Why Pluto is NOT a planet

First of all, I'm just going straight out and telling you. PLUTO IS NOT A PLANET. The International Astronomical Union made that clarification in 2006 and nothing will change that. I'm going to give you the reasons why I think Pluto is not a planet and why the IAU made the correction determination.

Secondly, when I was teaching college astronomy at the University of Pittsburgh, one of the things I also taught my students was that Pluto was not a planet. I last taught in 2005, a full year before the IAU made the announcement. Everything that I will talk about in here were the reasons I gave as to why Pluto is not a planet. Demoting Pluto did not diminish what Clyde Tombaugh accomplished in 1930 when he found Pluto at Lowell Observatory. There will be more of the discovery of Pluto in a later post.

One thing that argues against Pluto being a planet is its inclination to the equator of the Sun. In general, a planet should have a low orbital inclination as the Sun and planets were formed in the same nebula. As the nebula rotates and shrinks, all larger objects should stay in the same general plane. Here are the inclinations of the eight planets, Ceres, and Pluto

• Mercury 3.38°
• Venus 3.86°
• Earth 7.155°
• Mars 5.65°
• Ceres 17.75°
• Jupiter 6.09°
• Saturn 5.51°
• Uranus 6.48°
• Neptune 6.43°
• Pluto 11.88°

As you can see, Ceres (the largest asteroid or a dwarf planet in the Asteroid Belt) and Pluto both have orbital inclinations to the solar equator of more than 10 degrees. Earth has the largest of all the planets, but is inclined three degrees shallower. It would make sense since all the planets formed at the same time as the Sun and are the most massive bodies in the solar system after the Sun, that they would orbit within the equatorial plane of the Sun and not deviate much within that plane.

Another reason why Pluto is not a planet is it has a highly eccentric orbit. Objects that form in the same cloud as a star should be in a relatively circular orbit. (There really is no such thing as a perfect circle in science or nature. Variations in conditions can distort objects to make them less than perfect.) The nebula rotated which caused the cloud to collapse into a disk-like shape with the protosun at the center. Therefore, anything forming in that cloud will have a nearly circular orbit. Let's look at the different eccentricities of the same nine objects.

• Mercury 0.206
• Venus 0.007
• Earth 0.017
• Mars 0.093
• Ceres 0.076
• Jupiter 0.049
• Saturn 0.056
• Uranus 0.047
• Neptune 0.009
• Pluto 0.249

Looking at these eccentricities, all of them except Mercury and Pluto have eccentricities less than 0.1. Mercury's orbit is eccentric because of its proximity to the Sun and relativistic effects of the space that Mercury orbits in. Pluto is so much farther out, that relativistic effects do not affect it as much. Curvature of spacetime is a consequence of massive bodies that won't be explained here. You can always google the topic, or if you would like me to explain general relativity, comment below.

The density of Pluto is also a dead giveaway that Pluto is not a planet. 

Terrestrial Planets:

• Mercury 5.427 g/cm³
• Venus 5.243 g/cm³
• Earth 5.514 g/cm³
• Mars 3.934 g/cm³

Jovian Planets:

• Jupiter 1.326 g/cm³
• Saturn 0.687 g/cm³
• Uranus 1.27 g/cm³
• Neptune 1.638 g/cm³

Dwarf Planets:

• Ceres 2.077 g/cm³
• Pluto 2.03 g/cm³
• Eris 2.52 g/cm³ (estimate)
• Haumea 2.6 g/cm³ (estimate)
• Makemake 2.3 g/cm³ (estimate)

Pluto's density is too low to be terrestrial, but too high to be Jovian. Based on its density, we know that Pluto is a combination of both icy material and rocky material, with slightly more ice than rock.

The last argument I can make about Pluto not being a planet is its size relative to multiple moons in our Solar System.

 

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

 

If we were to include Ceres on this image, it would be smaller than Orcus (~800 km for Orcus and ~500km for Ceres).

 

The IAU has given a definition for a planet and a dwarf planet. 

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.

Callisto

Image Credit:

NASA/JPL/DLR

The farthest of the Galilean moons, and the second largest of all Jovian moons, Callisto has the smallest density of the four major moons of Jupiter of 1.81 g/cm³. From the density, we know that Callisto is mostly water and ice. It has a radius of 2410 km, giving it a total mass of 0.018 Earth masses. Orbiting at 1.883 million km from Jupiter, it takes 16.7 Earth days or 40 Jupiter days. Like Ganymede, Io, and Europa, it is tidally locked to Jupiter, so it rotates on its axis once ever 40 Jupiter days, as well. It does not share a resonance with Io, Europa, and Ganymede, which, if you recall, is 1:2:4. Callisto's resonance would be 9.4 if you would want to know that.

Since it is the farthest Galilean moon, it does not go as intense tidal friction as do the others. Because of this, it is not as active as the other three, and is not as well differentiated even though it is larger than both Io and Europa. It may have a liquid ocean below the ice crust, but it is less likely to harbor life than Europa.

The Four Galilean Moons of Jupiter. Clockwise from top left:

Io, Europa, Callisto, and Ganymede. Note how Callisto is the only one that has a uniform interior.

Image Credit:

NASA/JPL

 

Since it is not as strongly affected by tidal forces, Callisto's surface is slightly older than the surfaces of Io, Europa, and Ganymede. It is dark, dusty, and pockmarked with multiple craters. The dark and dusty material is from millennia of asteroid and comets impacts, disintegrating and covering the surface.

Callisto, however, is the best bet for human presence in the Jupiter system. Since it is farther from Jupiter than the rest, the magnetic field of Jupiter is less intense at the Callisto than the other Galilean moons. Also, the radioactivity from the moon itself is not as strong since the moon is geologically inactive. The thin atmosphere contains both carbon dioxide and molecular oxygen which can be used by the colonists or scientists on the surface, and with the liquid ocean below the crust, water is in abundance, as well. Though it is farther from Jupiter, it is still well within Jupiter's gravity well, so any asteroids or comets coming nearby will most likely be capture by Jupiter and crash into Jupiter (see Comet Shoemaker-Levy 9).

Callisto, like the other Galilean moons, formed at roughly the same time as Jupiter, but they were far enough away to be prevented from being accreted by the newly formed planet. They, obviously, are still under the gravitational influence of Jupiter itself.

Ganymede

Surface of Ganymede

Image Credit:

USGS Astrogeology Science Center/Wheaton/ASU/NASA/JPL-Caltech

 



Ganymede is the third closest Galilean Moon, and the largest of all Jupiter's moons. In fact, it is actually larger than Mercury and our Moon, and only three quarters the size of Mars. It has a density of only 1.9 g/cm³, about two times the density of water, implying that Ganymede is composed mostly of water and ice, though it does contain some rocky material and a small iron core. The iron core is inferred from Ganymede's size. It is large enough that in the past, radioactivity in the interior made the iron and rock molten, allowing for the heavier iron to sink to the center of the moon. Its ice crust is about 500 km thick, but it does have a 5-km thick liquid water layer about 170 km below the surface. Despite it being larger than Europa, its ice crust is too thick to allow life to exist, so we believe.

 

 Ganymede orbits at approximately 1.07 million km from Jupiter, giving it an orbital period of 7.15 days (twice that of Europa and four times that of Io). Its radius is 41.3% that of Earth, but its mass is only 0.025% of Earth, as Earth is mostly iron, nickel, and rocky material while Ganymede, as mentioned above, is mostly ice and water. Much like the Moon as it orbits the Earth, Ganymede is tidally locked to Jupiter. For that matter, so are Io, Europa, and Callisto. This means that for every orbit the Galilean moons make around Jupiter, each one rotates on its axis once. So much like the same face of the Moon is facing Earth, the same faces of Io, Europa, Ganymede, and Callisto are pointed to Jupiter.

 

Tidally locked moon orbiting a planet

Small line pointed to planet at all times

 

 

Ganymede has a surface that is not uniform. About a third of the surface is old, dark, and heavily cratered. We know that this makes the surface old as the early Solar System was heavily bombarded by asteroids and comets, though there are some impacts today, but not on the scale seen when the Sun was just being born. A way to look at this is to compare the surface of the Earth with that of the Moon. The Earth has a very active surface, from earthquakes, volcanoes, flowing water, and the weather, where the Moon is not active at all. A larger percentage of the surface of the Moon is cratered compared to the Earth. We know that we are not being impacted as much now as the Earth was in the beginning, as we do not see large meteors every day. However, Ganymede also has a large percentage of its surface that is much younger. Much like Io and Europa are affected by tidal forces from Jupiter and the other Galilean moons, Ganymede is as well. The tidal forces can cause fractures in the surface crust, allowing liquid water to come up to the surface and flow. This flowing water creates grooves on the surface, which flow through the craters. If the grooves formed before the craters, then the grooves would be broken, but we see the grooves as continuous.

"Old" surface vs. "New" surface

The left side of the image is the older surface of Ganymede, dark and cratered. The right side is younger, with less craters and grooves running along the surface

Image Credit:

NASA/JPL/DLR

 

Galilean Moons

Callisto, Ganymede, Europa, and Io

From Astronomy Online

 

 

The Galilean moons are the four largest moons orbiting Jupiter (though they are not the closest). They were first discovered by Galileo Galilei (and independently, by Simon Marius) in 1609 and 1610 after Galileo turned his telescope towards Jupiter. He noticed these bright dots near Jupiter and as he followed Jupiter in the sky over the course of weeks, he observed those dots staying with Jupiter as it moved in the sky. He correctly concluded that these dots, which he first called stars, were gravitationally bound to Jupiter. He was the first to discover satellites orbiting another body other than the Earth. As we learned in the post about Galileo, there could be objects that did not orbit the Earth. This put another nail into the coffin, successfully debunking the theory that everything orbited the Earth, the Geocentric model of the Solar System.

 

When Galileo discovered these four moons, he wanted to name them for his sponsor, Cosimo de Medici, calling them Cosmian stars, or the Medici family, the Medician stars. He labeled them Jupiter I, Jupiter II, Jupiter III, and Jupiter IV based on increasing distance from Jupiter. This convention held up until the mid 20th century until moons closer to Jupiter were discovered. The four names we use today (Io, Europa, Ganymede, and Callisto), were actually proposed by Johannes Kepler and adopted by Simon Marius who had also observed the moons. Galileo did not like the names and kept his convention, never using the names we use today.

 

All four moons are bigger than any dwarf planet, including Pluto. In fact, Ganymede has a larger diameter, though a smaller mass, than Mercury. The three inner Galilean moons also experience a 4:2:1 resonance. For every four orbits around Jupiter completed by Io, Europa completes two orbits and Ganymede completes one. Callisto has an orbital period of 9.4 times that of Io, so it doesn't really fit the pattern.

 

All the moons are actually visible with amateur telescopes and binoculars, as long as none are obscured by Jupiter's face. The next four posts will go into more detail about these fascinating moons.

Jupiter

Jupiter: Note the Great Red Spot to the lower right and the shadow of Europa on the lower left

Image is from the Cassini Spacecraft

 

Jupiter is by far the largest planet in the solar system in terms of mass and size. However, it is still less then 99% of the Sun's mass. It has a semi-major axis distance of 5.2 AU which gives it an orbital period of about 11 years. It rotates once on its axis every 10 hours, making it one of the fastest rotating bodies in the Solar System.

Previously, we had talked about Galileo and how he had discovered the first moons around a planet other than our Earth. This discovery helped prove the geocentric theory of the solar system was incorrect (Objects can orbit something other than the Earth). Galileo originally wanted to call them the Medician moons after his sponsors, the Medicis, but it was eventually agreed to name them the Galilean moons and to name them after four of the lovers of Zeus; Io, Europa, Ganymede, and Callisto.

Saturn is no the only planet with rings. In fact, all four Jovian planets have a ring system, though not quite as magnificent as Saturn's rings. Jupiter's rings are not as extensive as the rings around Saturn and were not discovered until 1979 by Voyager 1.

Jupiter is also home to the largest storm in the Solar System. The Great Red Spot is at least 183 years old and may be older. It may have been first observed in 1665, but it isn't clear if it was observed again until 1831. It is between 24,000 to 40,000 km east-west and 12,000 to 14,000 km north-south, making it capable of holding 2 to 3 Earths.

Jupiter also shares its orbit with a two groups of asteroids known as the Greek and Trojan asteroids. These asteroids were discussed previously here. These asteroids, much like the Amor asteroids, will never impact Jupiter. However, Jupiter's gravity is strong enough to capture both asteroids and comets. Many of Jupiter's moons are possibly captured asteroids, and back in 1994, Comet Shoemaker-Levy 9 entered Jupiter's gravitational field, broke up, and collided with Jupiter.

Jupiter has a very thick atmosphere, as the majority of its volume are the many layers of gas. There is a rocky core, approximately Earth-sized, but is only a small fraction of the entire diameter of Jupiter. The combination of its large volume and the majority of the composition of the planet give Jupiter the largest mass of all the planets, but also a low density, just a little bit above that of water.