Monday, 12 March 2012

Pluto and Charon – Binary Planet

Pluto Data
Mass: 1.32 x 1022 kg or 0.002 of Earth’s
Diameter: 2300 km or 0.18 of Earth’s
Surface gravity: 0.06 gee
Axial tilt: 119.6°
Mean surface temperature: about -230 Celsius
Rotation period: 6.39 days
Orbital period: 247.7 years
Inclination of orbit to ecliptic: 17.14°
Orbital eccentricity: 0.249
Distance from the Sun: 29.58–49.30 AU
Sunlight strength: 0.00041–0.0011 of Earth’s
Satellites: 1

Charon Data
Mass: 1.6 x 1021 kg or 0.0002 of Earth’s
Diameter: 1250 km or 0.098 of Earth’s
Surface gravity: 0.03 gee
Mean surface temperature: about -230 Celsius
Rotation period: 6.39 days
Orbital period: 6.39 days
Inclination of orbit to Pluto equator: 0.0°
Orbital eccentricity: 0.01
Distance from Pluto: 19 636 km or 8.5 Pluto diameters

Pluto is the furthest known planet. At the most distant point in its orbit, even sunlight takes nearly seven hours to get there – and a car journey at 70 miles per hour would take well over a quarter of a million years to cover the same stretch! It is for this reason, along with the planet’s very small size – 18 per cent that of the Earth – that astronomers still know very little indeed about this enigmatic worldlet. We do know at least that Pluto is primarily rocky with smaller quantities of ice. It also has a moon, called Charon. Fully half the size of Pluto itself, Charon is easily the largest satellite in comparison to its parent, and the pair has been called a binary planet. Pluto is an odd world – neither terrestrial nor giant. Indeed, some astronomers choose not to regard it as a planet at all on account of its diminutive size and its highly inclined and elliptical orbit compared with all the other planets. It seems more likely than Pluto is the largest of several icy worlds in the backwaters of the Solar System, out beyond Neptune.

Pluto is easily the smallest planet. At 2300 kilometres across it is less than half the size of Mercury and only 70 per cent the size of our Moon. With a relatively high density – surprisingly so, in fact, considering Pluto’s distance from the snow line – it must be mostly made of rock, with about 30 per cent ice. But no probe has ever been there, so very little is known about its surface. It is so distant that even the Hubble Telescope has great difficulty imaging the planet. Still, the best pictures show beyond reasonable doubt that Pluto’s surface displays great contrast. The equator is dark with bright patches while the poles are light-coloured. One interpretation of this is that the poles are bright because they are covered in enormous polar caps of frozen methane. Meanwhile, other research has shown that ices such as nitrogen and carbon monoxide are also present on Pluto – very similar, in fact, to Neptune’s moon Triton.

It is not only Pluto’s size that makes it unique. Its orbit is tipped at an angle of 17 degrees to the ecliptic and it is more elongated than that of any other planet, even Mercury. This means that its distance from the Sun varies from 30 AU to 49 AU – a difference ten times the size of Earth’s entire orbit. At its closest approach to the Sun, Pluto even crosses the orbit of Neptune. In fact from 1989 to 1999, Neptune was officially the most distant planet. But Pluto has now taken back its title and will hold onto it until well into the twenty-second century. Because of this bizarre meander, astronomers suspect that Pluto’s appearance changes dramatically as it journeys around the Sun. At present, for example, Pluto is almost as close to the Sun as it gets – it is ‘warm’ enough for some of its ices to have evaporated to form a tenuous but quite extensive atmosphere of nitrogen and methane, again as on Triton. This atmosphere extends at least 600 kilometres above the surface, but its pressure at the surface is very low – roughly what you’d encounter 80 kilometres above the Earth. Gradually, though, as the planet heads away from the Sun over the next few decades it is quite possible that most if not all of its atmosphere will freeze out and lightly dust the surface in a blue-white snow of nitrogen and methane.


One other thing that makes Pluto stand out is its companion, Charon. Over half the size of its parent and only 8.5 Pluto diameters away, Charon is even bigger in comparison to its planet than the Moon is to the Earth. Charon has a lower density than Pluto, so it must have a greater proportion of ice. Its surface properties are even more uncertain than Pluto’s, but water ice seems abundant while methane is not.

Pluto and Charon take 6.39 days to orbit each other. Charon revolves around Pluto’s equatorial plane. But, like Uranus, Pluto is tipped almost on its side. This means that the orbit of Charon is tipped more than 90 degrees relative to the Sun. Meanwhile, the orbital period of 6.39 days is also the amount of time it takes each world to spin on its axis. And so, just as the Moon keeps the same face to the Earth at all times, Pluto and Charon are forever locked in a similar gravitational embrace. From the surface of Pluto, Charon never appears to move in the sky. Instead, it just hangs there, forever presenting the same face but still going through a series of Moon-like phases. The same is true of Pluto seen from Charon. One way to visualise the pair is as a giant dumbbell with different sized masses on each end, endlessly tumbling in the vastness of space. If you happened to be on the hemisphere of Pluto turned away from Charon, or vice versa, you would never even know about your world’s faithful companion unless you journeyed to the other side.

History of Pluto and Charon
So why is Pluto such an oddball? Why does it have a highly inclined orbit and axial tilt? Why is it so tiny, when the planets before it are all giants? The answers to these questions are all very speculative. But astronomers have come up with a few explanations that paint a very colourful past for Pluto and its companion.

At first, Pluto was believed to be an escaped satellite of Neptune – a sister of Triton. Certainly Pluto and Triton are enough alike to make this an interesting idea. They have comparable surface compositions, densities, atmospheres and radii. But, despite this, the current consensus is that Pluto and Neptune have never been anywhere near each other – although their orbits do cross, the planets never actually interact because they are always in different parts of the Solar System from one another. Nevertheless, some scientists still think that Triton and Pluto do share a common origin. If this is true, Triton once orbited the Sun independently just as Pluto does today. Interestingly, since the 1990s astronomers have found a few hundred icy bodies orbiting near Pluto and beyond in a region that has become known as the Kuiper belt. Like the asteroids, the Kuiperbelt objects are essentially leftovers from the planet-building process – as we shall see in the next and final section of Part 3. Thus Pluto and Charon – and Triton before it was captured – may just be the largest members of a whole family of icy worlds that never quite made the grade in the race to become planets, back at the dawn of the Solar System. In a sense, Pluto and Charon could be little more than large icy planetesimals, not a real planet and moon at all.

This is all very well, but it doesn’t explain Pluto’s weird orbit. To do that, we have to invoke some sort of cataclysm long ago in Pluto’s deepest past. It should be obvious by now that the planets were frequent targets in the early Solar System’s cosmic pool table. The Earth, Venus, Mercury and Uranus all show evidence of having been hit by something very, very big. We have seen how Mercury lost much of its mantle and become an iron world; how the Earth gained a satellite; that Venus was knocked upside down while Uranus ended up on its side. In Pluto’s case, as with Uranus, perhaps a similar devastating collision with a neighbouring protoplanet knocked the world virtually on its side and also left it with its highly elongated and inclined orbit. Not only that but the event could also explain why Pluto has so much rock. It lost much of its icy mantle during its fatal encounter, just as Mercury lost its rock. Moreover, this scenario also offers an explanation for Charon’s presence. The Pluto collision would no doubt have left a lot of debris, and Charon could be the product of the accretion of that debris in orbit around Pluto. Strange to think that, so far from the Sun, Charon might be the outcome of the same mechanism that produced our own Moon.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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