why pluto is not considered as a planet for now?

Question

i'm wondering why scientist considered pluto as a planet for past long time but now,they tell that pluto is not a planet..! what is the thruth behind this?

Answer 1

The easy answer is already given by doug: there is a official definition set by the Internationl Astronomical Union, and Pluto does not fit the definition.

Pluto is in an area of the solar system where there appears to be lots of other objects just like Pluto, with orbits that cross each other's paths.

The same thing happened before: in the 17th century (Galileo's discovery of satellites around Jupiter) and in the 19th century (1801 to 1855).

The word planet comes from the Greek: aster planetes, meaning wandering star. Any (permanent) source of light that moved through the sky was called a planet (Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn -- although the Greeks would have used the Greek names, of course).

When Galileo discovered the satellites around Jupiter, he called them new planets. Soon after, as the dust settled around the new model of the solar system (putting the sun at the centre and making the Earth a "planet") old words took on new meaning. A satellite, in Latin-speaking Rome, was a person who hanged around someone of importance, doing tasks for him and hoping to get some favors in return. A bit like a groupie around a rock band.

So objects going around the Sun were planets while objects going around planets were called satellites (at first: the Moon plus 4 around Jupiter and a few around Saturn -- depending on the date when you decide the dust settled...)

When Ceres was discovered (1801) it was a new planet. Located between Mars and Jupiter, it had even been "predicted" by Titius-Bode Law.
http://en.wikipedia.org/wiki/Titius-Bode_law

Then came Pallas, Juno and Vesta. 4 planets in the same general area. Despite more searches, no new planets were discovered in that region of the solar system for some time. Astronomers were OK calling them planets.

Then came Astraea (1845), Iris (1847), Flora (1847), Metis (1848), Hygiea (1849)... There were 15 by 1851.

It became clear that the region of the solar system contained a collection of objects that belonged together but never really formed a planet (they were all much smaller than planets -- in fact, taken together they would have less mass than our Moon).

So Ceres, Pallas, Juno and Vesta were stripped of their "planet" status and became minor planets (a.k.a. asteroids -- because even in telescopes, they looked "point-like" like stars, not round like planets).

When Pluto was discovered, it was thought to be a massive planet responsible for perturbations in Neptune's orbit (astronomers thought that these perturbations "predicted" another planet beyond Neptune).

Turns out that Pluto is way too small to have been responsible for the perturbations and is only one of many similar objects in that region of the solar system. Wanting to avoid a repeat of 1801-1855, the IAU dusted off the definition of planet, refurbished it, and Pluto lost its status. It is now a "dwarf planet", somewhere between a "minor" planet and a "real" planet.

Could the definition of planet change again? Based on history, my guess is "yes". Could Pluto regain its full status as a planet? Possible but improbable.

Answer 2

Raymond's excellent answer gave most of the background info about the discovery of Pluto, how it and other objects were called "planets", what the current definition of planet is, and why it excludes Pluto.

However, your question also asked "why scientist[s] considered [P]luto as a planet for [a] long time" and there is a little bit more to the reasons behind this.

Firstly, Pluto probably should never have been deemed a "planet" in the first place, but it was named in honor of a great astronomer who had predicted a new "Planet X", beyond the orbits of the two outermost known planets, Uranus and Neptune (which are visible only via telescope).

Secondly, one important reason why the definition was revised by the IAU was that dozens of new "extra-solar" planets are now being discovered around other stars, and there must be must be consistent rules for telling which is which, among planets, asteroids, comets, binary stars, etc. (Under the new rules, a planet must be round, and big enough to "sweep" everything out of its path as is orbits a star. As a compromise, Pluto and similar objects are now called "dwarf planets".)

1. The renowned astronomer, Percival Lowell (1855-1916), devoted the last several years of his life searching for "Planet X". Lovell believed that certain discrepancies in the orbits of Uranus and Neptune were due to the gravitational pull of yet another body, further from the sun. In 1930 (14 years after Lovell's death), Clyde Tombaugh discovered a more-distant object which was then designated as a "planet."

Some people suggested "Lovell" as its name, but there was much opposition to using the name of a real person. The mythological names, "Pluto", was chosen partly because it began with the astronomer's initials "PL".

Later, it was found that the mass of Pluto was far too small to explain the discrepancies that had concerned Lowell, and the errors were actually due to incorrect figures for the masses of Uranus and Neptune. Much later, many other objects were discovered that were larger and/or more distant from the star they orbit, but these are not considered to be actual "planets".


2. Extra-solar planets are now being found, orbiting distant stars, but these are all very, very large. (Much larger than Jupiter.)
The Custer Institute (in Southold, NY) is leading an effort to have amateur astronomers, with small telescopes, pool their data together, for computer analysis, to discover new extra-solar planets that are smaller -- and therefore much more likely to support life.

Answer 3

The International Astronomical Union decided that Pluto was no longer a planet. The reason is that while Pluto is round, orbits the sun, and has three moons, it has not cleared (via gravity) the neighborhood of its own orbit of debris. Instead, they decided to classify it as a "dwarf planet".

There is a precedent for this. When Ceres was first discovered in 1801, it was considered to be a planet. After astronomers discovered several other objects in the same orbit, it was reclassified as an asteroid. And, in 2006, it was again reclassified as "dwarf planet" just like Pluto.

Neither Ceres nor Pluto have enough mass (gravity) to accrete all the junk from their orbits. They will be "dwarf planets" until we humans decide otherwise.

See the details below.

RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" [footnote 1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [footnote 2] , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [footnote 3] except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".


Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Answer 4

Dwarf Planet Eris Is More Massive Than Pluto

Science Daily — Aptly named after the Greek goddess of conflict, the icy dwarf planet, Eris, has rattled the general model of our solar system. The object was discovered by astronomer Mike Brown of Caltech in the outer reaches of the Kuiper belt in 2005.

Its detection provoked debate about Pluto’s classification as a planet. Eris is slightly larger than Pluto.

So if Pluto qualified as a full-fledged planet, then Eris certainly should too. Astronomers attending the International Astronomical Union meeting in 2006 worked to settle this dilemma. In the end, we lost a planet rather than gaining one. Pluto was demoted and reclassified as a dwarf planet along with Eris and the asteroid Ceres, the most massive member of the asteroid belt.

Answer 5

The scientists wanted planets to be a rare structure. There are dozens of unnamed objects supposedly larger than Pluto in our solar system. Either they would have to be declared planets or Pluto would have to be stripped of being called a planet.

Answer 6

They changed the rules which define what a planet is.

Doug