Pluto

Pluto is the largest dwarf planet in the solar system. (Eris, once thought larger, is now recognized not to be as large as first supposed.) Its name literally means "god of wealth" and is one of the two names for one of the brothers of Zeus; the other is Hades, or "god of the underworld." (The Romans used only the name Pluto for this particular Greek god.)

According to the International Astronomical Union, it no longer qualifies as a planet because it has not cleared its orbit of other objects.

Discovery
In 1930, Clyde W. Tombaugh at the Lowell Observatory surveyed the sky, looking for a planet beyond Neptune (called "Planet X") that other astronomers had predicted from calculations based on an erroneous value for Neptune's mass. Tombaugh knew nothing of the error, but found an object anyway. This object was later given the name Pluto.

But even he realized that Pluto was not large enough to be the predicted Planet X. Astronomers continued to search in vain for it, until Voyager 2 made its flyby of Neptune, and the Jet Propulsion Laboratory scientists determined from this flyby that Neptune was significantly heavier than previously supposed. Current theory predicts no more planets other than the eight now known, but still allows for many other objects, both in the classic asteroid belt and in the Kuiper Belt, essentially a second belt of asteroids and comets.

Orbital characteristics
Pluto lies in a highly eccentric (nearly 25 percent) orbit around the sun. In fact, its perihelion lies inside the semi-major axis of the orbit of Neptune.

Physical characteristics
Pluto has a density of 2,030 kg/m3, comparable to that of Ceres. It also has a brownish-red color, like that of iron rust. Significantly, Charon, its innermost moon, does not have that color.

Orbital characteristics of its moons
Pluto and Charon make a wide binary. The barycenter of the Pluto-Charon system lies above its surface, not within it. Remarkably, the eccentricity of the Pluto-Charon wide binary is nearly zero. The eccentricities of the orbits of Pluto's other known satellites are slightly larger than zero, but are all less than one percent.

The dwarf planet controversy
The discovery of Eris, a scatter-disk body 27% more massive than Pluto, caused a controversy concerning what does, and what does not, constitute a planet. If Pluto were to retain its historical designation of "planet," then Eris would also qualify. But Eris was held not to qualify because it was in a neighborhood with multiple other objects, a situation similar to that of a similar body in the asteroid belt, namely Ceres.

In 2006, the International Astronomical Union passed the following resolution:RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet"1 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.

(2) 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Under those rather strict criteria, Pluto does not qualify. For that reason, Pluto is no longer considered a planet. It shares the new "dwarf planet" category with Eris and Ceres.

Problems for uniformitarianism
Pluto was initially supposed to have been a moon of Neptune. But with the discovery of Charon in 1978, and the subsequent discoveries of its other moons Hydra and Nix, that theory became far less plausible.

Another hypothesis has held that Pluto entered the solar system from outside, and the Sun captured it. But if such a mass dived into the solar system from outside, then it would have followed a hyperbolic path. That it did not pass back out of the solar system to complete the hyperbola begs explanation.

But the physical characteristics of Pluto, and the orbital characteristics of its satellites, present the worst problems. The chief conventional theory to explain Charon, is that an impactor struck Pluto and caused it to eject Charon. This begs two questions:


 * 1) How could Charon and Pluto settle into nearly perfect circular orbits around their common barycenter?
 * 2) Why does Pluto have a rust color and Charon doesn't?

For that matter, why do the other moons (Styx, Nix, Kerberos, and Hydra) have nearly circular orbits, inclining only slightly from Pluto's equator?

The rust color alone presents a problem. Conventional theory says the rust color comes from tholins that form from solar ultraviolet light falling on an atmosphere of methane, nitrogen, and ammonia. More likely, Pluto has iron on its surface. The sun, acting on atmospheric water vapor, dissociated this and produced oxygen. Oxygen reacts with iron to form rust. But this change had to happen far closer to the sun than even the current perihelion of Pluto.

Conventional scientists infer the composition of the Plutonian atmosphere strictly from spectrographic data from Earth-bound telescopes. The New Horizons spacecraft (see below) carries its own spectrophotometer.

The Hydroplate Theory
The Hydroplate theory of the Biblical flood says that Pluto, Charon, and all the other moons of Pluto formed from material ejected from earth in the early stage of the Flood. As this material passed beyond the earth, it fell out of Earth's gravitational influence. Then and only then could it accrete. Pluto simply received a higher proportion of iron-containing rock than did Charon.

The two bodies were also moving through an environment with a large quantity of gas in it. Pluto especially acquired a vast atmosphere (by one account, more than 11,000 km in extent). The atmospheric pressure would set up a steep gradient between the day and night sides of Pluto. In addition, Pluto and Charon could act like solar sails, gaining momentum from light pressure. All these influences gave those bodies extra momentum to push them into a close encounter with a gas giant, most likely Jupiter. The two bodies, and probably the four other bodies that would form the rest of the system, passed close to Jupiter, then fell south of the ecliptic. That explains the argument of perihelion greater than 90 degrees.

The highly eccentric orbit of the Pluto-Charon system around the sun, is typical of the orbits of the largest trans-Neptunian objects.

Observation and Exploration
On January 19, 2006, the Visiting mission::New Horizons mission was launched. Pluto is its prime target, but mission planners are considering plans to study at least one other Kuiper belt object.

On 15 January 2015, New Horizons began its final approach to Pluto. The trajectory of New Horizons will carry it past Pluto; it cannot accomplish orbital insertion.

In popular fiction
Science fiction author Larry Niven once speculated on an "earlier generation" race of extraterrestrial beings that controlled the galaxy until it suffered mutual annihilation in a war with a revolting slave race. As part of that scenario, a member of that "slaver race" once had to make an emergency "landing" in a crippled spacecraft that could no longer brake to a safe approach speed. He then set a course for the earth, bailed out of his ship, and left the ship's autopilot with orders to crash-land on Neptune. Instead of that happening, the ship struck a moon of Neptune hard enough to knock it out of orbit; that moon became known as Pluto.

Pluto has also been a subject of speculation involving future efforts by humanity to colonize that body, efforts often complicated by the presence of extraterrestrial "campers" or even of pathogens, usually viruses, native to Pluto.

Related links

 * Pluto by Wikipedia
 * Hamilton, Calvin J. "Entry for 'Pluto'." Views of the Solar System, 2007. Accessed January 21, 2008.

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