Poor Pluto: A Very Far Outlier Demoted

Poor Pluto has been kicked off the world-famous Solar Planets for regularly failing to stay in its lane—or clear its own lane—during repeated orbits of the sun.

PRAGUE, Czech Republic (AP) — Leading astronomers declared Thursday that Pluto is no longer a planet under historic new guidelines that downsize the solar system from nine planets to eight.

After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is — and isn’t — a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one….

Much-maligned Pluto doesn’t make the grade under the new rules for a planet: “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighborhood around its orbit.”

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune’s….

Now, two of the objects that at one point were cruising toward possible full-fledged planethood will join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto whose discoverer, Michael Brown of the California Institute of Technology, has nicknamed “Xena.”

It’s a cold world out there at the far end of the solar system!

UPDATE: Matt of No-sword and his commenters offer some interesting observations about the etymologies of ‘planet’ and ‘dwarf planet’ in Japanese, Sino-Japanese, Chinese, and even Greek.

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2 Comments

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2 responses to “Poor Pluto: A Very Far Outlier Demoted

  1. This definition is a step backwards in that it does not define objects in the solar system more precisely. In fact, it does exactly the opposite by artificially narrowing the parameters for what is a planet and blurring the distinction between Pluto and other small planets and asteroids, a distinction that is crucial. Pluto has attained a state of hydrostatic equilibrium, meaning it has enough self-gravity to pull itself into a round shape. When this happens, objects become differentiated with core, mantle, and crust, like the bigger planets, and develop geological processes akin to those of larger planets–processes that inert, shapeless asteroids, “minor planets” and non-round KBOs do not have.

    These so-called new rules were adopted by four percent of the IAU, most of whom are not planetary scientists, in a procedure that violated the group’s own bylaws. It was immediately opposed by an equal number of professional astronomers in a petition led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Also, many planetary scientists had no say in this vote, as they do not belong to the IAU. And these rules make no sense in that they say a dwarf planet is not a planet at all and also in that they define objects solely by where they are while ignoring what they are. Under these new rules, Earth would not be a planet if it were in Pluto’s orbit.

    If we really want to be more precise in defining the solar system, we should keep the term planet as broad as possible to encompass any non-self-luminous spheroidal object orbiting a star. We can then distinguish between types of planets through subcategories such as terrestrial planets, gas giants, ice giants, dwarf planets, hot Jupiters, super Earths, etc. Gravitational dominance is significant, but not being gravitationally dominant does not preclude smaller objects in hydrostatic equilibrium from being planets. By that definition, our solar system now has 13 planets–eight gravitationally dominant planets and five smaller dwarf planets.

  2. Laurel,

    Thank you for a very nice and succinct summary of the case against demoting Pluto, and for broadening the definition of what constitutes a planet.

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