Must we refer to them as "brown dwarfs"? Shirley someone can come up with a more culturally acceptable term... ;) Quoting Wade Starks <StarksWC@ldschurch.org>:

You can blame Patrick for this posting. I had originally compiled these definitions from the internet for my own use. Yesterday, I decided to send my compilation to Patrick to see what he thought. He suggested I post it here. With some trepidation, I do so. I continue to be perplexed by all the confusion amongst astronomers about the definition of a planet. In this age where scientists are seeking the holy grail of a unified theory of everything, it seems there should be some very simple and discrete criteria which can define the difference between stars, brown dwarfs and planets. If certain definitions become complex, or inject ambiguity, should they be modified or discarded?

Like everyone else, I have grown up thinking that our solar system is comprised of 9 large rocky or gaseous objects called planets and other debris which orbit our Sun together with which orbit many of these planets we call moons. Is that view too antiquated? As simple as it sounds, is it really overly complex?

Anyway, see what you think.

Wade Starks

Stars, Brown Dwarfs and Planets A Proposed Definition

In an era when scientists are trying to find a simple unified theory of everything, it should be surprising that they cannot seem to come up with a simple definition of planet. It is with this in mind that I have put together this document a compilation of opinions expressed by various astronomers and astronomical organizations as made available on the Internet. It is based on two more or less subjective criteria. If a rule seemed to simplify or clarify a definition, I included it. If a rule seemed to complicate or make the definition ambiguous, I excluded it. As it turns out, the remaining definitions are based solely on certain limits imposed by gravity and heat on the internal mechanics of the astronomical body. They come to define very discrete and unambiguous boundaries between stars, brown dwarfs, planets and those remaining objects which, by these definitions, are less than planets.

Definition of a star Rule 1: A star must contain enough mass to support and sustain hydrogen fusion.

Rule 2: Or it must contain degenerate matter, such as solid neutronium (the remains of a star).

Types of stars determined by size, age and activity include: Yellow Dwarfs, Red Dwarfs, Super Giants, Variable, Pulsars, Wolf-Rayet, Red Giants, Novas, Super Novas, Hyper Novas, White Dwarfs, Black Dwarfs, Neutron Stars, Magnetron Stars, Quasars, Black Holes.

Definition of a Brown Dwarf, AKA failed star Rule 1: The object does not have sufficient mass to sustain hydrogen fusion. Rule 2: The object has sufficient mass to sustain deuterium fusion.

Note: Like planets Brown Dwarfs do not maintain equilibrium as do stars and they cool over their lifetime. The minimum mass of a Brown Dwarf is considered to be about 13 Jupiter masses; a mass considered sufficient to sustain deuterium fusion. Larger Brown Dwarfs may have supported hydrogen fusion for a brief time, but could not sustain it and consumed little of their hydrogen fuel. The upper limit of the mass of a Brown Dwarf is considered to be less than about 75 Jupiter masses. It is generally accepted that Brown Dwarfs usually form from the contraction of rotating gas as do stars. It seems quite possible for a Brown Dwarf to have formed from the merger of two large gaseous planets whose combined mass would exceed the minimum Brown Dwarf mass limit. Such a Brown Dwarf might then have ultimately formed from an accretion disk.

Definition of a planet Rule 1: A planet must not be so massive that it can support any fusion reactions, even briefly. Rule 2: A planet must not contain degenerate matter, such as a core of solid neutronium that might be left over from the death of a star. Rule 3: A planet must have sufficient mass to have a shape determined by gravity and not by the strength of its materials. This means it is spheroid, i.e., nearly spherical.

Note: These rules allow certain objects in our Solar System such as moons, Ceres and some Kuiper Belt objects to be classified as planets. How the object formed, or whether or not it is in orbit around a star is not part of this definition of a planet. However, it is generally accepted that most planets do form by accretion in the disc of gas, dust and rocks left behind by a proto star and therefore do orbit stars or Brown Dwarfs. Is seems quite reasonable that some planets might be ejected from their birth stellar systems and become wandering planets in interstellar space. Such objects would not by such action somehow metamorphose and cease to be planets. So also it is reasonable that some larger gas giant planets might have formed by the contraction of rotating gas as do stars and brown dwarfs.

Roles of planets (special meanings applied to certain terms) 1. Major Planet a planet that is in orbit around a star. 2. Minor Planet i.e., a natural satellite, or moon a planet which is in orbit around another planet. This definition allows moons in turn to have moons themselves. 3. Wandering Planet This is a planet which is not in orbit around any other object, except perhaps, though not necessarily, around a Galaxy.

Definition of a sub-planet, i.e., a planetoid (again exapting a word for this definition) Planetoids would be any objects in space too small to fit the definition of a planet, e.g., asteroids, comets and other space debris larger than gas and dust. These may exist in orbits around a star, a brown dwarf, or a planet.

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