{"id":54632,"date":"2024-02-08T08:33:09","date_gmt":"2024-02-08T08:33:09","guid":{"rendered":"http:\/\/e011e0ca-b0dd-4ce5-8270-0d803ed070df"},"modified":"2024-02-08T12:34:23","modified_gmt":"2024-02-08T12:34:23","slug":"why-pluto-is-not-a-planet","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/why-pluto-is-not-a-planet\/","title":{"rendered":"Why Pluto is not a planet"},"content":{"rendered":"

Is Pluto a planet? If not, why not? What makes a planet a planet, and why was the 9th in the Solar System reclassified as a dwarf planet? <\/p>

By Paul Abel\n <\/p>

Published: Thursday, 08 February 2024 at 08:33 AM<\/p>

\n\n

Pluto is not a planet. You’ve probably heard this statement many’s a time, and maybe even wondered why.<\/p>

Many of us remember being taught at school that Pluto was the 9th planet of the Solar System. So what changed?<\/p>

In a nutshell, Pluto is considered to not be a planet because of its size, its shape, its orbit and the discovery of many other bodies just like it.<\/p>

But let’s take a look more closely at the history of Pluto, is re-classification as a dwarf planet and how this whole debate got started.<\/p>

The most accurate natural colour image of Pluto taken by NASA’s New Horizons spacecraft in 2015. Credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute\/Alex Parker<\/figcaption><\/figure>

Pluto becomes a dwarf planet<\/strong><\/h2>

Pluto’s status was re-classified from ‘planet’ to ‘dwarf planet’ on 24 August 2006 by the International Astronomical Union following a vote of its members.<\/p>

The official act that took planetary classification from Pluto was IAU Resolution 5A, \u2018Definition of a Planet in the Solar System\u2019.<\/p>

But why was Pluto re-classified, what led to the vote ever taking place, and is Pluto indeed a planet?<\/p>

Pluto’s journey from its discovery as the 9th planet in our Solar System to its reclassification as a dwarf planet and Kuiper Belt<\/a> Object is a fascinating tale.<\/p>

What started off as a curious orbital anomaly in the 18th century turned into the discovery of an object that forced us to rethink our ideas of what a planet should be, and ultimately how our Solar System began.<\/p>

Let’s take a look back at how Pluto was first discovered.<\/p>

\"Once
Once the 9th planet in the Solar System, Pluto is now classified as a Kuiper Belt Object. Credit: NASA\/JHUAPL\/SwRI<\/figcaption><\/figure>

The discovery of Pluto<\/strong><\/h2>

By the mid 17th Century, a scientific revolution was in full flow. Johannes Kepler<\/a>\u2019s laws of planetary motion, combined with Newton\u2019s law of gravity<\/a>, provided astronomers with a powerful tool.<\/p>

The motions of comets and planets could be determined with absolute certainty. The prevailing view was of a Solar System that ran like clockwork.<\/p>

By 1821 a curious anomaly had arisen: the recently discovered planet Uranus was not behaving as it should.<\/p>

French astronomer Alexis Bouvard published tables of Uranus\u2019s position, but found that the planet frequently strayed from its predicted path.<\/p>

He realised these discrepancies implied the presence of a large unseen body, tugging Uranus off course.<\/p>

\"A
A view of Uranus captured by the Hubble Space Telescope in 2006. Credit: NASA\/Space Telescope Science Institute<\/figcaption><\/figure>

Both John Couch Adams and Urbain Le Verrier predicted where this new planet might be, the latter also having theorised the existence of a planet called Vulcan<\/a> in order to explain Mercury’s orbit.<\/p>

But it was Johann Galle and Heinrich d\u2019Arrest who claimed victory, and the discovery of Neptune on the night of 23 September 1846 went to them.<\/p>

Mathematicians set to work finalising the details of Neptune\u2019s orbit, but they found the discrepancies in Uranus\u2019s path around the Sun didn\u2019t completely vanish as expected.<\/p>

Something was still causing Uranus to wander. The problem was of keen interest to Percival Lowell, who had established a large observatory in the frontier town of Flagstaff, Arizona, in 1894.<\/p>

Lowell concluded that Uranus and Neptune were being drawn off course by another more distant planet, which he named \u2018Planet X\u2019.<\/p>

\"American
American astronomer Percival Lowell was one of the prominent figures arguing for a planet beyond Neptune that was affecting the orbit of Uranus. Credit: Mondadori via Getty Images<\/figcaption><\/figure>

In 1906 he began searching for it, but after a decade Planet X had eluded him and he died in 1916. The new director of the observatory, Vesto Slipher, continued the search.<\/p>

In 1929, the task of searching for Planet X was given to a recent addition to the Lowell Observatory: Clyde Tombaugh<\/a>.<\/p>

The work was hard \u2013 at night careful photographs were taken of the sky where Lowell had predicted the new planet should be.<\/p>

During the day, the plates were analysed using a \u2018blink comparator\u2019 \u2013 a device that switched back and forth between two photographs taken at different times.<\/p>

Looking through the comparator, stars remained fixed in position but a planet moving in its orbit would appear in a different location in each plate.<\/p>

On 18 February 1930, after months of searching, Tombaugh made a discovery: he noticed a tiny dot moving among the background stars of Gemini.<\/p>

The long search was over \u2013 Planet X had been found at last.<\/p>

\"Clyde
Clyde Tombaugh, the astronomer who discovered Pluto, pictured at a Zeiss Blink Comparator, a machine used in the discovery of Pluto. Credit: Lowell Observatory Archives<\/figcaption><\/figure>

How Pluto got its name<\/strong><\/h2>

Planet X was named Pluto after the classical god of the underworld, but almost immediately there were doubts as to whether Pluto could really be Planet X.<\/p>

It was very faint, and its disc remained unresolved even in the world\u2019s largest telescopes.<\/p>

Pluto was obviously small, perhaps only the size of Earth or even Mars. It seemed unlikely it could have much influence over Uranus and Neptune.<\/p>

Observing the planet was extremely difficult, but a handful of dedicated astronomers continued the work.<\/p>

Gradually, larger telescopes were constructed and observations showed that Pluto was probably covered with methane-ice.<\/p>

In 1978, astronomer James Christy’s work led to the discovery of Charon<\/a>, Pluto\u2019s largest moon.<\/p>

Charon enabled astronomers to better estimate the mass of Pluto, which was much lower than expected \u2013 smaller even than Earth\u2019s own Moon.<\/p>

\"Astronomer
Astronomer James Christy pictured with the photographic plate on which he and Robert Harrington discovered Pluto\u2019s largest moon Charon, in 1978. Credit: U.S. Naval Observatory<\/figcaption><\/figure>

The small size, combined with its unusual orbit, began to cast serious doubt as to whether Pluto could really be a planet at all.<\/p>

The 1990s saw the arrival of CCD cameras<\/a>, which were much more sensitive to light than photographic film.<\/p>

They allowed astronomers to see fainter objects and peer deeper into the Solar System.<\/p>

Using CCDs it became apparent that there were many Pluto-sized bodies out beyond the orbit of Neptune.<\/p>

These objects belong to the Kuiper Belt, a vast field of debris left over from the formation of the Solar System, and Pluto was its largest member.<\/p>

Kuiper Belt Objects are like fossils: their ancient surfaces are like postcards from the past.<\/p>

From 2002 a host of large bodies was found: Quaoar, Sedna, Makemake and Eris.<\/p>

Loud astronomical voices began to debate Pluto\u2019s status. It was argued that if Pluto was a planet, were these other objects planets also?<\/p>

In 2006, the International Astronomical Union (IAU) settled the matter and defined what was required for an object to be classed as a planet.<\/p>

\"A
A computer model of the Kuiper Belt. Credit: NASA<\/figcaption><\/figure>

What makes a planet a planet<\/strong><\/h2>

According to the International Astronomical Union, a planet is a celestial body that:<\/p>

  1. is in orbit around the Sun<\/li>
  2. has sufficient mass for its gravity to overcome rigid body forces so it assumes a hydrostatic equilibrium (nearly round) shape<\/li>
  3. has cleared the neighbourhood around its orbit.<\/li><\/ol>

    One of the conditions is that all material must be cleared from the object\u2019s orbit, something Pluto hasn\u2019t done due to its small size.<\/p>

    Pluto received a new classification: it was a dwarf planet, but this decision is not without its critics.<\/p>

    Pluto may have been demoted, but it plays a vital role in understanding the conditions of the early Solar System.<\/p>

    \"IAU
    IAU members cast their votes at the 2006 General Assembly, and Pluto’s fate is sealed. Credit: IAU\/Robert Hurt (SSC)<\/figcaption><\/figure>

    Arguments for Pluto as a planet<\/strong><\/h2>

    Many argue that if Pluto were to be classed as a planet then so too would many other objects in the Kuiper Belt, and the planet count in the Solar System would rise dramatically.<\/p>

    But what about the arguments in favour of Pluto being recognised as a planet once more?<\/p>

    Pluto was named a planet when discovered<\/strong><\/h3>

    Some argue to keep Pluto as a planet for sentimental reasons: for much of its history, it has been thought of as a planet and therefore should continue to be so.<\/p>

    Issues with the IAU Resolution<\/strong><\/h3>

    Some astronomers point out that there are inconsistencies in the wording of Resolution 5A \u2013 while it is true that Pluto hasn\u2019t cleared its orbit, neither has Earth or Jupiter.<\/p>

    Earth orbits with 10,000 near-Earth asteroids, while 100,000 Trojan asteroids lie within Jupiter\u2019s orbit.<\/p>

    One could argue that both Jupiter and Earth also fail the IAU definition.<\/p>

    It’s large enough to be a planet<\/strong><\/h3>

    Those who hold this view say that a better definition of what a planet is might be to say that any object orbiting the Sun that has a surface area greater than 1,000km can be called a planet.<\/p>

    If this definition were to be adopted both Pluto and Eris would be classed as planets.<\/p>

    This article originally appeared in the August 2015 issue of BBC Sky at Night Magazine.<\/strong><\/em><\/p> <\/body><\/html>\n

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