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Planets: Dwarf planets

Pluto used to be classified as a planet. Now it is classified as a dwarf planet. Why?

 

Pluto New Horizon July 13 NASA JHUAPL SwRI

Here are some recent photographs of Pluto, and it’s largest moon, Charon.

“This animation was inspired by two iconic images of Pluto, the global true color mosaic and the lookback image with atmospheric haze. It represents a fly-around from the day side to the dark side and back (not the rotation of Pluto itself).
Image Credit: NASA / JHUAPL / SwRI; Animation Credit: Christian Fröschlin”

How big is Pluto compared to Earth?

Why is Pluto no longer considered a planet? Actually, many bodies in space have been added to, and then removed from, the planet category. The number of planets has changed over time, as people have changed the definition of the word.

Notice how many “planets” were removed, as we learned that they more properly placed in the newly developed asteroids category.  The same is true for Pluto, Eris, and Sedna – now they’re in the dwarf planet category.

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The International Astronomical Union (IAU) defines a dwarf planet as a celestial body that:

* is in direct orbit of the Sun

* is massive enough for its gravity to turn it into a nearly round shape

* has not cleared the neighborhood around its orbit.

It is estimated that there are hundreds to thousands of dwarf planets in the Solar System, most of which lie beyond the orbit of Pluto.

The IAU currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris.

Hundreds more dwarf planets may be found when the entire region known as the Kuiper belt is explored.

The number of dwarf planets may exceed 10,000, when objects scattered outside the Kuiper belt, out in the Oort cloud are considered.

The Oort cloud – named after Dutch astronomer Jan Oort – is a cloud of icy planetesimals surrounding our Sun at a distance of up to 50,000 AU (0.8 light years.)

{Adapted from Wikipedia, Dwarf Planet}

This picture shows the sizes of the original three dwarf planets (Eris, Ceres, and Pluto) as compared to Earth. It also shows Pluto's large moon Charon (and its two small moons Nix and Hydra) and Eris's moon Dysnomia to scale. The image also shows Earth's Moon (Luna) and the planet Mars for comparison. None of the distances between objects in this image are to scale. Images courtesy of NASA, ESA, JPL, and A. Feild (STScI).

This picture shows the sizes of the original three dwarf planets (Eris, Ceres, and Pluto) as compared to Earth. It also shows Pluto’s large moon Charon (and its two small moons Nix and Hydra) and Eris’s moon Dysnomia to scale. The image also shows Earth’s Moon (Luna) and the planet Mars for comparison. None of the distances between objects in this image are to scale.
Images courtesy of NASA, ESA, JPL, and A. Feild (STScI).

What does it mean, “cleaning out the neighborhood?

cleaning out the neighborhood orbits Pluto

Formation of Solar System

Did Pluto clear out the Kuiper belt?
Kuiperbelt-1

Does it look like Pluto has cleared out it’s neighborhood?

Pluto did not clear out its neighborhood

Oort cloud

*
Oort cloud

What specifically does it mean for a planet to clear out it’s neighborhood?

Should Earth get demoted from planet status just like Pluto?, Alasdair Wilkins, Io9.Com, 3/3/11

As Ray Villard over at Discovery News points out, Earth is surrounded by so-called Near-Earth Asteroids. The prototype telescope Pan-STARRS (Panoramic Survey Telescope & Rapid Response System), located in Hawaii, can detect more than a dozen new asteroids on any given night, and some estimates tab as many as 20,000 asteroids in the same orbit as Earth.

The presence of so many asteroids seems like a serious problem for Earth’s claim that it has cleared its neighborhood. And Earth isn’t alone in this problem – Jupiter is surrounded by some 100,000 Trojan asteroid, and there’s similar clutter around Mars and Neptune. Indeed, one object that Neptune has categorically failed to clear from its orbit is Pluto itself. Alan Stern, the head of NASA’s New Horizons mission to Pluto and a critic of the Pluto reclassification, points out quite simply, “If Neptune had cleared its zone, Pluto wouldn’t be there.”

This seems like an untenable situation. After all, what sensible definition for a planet would exclude the largest in our solar system (Jupiter), the third-largest (Neptune), and, in Earth, the biggest rocky planet…not to mention the only one to support life?

Thankfully, there’s a very simple way out of this. We’ve been talking a lot about the sheer number of objects around Earth and Jupiter, but let’s instead consider those objects relative to the planets themselves. Pluto, for instance, is just .077 times the mass of all the other objects in its orbit, meaning it makes up roughly 8% of the mass found in its orbit. Earth, on the other hand, is 1.7 million times the mass of all the other objects in its orbit. Earth may be cluttered, but all the asteroids around it amount to less than nothing.

This figure is known as the planetary discriminant, an idea put forward by astrophysicist Steven Soter as a simple way of measure just how clean a planet’s orbital neighborhood really is. As it turns out, Earth has the cleanest neighborhood of any planet, with Venus the closest behind with 1.35 million. Jupiter is the next cleanest, with a planetary discriminant of 625,000. As it happens, Neptune has the smallest discriminant, at just 24,000.

Even so, the difference in the size of discriminants between the eight planets and the five dwarf planets is massive. None of the dwarf planets – which currently includes Ceres, Eris, Pluto, Makemake, and Haumea – have discriminants greater than 1. The asteroid belt object Ceres has the largest, at .33, but that really is nothing compared to those of the eight planets. So then, Earth is definitely a planet, and the argument for its demotion rather resoundingly falls apart.

The IAU didn’t include a strict cut-off for how clean a neighborhood has to be for an object to be considered a planet. Obviously, when the gap between the least clean planet and the cleanest dwarf planet is separated by a factor of more than 72,000, there isn’t much need for one. So, barring the discovery of something very unusual in the outer reaches of our solar system (like, for the sake of argument, the Oort Cloud planet Tyche) that would seriously blur the line in terms of what it means to clear one’s orbit, we can safely say that Earth is definitely a planet, Pluto is definitely a dwarf planet, and only one of them is capable of keeping its neighborhood clean.

Resources

The Science Has Spoken: Pluto Will Never Be A Planet Again

Learning Standards

ESS1. Earth’s Place in the Universe

8.MS-ESS1-1b. Develop and use a model of the Earth-Sun system to explain the cyclical pattern of seasons, which includes Earth’s tilt and differential intensity of sunlight on different areas of Earth across the year.

8.MS-ESS1-2. Explain the role of gravity in ocean tides, the orbital motions of planets, their moons, and asteroids in the solar system.

ESS1.B Earth and the solar system – The solar system contains many varied objects held together by gravity. Solar system models explain and predict eclipses, lunar phases, and seasons.

 

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