Thinking About Making Pluto Planet Again

With the New Horizons probe set to go the first spacecraft to fly past Pluto on Tuesday, the debate over its official status has flared up once more — and there are several good reasons most astronomers no longer consider Pluto a planet.

For one, in terms of size, Pluto is much closer to Eris, Haumea, and Makemake — small objects classified as dwarf planets — than to Mercury or Mars. Like those other dwarf planets, Pluto also orbits the sun within a fairly crowded band of droppings called the Kuiper belt, rather than as a relatively isolated object in infinite.This is why the International Astronomical Union decided to formally reclassify Pluto as a dwarf planet back in 2006.

Merely recently, after I laid out the differences between Pluto and the other eight planets, I heard from several scientists and laypeople who made a unlike sort of statement: that we should just classify all dwarf planets as planets. Not just Pluto, butEris, Haumea, and Makemake, also.

(Lexicon)

In 2006, the IAU ruled that in order to be a planet, an object must orbit the sun*, be massive enough that the force of its gravity has pulled it into a round shape, and have cleared the neighborhood of its orbit of other debris. These Pluto supporters are arguing that we should only eliminate the 3rd requirement and have three classes of planets: rocky inner planets (like Earth and Mars), gas giants (like Saturn and Neptune), and dwarf planets (like Pluto and Eris).

In that location'due south no indication that the IAU has any plans to prefer this sort of scheme. For now, the formal debate appears to exist expressionless, and Pluto is non officially a planet. But there are some compelling arguments for why the IAU might want to reopen the discussion.

i) Pluto is tiny — only the Earth is tiny too, compared with Jupiter

From left: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the dwarf planets Pluto, Haumea, Makemake, and Eris. (Illustration past Roberto Ziche)

In terms of mass and diameter, Pluto and the other dwarf planets are an lodge of magnitude smaller than World and the other rocky planets of the inner solar organisation. As I argued, this is a good reason to put them in a different category of astronomical objects.

Just every bit Philip Metzger, a planetary scientist at the University of Central Florida, points out, Earth and the other rocky planets are likewise an order of magnitude smaller than the gas giants:

Ultimately, the basic concept of "planet" is always going to exist somewhat arbitrary — there's a continuum of differently sized pieces of debris orbiting the dominicus, and we simply choose to draw the line between "planets" and "not planets" somewhere.

Metzger's point is that nosotros could just as easily describe this line to include Pluto, Eris, and other dwarf planets — or draw it to exclude Earth, Venus, and the other terrestrial ones. Where y'all choose to draw the line depends on the assumptions you bring to the fence.

2) Pluto and other dwarf planets are dissimilar from about Kuiper belt objects

Pluto is orbited by at to the lowest degree five moons. (NASA, ESA, and One thousand. Showalter [SETI constitute])

The master evolution that led to Pluto's reclassification was the discovery, over the by few decades, that it's part of a cloud of debris called the Kuiper belt — a region of the solar organization, out past Neptune, that'southward filled with thousands of chunks of rock and ice (including Eris, which is actually more than massive than Pluto).

This is similar to what happened with Ceres. When that dwarf planet was offset discovered in the 1800s, it was idea to be a unique object — and considered a planet — simply the later discovery that it was part of the asteroid belt led scientists to reconsider.

Yet at that place's one pretty big planet-similar characteristic that Pluto, Eris, and the other dwarf planets have, and which the other Kuiper belt objects (KBOs) don't. "The majority of asteroids and KBOs are tiny, shapeless rocks and ice balls," says Laurel Kornfeld, a scientific discipline writer who blogs about Pluto. The handful of dwarf planets, on the other hand, are massive enough for the force of their ain gravity to pull them into rounded shapes. That is, they satisfy the second requirement in the IAU's definition of planet.

Pluto also has a few other key qualities that most KBOs don't. Information technology's orbited by 5 moons, has a sparse atmosphere, and, equally Kornfeld points out, is believed to have differentiated geological layers: a rocky core, surrounded past a drapery made of ice. These distinguishing features, she argues, hateful Pluto has more in common with rocky planets like Earth than Kuiper belt objects.

iii) The Kuiper belt is less crowded than y'all might imagine

A plot of all known Kuiper chugalug objects, shown in green. (WilyD)

The IAU'southward definition of planet excludes Pluto and the other dwarf planets through its tertiary, "clearing the neighborhood" requirement. Considering Pluto and the other dwarf planets are role of a cloud of debris they haven't cleared (the Kuiper belt), they aren't planets.

Merely lots of Pluto supporters criticize this requirement as being excessively vague — and in some ways, they say, you could even argue that it has cleared its orbit. "Pluto gets striking by virtually the same rate of infalling bodies (asteroids, comets) every bit the Earth," Metzger says. "That's considering the distances are more vast between bodies so far from the dominicus." The Kuiper belt is more debris-dense than neighboring areas of deep space, but it's not all that crowded compared with our orbit here in the inner solar arrangement.

What's more, he and others betoken out, the "clearing the neighborhood" requirement means that in its electric current location Pluto isn't a planet, simply if it were moved to a different spot it suddenly would exist. The classification has every bit much to practice with an object'southward location as its intrinsic properties.

So will Pluto ever become a planet once again?

The first colour image of Pluto sent back by the New Horizons probe. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Establish)

David Aguilar, who served every bit the director of media at the Harvard-Smithsonian Center for Astrophysics before retiring in January, suggests that all this can be fixed by a slight change to the IAU's planet definition.

Currently, objects like Pluto and Eris that are massive plenty to be round and orbit the sun, just haven't cleared their neighborhoods, are technically dwarf planets, only non planets. Aguilar recommends merely eliminating the controversial third requirement — so that dwarf planets become a type of planet.

"Information technology's as elementary as that, and it neatly eliminates 99.9999 percent of all the small, irregular, lumpy rocks floating out in the Kuiper belt," he says. Sure, information technology would mean the listing of planets will likely grow significantly in future years — as more dwarf planets are found in the Kuiper chugalug and beyond — only that shouldn't terminate us from adopting the best definition possible.

But unfortunately for the Pluto supporters, there's no indication that the IAU plans to pick this topic back up anytime before long. The original reclassification stirred up a ton of controversy that continues to this day, and the astronomers who support the original determination accept no wish to stoke it further. For ameliorate or worse, when the New Horizons probe visits Pluto for the first fourth dimension this coming Tuesday, information technology'll be visiting a dwarf planet.

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*An unrelated but substantial criticism of the IAU'south definition is that it requires an object to orbit the sun to be a planet — then it totally excludes all exoplanets. As we discover more and more planets orbiting other stars, this is becoming a bigger problem, and in that location are plenty of astronomers who would like this aspect of the definition changed, too.

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Source: https://www.vox.com/2015/4/21/8458129/pluto-planet

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