The current definition, established by the International Astronomical Union in 2006, states that to be considered a planet, an object must be in the solar system and orbit the Sun. However, this definition is problematic in that it is not quantitative and excludes exoplanets. A proposed new definition states that an object can orbit one or more stars, brown dwarfs, or stellar remnants, and sets a mass limit that should apply to any planet.
Artist's impression of an exoplanet and its host star. Image courtesy of Sci.News.
In 2006, the International Astronomical Union (IAU) adopted Resolution B5, which defines a planet as a celestial body that (a) orbits the Sun, (b) has sufficient mass so that its own gravity overcomes the forces of a rigid body, has a shape in hydrostatic equilibrium (approximately circular), and (c) is swept around its orbit.
“The current definition specifically mentions orbiting the sun,” said Professor Jean-Luc Margot of the University of California, Los Angeles.
“Thousands of planets are currently known to exist, but the IAU definition only applies to planets in our solar system.”
“We propose a new definition that can be applied to objects orbiting any star, stellar remnant, or brown dwarf.”
Prof Margot and his colleagues argue that the requirement to orbit the Sun is too specific, while other criteria in the IAU definition are too vague.
For example, it says that the planet has “gone out of orbit,” but doesn't say what that means.
The proposed new definition includes quantifiable criteria that can be applied to define planets inside and outside the solar system.
Under the new definition, a planet is (a) an object that orbits one or more stars, brown dwarfs, or stellar remnants; (b) a mass greater than or equal to 10twenty three kg, (c) is 13 times the mass of Jupiter (2.5 × 1028 kg).
The authors ran mathematical algorithms on the properties of objects in the solar system to determine which ones are densely populated.
This analysis reveals groups of distinct properties common to planets in our solar system, which can be used as a starting point for creating a general classification of planets.
For example, if an object's gravitational force is sufficient to accumulate or eject smaller objects nearby to clear a path, the object is said to be dynamically dominant.
“All planets in the solar system are dynamically dominated, but other objects, including dwarf planets like Pluto and asteroids, are not, so this property can be included in the definition of a planet,” Prof Margot said.
Dynamic dominance requirements dictate a lower mass limit.
But it's also possible that the potential planet is too large to fit the new definition.
For example, some gas giants can grow so large that thermonuclear fusion of deuterium occurs, and the object becomes a star known as a brown dwarf, no longer a planet. This limit has been determined to be more than 13 Jupiter masses.
On the other hand, the current requirement that it be spherical is more problematic.
Distant planets are rarely observed in enough detail to determine their shapes with certainty.
The researchers argue that even though planets are generally round, the shape requirements would be so difficult to implement that they are virtually useless for definitional purposes.
“Fixing the definition to mass, the quantity that is most easily measured, eliminates debate about whether a particular object meets the criteria – this is a weakness of the current definition,” said Dr Brett Gladman, a researcher at the University of British Columbia.
“The good news is that in the solar system,twenty one kg looks round.”
“Thus, any object that meets the proposed lower mass limit of 10twenty three kg is expected to be spherical.”
Team paper Published in Planetary Science Journal.
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Jean-Luc Margot othersA quantitative criterion for defining planets. Planet Science Journal 5,159; doi: 10.3847/PSJ/ad55f3
Source: www.sci.news
