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.

Although a widely accepted taxonomy of extrasolar planets does not yet exist, much theory and observation have established its foundations. A few key factors are typically 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 50 exoplanets have now been characterized in this way (Torres et al. 2008, Extrasolar Planets Encyclopaedia). They range in mass from Gliese 436 b at 0.073 MJUP to XO3-b at 11.79 MJUP. For these planets, astronomers can determine radius as well as mass, providing 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 giants, and gas giants. Together they span at least five orders of magnitude in mass (see Fortney et al. 2007).

terrestrial planets

At the bottom of the range 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 M dwarf and K-type stars (GJ 876 d; GJ 581 c, d; HD 40307 b, c, d; HD 181433 b) invites a new perspective on this question. Although their host stars are relatively lightweight and metal-poor, indicating a shortage of heavy elements in their native environments, the planets themselves have minimum masses ranging from about 4 MEA to 9 MEA. The most likely explanation for the enhanced mass is that these planets contain a large proportion of volatiles, having formed by accreting icy planetesimals and migrating 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 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).

Once assembled, these “ice dwarfs” could be transported into the inner system by various mechanisms, including planet-planet scattering, Type I migration, and Type II 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” (aka “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 mass in volatiles 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/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).

ice giant planets

The defining criterion for ice giants seems to be the accretion of enough mass in ice to retain a substantial hydrogen-helium atmosphere. This condition is satisfied in the Solar System by Uranus and Neptune, whose masses are 14.5 and 17.2 MEA, respectively. Among extrasolar planets, more than a dozen have been detected in the range of 11 MEA to 50 MEA. Their composition may be inferred from transit studies of the classic Hot Neptune, Gliese 436 b, which resembles Uranus and Neptune: 25% metals and silicate rock, 60%-70% water/ammonia ice, and 5%-15% hydrogen and helium (Gillon et al. 2007; see also Guillot 2005). Planets of this type necessarily form outside their systems' ice lines, where sufficient quantities of icy planetesimals are available to ensure the assembly of massive cores and the capture of hydrogen envelopes before the protoplanetary nebula disperses.

The upper mass limit for icy planets is 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, leaving few stragglers in the interval (Ida & Lin 2004). Their model suggests that the upper limit for ice giants is around 20 MEA, while the lower limit for gas giants is around 95 MEA (i.e., the mass of Saturn).

Discoveries in the meantime have failed to support this prediction, as illustrated by the graph below. It uses a sample of 54 exoplanets, representing all reported radial velocity discoveries with m sin i < 150 MEA (0.47 MJUP) tabulated in the Extrasolar Planets Encyclopaedia as of September 2008. This range was selected to highlight any potential boundary separating ice giants from gas giants. Although a spike in the distribution is apparent between 10 and 20 MEA, coinciding with the mass range of Uranus and Neptune, no further pattern emerges as planet mass ascends into the gas giant range.

We may tentatively conclude that 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 ends well short of 100 MEA – possibly around 60 MEA.

gas giant planets

Gas giants occupy the top of the range, beginning somewhere below 100 MEA (0.3 MJUP) and extending well beyond 1000 MEA (3 MJUP). Current data on more than 300 extrasolar planets, as tabulated in the Extrasolar Planets Encyclopaedia, place the median mass of this population at about 555 MEA (1.75 MJUP), counting the minimum gas giant mass as 65 MEA (0.2 MJUP). Even for gas giants, however, the upper boundary is as uncertain as the lower. 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 small population of objects on planet-like orbits (currently about a dozen) 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?

With regard to size, both theory and observation indicate that a gas giant's radius is determined 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. Around a Sun-like star, a slightly sub-Saturn-mass planet (0.24 MJUP) whose core is 10 MEA will have a diameter very similar to Jupiter's (0.946 DJUP) if it travels in a “hot” orbit of 0.045 AU. The same planet will shrink to 0.844 DJUP if it orbits at the actual semimajor axis of Saturn (9.5 AU). If we amplify the planet's mass to 11.3 MJUP (just below the brown dwarf limit), its diameter will increase only to 1.050 DJUP at a semimajor axis of 0.045 AU and to 1.021 DJUP at 9.5 AU. Thus a gas giant’s diameter may vary by a factor of less than 2 while its mass varies by a factor of 47. Also, as Fortney and colleagues show, 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, coolest, 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 September 2008