Pluto

Pluto is a dwarf planet

Discovered in 1930 by the American astronomer Clyde Tombaugh, Pluto is out of the ordinary. Of an equatorial diameter lower than 2 500 km, it represents only tenth two of the Earth. Not satisfied to be, so far, the smallest member of the solar system, it also possesses an eccentric and very oblique orbit with regard to the plan of the ecliptic.
Sometimes even, Pluto is closer of the Sun that Neptune. It is crossed in the perihelion in 1989, its passage in the aphelion is planned for 2113. Pluto is the second dwarfish planet of the solar system by the size.
It orbits around the Sun at a distance varying between 29 and 49 astronomical units and belongs to the belt of Kuiper. It is about the first trans-Neptunian object of having been discovered. The solar system, which counted nine planets since 1930, contains no more than eight since August, 2006.

Further to this modification of the nomenclature, Pluto was added to the list of the minor objects of the solar system and saw attributing the number 134340 in the catalog of the minor objects.

Image: Composite photograph of Pluto and Charon photographed July 11, 2015 by the New Horizons probe. Image Credit: NASA/JHUAPL/SWRI

Pluto lost his status of planet

The solar system has lost a member of the family of planets. "A planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to outweigh the cohesive forces of the solid body and assumes a hydrostatic equilibrium (spherical), which eliminated any body moving in a close orbit."
This definition was approved on August 24, 2006, at the 26th General Assembly of the IAU (International Astronomical Union) by a show of hands of about 400 scientists and astronomers after ten days of discussions. In addition, the IAU has created a new class of objects: the dwarf planets. The first suspicions about the true nature of Pluto were born in 1992. This year, Americans David Jewitt and Jane Luu observed for the first time an object in the Kuiper Belt.
The existence of such objects, very faint and gravitating beyond Neptune, was suggested by Gerard P. Kuiper in 1951. With a diameter of a few hundred miles, it would be the remains of the original nebula that gave birth to the solar system. However, the advent of increasingly powerful in recent year's telescopes led to the discovery of some seventy objects of this type. The largest trans-Neptunian objects (OTN) have obtained the status of dwarf planet.
The four largest are: Eris (≈ 2400 km), Pluto (≈ 2300 km), Makemake (≈ 1600 km) and Haumea (≈ 1600 km).

Image: Diameters compared, big dwarf planets, relative to that of the Moon and Earth. Earth (≈12756 km), Moon (≈3474 km), Eris (≈2300 km), Pluto (≈2300 km), Makemake (≈1600 km) and Haumea (≈1600 km).

Image: The Pluto system, with three satellites seen by Hubble, 15 May 2005. In June 2011, another satellite was discovered by Hubble. The fourth is called Kerberos. The tiny moon Kerberos orbits Pluto in 32.1 days. The fifth, Styx, revolves in 20 days, it is between 10 and 25 km.

Satellites of Pluto

Charon, discovered by James Christy in 1978, is a big satellite with regard to the planet mother (diameter 1270 km). Charon has a mass about ten times lower than Pluto and the report of diameters is from 1 to 2.
The infernal couple turns around its common center of gravity, as two objects connected by a stiff bar such a barbell. Charon is situated in only 19000 km of the dwarf planet and moves around this planet with a period of 6.4 days (equal to the period of rotation of Pluto, what establishes a synchronous orbit).
Hydra Hydra is the mythological snake in nine heads which kept the realm of Pluto which is the Roman name of the Greek god of the Hells, Hades. Besides the relation with Pluto, names were chosen because of their initials "N" and "H", which are also the first letters of New Horizons, the probe of the NASA launched in January, 2006 towards the system of Pluto.  and Nix In the Greek mythology, Nyx was the night goddess and the mother of Charon, the man who transported in his boat souls through the river the Styx towards the underground world governed by Pluto. THE UAI modified its name in "Nix" according to the Egyptian spelling of the name of the goddess to avoid any confusion with two asteroids which already carry the name "Nyx".  are two small satellites discovered on orbit around Pluto, thanks to the spatial telescope Hubble in May and June, 2005 by two American astronomers Alan Stern and Hal Weaver. Two new moons orbit to 44000 km of Pluto, is twice farther than Charon. They make between 45 and 160 km in diameter and are 5000 times less brilliant than Charon. Both satellites circulate in the plan of Charon. They were cataloged under the temporary references S/2005 P1 and S/2005 P2, before becoming Hydra and Nix in 2006.
The Hubble Space Telescope has discovered a fourth moon orbiting in 31 days, around Pluto, it is tentatively named P4. It is the smallest moon, with an estimated size between 13 and 34 km. The natural satellite was discovered on Hubble images taken 28 June 2011 and confirmed on 3 and July 18, 2011. In preparing the overview by the New Horizons mission in 2015, this discovery has been done. As for P5 named Styx, astronomers think it has a potatoid form 10 to 25 km long. On the table satellites of Pluto (right), Pluto was added to the list because it revolves around the same center of mass than Charon.

Image: Simulation rotations of Charon, Nix, Hydra, Kerberos and Styx around the icy dwarf planet Pluto. The tidal force of most of the inner solar system moons blocks the satellite, a side facing their central planet. This animation shows that this is not the case with small moons of Pluto, which behave like tops while Pluto and Charon are face to face. Pluto is shown in the center with, in order, from the smallest to the largest orbits Charon, Styx, Kerberos Nix and Hydra. The rotation of two nearby objects like Pluto and Charon (12% of the mass of Pluto) is around centroid or center of mass of the two objects, Pluto and Charon. The barycenter is located outside the main body, ie Pluto. Credits: NASA/JHUAPL/SwRI/M. Showalter

The surprising Pluto Mountains

The latest news reaching us from the New Horizons probe that flew Pluto in July 2015 and the first images reveals a surprise.
In the region near the equator of Pluto, a chain of young mountains of 3500 meters high, rise above the ice surface.
These Mountains were probably formed there are no more than 100 million years, they are very young compared to the age of 4.56 billion years the solar system. In addition, they can still be evolving, according to the team of Jeff Moore of the Ames Research Center of NASA in Moffett Field, California.
This region, which covers less than one percent of the surface of Pluto, may still be geologically active today. Moore and his colleagues based the estimate of young age by the absence of crater in this view.
Like the rest of Pluto, this region would probably have been pummeled by space debris during the billions of years past and would be filled craters. It seems that recent activity has erased the scars. "This is one of the youngest surfaces we have seen in the solar system," says Moore.

Unlike the icy moons of the giant planets, Pluto can not be heated by gravitational interactions with a much larger planetary body. Another process of generating this mountainous landscape.
"It can bring us to rethink geological activity on many other icy worlds," says Deputy Team Leader GGI John Spencer of the Southwest Research Institute in Boulder, Colorado.
The mountains are probably composed of water and ice, the "bedrock" of Pluto.
Although the methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to build mountains, it is more likely that a more rigid material, such as water ice has created peaks.
"At temperatures of Pluto (the average ground temperature is measured at -223 ° C), water ice acts like a hard rock" said Bill McKinnon of Washington University, St. Louis.
The image was taken about 1.5 hours before New Horizons is closer to Pluto, the craft was 77,000 km from the surface of the dwarf planet.

Image: The surprising Pluto Mountains. Unlike the icy moons of the giant planets, Pluto can not be heated by gravitational interactions with a much larger planetary body. Another process of generating this mountainous landscape. Scientists suspect a geological activity, which would be very surprising.