It was discovered on February 18, by astronomer Clyde W. It is the largest object in the Kuiper belt but less dense than Eris. It was originally referred to as the ninth planet, but was demoted into being classified as a dwarf planet in because of its size.
Pluto's natural satellite, Charon , is half the size of Pluto. In fact, it is so big that Pluto orbits Charon too. But the IAU has yet to formalize a defintion for a binary dwarf planet system, so Charon is classified as a satellite for now. Pluto has 5 natural satellites. Charon, Nix, Kerberos, Hydra, Styx. Charon is half of Pluto's diameter. Nix and hydra will de-orbit Pluto and drift away.
Pluto was formed from a clump of rocky materials and particles around 4. Because Pluto shares its orbital neighbourhood with other icy Kuiper Belt Objects, the resolution effectively stripped the distant world of a planetary designation it had held for some 76 years. It was immediately relegated it to the distinct category of "dwarf planet", alongside the biggest body in the asteroid belt, Ceres, and other large Kuiper Belt Objects such as Eris, Quaoar and Sedna. Commenting at the time, the IAU's president of planetary systems science Prof Iwan Williams said: "By the end of the decade, we would have had planets, and I think people would have said 'my goodness, what a mess they made back in '.
In a word, no. Some experts immediately questioned the part of the definition about a planet clearing its orbital neighbourhood. This is because Earth shares its cosmic turf with more than 12, near-Earth asteroids. Thus, some have argued that Earth, Jupiter and other planets also fail to meet the IAU's definition.
Speaking just after the vote, Prof Alan Stern, chief scientist for the New Horizons mission, called the outcome "an awful decision" and described the new definition as "internally inconsistent".
The low turn-out has been blamed on timing; the vote was held on the last day of the General Assembly when many participants had left or were preparing to fly out from Prague. The debate has rumbled on ever since, on television, in the pages of books and in public talks.
But the latter expert turned down the offer, stating: "I don't have opinions that I require other people to have. The flyby of Pluto is unlikely to provide any information relevant to a change in Pluto's status. But it will bring into clear focus once more what is, and what isn't, meant by the term "planet".
Follow Paul on Twitter. Image source, IAU. In , the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0. In , Myles Standish used data from Voyager 2 ' s flyby of Neptune , which had revised the planet's total mass downward by 0. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished. Brown concluded almost immediately that this was a coincidence, [37] a view still held today. Orbit of Pluto—ecliptic view.
This 'side view' of Pluto's orbit in red shows its large inclination to Earth's ecliptic orbital plane. This diagram shows the relative positions of Pluto red and Neptune blue on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance between them to emphasise the closest approach in Pluto's orbital period is Earth years.
Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than Neptune 's. The Pluto—Charon barycentre came to perihelion on September 5, , [38] [lower-alpha 2] and was last closer to the Sun than Neptune between February 7, and February 11, In the long term Pluto's orbit is in fact chaotic.
While computer simulations can be used to predict its position for several million years both forward and backward in time , after intervals longer than the Lyapunov time of 10—20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit. This does not mean Pluto's orbit itself is unstable, but its position on that orbit is impossible to determine so far ahead.
Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering. Orbit of Pluto—polar view. This 'view from above' shows how Pluto's orbit in red is less circular than Neptune's in blue , and how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic. Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely.
There are several reasons why. At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision.
This alone is not enough to protect Pluto; perturbations from the planets especially Neptune could alter aspects of Pluto's orbit such as its orbital precession over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the mean motion resonance with Neptune : for every three of Neptune's orbits around the Sun, Pluto makes two.
The two objects then return to their initial positions and the cycle repeats, each cycle lasting about years. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto.
Pluto and Neptune's minimum separation is over 17 AU. Pluto comes closer to Uranus 11 AU than it does to Neptune. The resonance between the two bodies is highly stable, and is preserved over millions of years. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change. These arise principally from two additional mechanisms besides the mean motion resonance. This is a direct consequence of the Kozai mechanism , [42] which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. The closest such angular separation occurs every 10, years.
Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the superresonance , and is controlled by all the Jovian planets. To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise.
After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to Kepler's third law.
As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto and, to a lesser degree, of Neptune. After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit near the opposing node to where we began. The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node.
The whole process takes about 20, years to complete. Pluto's rotation period , its day, is equal to 6. Hubble map of Pluto's surface, showing great variations in color and albedo. Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until , when the New Horizons spacecraft is expected to arrive there. Pluto's visual apparent magnitude averages The earliest maps of Pluto, made in the late s, were brightness maps created from close observations of eclipses by its largest moon, Charon.
Observations were made of the change in the total average brightness of the Pluto—Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot.
Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time. Current maps have been produced from images from the Hubble Space Telescope HST , which offers the highest resolution currently available, and show considerably more detail, [55] resolving variations several hundred kilometres across, including polar regions and large bright spots.
These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour. Pluto's surface has changed between and the northern polar region has brightened and the southern hemisphere darkened. Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.
Theoretical structure of Pluto [62] 1. Frozen nitrogen [60] 2. Water ice 3. Observations by the Hubble Space Telescope place Pluto's density at between 1. Pluto's mass is 1. Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In Pluto was calculated to be roughly the mass of the Earth, with further calculations in bringing the mass down to roughly that of Mars.
Template:Sfn In , Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth. Template:Sfn Pluto's albedo is 1. The discovery of Pluto's satellite Charon in enabled a determination of the mass of the Pluto—Charon system by application of Newton's formulation of Kepler's third law.
Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, while the invention of adaptive optics allowed them to determine its shape more accurately.
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