Solar and extrasolar planets of various masses, from rocky terrestrial planets (Mars, Earth) through ice giants (Uranus) to gas giants (Saturn, HD 128311 c). Numbers above the red line indicate approximate mass in units of Earth mass (MEA); numbers below the red line indicate mass in units of Jupiter mass (MJUP). Planets are represented at their approximate relative sizes. HD 128311 c is assumed to have a diameter of 1.16 DJUP, given a mass of 3.22 MJUP, a semimajor axis of 1.76 AU, a system age of 300 million years, and a refractory core of 10 MEA.

Taxonomy refers to any orderly system of classification. Perhaps the most familiar example of a taxonomy is the assignment of genus and species names to biological organisms, such as Homo sapiens (people), Ursus maritimus (polar bears), or Triticum aestivum (wheat).

Although a widely accepted taxonomy of extrasolar planets does not yet exist, theory and observation have established its foundations. A few key factors are usually regarded as significant: planet mass, internal composition, and formation history.

The most reliable data on exoplanets derive from radial velocity and photometric transit observations. When a system can be observed by using both methods, a wealth of evidence is available. More than 60 exoplanets have now been characterized in this way (Torres et al. 2008, Extrasolar Planets Encyclopaedia). They range in mass from CoRoT-7b at 0.015 MJUP (4.8 MEA) to XO3-b at 11.79 MJUP. For these planets, astronomers can determine radius as well as mass, and thus provide important constraints on composition.

In combination with evidence from the Solar System, whose planets we know best, radial velocity and transit data suggest a basic planetary taxonomy that includes at least three major species:

• terrestrial planets
• ice giant planets
• gas giant planets

All three consist of the same basic ingredients mixed in different proportions: metals (mostly iron and nickel), silicate rocks, ices (water, methane, ammonia), and hydrogen (typically accompanied by a small proportion of helium). Together these three planetary species span at least five orders of magnitude in mass (see Fortney et al. 2007).

terrestrial planets

At the bottom of the mass distribution are the terrestrial planets. The Solar System has four, ranging from Mercury – a dense, airless, highly metallic world with only 6% of Earth's mass – to the Earth itself, which has a substantial atmosphere and a very modest endowment of ice in addition to its metallic core and silicate mantle. The upper mass limit for such planets is poorly understood. Although 10 MEA is usually given as the boundary (e.g., Ida & Lin 2004), numerical simulations of planet formation typically yield smaller maximum masses (Raymond et al. 2004, Kenyon & Bromley 2006), in the range of 2 MEA to 4 MEA for planets composed primarily of refractory elements (i.e., metals and silicates).

The recent discovery of several “Super Earths” in short-period orbits around nearby G, K, and M dwarf stars invites a new perspective on this question. Although their host stars are often metal-poor, indicating a shortage of heavy elements in their native environments, the planets themselves have minimum masses ranging from about 2 MEA to 9 MEA. In many if not most cases, the likeliest explanation for their enhanced mass is a large endowment of volatiles (water, methane, ammonia). Most such objects probably formed by accreting icy planetesimals relatively far from the host star and then migrated inward to their present hot orbits (Raymond et al. 2007).

Accordingly, several studies propose a subdivision of low-mass planets into two species: (1) rocky planets that resemble those in the inner Solar System and (2) icy planets that resemble scaled-up versions of Ganymede and Callisto, the two most massive and volatile-rich moons of Jupiter (Kuchner 2003, Leger et al. 2004, Sotin et al. 2007, Selsis et al. 2007). Presumably the rocky planets will form inside the system ice line and the icy planets will form outside it. The latter are likely to be 50% refractory and 50% ice (Leger et al. 2004, Selsis et al. 2007).

Earth and Super Earths. Comparative sizes and bulk compositions of three terrestrial-mass planets. Left: Earth, mass 1 Mea, diameter 7900 mi (12,755 km). Center: Rocky Super Earth, mass 3 Mea, diameter 10,665 mi (17,220 km). Right: Icy Super Earth, mass 6 Mea, diameter 15,010 mi (24,235 km). Mass-radius relationships follow Fortney et al. 2007.

Once assembled, these “ice dwarfs” or “min-Neptunes” could be transported into the inner system by various mechanisms, including planet-planet scattering and Type I migration. Their subsequent evolution would depend on their final semimajor axes and on the luminosity of their host stars. In the habitable zone, the outer layers of an icy planet would melt, resulting in an “Ocean Planet” with a rocky core surrounded by a thick layer of ice supporting a global ocean 100 kilometers deep. Starward of this region, water cannot be sustained in liquid form, so the result would be a “Steam Planet” or “Sauna World” in which a thick, hot atmosphere of water vapor is in direct contact with a high-pressure mantle of ice. Remarkably, a sufficiently massive Steam Planet is likely to maintain its volatile content even on a star-grazing orbit. Selsis and colleagues have found that a planet of 6 MEA whose composition is 50% ice and 50% rock and metal can maintain its reservoir of water for more than 5 billion years, even at a semimajor axis of 0.04 AU (Selsis et al. 2007).

Among stars, low-mass objects vastly outnumber high-mass objects. A similar relationship holds among the objects in our Solar System, which contains two gas giants, two ice giants, four terrestrial planets, well over 100 moons, and hundreds of thousands of meter-sized bits of debris. Although conclusive proof is still lacking, the same situation very likely prevails in extrasolar systems. Given the limitations of all current detection methods, announced gas giants outnumber all announced ice giants and Super Earths combined by a factor of 10 to 1. Nevertheless, Michel Mayor and colleagues have recently estimated that about 30% of G and K stars in the Solar neighborhood harbor ice giants or Super Earths with periods shorter than 50 days -- in other words, closer to their host stars than Mercury is to our Sun (Mayor et al., in press 2009). This figure substantially exceeds the proposed population of nearby extrasolar gas giants. Andrew Cumming and colleagues have estimated that 17% of nearby Sun-like stars have gas giant planets orbiting within 20 AU, the approximate semimajor axis of Uranus (Cumming et al. 2008).

Super Earths, Ice Giants, Gas Giants
mass distribution of 89 exoplanets < 170 Mea (0.535 Mjup)
data from Extrasolar Planets Encyclopaedia as of 19 October 2009

ice giant planets

The defining criterion for an ice giant seems to be the accretion of enough frozen volatiles (water, methane, ammonia) around a heavy-element core to retain a hydrogen-helium atmosphere comprising about 10% of the object’s total mass. Theoretical studies indicate that about 10 MEA is the minimum mass needed to sustain such an atmosphere (Pollack et al. 1996). In the Solar System, this condition is satisfied by Uranus and Neptune, whose overall masses are 14.5 and 17.2 MEA, respectively. Each of these planets has an extensive rock/ice core enveloped by more than an Earth mass in gaseous hydrogen (Guillot 1999).

Among extrasolar planets, two similar objects have been studied both by radial velocity measurements and by transit photometry: Gliese 436 b and HAT-P-11b. Like most other transiting planets known to date, these two orbit very close to their host stars, with respective periods of 2.6 and 4.9 days. Their masses are well constrained: Gliese 436 b at 23.2 MEA and HAT-P-11b at 25.8 MEA (Torres et al. 2008, Bakos et al. 2009). Precise data on the two planets' radii indicate that each contains at least 10% of its mass in gaseous hydrogen, enveloping a core of rock and ices comprising the remaining 90% (Gillon et al. 2007, Bakos et al. 2009). Thus both planets are generally described as Hot Neptunes.

Objects of this type, nevertheless, must form outside their systems' ice lines, where sufficient quantities of icy planetesimals are available to ensure the assembly of massive cores capable of retaining hydrogen atmospheres. Their upper mass limit remains uncertain. Ida & Lin predicted a “planet desert” between 10 and 100 MEA, on the grounds that gaseous accretion is so rapid once a planetary core reaches 10 MEA that it will quickly expand to the mass of Saturn (95 MEA), leaving few stragglers in the interval (Ida & Lin 2004).

Discoveries in the meantime have not supported this prediction, as illustrated by the graph above. It uses a sample of 89 exoplanets, representing all reported radial velocity discoveries with m sin i < 170 MEA (0.535 MJUP) listed in the Extrasolar Planets Encyclopaedia as of October 2009. This range was selected to highlight any potential boundary separating ice giants from gas giants. Although numbers are high between 10 and 20 MEA, coinciding with the mass range of Uranus and Neptune, no corresponding spike appears around the mass of Saturn. Instead, one or more "valleys" appears between 40 and 140 MEA, where we might expect to see high-mass ice giants and low-mass gas giants. The lowest point in the distribution appears around the mass of Saturn.

Evidently rocky planets span almost three orders of magnitude, from less than 0.1 MEA to the vicinity of 10 MEA, whereas ice giants seem to be a transitional species. Their distribution begins around 10 MEA and then blends into the range of the gas giants, somewhere between 50 and 70 MEA.

Mass Distribution of 313 Exoplanets < 5.35 MJUP
in units of Earth and Jupiter mass (100 Mea = 0.314 Mjup)
data from Extrasolar Planets Encyclopaedia as of 19 October 2009

gas giant planets

Gas giants occupy the top of the mass range, beginning at about 60 MEA (0.2 MJUP) and extending well beyond 1000 MEA (~3 MJUP). Their name reflects the assumption that their overall mass is about 75% to 95% hydrogen, like the bulk composition of Jupiter and Saturn.

The recent discovery of the transiting planet HAT-P-12b presents the most lightweight gas giant ever observed. This object has a mass of only 0.21 MJUP (67 MEA), substantially less than Saturn. Measurement of its radius establishes that the planet's bulk composition is dominated by hydrogen and helium, while its mass in heavy elements is no greater than 10 MEA (Hartman et al. 2009). Thus HAT-P-12b is at least 85% hydrogen-helium and no more than 15% rock, metal, and ice.

Current data on more than 400 extrasolar planets, as listed in the Extrasolar Planets Encyclopaedia, place the median mass of the gas giant population at about 555 MEA (1.75 MJUP). Even for this population, however, the upper limit is imprecise. Uncertainty stems from the fact that any substellar object above 13 MJUP may be classified as a brown dwarf: a minimally luminous object that forms like a star through the collapse of a molecular hydrogen cloud. Although gas giants become progressively scarcer above 5 MJUP, a population of massive objects on planet-like orbits (more than 20 so far) has been reported between 10 and 25 MJUP, coinciding with the brown dwarf range. For this group of “super planets,” data on mass and composition may be insufficient to establish their true nature; we may need to know their formation histories, which so far remain out of reach. See also Super Jupiter or Brown Dwarf?

Mass Distribution of 358 Exoplanets < 13 MJUP
data from Extrasolar Planets Encyclopaedia as of 19 October 2009

With regard to size, both theory and observation indicate that a gas giant's radius is determined primarily by its temperature and composition rather than its mass. Fortney and colleagues (2007) provide theoretical tables of planetary radii for various combinations of age, semimajor axis, mass, and internal structure. All gas giant planets with similar ages and bulk compositions have very similar diameters, regardless of their variation in mass. Also, as Fortney and colleagues demonstrate, gas giants are systematically larger when their cores are less massive, and smaller when their cores are more massive (Fortney et al. 2007).

According to these models, if we consider all gas giants at all ages, masses, core masses, and semimajor axes, the potential range in radii extends from 1.656 RJUP for the youngest, hottest, least massive, and least dense, down to 0.558 RJUP for the oldest, coldest, least massive, and most dense (Fortney et al. 2007). Note that, somewhat counterintuitively, the least massive gas giants present both the largest and the smallest radii.

Observational data show good but not perfect agreement with the models of Fortney and colleagues. A recent study by Guillermo Torres and colleagues (2008) presents self-consistent parameters for 23 well-characterized transiting exoplanets. Among gas giants in this population – all of which are extremely hot, traveling in extremely tight orbits – the observed range of radii extends from 1.751 RJUP for TrES-4 (0.92 MJUP) down to 0.654 RJUP for HD 149026 b (0.359 MJUP). The top of this range exceeds theoretical predictions, posing many unresolved questions.

Last update: October 2009