One of the basic activities of science is the categorization of objects being observed based on common properties. Whether the objects be plants, rocks, subatomic particles or some physical phenomenon, categorization helps to illuminate the relationships between things providing new insights into their nature and the forces that shape them. Astronomy is no different with categories of observed objects including galaxies, nebulas, stars and planets among many more.

When a sufficient number of objects have been categorized, it is frequently possible to recognize the existence of smaller groupings or sub-categories. Early on astronomers recognized that the planets in our solar system could be subcategorized as being either rocky planets like the Earth or volatile rich giants like Jupiter. With the discovery of thousands of extrasolar planets over the last two decades, astronomers are beginning to create new categories of planets as well as refine their existing definitions. American astronomer Artie T. Hatzes (Thuringian State Observatory in Tautenberg, Germany) and German astronomer Heike Rauer (Institute for Planetary Research in Berlin) have submitted a paper for publication in which they propose a new definition for “giant planets” based on the observed properties of the not only the largest planets but substellar objects known as brown dwarfs – objects that presumably form like main sequence stars but have insufficient mass to maintain energy producing fusion reactions like normal stars.

As the first step in their analysis, Hatzes and Rauer constructed a mass-density diagram for the full range of objects with masses from planets through main sequence stars. Data for the extrasolar planets in their sample were largely restricted to that derived from transit observations from to ESA’s CoRot and NASA’s Kepler missions for radius information with follow up precision radial velocity observations providing measurements of the mass. Information on the much less numerous brown dwarfs came from a range of published ground-based observations with data on main sequence stars coming from yet other published sources.

An inspection of the resulting mass-density diagram clearly shows three broad categories of bodies. Main sequence stars fall tightly along a trend line of decreasing density, ρ, with increasing mass, M, as had been previously observed by astronomers. Objects with masses between 0.3 and about 60 times the mass of Jupiter (or M J ), however, fall tightly along a distinctly different line in mass-density space of increasing density with increasing mass. Hatzes and Rauer found that this trend can be described by the best fit linear equation logρ = (1.15±0.03) logM – (0.11±0.03), where M is in units of M J , with a correlation coefficient of 0.976 (where 1 is defined as a perfect fit). Still smaller bodies display a significantly greater amount of scatter with a general trend of increasing density with decreasing mass (see “A Mass-Radius Relationship for ‘Sub-Neptunes‘“). These smaller bodies, which Hatzes and Rauer call “low mass planets”, would include bodies like Neptune, mini-Neptunes and rocky planets which display a large range in the relative amounts of hydrogen/helium, water and rock/iron (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”).

Based on this analysis, Hatzes and Rauer propose that bodies in the 0.3 to about 60 M J mass range be designated “gaseous giant planets”. While this mass range covers planets like Jupiter and Saturn which are believed to have formed by core accretion (i.e. a large rocky protoplanetary core draws gas directly from the disk of gas and dust encircling a young star), it also includes brown dwarfs which presumably form by the direct collapse of a cloud of gas just like main sequence stars. While there is sparse data available, there does appear to be a transition in the trend line in the 60 to 80 M J mass range (the upper end of which represents the lowest mass main sequence stars). While more data will be needed, bodies in this mass range may represent a different class of object.

Hatzes and Rauer argue that the distinction between large planets and brown dwarfs is meaningless in terms of the properties of these bodies since there is no clear change seen in the mass-density trend line. For example, the generally accepted 12 M J boundary between large planets and brown dwarfs is set by the fact that more massive bodies would briefly fuse deuterium during their earliest history. But this brief deuterium-burning phase early on has little discernable impact on the observed properties much later in life. There has been recent work on the population distribution of substellar bodies which suggests a dividing line near 25 M J . Neither of these limits are reflected in an obvious change in the mass-density trend. Given the large mass range, the authors merely recommend that there be two broad sub-categories – “low mass giant planets” and “high mass giant planets”. They further speculate that maybe objects in the 60 to 80 M J mass range might represent “true” brown dwarfs.

While there are astronomers who insist on there being a distinction made between large planets and brown dwarfs based solely on the differing modes of formation, these differences are not expected to have much impact in the observed properties of these bodies. In other words, a 12 M J planet will have essentially the same properties as a 12 M J brown dwarf and it is unlikely that the two could be differentiated by any sort of measurements that can be made in the foreseeable future. Besides, Hatzes and Rauer point out that stars can form by methods other than the simple collapse of a cloud of gas, such as the merger of two smaller bodies, yet astronomers still categorize all such bodies as “stars” despite the differences in the details of their formation mechanism since their properties are largely indistinguishable.

As summarized by Hatzes and Rauer, “we propose a new definition of planets, brown dwarfs, and stars based not on arbitrary separation of distributions, or whether short-lived deuterium burning has occurred, or just because we are biased in thinking that giant planets should all have masses close to that of our Jupiter.” While this will hardly be the last word on this subject and there will be those who insist on differentiating these sub-stellar bodies based solely on their formation mechanism, scientists have finally reached a point where observational data from a large number of bodies can be used to define meaningful categories for a variety of bodies unknown to science only a generation ago.

A French translation of this article by Alexandre Lomaev is also available: “Nouvelle définition des géante gazeuse?”, Extrasolar.fr – Encylopédie des Mondes Extérieurs, August 7, 2015 (in French) [Post]

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Related Reading

“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]

“The Transition from Rocky to Non-Rocky Planets”, Centauri Dreams, November 14, 2014 [Post]

“The Composition of Super Earths”, Drew Ex Machina, January 3, 2015 [Post]

“A Mass-Radius Relationship for ‘Sub-Neptunes'”, Centauri Dreams, May 22, 2015 [Post]

General References

Artie P. Hatzes and Heike Rauer, “A Definition for Giant Planet Based on the Mass-Density Relationship”, arXiv 1506.05097 (submitted for publication in The Astrophysical Journal), June 16, 2015 [Preprint]