Why is Pluto not a planet anymore ?

Officially, as of 24 August 2006, Pluto is no longer considered a true planet, but a dwarf planet. Why is Pluto not a planet anymore ?

It was approved by the General Assembly of the International Astronomical Union, which also set a series of guidelines on whether a celestial body could be defined as a planet or a dwarf planet.

Let’s start with these guidelines.

A planet can be defined as a celestial body that:

  • Orbiting the Sun
  • Its mass is such that its gravity can overcome the rigid forces of the body and allows it to assume a nearly spherical shape.
  • He was able to “clean” his orbital belt of other objects of comparable size.

Let’s see how Pluto behaves

Since its discovery in 1930, there has been some doubt that Pluto was a real planet. For one thing, it was very small, much smaller than our Moon.

On the other hand, its rotation around the Sun follows an unusual trajectory and is more like that of an asteroid.

With advances in technology, measurements of Pluto have become increasingly accurate and precise since the 1960s, so much so that it has been “lighter and smaller” from a decade to a decade.

We now know that Pluto, with a diameter of about 2,280 kilometers, is six times smaller than Earth, with a moon about half the size, and even more bizarre, that it travels in an elliptical orbit and orbits Neptune.

Despite its small size, until 1992 it was for all intents and purposes considered the most distant planet in the Solar System.

But it was in that year that some discoveries awoke the eyes of many astronomers.

Study to proof Pluto a dwarf planet

In 1992, David Jew of the University of Hawaii and J. Luu discovered a strange object, named QB1, in 1992, a small icy celestial body similar in size to an asteroid, orbiting the Sun at a distance equal to one and a half times.

Neptune. QB1 was the first clue that other Pluto-like objects (in terms of ice and rock composition) populate the outer reaches of the Solar System.

Since then, about a hundred objects such as the QB1 have been found. This group of objects is now known as the Kuiper Belt, named after Gerard Kuiper, who first proposed the existence of such a belt.

To date, it is estimated that about 35,000 objects may populate the Kuiper Belt, which is smaller than Pluto but practically identical to it. Then came a turning point in 2003, thanks to a discovery by astronomer Mike Brown, who discovered a celestial body larger and more massive than Pluto.

Pluto in regions where it was found: a possible tenth planet provisionally named 2003 UB 313. And later its name was changed to Eris.

Therefore, Pluto, which weakened at the third point as a result, was actually downgraded to a dwarf planet. There are other clues to be confirmed that have prompted researchers to deprecate Pluto.

Some more facts about Pluto

In fact, it was observed that Pluto’s reflectivity varies from time to time, indicating the presence of another celestial body in its vicinity.

It is Charon, another celestial body, somewhat larger than half that of Pluto, that does not act like a normal satellite, but forms a true planet-planet binary system.

In fact, they are affected by a mutual gravitational attraction.

Gravity was detected in the space between the two bodies at a distance of 950 km from the surface of Pluto.

The study of the motion of this binary system has also shown that the masses of Pluto and Charon are about one-eighth the mass of the Moon.

So all of this has suggested that Pluto is not a true planet, but may be a satellite that has escaped the gravitational pull of another planet or asteroid, for example.

The final conclusion:

Its composition is 70% rock and the rest (believed to be) ice more similar to the satellites of Neptune than to the gaseous outer planets (Jupiter, Saturn, Uranus and Neptune).

As mentioned above, the square also works against this.

  • Pluto completes its revolution in about 247.9 Earth years (so it has not yet completed a full orbit since its discovery), with a distance of 7.37 billion kilometers from the Sun and 4 The range is from perihelion to 0,43 billion km.
  • Therefore, at a certain time of year, it is closer to the Sun than Neptune. Its orbit still has two features that distinguish it from the other planets.
  • It has a marked eccentricity (0.24, about 15 times that of Earth) and it is greatly inclined with respect to the plane of the ecliptic, which is Earth’s orbit. (17.14 degrees). , for example, compared to 2.48° for Saturn).

Why is Pluto no longer a planet?

The problem dates back to the 1970s, when astronomers began to use more advanced techniques to estimate the size and mass of celestial objects found in space.

From that point on, with each subsequent measurement, Pluto went out of the norm, getting smaller and smaller and lighter. Scientists now know for sure that it has a diameter of 2,280 km, a moon that is half its size and follows an elliptical orbit, crossing the orbit of Neptune.

Pluto was considered a planet until 1992. In the early 1990s, David Jewish and J.J. of the University of Hawaii. Luu identified an icy celestial body, called QB1, which is similar in size to that of an asteroid.

This discovery allowed astronomers to understand that there were other celestial bodies similar to Pluto in space. These objects are called the Kuiper Belt in honor of Professor Gerard Kuiper, who first identified this belt as composed of 35,000 celestial objects with a diameter of about one hundred kilometers.

Pluto is very similar to all of the objects found in this band, the only difference being its reflectivity, which allows it to be much brighter. In 2005, scholar Mike Brown discovered a celestial body more massive and larger than Pluto.


The potential new planet, later called Eris, lowered Pluto’s status. In 2006, the General Assembly of the International Astronomical Union made a final decision, declaring that it would be reclassified exclusively as a dwarf planet, along with Eris, Ceres, Haumea and Makemake.

On this occasion, the International Astronomical Union decided to create guidelines for the division of celestial bodies between planets and dwarf planets. A planet is defined as if: It orbits around the Sun, its mass is such that its gravity is able to overcome the forces of a rigid body and its shape is spherical.

He is able to free his orbital bar from any other comparable object.

Because it did not meet these criteria, Pluto was considered a dwarf planet and the first in the category of transneptunian celestial bodies.

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