Is Pluto a planet or a dwarf planet, that is the question.
We all grew up learning that Pluto was the ninth planet in the solar system. However, discovery of several objects of similar size in the Kuiper belt led the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a Dwarf Planet, thereby reducing the number of planets in the Solar System to 8.
Now, advocates of Pluto's planethood led by Alan Stern, principal investigator of the New Horizons mission to Pluto, will officially propose a new definition of "planet", at the 48th Lunar and Planetary Science Conference in The Woodlands, Texas, to be held during March 20-24, 2017. The new definition will restore planetary status to Pluto.
The IAU Definition of Planet
The definition of planet set in Prague, Czech Republic in August 2006 by the IAU states that, in the Solar System, a planet is a celestial body which:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has "cleared the neighborhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet".
Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (part of the Kuiper belt); Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit. Hence, it was re-classified as a dwarf planet.
The following diagram shows the naming conventions and relationships for objects in the Solar System. Pluto resides in the “plutoids” category in the diagram. A plutoid is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is massive enough to be rounded in shape.
Reason for the 2006 Definition
In the 1990s, astronomers began finding other objects in the Kuiper Belt. Many of these shared some of Pluto's key orbital characteristics and are now called plutinos. Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit, it became clear to many that either they all had to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. Similar issues would arise with objects in other planetary systems.
Reaction to the “Demotion” of Pluto
Alan Stern publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons". He contended that Earth, Mars, Jupiter, and Neptune have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. Stern said that the reclassification largely stemmed from the opinions of a small fraction of astronomers, not planetary scientists.
However, other astronomers have countered the argument that Earth, Mars and other planets have not “cleared the neighborhood” by saying that, the major planets, unlike Pluto, completely control the orbits of the other bodies within their orbital zone, such as asteroids at their Lagrangian points.
Astronomer Mike Brown, the discoverer of rival Eris and several other large trans-Neptunian worlds states - "If you think of the solar system as a place consisting of eight planets ... and then a swarm of asteroids and a swarm of Kuiper belt objects, you have a profound description of the local universe around us. Understanding how such a solar system came to be is one of the major tasks of a wide range of modern astronomers. If, on the other hand, your definition simply requires a spherical shape and you think of the solar system as a place with large things that are round and smaller things that are not quite round, you have a relatively trivial description of the universe around us. There is nothing important to study here: We've known for hundreds of years that gravity pulls things in space into the shape of a sphere."
The New Proposed Definition of a Planet
Stern at al propose the following definition of a planet in their paper to be presented next month —
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.
In simpler words, planets are “round objects in space that are smaller than stars”.
The authors state that “In keeping with emphasizing intrinsic properties, our geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects.”
The new definition classifies dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth’s Moon as “full fledged” planets. With this definition of a planet, the Solar System contains at least 110 known planets.
The author’s address the obvious concern of naming 110 planets as follows -
Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn only a few (9? 12? 25?) planets of interest. For analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets.
Dwarf Planets
The IAU currently recognizes five dwarf planets - Ceres, Pluto, Haumea, Makemake and Eris.
Another hundred or so known objects in the Solar System are suspected to be dwarf planets. Estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.
Dwarf planet Eris is 27% more massive than Pluto although Pluto is slightly larger by volume; its high elliptical orbit takes it 3 times farther from the Sun than Pluto.
Pluto Information
Feature |
Value |
Discovery date |
Feb 18, 1930 |
Mass |
0.218% of Earth
17.7% of Moon
4% of Mercury
|
Orbital radius |
29 - 39 times the Earth-Sun distance
|
Distance from Earth on Feb 27 |
5.071 billion km |
Time for light to travel from Pluto to Earth on Feb 27 |
4.7 hours |
Orbital period |
248 years |
Surface Temp |
-375o F (-225o C). Brrr. |
Number of Moons |
5
More than Mercury, Venus, Earth (1) and Mars (2) combined
|
Unlike other planets, Pluto's orbit is moderately inclined relative to the ecliptic — over 17°. Pluto’s orbit is the most eccentric of any planet in our Solar System.
Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding (Pluto orbits the Sun twice for every three Neptune orbits).
The New Horizons mission has revealed Pluto to be an unusually active geological world with flowing glaciers, cryovolcanoes, ice mountains, canyons, cliffs, and a large nitrogen glacier.
Pluto is thought of to have a rocky core surrounded by a mantle of water ice. Mountains are seen rising 2 to 3 kilometers above their surroundings. The mountains are likely made of "water ice-based bedrock". Frozen gases on Pluto's surface include nitrogen (N2), carbon monoxide (CO) and methane (CH4).
When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than 5% of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. www.space.com/...
Here is a short video of close-up images of Pluto taken by the New Horizons spacecraft last year.
The following set of naming themes for features on Pluto and its five moons, informally used by the New Horizons mission, was approved by the IAU last week-
- Gods, goddesses, and other beings associated with the underworld in folklore, mythology, and literature.
- Names for underworld locations from various cultures around the world.
- Mythical heroes and explorers of the underworld.
- Scientists and engineers involved in the study of Pluto and the Kuiper Belt.
- Pioneering space missions and spacecraft.
- Historic pioneers in the exploration of the Earth, ocean, and sky.
Note that in mythology, Pluto is the god of the underworld; the name was proposed by Venetia Burney (1918–2009), a then eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.
Moons of Pluto
Pluto’s five moons, in order of distance from Pluto, are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest of the five moons, is mutually tidally locked with Pluto, and is massive enough (12.5% of Pluto’s mass) that Pluto–Charon is sometimes considered a double dwarf planet.
Here is an animation of Pluto’s five moons. Charon is the innermost moon.
Here is a simulated flyover over Charon, made using data taken by the New Horizons spacecraft.
References
- “A Geophysical Planet Definition”, Stern at al, www.hou.usra.edu/...
- IAU definition of planet — en.wikipedia.org/…
- Pluto and the Developing Landscape of Our Solar System (IAU 2006 paper) — www.iau.org/…
- “How I Killed Pluto and Why It Had It Coming”, Mike Brown — www.amazon.com/...
- Pluto wiki — en.wikipedia.org/…
- NASA New Horizons mission page — www.nasa.gov/…