Posts Tagged ‘Pluto’

It may be small but Pluto is still a planet

July 13, 2015

It orbits the Sun. It has five moons. It is the tenth most massive object orbiting the Sun. It is not that Pluto is not a planet, but that there are many more planets than the “big ten”. The asteroid belt and the Kuiper belt objects are all also planets. Inventing new definitions and calling them “dwarf planets” or “planetoids” doesn’t change the fundamentals.

The International Astronomical Union (IAU) defines a planet thus:

A planet is a celestial body which:

  1. is in orbit around a Star,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has “cleared the neighbourhood” around its orbit.

Calling Pluto a “dwarf planet” rather than a “planet” has nothing to do with its properties or the properties of the Sun. It is not even a matter of language or semantics. It is merely for the convenience of a bunch of lazy astronomers who were afraid of having too many planets to classify.

Ultimately it is just a matter of usage. For me any celestial body orbiting another is and remains a planetary body. And every body which orbits the Sun directly (and not by virtue of orbiting a planetary body first) is a planet. Every asteroid is a planet of the Sun. Jupiters moons remain planets of Jupiter. And that makes Pluto a planet. New planets may well have been found in the Kuiper belt – but Pluto remains a planet for me, in spite of the failings of the IAU.

Pluto is a planet with moons after all

July 4, 2013

I grew up in a solar system which had nine planets.

And then poor Pluto was stripped of the honour of being a planet and we had only eight.

But then the “dwarf planets” were defined and Pluto joined this group of 5.

… the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects.

The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris.

It is suspected that another hundred or so known objects in the Solar System are dwarf planets.

And we now have at least 13 known planets. Pluto is not only a dwarf planet it also has at least 5 moons and the last 2 have just been named. The 5 are Charon, Styx, Nix, Kerberos and Hydra.

Pluto and its moons: IAU


The International Astronomical Union (IAU) is announcing that the names Kerberos and Styx have officially been recognised for the fourth and fifth moons of Pluto, which were discovered in 2011 and 2012. The names were submitted to the IAU by the leader of the team responsible for the discovery, who had called for the help of the general public in an open contest that attracted a substantial number of participants.

The IAU is pleased to announce that today it has officially recognised the names Kerberos and Styx for the fourth and fifth moons of Pluto respectively (formerly known as P4 and P5). These names were backed by voters in a recently held popular contest, aimed at allowing the public to suggest names for the two recently discovered moons of the most famous dwarf planet in the Solar System.

The new moons were discovered in 2011 and 2012, during observations of the Pluto system made with the NASA/ESA Hubble Space Telescope Wide Field Camera 3, and increasing the number of known Pluto moons to five. Kerberos lies between the orbits of Nix and Hydra, two bigger moons discovered by Hubble in 2005, and Styx lies between Charon, the innermost and biggest moon, and Nix. Both have circular orbits assumed to be in the plane of the other satellites in the system. Kerberos has an estimated diameter of 13 to 34 kilometres, and Styx is thought to be irregular in shape and is 10 to 25 kilometres across.



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