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astronomytwins (Uranus and Neptune ** )

Astronomy — The Twin Planets, Uranus and Neptune

The information on this page was obtained from a fascinating article in the December 2019 edition of ‘Sky and Telescope’ magazine.  The article was entitled ‘The Neglected Planets’, and was written by Ken Croswell.

  • With similar colors, diameters, masses, densities, and rotation rates, Uranus and Neptune are twin worlds.
  • Both planets owe their distinct colors (green and blue) to methane gas.  Methane absorbs red light but reflects green and blue.
  • These planets, like Jupiter and Saturn, have ice, rock, and metal at their centers.
  • Both of these planets are only about 5% as massive as Jupiter.
  • Uranus and Neptune are ‘ice giants’, so named because they have large quantities of three compounds that were frozen solid in the cold outer solar nebula (water, methane, and ammonia).
  • Both planets have tilted magnetic fields that arise from the center of each planet.
  • These two planets have the same effective temperature, that being -353 degrees Fahrenheit.
  • Neptune is much more ‘active’ than Uranus, with plenty of clouds and storms and internal heat.  Fast winds, however, race across both planets.
  • Both planets have hydrogen sulfide in their atmospheres, a poisonous gas that smells like rotten eggs.
  • Both of the planets spin at a rapid rate, but they have very different axial tilts.  They both rotate once in about 16 or 17 hours.  Uranus lies on its side as it spins, with an axial tilt of 98 degrees.  Neptune’s axial tilt is 28 degrees, similar to Earth’s tilt.
  • Uranus has 13 rings and Neptune has six.  Whereas Saturn’s rings glisten with water ice, those of Uranus and Neptune are dark, probably due to carbon compounds.
  • Uranus has 27 known moons and 13 rings.  The 18 inner-most moons orbit the planet’s equator.  The others follow highly inclined paths, suggesting that they are captured objects.
  • Neptune has 14 moons. The stand-out moon for this planet is Triton.  It is nearly twice as large as Uranus’s largest moon and slightly larger than Pluto!  Yet Triton revolves backward, a sign that it too is a captured object.  Triton is about as close to Neptune as the Moon is to Earth
  • The moon Nereid, a moon of Neptune, has the most elliptical orbit of any moon in our solar system, with an orbital eccentricity of a whopping 75%.
  • The most distant moon of Neptune is Neso.  It ventures farther away from its planet that any other moon in the solar system.  At its extreme, Neso is 72 million kilometers from Neptune, nearly half the distance of the Earth from the Sun.  So distant is Neso that it takes 26 years to orbits Neptune one time, nearly as long as it takes Saturn to circle the Sun.  Neptune can retain this distant moon because the planet is so far removed from the gravitational influence of the Sun.

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