Pluto is the last of the currently acknowledged planets of our Solar System. In Greek mythology Pluto was the king of the Underworld known as Hades, and is aptly named because of its huge distant. The actual name is thought to mean “The Wealthy One”, derived from the Greek word, ploutos, hence the use of modern words like plutocracy - meaning the government ruled by the wealthy elite. Pluto is seen as wealthy because of the living’s life debt - everything born of the earth will eventually return the underworld. In Greek and Roman times this was ritualised by placing a coin or coins on the tongue of someone who had died before burial. It was believed the enacting the needed payment to the ferryman, Charon, so that the person who died could cross the river Styx and enter the underworld and the afterlife.
Pluto was discovered by the American astronomer Clyde Tombaugh as a faint 15th (17.0p) magnitudde ‘star’ at 4pm. on 18th February 1930. It was found by accident within a field 40 arc minutes from the yellowish 3.5 magnitude star Wasat / δ Geminorum. Almost a week later, after carefully rechecking the results, taking more images, and confirming the planet visually, an announcement was made to the world of Pluto’s discovery on 25th February 1930. Pluto was first imaged here on the 23rd and 29th January 1930 using two different photographic plates taken with the 35cm (13-inch) f/5 astro-camera at Lowell Observatory in the high desert country in Flagstaff, Arizona.
PLUTO DATADiscoverer : Clyde Tombaugh Discovery Date : 18th February 1930 Satellites : 3 Diameter : 2 302 km. Period (P) : 247.92065 years Synodic Period : 366.7 days Orbital Velocity : 4.749± km.s-1 Eccentricity (e) : 0.248 83 Inclination (i) : 17.140° Mass : 1.314±0.018 × 1022 kg. Mean Density : 1.85 g.cm-3 Mean Distance (a) : 5.906 4 ×1012 km. / 39.482 A.U. Min. Distance : 4.44 4 ×1012 km. / 29.7 A.U. Max. Distance : 7.38 4 ×1012 km. / 49.3 A.U. Sidereal Rotation : -6.3867 days Day Length : 6.387 days Maximum Diameter : 0.13 arcsec Minimum Diameter : 0.08 arcsec Maximum Magnitude : 13.7 Atmosphere : N2 CH4, CO |
Although the Pluto’s discovery was really being ‘in the right place at the right time’, in truth it was found under a specific planetary search program for any possible Trans-Neptunian planets. At the time, there was the suspicion of the existence of one main planet or planets beyond the orbit of Neptune, that were called Planet-X (and other letters of the alphabet for any additional unknown planets.) This belief was based on the small effects of perturbations - being significant influences on the planetary gravitational forces in the solar system. These small anomalies caused major problems in predicting, as in this case, by the small gravitational influences of Uranus and Neptune. For example, understanding these small perturbations was pivotal evidence for the discovery of Neptune, which was found by mathematical calculations of observations of Uranus. At the time of Neptune’s discovery this prediction was heralded as the greatest achievement of science in the mid-19th Century.
On the day of the discovery, Tombaugh was using the image device known as a blink-comparator, which alternatively blinks between the two different images. Any object remaining fixed in position was likely a star, and those which showed changed position were real moving solar system bodies. It soon became evident from the very pedestrian movement against the background stars that this body was placed in the farthest depths of our Solar System.
Pluto takes some 247.9 to just orbit once - two years shorter than the often quote period of 249.9 years that once stood for decades. At an average distance of some 39.5 A.U. or 5.7 billion kilometres from the Sun moving at the slow orbital velocity of about 17.1 km.sec-1. Also because of the highly elliptical orbit can be as close as 29.7 A.U. at perihelion (05th September 1989 AD in Libra), or as far away as 49.3 A.U. at aphelion (21st August 2113 AD in Cetus). During the aphelion, Pluto drops by about 2 to 2&189; magnitudes and is about two-thirds the size compared to when it is closest to the Sun. Observers today have been very lucky to live during a time when Pluto is easiest and brightest to observe, for about 50 years around 2113 AD, the planet will require at least an aperture of 40cm to see.
For many years there was much uncertainty on the true diameter, as Pluto appeared stellar-like even in the largest of telescopes. Presently Pluto (2006) has been estimated by JPL to be some 2 302 kilometres in diameter, which very slightly larger than the often earlier quoted 2 274 km. Some other texts sometimes quote the rougher estimate of 2 500 km.
Unlike most of the planets, Pluto is also highly inclined to the ecliptic, who tilt is 17.1°. Temperatures in this region of space are very cold without the warmth of the Sun, so we find the surface temperature is a chilly -233° to -223°C or just 40K to 50K above absolute zero.
Very little is known about Pluto since first discovered, but we expect that the dedicated interplanetary spacecraft, called New Horizons will certainly give far more information than the rather poor knowledge we have today (2006). Sadly this spacecraft was only launched on 19th January 2006 and isn’t expected to arrive until 14th July 2015 - so we have sometime to wait till further good images or new secrets are revealed.
Spectroscopy and rare planetary occultations by stars have shown Pluto does have a weak tenuous methane atmosphere extending perhaps to almost 40% of the planet’s radius. At present this tiny planet lies nearest to the Sun (perihelion) and at this distance from the Sun the temperatures reach about-220°C, but when the planet is placed at aphelion, this methane is likely freeze as a solid. So perhaps this atmosphere may be only temporary. Discovery of this was made late in 1988 by carefully making photoelectric observations during the time when the planet was being occultated by a star. As for the surface features, the best so far has been obtained by the Hubble Space Telescope (HST.) These were made between late June to early July 1994 and imaged nearly the entire surface using the European Space Agency’s (ESA) Faint Object Camera (FOC). (See one of these images of Pluto in pixelated above.) Although the resolution is poor, down to about 200 kilometres, there is clear evidence of a dozen or so patches of brighter regions intermixed with the slightly darker surface.
When at opposition, Pluto is looks like a faint 14th magnitude star, and so something like a 25cm (10-inch) telescope is needed to observe it. However, some keen-eyed amateurs have claimed to have glimpsed it with 20cm. Although the view of Pluto is frankly disappointing, any amateur that see the planet once or twice in their observing career should considered themselves as reaching a desired personal goal. The planet presently subtends somewhere between 0.13″ and 0.14″ arcsecs, and for all intents and purposes remains unresolvable through amateur telescopes. Pluto itself now has the average polar tilt of -34.5° to the ecliptic, but is well positioned for southern observers over the next few decades.
There has been in the last decade some significant debate whether Pluto should be classified as a true planet, an asteroid, even a comet; mainly because of it relatively small size. This question has now extended if Pluto should be considered a planet at all. The matter has been raised mainly because of the many hundreds of trans-Plutonian bodies or KBO’s (Kuiper Belt Objects) that have been discovered since 1996. Pluto is now considered as an example of a KBO. During 2003 this story has been made far more complicated with the discovery of the trans-plutonian KBO 2003 UB 313, which itself is slightly larger than Pluto. Some have already labelled it as the 10th Planet.
Although now popularly nicknamed Xena, this name is almost certainly tentative. The body will almost certainly retain a true classic Greek mythological name, which I likely guess it shall be known as Persephone. Suitably, she is in Greek mythology the goddess of the Underworld, being the daughter of Zeus and Demeter. In Roman mythology she is known as Proserpine. Her name is also apt for astronomers, as the derivation implies the “one who destroys all light.”
Like Pluto, however, the accepted planetary status of these bodies may be also be changed in August 2006. Here the I.A.U. who controls the naming of all celestial bodies will decide their fate during a scheduled meeting in the Czech capital Prague. Soon we may have either eight or ten planets in the Solar System. If this same international jury rules against this possibility, then Pluto prominence as a planet may be simply stripped - leaving just eight official planets in our Solar System. In the meantime, the actual number of planets does remain in flux - making difficulties for those teaching astronomy to children or those writing astronomical books!
On 07th June 1978, Charon was discovered J.W. Christy of the U.S. Naval Observatory while imaging Pluto with the 1.8m telescope at Flagstaff Arizona Later, previous images were found, showing the unrecorded new moon. Clearly, one of the major problems in observing Charon is that the separation is only small. It averages just under 1 arcsec, requiring advanced techniques to see it, and is all but invisible to amateur’s eyes. Throughout Pluto’s long elliptical orbit around the Sun, the separation can be anywhere between 0.6 and 1.1 arcsec. Charon was later first clearly resolved by the planetary camera of the Hubble Space Telescope in October 1990. This unexpected faint 17.8v magnitude moon was soon named after the Greek mythological ferryman who transports the dead across the river Acheron and into the underworld of Hades.
Orbiting in 6.2 days at the distance of 19 570 kilometres, Charon’s diameter was first estimated to be small, but when Charon was observed during a rare star occultation on 07th April 1980, this measure of Charon’s diameter proved to be much larger than expected, being some 1 210±10 kilometres across being frequently quoted today (2006). More recently, the JPL estimates find a slightly smaller 1186 ±26 km across. Charon roughly contains about 12% the mass of Pluto’s and similarly has a low density of about 1.85 5.5 g.cm-3. Earth is about 5.5 g.cm-3.
Charon, like the Earth’s Moon is locked in synchronous orbit, always displaying the same face towards Pluto. Hence the rotation of Charon equals the 6.2 day rotation period of Pluto. As the mass ratio is about 12%, the gravitational pivot of the orbit between the two bodies occurs somewhere between Pluto and Charon. Due to this proximity this would make the system more like a ‘double planet’, than the usual planet/moon system.
Charon’s orbit is significantly inclined at 94.9° ecliptic, and like the Uranian axis, rings and moons can be seen edgewise to the observer or wide open. Edge-wise orbit crossings, when occultations and eclipses are possible, and this will happen twice during the 247-odd year orbital period of Pluto. Previous crossings occurred on 26th December 1987 and 18th April 1988, and the next will not be until 05th April 2110 and 30th November 2111 AD. The orbit appears more oval between these times, being next at its widest in 2048 AD.
Both these new Plutonian moons were announced on the 31st October 2005, being found by a collaboration of astronomers using the HST planetary camera while imaging Pluto. They were known under the provisional named IAU’s designations as S/2005 P1 and S/2005 P2, until the 23rd June 2006 when after a spirited debate they were named by the International Astronomical Union (IAU) as Nix and Hydra whose announcement was made in the IAU Circulars No. 8625. Nix in Greek mythology was the goddess of night, who bore the brighter moon Charon, who was the ferryman that transports the dead across the river Styx to reach the Underworld. Hydra was a nine-headed serpent creature who carefully guards the main entrance to the Underworld from intruders or those wishing to escape.
Once astronomers looked at the earlier HST images taken on 14th June 2002, they found these same moons. This proved helpful in establish the orbital parameters of these new moons, whose respective distances are 48 700 and 64 800 kilometres from Pluto, whose mutual orbital periods are 24.86 and 38.21 days. These two new outer moons are very similar in size, about 90 km. in diameter. Like Charon, they orbit in the same orbital plane, whose inclinations are c.96° to the ecliptic.
Discovery of these moons may have implications for Pluto. As no other of the Kuiper belt bodies has more than one moon, this might finally save Pluto its planetary membership. No doubt we will learn much about these new moons during the New Horizons one billion ($A) spacecraft fly-by in 2015 now en-route.
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