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Pluto and Outer Solar System

Our knowledge of the outer reaches of the solar system has increased greatly in recent years, with probes, orbiting telescopes and computer modelling revealing previously unknown objects, and even new planets far beyond the orbit of Neptune.


Pluto (134340 Pluto) was demoted from a planet to dwarf planet in 2005, but is nevertheless a fascinating little world, which the New Horizons spacecraft photographed in a flyby in July 2015. It is located outside the orbit of Neptune for nearly all of its orbit, and its orbit is inclined to the solar plane, and is within the Kuiper Belt.

    Vital statistics:
  • Mass: 1.30 x 1022 kg = 0.0022 Earths, 0.177 of the Moon's mass
  • Diameter: 2374 km = 0.18 Earth's
  • Density: 1.86 g/cm3 = 0.337 Earth's density
  • Surface gravity: 0.620 m/s2 = 0.063g
  • Distance from Sun: 4.44 - 7.31 billion km (5.87 Gkm semi-major axis) = 29.7 - 49.3 AU (Earth-Sun distances)
  • Period of orbit: 248 Earth years
  • Period of rotation: 6.4 days
  • No. of moons: 5
  • Atmosphere: Practically non-existent, 0.3 - 1.0 Pa; N, CH4, CO
  • Discovery: Clyde Tombaugh, February 18, 1930

New Horizons

The NASA probe New Horizons passed Pluto on Bastille Day, July 14 2015!

Pluto seen from the NASA New Horizons probe, 14 July 2015

July 14 2015: Flyby of Pluto, Charon, Hydra, Nix, Kerberos and Styx. The closest approach to Pluto was 13,695 km, at which point New Horizons was cruising at 13.78 km/s (49,600 km/h).

New Horizons instruments
Instruments of the New Horizons probe, which passed by with closest approach to Pluto, 14 July 2015

NASA probe New Horizons was launched on January 19, 2006, and flew by Jupiter, with nearest approach on February 28, 2007. With this slingshot assist, it passed Saturn's orbit (but Saturn was not at home) on June 8, 2008, and has taken another 7 years to reach Pluto, at 32.9 AU (an AU is the distance from the Earth to the Sun) from the Sun. (March 18, 2011: passed Neptune's orbit; August 25, 2014: passed Neptune's orbit).

Ice mountains on Pluto! This was a totally unexpected discovery, and has scientists hotly debating the mechanisms which caused them

The probe has been sending images of Pluto and its companion Charon, but it was not until May 15, 2015 that its images exceeded the resolution of the Hubble telescope's best efforts (which is testament to the excellence of Hubble, rather than a detraction from NASA's probe specs).

Charon has a dark path of hydrocarbon (methane), which indicates that it exchanges atmosphere with Pluto

The probe will continue towards the outer reaches of the solar system, passing into the Kuiper Belt by 2020, where it will attempt to answer questions about the mysterious bodies which inhabit it, about their surface geologies, interior structures and any atmospheres.

It will then continue on to enter the Heliosphere, where Voyager 1 is now, only around 2040!

Kuiper Belt

The Kuiper Belt is named after Gerard Kuiper, the famous Dutch-American astronomer. It is a disc of small bodies, similar to but much larger than the asteroid belt, and located outside the orbit of the planets. Neptune is the outermost planet, at 30 AU (Astronomical Unit = 150 million km, the distance Earth - Sun). The Kuiper Belt begins at around 32 AU and extends to around 45 AU.

Most of the estimated Kuiper Belt Objects (KBO) consist of frozen methane, ammonia and water. There are at least three dwarf planets: Pluto, Haumea, and Makemake. There are an estimated 100,000 KBOs which have a diameter greater than 100km.

A zone of material is called the Scattered Disc, and includes the periodic comets, and objects with eccentric orbits, such as Eris, with orbits with perihelions as much as 100 AU.

The Big Six

There are 6 objects which have been identified to have trans-Neptunian orbits (orbits that are outside the orbit of Neptune, the furthest planet from the Sun), but are not large enough to be ranked planets.

Transneptunian orbits
Transneptunian objects have very elliptical orbits


Eris was discovered by Mike Brown et al., at the Palomar Observatory, California, in 2005. It is a minor planet (136199 Eris), the second-largest dwarf planet after Pluto. It is, however, the most massive, at 127% the mass of Pluto, and 0.27% of Earth's mass.

    Vital statistics:
  • Mass: 1.66 x 1022 kg = 0.0028 of Earth's mass, 0.23 of the Moon's mass
  • Diameter: 2326 km = 0.18 Earth's
  • Density: 2.52 g/cm3 = 0.46 Earth's density
  • Surface gravity: 0.82 m/s2 = 0.083g
  • Distance from Sun: Aphelion = 97.7 AU (14.6 billion km), Perihelion = 37.9 AU (5.7 billion km)
  • Period of orbit: 558 Earth years
  • No. of moons: 1 (Dysnomia)

The New Planet Nine

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Professor Mike Brown, discoverer of Eris, points out the location of Planet 9

With Pluto's demotion in 2005, the post of Planet 9 has been vacant... until now. On 20 January 2016, Mike Brown and Dr. Konstantin Batygin at California Institute of Technology announced the theoretical discovery (with quadrillion percent certainty, according to Batygin!) of a planet. This object could be about ten times the mass of the Earth, and has a very eccentric orbit (very elliptical) ranging from 200 - 1200 AU (30 - 180 billion km) from the sun, and its orbit has an incline of 30°, and a period of 10-20,000 years.

Currently, it is at its furthest from us, and is located somewhere near Orion.

The planet is at its aphelion, the furthest from the Sun, somewhere near Orion in our sky, and so faint it cannot be seen. Its existence is a theoretical necessity to fit the computer model describing the orbits of the trans-neptunian bodies. Given that this type of indirect detection through the effect of the body's gravity has on visible objects is the means by which William Herschel discovered Neptune and Clyde Tombaugh discovered Pluto in 1930, provides encouragement that the existence of the object will be confirmed soon through more direct means.

Content © Renewable.Media. All rights reserved. Created : July 23, 2015 Last updated :February 20, 2016

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