Pluto and Charon
Images of Pluto and Charon from the Hubble Space Telescope

Is Pluto A Planet?

Is there really a simple answer to the thorny question of the status of Pluto?

Yes there is!  But there's more to it than that ...


Pluto - Earth's distant cousin

The planet Uranus was discovered in 1781, but with time it was noticed that it wasn't behaving exactly as predicted. Careful calculations suggested this was due to the gravitational pull of a previously unknown planet, and in 1846 the planet Neptune was discovered as a result. However it was felt that another planet was required to fully explain the movement of Uranus, and Percival Lowell founded an observatory in Flagstaff, Arizona, and began a project to search for this more distant planet.

In 1929 the task of searching for the mysterious "Planet X" was handed to 22-year-old Clyde Tombaugh. He took thousands of photographs of the night sky, repeating pictures of each given area after a few days.  He then inspected each pair of images in a device called a "blink comparator".  By switching back and forth between two images, Tombaugh could see if there was anything that had changed its position relative to the stars.
On 18 February, 1930, Tombaugh examined two photographic plates that he had made on Januray 23 and 29, and discovered a faint point of light which had indeed changed position. Knowing the angular field of view it was possible to calculate the angle through which the object had moved over the time between the pictures were taken, and it was confirmed that this was an object further out than Neptune.
Clyde Tombaugh

Clyde Tombaugh

Pluto dicsovery plates

Pluto discovery plates. Pluto is at the tip of the arrow in each image.

Venetia Burney

Venetia Burney

What's in a name?

On March 14, 11-year-old Venetia Burney was having breakfast at her house in Oxford with her mother and grandfather, Falconer Madan, who was a former librarian at Oxford's Bodlean Library. Madan was reading The Times newspaper, which included an article on page 14 reporting that astronomers in the United States had discovered a new planet.

Venetia had studied the Greek myths at school, and suggested that Pluto, the ruler of the underworld, would be an appropriate name for this new planet out in the cold darkness at the edge of the solar system. Her grandfather agreed and passed the suggestion to professor of astronomy Herbert Hall Turner, who sent a telegram to colleagues in America.

They also agreed, and a vote on the name was carried unanimously.

When word was sent back to England and to
Falconer Madan, he gave the news to Venetia - along with 5 - the equivalent of around 300 today!

A very small planet

At the time of its discovery, Pluto was thought to have a mass that was about the same as the Earth.  Subsequent observations reduced this, and in 1978, astronomer James Christy discovered Pluto's moon, Charon. This enabled an accurated measurement, and Pluto was calculated to have a mass that is only 1/500 that of Earth, far too small to have any real effect on the orbit of Uranus. 

data from Voyager 2, which had flown past Neptune in 1989, had been used to re-calculate the mass of Neptune as being 0.5% less than previously. In 1992 E. Myles Standish had used this data to re-calculate Neptune's gravitational effect on Uranus, and showed that the new value accounted for the differences in the orbit of Uranus, and so no additional planet was required.

That meant that the discovery of Pluto had been a fortunate coincidence.

New discoveries beyond Pluto

From 1992 onwards, many objects have been discovered beyond Pluto.  These are generally members of what has become known as the Kuiper Belt, similar to the asteroid belt that lies between Mars and Jupiter.

In late 2003, a team led by Mike Brown discovered 2003 UB313, which was estimated to be slightly larger than Pluto. If Pluto was a planet, didn't that mean that this object was a planet as well?

With the discovery of various objects beyond Pluto, the question arose of which of these should also be regarded as planets.

The IAU conference in 2006

The International Astronomical Union voted at its 2006 conference to re-classify Pluto as a Dwarf Planet, along with Ceres - the largest asteroid orbiting between Mars and Jupiter - and 2003 UB313, now know as Eris.

The subsequent headlines that Pluto was no longer a planet created a huge outcry, among both the astronomical community and the general public, and this controversy continues today ... but does it need to?

Is Pluto a planet?

I have a range of presentations on astronomy and space exploration, and "Is Pluto A Planet?" looks at the question of the status of Pluto.  I examine the IAU's definition of a planet, and some of its unexpected - and unintended - consequences, and give a definitive answer to this long-running question.

Public presentations

With the New Horizons spacecraft reaching Pluto in July 2015, this has become my most popular presentation at the moment.  I have 24 bookings (so far) at the following locations:

2014 April
Astronomical Society of Haringey   
"Is Pluto A Planet?" flyer
2015 January Cafe Scientifique, Harpenden

February Hertfordshire U3A Network

March Droitwich Space Day

Knights Templar School, Royston

Meridian School, Baldock

May West of London Astronomical Society

Farnham Astronomical Society

Hertford Astronomy Group

Rugby & District Astronomical Society

Wakefield & District Astronomical Society

June Breckland Astronomical Society

The British Interplanetary Society

The Welwyn Festival

July The Society for Popular Astronomy

October The International Astronomy Show

Liverpool Astronomical Society

November Herts Engineers Luncheon Club

December Institute of Physics, University of Hertfordshire

Institute of Physics, Atomic Weapons Establishment - Aldermaston
2016 January Carolian Astronomy Society

April Hampstead Scientific Society

April South-West Herts Astronomical Society

October Cardiff Astronomical Society

Click on the image for a full-sized version of the flyer


If you would like to book an entertaining presentation of "Is Pluto A Planet?" then please contact me.


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