Comparative sizes of four dwarf planets, which may be joined by up to 50 more objects.
Credit: Hubble Space Telescope
SYDNEY: The status of former planet Pluto has taken another blow, with new research suggesting up to 50 known objects may also meet the criteria to be dwarf planets.
To be labelled as a dwarf planet, an object must meet two criteria, as determined by the International Astronomical Union (IAU): they must be 'nearly round' and they must orbit the Sun.
Until now, however, the point at which 'potato-shaped' planets become 'nearly round' has not been formally defined, said astronomer Charley Lineweaver from Australian National University (ANU) in Canberra.
Potato radius
Lineweaver used the phrase 'potato radius' to describe the point at which objects shaped like potatoes are pulled into nearly spherical shapes by their own gravity, in a paper published in the Proceedings of the 9th Australian Space Science Conference.
Informally, 400 km has been used as the potato radius. However, this is not appropriate for most objects.
"The strength of the material is an important feature that determines how big you have to be to be a sphere," said Lineweaver.
Icy objects have smaller potato radii
"If you're ice you're kind of weak and you only have a radius of 200 km to make you spherical. If you're rocky, like an asteroid, you need about 300 km or more."
'Trans-Neptunian Objects', or TNOs, are those that orbit the Sun at a greater average radius than Neptune.
Astronomers only know of five that have radii larger than 400km, but there are many more between 200 and 400km.
Growing number of dwarf planets
"Of all those 50 objects that will be let into the club, all of them are icy objects," said Lineweaver, "so the relevant potato radius will be 200 km."
According to Lineweaver, Pluto's family is set to get even bigger. "The whole project of finding trans-Neptunian objects didn't even start until 1995 or so," he said. "In 10 years there might be another 50 or 100."
Dwarf Planets Are Planets Too
Adding more dwarf planets is in no way another demotion for Pluto. The reason is that in spite of the controversial IAU decision, dwarf planets are planets too. Dr. Alan Stern, who coined the term, intended it to refer to a subclass of planets large enough to be in hydrostatic equilibrium (pulled into a round shape by their own gravity) but not large enough to gravitationally dominate their orbits. He never intended dwarf planets to be designated as not planets at all. And he said he anticipates there being hundreds of these small planets in our solar system.
Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet. Under this definition, our solar system has 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
Laurel Kornfeld
Dwarf Planets Are Planets Too
All modern books contain the 13 planets of the sol system.
Nobody said that dwarf planets are not planets. ie. the term dwarf 'planet', planet, notice the word planet.
Just like nobody thinks dwarf humans are not humans.
Categorising, is what the human mind does, it is an inherent part of a thing called language.
There is nothing wrong with it, except for the fact it we form judgemental assumptions based upon it.
Peace,
no quizzle
Still waiting for something bigger
As cute as the dwarf planets are I'm still hanging out for that runaway moon of Uranus to turn up in the reaches beyond the Kuiper Belt. It may mass ~150% of Mars' mass, but will doubtless be covered in ice. Or will it? It's almost big enough to have drawn in a moderate layer of H/He like its Ice Giant parents. If Uranus's ice/rock core, massing ~11 Earths, captured three Earth masses of H/He, then its rogue moon, massing ~1/100th of its primary might've captured ~0.03 Earth masses of H/He - and not lost it! On a planet ~7500 km across that's a column mass of ~1 million tons per sq. metre (Earth's atmosphere masses ~10 tons per sq. metre.) A Mini-Ice Giant planet!
A bit unlikely, but why not? The Outer Solar System has taught us to expect the unexpected...