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Today in science: Pluto the dwarf planet

August 24, 2016 is the 10th anniversary of astronomers’ announcement of Pluto’s demotion to dwarf planet. The decision caused a shock wave around the world.

Pluto via New Horizons spacecraft on July 14, 2015.

Pluto backlit, via New Horizons spacecraft on July 14, 2015. Image via NASA/JHUAPL/SwRI.

August 24, 2006. Today is the 10th anniversary of Pluto’s demotion to dwarf planet status. Our solar system went from having nine major planets to having eight major planets, with the outermost planet being Neptune, according to astronomers’ new definition of what it means to be a planet. The XXVIth General Assembly of the International Astronomical Union (IAU) formalized the decision. The public and many astronomers didn’t take it lightly, with some declaring they would still consider Pluto a planet and with the word Plutoed – meaning to demote or devalue something – entering the global lexicon.

Prior to 2006, astronomers hadn’t gotten around to establishing clear standards – such as a minimum size or mass, or other considerations – by which an object might be categorized as a solar system “planet” versus “dwarf planet.”

They began to see a need when many small bodies – such as Haumea and Makemake – began being discovered in the outer solar system. Eris, also considered a dwarf planet, is even more massive than Pluto! So if Pluto is a planet, why shouldn’t Eris be granted planet status as well? That was the question the IAU asked itself, which led to its formation of a Planet Definition Committee and ultimately the 2006 decision.

From Upper Left... and a prize-winning educator.

Meet the Planet Definition Committee of the International Astronomical Union. This group made the final decision to demote Pluto to dwarf planet status. But, even within the committee, not all agreed.

The committee had a few possible roads to travel down. One would be to make the decision by size (or mass) so that Pluto would remain a planet, and therefore Eris and Ceres – the largest body in the asteroid belt in the inner solar system – would become planets, too. For awhile, it looked as if that might happen with some IAU committee members favoring that decision.

Another option for the IAU might have been to define the concept of a planet without any specific logic: Earth is a planet, Pluto is a planet, but Eris is not just because.

On August 24, 2006, the IAU announced its solution to the questions raised by having so many new objects in the outer solar system. It had decided to create a scientific definition of what it means to be a planet. Unfortunately, the definition excluded Pluto from major planet status. Here’s the definition:

A 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, and
(c) has cleared the neighbourhood around its orbit.”

It’s #3 that causes Pluto to fail as a planet, according to the IAU. For an object to be a major planet, according to this definition, it must be the dominant gravitational object in its orbit. It must either sling other objects away or merge with them.

Pluto is only 0.07 times the mass of the objects in its orbit. Meanwhile, Earth is 1.7 million times the mass of the objects in its orbit.

View larger. | Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.) The images, taken when the spacecraft was 280,000 miles (450,000 km) away, show features as small as 1.4 miles (2.2 km). Image via NASA/JHUAPL/SwRI.

When Pluto lost its full planet status, it was revealed as one of the world’s most beloved astronomical objects. So it was ironic when New Horizons – first spacecraft ever to visit Pluto – discovered a heart-shaped region on it in 2015. Image taken 280,000 miles (450,000 km) from Pluto, via NASA/JHUAPL/SwRI.

Just 15 minutes after its closest approach to Pluto on July 14, 2015 ...Image credit: NASA/JHUAPL/SwRI

New Horizons captured this image just 15 minutes after its closest approach to Pluto on July 14, 2015, as the spacecraft looked back toward the sun. This near-sunset view shows Pluto’s rugged, icy mountains and flat ice plains, plus haze layers in Pluto’s tenuous but distended atmosphere. The image was taken from a distance of 11,000 miles (18,000 km); the scene is 780 miles (1,250 km) wide. Image via NASA/JHUAPL/SwRI.

On that fateful day – August 24, 2006 – the IAU also created a new category of celestial objects for Pluto and all Pluto-like objects:

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,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

According to the IAU, there are currently five dwarf planets: Pluto, Ceres, Eris, Haumea, and Makemake. However, there is a likely potential sixth dwarf planet candidate temporarily designated as 2007 OR10, and many other known dwarf planet candidates.

Astronomers believe there may be hundreds of undiscovered dwarf planets in the Kuiper belt of the outer solar system. There may be up to 10,000 in the region beyond.

Credit: Hadria Beth, Quark Tees.

Still love Pluto? You might like this tee from Hadria Beth, Quark Tees.

By the way, it’s not common knowledge that many astronomers started out quite careful about their use of the word “planet” with respect to Pluto. In 1932, for example, only two years after American astronomer Clyde Tombaugh discovered Pluto, another astronomer, Armin Otto Leuschner, wrote in a journal article:

You may observe that with extreme conservatism I am still referring to Pluto as an object rather than as a planet. There is every probability that it is a planet, as is now universally concluded, from available material … So far only an upper limit for the mass of Pluto … has been established, and such a mass is believed from gravitational considerations to be too small to affect the motions of Uranus and Neptune sufficiently … There is also a remote chance that later investigations will render its mass comparable to that of comets.

Recall why astronomers began searching for Pluto in the first place. They expected to find an object large enough to gravitationally disturb the orbit of Neptune. If Pluto would have been discovered a decade or so later, when Edgeworth speculated of the existence of the Kuiper Belt, it might have never been awarded the status of planet.

As it was, like it or not, Pluto became the world’s eye-opener when it came to the classification of solar system objects.

Bottom line: August 24, 2016 is the 10th anniversary of Pluto’s demotion to dwarf planet status. According to a new definition by the International Astronomical Union, Pluto became a dwarf planet because it is has not “cleared the neighborhood around its orbit.”

Read more about Pluto here

The IAU talks about its decision to demote Pluto here

Daniela Breitman

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