical (and computer) equipment became better, so in 1992 we discovered the first trans-Neptunian object (TNO; a general name for an object more distant than Neptune). In the years that followed, we discovered many more (by now, we know more than 1,000). And as we discovered more and more, the debate among astronomers started: can we really call Pluto a planet, or can we degrade it to call it only the biggest TNO? On the congress of International Astronomical Union in 2000, it was decided to call it a planet, simply because of tradition.
But things soon changed. In 2004, an object later named Eris was discovered, and it turned out to be bigger than Pluto (then it was thought that it was some 300 km in diameter bigger; today we know that it is only *very slightly* bigger, but still!), and debate arose again. Better to say, it never stopped. But then, there were new things to discuss. Is Eris to be called a planet? How many planets are there in Solar system? Finally, on 2006 congress of IAU, the definitions were set and the new category in Solar system was introduced.
The planet is an object that:
a) is in orbit around the Sun;
b) is massive enough to be in a hydrostatic equilibrium - that is, spherical - as a result of its own gravity;
c) has cleared the neighbourhood around its orbit (because it is massive enough to do that).
The new category is dwarf planet. It is an object that:
a) is in orbit around the Sun;
b) is massive enough to be in a hydrostatic equilibrium - that is, spherical - as a result of its own gravity;
c) has not cleared the neighbourhood around its orbit (because it is not massive enough to do that);
d) is not a satellite.
So, Pluto has since been called dwarf planet. There are four other objects that match the criteria: one is Ceres, formerly classified as asteroid, and three are TNOs (Eris being one of them, of course). It is considered that several dozens TNOs can also be classified as dwarf planets, but they are not studied enough for now and we don't have enough data about them to be sure about their masses and other characteristics like size and shape. It will probably change after many future explorations.
And that's it... a short story about Pluto and how we came to know it better. Of course, we still don't know as much as we'd like. All planets were visited by at least one space-probe, so we discovered many things about them that way. No probe visited Pluto, so we don't have detailed pictures nor we know as many things as about some other objects. But that is about to change! A probe called New Horizons, equipped with many scientific instruments, will arrive to Pluto in 2015. And that's not all; New Horizons will go forward, to outer parts of Solar system, and will explore some TNOs. We are really looking forward to those new discoveries; we will deepen our knowledge!
A short story about Pluto? *g* Not so short, perhaps; I can talk *a lot* when it comes to astronomy. *g* I hope you are satisfied with the answer, Liz. :)