why is pluto kicked out of the planet club?

Answer 1

The basic probem Pluto has and had from the outset is that it is smaller than 7 moons in the Solar System: Ganymede, Io, Europa and Callisto (the 4 Gallilean moons of Jupiter) Titan (Saturn's largest moon) Triton (Neptune's largest moon) and our own Moon, all of which were discovered before Pluto.

There is understandable and widespread dismay at Pluto being demoted in status but people need to understand the reasons the IAU had to grapple with definitions and categories at this time:

(1) in 1930 we knew of just one body lying beyond the orbit of Neptune. Now we know of more than 1000

(2) we are discovering asteroids at a rate of 5000 a month

(3) we now know of 200+ extra-solar planets orbiting 170+ other stars, some of which we now know to have asteroid belts

It is conceivable the IAU may create more categories in the future in the light of more discoveries, The moment we find an extra-Solar System planet with extra-terrestrial life on it, for example, I would expect Habitable Zone Planet to be a new category and only Earth and Mars of our local 8 planets to be in it.

We already have the distinction between a terrestrial planet (the inner 4 planets) and a gas giant (the outer 4 planets) and are assessing new extra-Solar-System planets in the light of that distinction and a new category name for the informally-named "hot Jupiters" (i.e. large planets orbiting near to their star at less than 1 AU distance) of which we know several, may not be far away,

As science expands its knowledge, it needs more concepts and categories with which to describe and classify that knowledge, That is perfectly normal and should neither surprise nor alarm us,

Creating new categories and reclassifying known objects in the light of them has happened before: in the 19th Century when the number of planets was pruned from 11 to 7 out of concern that being consistent and admitting other, newly discovered bodies to the planetary club that were similar to the ones they chose to kick out instead would have meant the number of planets could rapidly start to escalate and mushroom out of control,

To understand what is going on now, it helps to understand what went on then,

The number of bodies in the Solar System known to astronomers has been burgeoning for a long time now, but the general public seems unaware of this, given the way people blithely talk of Ceres (discovered 1801) Charon (discovered 1978) and Xena (discovered 2003) having "just been discovered",

There was a similar definitions crisis in the early 19th century and again in the mid-19th Century as the number of known objects in the Solar System started to grow and grow,

By 1807 the 8 Solar System bodies known to classical astronomy (the Sun, the Earth, our Moon and the 5 classical planets known from antiquity, Mercury, Venus, Mars, Jupiter, and Saturn) (1 star, 6 planets, 1 moon) had grown to 26. Uranus was found in 1781 making 7 planets. There were 4 Jovian moons, 7 Saturnine moons and 2 Uranian moons, 14 in all

And then there was the discovery of the first four asteroids. These were 1 Ceres on January 1, 1801, 2 Pallas on March 28, 1802, 3 Juno on September 1, 1804, and 4 Vesta on March 29, 1807,

What were astronomers to call these new objects? They weren't moons as they rotated around the Sun, so they had to be planets, didn't they? As there was, initially, no other category but moons or planets to put them in.

After 2 Pallas was discovered though, Sir William Herschel (the discoverer of Uranus) coined the term "asteroid" meaning "star-like"), in 1802.

But Ceres was meantime assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century until further asteroids were discovered.

So we now had 1 star, 11 planets and 14 Moons, the beginnings of a distinction between major and minor planets and a sense of unease as to what we would do if more asteroids were discovered as the first four were all disappointingly small in size, so did they really belong in the planetary club? (Similar doubts were expressed about Pluto, right from the outset in 1930,)

38 years pass and then in 1845 the asteroid 5 Astraea is discovered and on September 23, 1846 the planet Neptune and a mere 17 days later on October 10, 1846, Neptune's moon, Triton. (We now have 1 star, 12 Planets 15 Moons and 1 non-planetary Asteroid.)

The pace of discovery then starts to really hot up. Four more asteroids in nine months: 6 Hebe on July 1, 1847, 7 Iris on August 13, 1847, 8 Flora on October 18, 1847, and 9 Metis April 25, 1848

Then on September 16, 1848 an 8th moon of Saturn called Hyperion is discovered,

Plus a further 6 asteroids are found in just over two years: 10 Hygiea on April 12, 1849, 11 Parthenope on May 11, 1850, 12 Victoria on September 13, 1850, 13 Egeria on November 2, 1850, 14 Irene on May 19, 1851 and 15 Eunomia on July 29, 1851.

And on October 24, 1851 a 3rd and a 4th moon of Uranus: called Ariel and Umbriel were discovered.

So now we had 42 objects: 1 star 12 planets 18 moons and 11 asteroids. If the latest asteroids were all to be regarded as planets, making a total of 23 planets (and 10 Hygiea was bigger than 3 Juno, just like Xena is bigger than Pluto), it was likely to start getting silly (by 1868 the number of asteroids was to rise to 107 and Victorian schoolchildren would have needed a massive 115-word mnemonic to remember all the names).

The unease grew to a crisis, a redefinition was clearly necessary and an inevitable decision was taken to regard all 15 asteroids as a separate category from planets and Ceres, Pallas, Juno and Vesta were kicked out of the planetary club, just like Pluto has been kicked out now.

There are some clear parallels between the situation in the 1850s and the situation now, Four under-sized runts had obtained planetary status, with seemingly more to follow as they were discovered, creating an overwhelming feeling among astronomers that the currency would be devalued if all these further objects were to then be automatically awarded planetary status. So they cried Whoa! And called a halt. And created a new category, Just like the IAU has now done,

SO HOW MANY OBJECTS HAVE WE GOT IN THE SOLAR SYSTEM NOW?

Stars: 1

Planets: 8

Moons: over 80 known moons of the dwarf planets, asteroids and other small solar system bodies.

(The asteroid 87 Sylvia has 2 moons for example as does the Kuiper Belt Object KBO 2003 EL61.)

AND another 162 moons orbiting around planets: Mercury has none, Venus has none, Earth has 1, Mars has 2, Jupiter has 63, Saturn has 56, Uranus has 27, Neptune has 13.

Kuiper Belt Objects: over 800 (all discovered since 1992).

Trans-Neptunian Objects: over 1000 (includes the 800+ KBOs) i,e, there are 200+ in the Scattered Disk and the Oort Cloud.

Asteroids: Hundreds of thousands of asteroids have been discovered within the solar system and the present rate of discovery is about 5000 per month. As of July 23, 2006, from a total of 338,186 registered minor planets, 134,339 have orbits known well enough to be given permanent official numbers. Of these, 13,242 have official names.

Current estimates put the total number of asteroids above 1 km in diameter in the solar system to be between 1.1 and 1.9 million

So you can see

(a) why some definitions are needed and why reclassification is necessary

(b) how totally unaware of the state of scientific knowledge the general public is and how uninformed people are when they get excited at tales of "3 new planets being discovered" and wonder if there might perhaps be more where those came from,

Finally, these issues need to be seen in the context of the 205 extra-solar planets we now know to exist and the asteroid belts that have now been detected in some of those stellar systems,

Consistency being a desirable thing to achieve in science, whatever definitions and categories the IAU now adopt, they need to be applicable to every star with other objects in orbit around it, throughout the entire universe, That is the context in which Pluto's status is now being discussed,

Answer 2

Short answer - Because science allows you to change or refine your answer if it is incorrect.

Long answer - Most ancient societies thought there were 7 planets: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Then Galileo, Copernicus, and Kepler came along and although it took a while, proved that the Earth and the other planets except the moon revolve around the sun.

So the idea about what a planet is changed and Western society recognized Mercury, Venus, Earth, Mars, Jupiter, and Saturn as the 6 plantes. Later Uranus, and Neptune were discovered and added to the list.

But the asteroid Ceres was also considered a planet at first. After astronomers realized how small it was and that there were thousands of similar objects in the same orbit it was reclassified as an asteroid in a region that was then called the asteroid belt.

The situation with Ceres is almost exactly like the current situation with Pluto. When it was first discovered it was hailed as a new planet but over time we learned it was a much smaller size and more like other non-planetary objects than planets. The other big fact that there is no debate on is that if Pluto was discovered today, it would not be a planet.

So the reason that Pluto was kicked out as well as the sun, the moon, and ceres is because as our knowledge of the universe increases there is a need for us to change and refine the details.

Answer 3

why they think pluto is not a planet

In a dramatic about-turn, the American space agency Nasa has announced that it is considering a mission to Pluto.

The statement comes just months after the agency cancelled all proposals to visit the outermost member of the Solar System.

Tiny Pluto, just 2,274 km (1,412 miles) in diameter, is the only planet that has not been visited by a space probe

Nasa is seeking proposals from principal investigators and institutions around the world to develop a mission. It says there are no restrictions on the launch date but the goal should be to reach Pluto by 2015.

The spacecraft should also fly by the planet's large moon Charon before continuing its mission to the swarms of smaller worlds that comprise the Kuiper Belt (KB), a ring of icy objects beyond the large planets.

Valuable opportunity

When Nasa ditched plans in September to visit Pluto, planetary scientists protested, saying that unless a mission reached Pluto by 2020 at the latest, a valuable opportunity to study the tiny world's atmosphere would be lost for 230 years.

This is because Pluto has a thin atmosphere for only 20-30 years out of its 248-year orbit of the Sun. For the rest of the time, the atmosphere freezes out.

Scientists would dearly love to get a mission off the ground by 2004.

It would normally take about 12 years to travel the 6 billion km (3.7 billion miles) to Pluto but if a mission is launched soon that travel time could be cut to just eight years.

Answer 4

The size is not considered actually. It is because it crosses the path of another planet, Neptune, in its orbit and does not "clear the neighborhood," meaning exactly that. If they did not consider that specific criterion, then comets and other orbital bodies of substantial size would have to be considered as planets. It is a shame that they could not just make an exception, being that it has three moons and all. Even though it is just a technicality, a lot of people take it seriously. Pluto will still be there and will still have its name. It didn't change, just our views on it. Just like French/Freedom fries. It's still the same thing.

Answer 5

Pluto is no longer classified as a planet due to several factors. As the size of it is considerably smaller than other planets, its size is more comparable to other objects in the solar system. It is, afterall, 1/3 the size of our moon. The new criteria for planet classification states that a planet must be able to clear a path through its orbit. That is to be able to order debris. Due to the size of Pluto it is not able. The americans will be gutted its the only one they discovered.

Answer 6

Because a group of scientists decided they needed to re-write the book. I realize that science is constantly evolving & needs to redefine things from time to time. But what difference does it make in the general scheme of things whether or not Pluto is a true planet? Why not let it be a planet? I'll always think of it that way.

Answer 7

Pluto was kicked out because the people in NASA and other space clubs decided it was too small to be a planet, but I disagree. If Pluto got kicked out then Jupiter, Saturn, and Neptune should be kicked out because their all made of gas.

Answer 8

Because a group of people who think they know what they are talking about have decided pluto is not a real planet...
Who cares??:? Not me, not pluto nor most people on our planet...
It's totally meaningless.. just like the people who are willing to waste their time thinking about this and coming to a "momentous" decision...
They should go get a life.

Answer 9

It is not just because of Neptune being in it's orbital range, there are many other bodies, of similar size to Pluto, that are also in the same area as Pluto.

Answer 10

that disturbs me.. I have always learned that pluto is a planet.. now it is a dwarf planet.. what does that mean.. It is still a planet or do they plan to call it a comet... a star.. a meteorite? To me, it is still a planet... they said it was not a planet because it is too small.

Answer 11

It's because Pluto does not 'dominate the area of its orbit', which is the new criterion. The other eight planets are the largest objects in their orbital range, but Pluto's eccentric orbit means it passes close to Neptune's orbit; therefore it is not the dominant object.