Radial velocity observations are the most reliable and consistent method of detecting extrasolar planets (see Detection Methods). Even under the most favorable circumstances, however, they yield very limited data: orbital period and eccentricity, approximate semimajor axis, minimum mass. Moving from this stark list of numerical values to a three-dimensional view of a planet requires considerable extrapolation, much of which is informed by theoretical speculations that remain in flux.

Early on, it was recognized that extrasolar planets vary widely in terms of the shapes and sizes of their orbits. In this regard they bear little resemblance to those in the Solar System. Seven out of eight planets circling our Sun, with semimajor axes ranging from 0.7 to 30 AU, travel in orbits that are almost perfect circles. Their orbital eccentricities - the fraction by which they deviate from circularity - are all smaller than 0.1. Even the most elliptical orbit, that of Mercury, has an eccentricity of only 0.206. Although Mercury's semimajor axis is 0.39 AU, it approaches the Sun as close as 0.31 AU and retreats as far as 0.47 AU over the course of a single orbit.

Extrasolar planets tend to follow orbits that are either much tighter or much more elliptical. More than 40% have semimajor axes smaller than Mercury's, and about 40% have eccentricities more extreme. Eccentricity varies by distance from the host star. Among planets with semimajor axes smaller than 0.05 AU, more than 90% have orbital eccentricities (e) smaller than 0.10, and none have e > 0.30. However, fewer than 20% of planets orbiting beyond 0.39 AU (equivalent to Mercury's orbit) have e < 0.10, while more than 40% have e > 0.30.

A small group – less than 5% of the total sample – have extreme eccentricities of 0.7 or higher, like the orbits of comets rather than those of planets as we usually imagine them.

By themselves, orbital eccentricity and semimajor axis tell us little about an exoplanet. They must be interpreted in the context of the planet's mass as well as of the mass, luminosity, spectral type, and evolutionary history of its parent star. From the various permutations of these values has emerged an ad hoc bestiary of planets, including these exotic species:

Hot Jupiters   Hot Neptunes   Super Earths   Super Jupiters   Cool Giants

All but the last were unknown and largely unsuspected until the modern era of exoplanetary studies began in the 1990s.

hot jupiters

Among exoplanets located within 100 parsecs of the Sun, about 20% have semimajor axes smaller than 0.1 AU and orbital periods of 10 days or less. For the complete sample, the figure is 30%, because detections beyond 100 parsecs become increasingly biased in favor of short-period planets. Most of this star-hugging population consists of gas giants, meaning planets at least 20% as massive as Jupiter whose bulk composition is about 80%-90% hydrogen. Proximity to their host stars makes these objects hotter than Venus, the planet with the highest surface temperature in the Solar System. As a result such exoplanets are usually known in English as Hot Jupiters. In French they may be called pegasides (Pegasids), referring to 51 Pegasi, the first such object to be detected, but also recalling a well-known meteor swarm.

evolutionary history

Despite their current “torch” orbits, astronomical consensus favors a cooler and more distant location for the birthplace of Hot Jupiters, in the vicinity of their home system’s ice line (see also Evolution and System Architectures). Under favorable conditions, planetary accretion in this region can proceed very quickly, encouraging newborn gas giants to capture massive atmospheres of hydrogen, open a gap in the circumstellar nebula, and spiral inward to the vicinity of the central star. This inward journey (known as Type II migration) necessarily halts when the migrating planet reaches the inner edge of the nebula, which is truncated by the magnetosphere of the central star at an approximate radius of 0.05 AU. Once migration ends, stellar tides modify the shape of the planet's final orbit in a process known as tidal circularization. Therefore, Hot Jupiters typically have orbital eccentricities approaching zero.

Intense stellar irradiation causes these gaseous bodies to expand in diameter and become increasingly tenuous, as confirmed by photometric transit observations (Charbonneau et al. 2007). Nevertheless, Hot Jupiters are too heavy to evaporate, and they appear to retain their large masses and distended hydrogen envelopes over billions of years (Melo et al. 2006, Hubbard et al. 2007, Lammer et al. 2009).

In fact, the most menacing aspect of a Hot Jupiter's orbital environment is not heat but tides. Recent theoretical studies argue that planets with very short periods (e.g., less than 3 days) are likely to undergo orbital decay through the action of stellar tides and be engulfed by their parent stars before those stars leave the main sequence phase of their evolution (Levrard et al. 2009, Jackson et al. 2009). As Brian Jackson and colleagues conclude, "Tidal destruction of close-in exoplanets is common."

Hot Jupiters are evidently a typical outcome of the evolution of Sun-like stars (those of spectral types F, G, and K), but not of A-type stars or M dwarfs. Their host stars range in mass from 0.71 Msol to 1.52 Msol, similar to the population of Sun-like stars that harbor gas giant planets at larger radii. The distribution of host star metallicities is also unexceptional, ranging from -0.3 to +0.5. Nevertheless, the current sample of Hot Jupiters is slightly biased in favor of high mass and enrichment in metals, given a median host star mass of 1.1 Msol and a median metallicity of +0.11. At the same time, almost 40% of the sample orbit stars whose metal content is less than or equal to that of our Sun, a star of average metallicity in this region of the Milky Way. As a result, it is uncertain why some stars produce Hot Jupiters while others do not.

distinctive features

Regardless of their origins and fate, Hot Jupiters still manage to survive on timescales of a few billion years or more. David Sudarsky and colleagues have generated theoretical spectra for gas giants at a range of temperatures, which correspond to various combinations of semimajor axis and mass (Sudarsky et al. 2003). For orbits within a few tenths of an AU (corresponding to Hot and “Warm” Jupiters) they suggest more than one possibility. The very closest planets (in their terminology, “roasters”) would be covered by dark clouds of silicate and iron compounds. At somewhat cooler temperatures (but still in the vicinity of 500-1000 K) no clouds at all would be able to form, and planets in this range would present clear spheres of indigo or deep blue (“Gaseous Class III and Class II”).

Despite their nickname, then, Hot Jupiters do not resemble the Solar System’s giant planets. Their tight orbits will prevent the retention of moons and stall their rotation through tidal drag. Thus they always turn the same hemisphere to their host stars, as the Moon does to the Earth. For the same reason, they will lack the colorful cloud bands that we see on our own Jupiter and Saturn, since such features are a direct consequence of rapid rotation (Jupiter and Saturn each rotate in about 10 hours). Nor are they likely to preserve rings in the face of stellar tides.

The known Hot Jupiters comprise a distinct group. Extrasolar gas giants are actually a little more numerous inside a radius of 0.1 AU, the conventional boundary for "hot" planets, than they are between 0.1 AU and 1 AU. This distribution implies that, once initiated, Type II migration is fast and efficient. Migrating planets are more likely to reach the primordial nebula's inner cavity than they are to be stranded at intermediate radii. Even among Hot Jupiters, smaller orbits are more common than larger ones, as almost 70% are found inside 0.05 AU with periods of 4 days or less.

A second distinguishing feature of this population is mass. Although the complete sample covers the full mass range of extrasolar gas giants (0.211-13.75 MJUP), numbers peak around the mass of Jupiter. The median Hot Jupiter mass is 0.94 MJUP, in contrast to the median of 1.75 MJUP for all known gas giants. Eighty-five percent are lighter than 3 MJUP, while 90% are at least 0.4 MJUP.

transiting hot jupiters

More concrete data on hot giant planets have resulted from studies of the select group that are observed in transits of their host stars. By October 2009, more than 60 transiting exoplanets had been characterized through a combination of radial velocity and photometric studies (Torres et al. 2008, Extrasolar Planets Encyclopaedia). Among them, 58 are gas giants, and all but 2 of these 58 orbit within 0.1 AU of their host stars. Thus, transiting giants now comprise a majority of known Hot Jupiters.

The median mass of all transiting Hot Jupiters is 1.03 MJUP. Since the masses of these planets are known precisely, whereas only lower limits are known for non-transiting giants, this is actually a more reliable value for the median mass of Hot Jupiters than the figure of 0.94 MJUP, quoted above, which is derived from combining data on both populations of hot giants. In any case, both analyses agree that the typical Hot Jupiter is more or less identical in mass to our own cold Jupiter.

Transit observations have also been definitive in establishing that gas giants vary widely in terms of composition and structure. On the basis of mass, radius, and inferred density, it seems clear that some are composed almost entirely of hydrogen, like stars, while others have large percentages of heavy elements, more like ice giants or terrestrial planets.

An example of the former is TrES-4, which orbits an F8 star located about 440 parsecs away. The planet's semimajor axis of 0.05 AU corresponds to a period of less than 4 days. The mass of TrES-4 is unexceptional, at about 0.925 MJUP, but its radius is a remarkable 1.783 RJUP. This value indicates the largest volume and lowest density of any exoplanet so far detected (Torres et al. 2008, Sozzetti et al. 2009). Its discovery team speculated that the planet may be enveloped in an extended comet-like tail (Mandushev et al. 2007).

At the other end of the scale is HD 149026 b, which circles a highly metallic G-type subgiant located at a distance of about 79 parsecs. Its semimajor axis of 0.04 AU corresponds to an even shorter period of 2.9 days. At 0.359 MJUP, this planet is slightly more massive than Saturn. Yet the radius of HD 149026 b is anomalously small, measured at 0.725 RJUP (Nutzman et al. 2009). This value implies that the planet has a heavy-element core about 70 times as massive as Earth, comprising about 60% of its overall mass (Nutzman et al. 2009). This is much larger than the heavy-element fractions usually predicted for gas giant planets (5%-25%), posing problems for the core-accretion theory of planet formation (see Broeg & Wuchterl 2007, Dodson-Robinson & Bodenheimer 2009).

HD 209458 b, a more typical example of the species, is relatively well known through spectral analysis (Grillmair et al. 2007, Richardson et al. 2007, Swain et al. 2008). Its mass is 0.685 MJUP; its radius is 1.359 RJUP (Torres et al. 2008). It orbits a hot G0 star located 47 parsecs away, at a semimajor axis of about 0.05 AU, in less than four days. The planet's spectrum indicates that it reflects very little light, and its extended atmosphere appears to contain silicate compounds, as predicted by Sudarsky and colleagues. Evidently HD 209458 b and similar planets are “cloaked in black silicate clouds,” so that they display dim, dark spheres unlike any planets in the Solar System (Sanderson 2007).

Greg Laughlin and David Langton have modeled the atmospheric dynamics of individual transiting Hot Jupiters by generating animated simulations (Langton & Laughlin 2007, Laughlin et al. 2009; see also Laughlin’s blog entry on Vorticity). They focus on the minority of transiting planets that follow eccentric orbits and thus experience substantial temperature variation. Their models reveal extremely turbulent atmospheres, with enormous vortices rolling around the planets’ equatorial zones and in some cases devolving into regions of swirling chaos.

hot neptunes

One distinct population of short-period planets consists of Neptune-mass objects on orbits that are typically smaller than Mercury's. The mass range of these planets begins at about 10 MEA, the minimum needed to accrete and sustain a substantial hydrogen atmosphere, and may rise to the vicinity of 60 MEA. Nevertheless, most are less massive than 35 MEA. Thus they are somewhat heavyweight cousins to the ice giants in our Solar System. Given their warmish to hot orbits, this kinship inevitably suggests the name Hot Neptunes.

To date, two such planets (GJ 436 b and HAT-P-11b) have been observed in transit across the face of their host stars. As with the Hot Jupiters, these observations provide insights into their internal structure. Both of the transiting Hot Neptunes are about 10% hydrogen and 90% ice and rock, just like Uranus and Neptune (Gillon et al. 2007, Bakos et al. 2009).

orbital environment

All but one of the known extrasolar Neptunes orbit within 0.8 AU of their primaries, and about 85% orbit within 0.3 AU. Such an orbital distribution is no surprise. Because ice giants tug less strongly on their host stars, they are more difficult to detect by radial velocity observations than their more massive gaseous cousins. Thus they tend to be observed quite close to their stars, where the effects of their orbital motion are most evident. Yet unlike gas giants, the population of extrasolar ice giants does not exhibit any particular pile-up inside 0.05 AU or even 0.1 AU. Instead, these planets are more evenly dispersed over a range of short- to medium-period orbits. Their distribution surely provides clues to their formation and evolution, if only we knew how to interpret them.

Like most exoplanets of all species, the typical extrasolar Neptune is the only detected planet of its host star. Nevertheless, more than one-third are found in systems with one or more additional planets, and their companions run the gamut of exoplanetary species. The only notable exception to their companionability is the fact that no ice giants have been observed in systems containing a Hot Jupiter, or indeed any gas giant at all on an interior orbit. Apart from this exception, Hot and Warm Neptunes are remarkable for their ability to play well with others. Their sociability is undoubtedly related to their low masses (since more massive planets would be likely to eject other objects from their vicinities) and moderate eccentricities. To date, just one planet in this mass range (HD 117618 b) has a published eccentricity higher than 0.3. More extreme eccentricities are typically understood as the outcome of planet-planet scattering, suggesting that most extrasolar Neptunes originate in relatively placid systems.

Hot and Warm Neptunes are also found around a diverse range of host stars, from lightweight M dwarfs to massive G-type stars. To date, however, they have not been observed around hotter stars of spectral types F or A. Given the difficulties of planet detection around A-type stars, the absence of Neptune-mass planets in this environment is no surprise; detection methods are simply not yet sensitive enough either to confirm or exclude their presence. Their non-detection around F-type stars may be more significant. Again, however, if this is a clue to the origin of extrasolar ice giants, we do not yet know how to read it.

composition & structure

Detailed simulations of the formation of GJ 436 b suggest that Hot Neptunes may be a unique subspecies and not simply hot versions of the Solar System’s ice giants. Pedro Figueira and colleagues conclude that the bulk composition of GJ 436 b must be 10%-20% gaseous hydrogen/helium, 45%-70% rock/metal, and 15%-40% ice (Figueira et al. 2009). By contrast, Neptune and Uranus are about 5%-15% hydrogen/helium, 25% rock/metal, and 60%-70% ice (Guillot 1999, Figueira et al. 2009). Figueira’s group argues that the enrichment of GJ 436 b in metals and silicates is a result of the planet’s formation history. They propose that it initially accreted an icy core beyond the system’s ice line (between 1 and 5 AU; see Kennedy et al. 2006) and then traveled to its present short-period orbit by Type I migration, a mechanism by which planets substantially less massive than Saturn undergo orbital decay in a gas-rich protoplanetary disk. In the process, GJ 436 b accreted additional mass in rock, metal, and hydrogen.

A similar formation process and composition are predicted for the other known Hot Neptunes, which have masses and orbits resembling those of GJ 436 b (Figueira et al. 2009). In fact, Figueira’s group argue that such planets represent a “typical population” rather than an unusual outcome of system evolution. They evidently form just outside the ice line and then assume tight orbits through Type I migration, gas giant shepherding, or inward scattering.

By contrast, long-period ice giants like Uranus and Neptune assemble at the outer edge of the optimal planet forming region, which in the Solar System corresponds to heliocentric radii of about 10 to 15 AU (Tsiganis et al. 2005). Planetesimal accretion occurs so slowly in this orbital space that Jupiter-mass planets have no time to coagulate before the primordial gases disperse and thereby close the window on gas giant accretion. Icy planets, however, assemble more readily here and are able to retain relatively modest hydrogen envelopes. After accretion they may be carried still farther outward, either by orbital relaxation or by perturbations from more massive planets (e.g., Jupiter and Saturn) on smaller orbits.

The recent discovery of HAT-P-12b, a transiting planet of sub-Saturn mass, provides a convenient boundary to distinguish ice giants like Uranus and GJ 436 b from gas giants like Saturn and the vast majority of extrasolar planets so far detected. HAT-P-12b has a true mass of about 67 MEA (0.21 MJUP) and a diameter about 96% of Jupiter's (Hartman et al. 2009). These values correspond to an object with a thick hydrogen-helium atmosphere surrounding a refractory core of 10 MEA or less.

HAT-P-12b, therefore, is plainly a gas giant, and it most probably reached its present star-grazing orbit through Type II migration. Previous calculations suggested 0.20 MJUP as the likely minimum mass for planets capable of opening a gap in their system's primordial nebulae and thus initiating such migration (Armitage 2007, Raymond et al. 2008). HAT-P-12b neatly fulfills these predictions.

gas versus ice

A few years ago, theorists also predicted a “planet desert” between then-known ice giants (Uranus- to Neptune-mass objects of 10-30 MEA) and gas giants of at least Saturn's mass, or 95 MEA (Ida & Lin 2004, 2005). But the desert has turned out to be fertile. As of October 2009, the Extrasolar Planets Encyclopaedia lists 21 objects distributed in the range between the ice giant HAT-P-11b (~26 MEA) and Saturn. It seems likely that most planets between about 10 MEA and 65 MEA are ice giants, while planets more massive are probably gas giants.

An alternative would be to define the boundary between these two types in terms of composition rather than mass. The well-constrained ice giants are about 10% hydrogen and 90% ice and rock. The well-constrained gas giants are much more diverse (Torres et al. 2008), ranging from planets that are almost 100% hydrogen (HD 209458 b, HAT-P-1b) to those that are only 40% hydrogen (HD 149026 b, HAT-P-3b). By comparison, Saturn is about 15%-25% rock/ice and 75%-85% hydrogen (Saumon & Guillot 2004, Anderson & Schubert 2007), while Jupiter is 5%-10% rock/ice and 90%-95% hydrogen (Saumon & Guillot 2004, Fortney 2007, Militzer et al. 2008).

Can a planet that is less than 75% hydrogen qualify as a gas giant? Jonathan Fortney has speculated that HD 149026 b "may be a hybrid of the ice giants and gas giants," since its "bulk mass fraction of heavy elements" is more reminiscent of Uranus and Neptune than of Jupiter and Saturn (Fortney 2007). It might even be possible for two planets to have exactly the same mass (~70 MEA ), yet one could be a large ice giant with 20% hydrogen and the other a small gas giant with 90% hydrogen.

super earths

Yet another of the exotic bodies known or anticipated only in exoplanetary systems is the class of Super Earths, with masses between 2 and 10 times that of our own (non-Super) Earth. Radial velocity search programs have so far proposed well over a dozen candidates in this mass range, and microlensing searches have nominated 2 more. Given the sparse data available through microlensing, the present discussion is limited to radial velocity detections.

To date, the smallest Super Earth candidate is Gliese 581 e, whose minimum mass is estimated at 1.94 MEA. With current techniques, this value approaches the detection limit for low-mass objects orbiting low-mass stars. Dynamical considerations indicate that the maximum possible mass for Gliese 581 e is only 3.1 MEA (Mayor et al. 2009), enabling more detailed hypotheses to be formulated on its internal structure.

Unfortunately, the masses of most Super Earths are less well constrained, because a planet’s true mass cannot be calculated without data on the inclination of its orbit against the plane of the sky. This angle is undetectable by radial velocity methods. Nevertheless, we have good information on 3 additional Super Earth candidates. Gliese 876 d, the first to be discovered, has a minimum mass of 5.89 MEA. Through dynamical arguments, an approximate inclination of 50 degrees has been estimated, implying a true mass of 7.5 MEA (Rivera et al. 2005). Astronomers can offer more precise data for the 2 detected planets of the distant G-type star CoRoT-7. This system's inner planet ("b") is observed to transit its primary, enabling both its radius and its inclination to be measured. These data indicate a mass of about 4.8 MEA. Assuming that the second planet ("c") travels in the same orbital plane yields a mass of about 8.5 MEA for this object (Queloz et al. 2009).

Whether any given Super Earth represents a larger version of the Solar System’s terrestrial planets – Mercury, Mars, Venus, and Earth – or a smaller version of our ice giants – Uranus and Neptune – depends on its mass, orbital environment, and formation history. Several recent studies have explored each alternative, presenting model planets composed of varying proportions of iron, silicate rocks, and ices (Valencia et al. 2006, Fortney et al. 2007, Baraffe et al. 2008).

The likeliest rocky Super Earths are CoRoT-7 b, Gliese 581 e, and Gliese 876 d. This conclusion is supported both by their low masses and by their hellishly hot orbits. CoRoT-7 b, for example, circles its Sun-like host at a distance of less than 2 million miles in a period of only 20 hours. Given a mass under 5 Mea, along with blazing surface temperatures, this planet is most probably composed of metals and silicate rocks. If it is, then some or all of its surface must be molten (Valencia et al. 2009). Similarly, Gliese 581 e, with a period of about 3 days, is “almost certainly rocky,” according to its discovery team, especially given its status as the least massive exoplanet yet discovered (Mayor et al. 2009). Magma ponds or oceans are likely on this planet. Even the heavier Super Earth orbiting Gliese 876 must be rocky in composition, and a combination of tidal stress and stellar irradiation may have rendered this world more or less molten all the way to the core (Jackson et al. 2008).

Other candidate Super Earths, such as Gliese 581 c, d and HD 40307 b, c, d, are more likely to be ice planets (AKA “Ocean Planets” or “Mini-Neptunes”), a species originally hypothesized by Marc Kuchner and elaborated in subsequent studies by other investigators (Kuchner 2003, Leger et al. 2004, Sotin et al. 2007, Selsis et al. 2007). This conclusion is supported by two factors: the low masses and metallicities of their host stars, indicating a shortage of refractory materials (rocky and metallic) in their primordial environments; and the absence of a shepherding mechanism to focus these materials in small orbits. In the specific cases of HD 40307 b and c, dynamical considerations involving the effects of the host star's tides on the two planets' interiors provide further evidence in favor of icy rather than refractory compositions (Barnes et al. 2009).

Such icy Super Earths probably form outside their system’s ice lines and are then transported to smaller orbits by planet-planet scattering or Type I migration. Given their native environments, they are likely to be 50% rock/metal and 50% ice (Leger et al. 2004, Selsis et al. 2007). 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.

super jupiters

The median mass for all extrasolar gas giants is about 1.75 MJUP (assuming 0.2 MJUP as the lower mass limit). At least 15% of these giant planets are extreme heavyweights, with minimum masses of 5 MJUP or more. Beginning with the first batch of detections in the 1990s, astronomers have expressed concern that some of these heavyweights – known as “Super Jupiters” or “Megajovians” – might actually be brown dwarfs or failed stars (Marcy & Butler 1996, Baraffe et al. 2008). Recently, however, the trend has been to accept that gas giants can form at a wide range of masses, even as high as 25 MJUP. This is almost double the limit of 13 MJUP that is often proposed as the boundary between gas giants and brown dwarfs (e.g., Butler et al. 2006, Udry & Santos 2007). See Super Jupiter or Brown Dwarf?

Regardless of which upper limit we accept, the current sample of Megajovians stands apart from the general exoplanet population in terms of native environment and orbital configuration. The present discussion refers to a sample of 44 high-mass planets detected within 200 parsecs of the Solar System; all are at least 5 times Jupiter's mass. The complete list is available here.

Megajovians have been confirmed only around relatively heavy stars of 0.75 Msol or more, in keeping with the hypothesis that exoplanet mass scales with host star mass. Except for the ambiguous case of HD 41004 Bb, none have been reported in orbit around M dwarfs, and only three are known to orbit K dwarfs (14 Herculis, BD-17 63, HD 162020).

All other Megajovian hosts are either main-sequence stars of spectral classes G or F, or evolved stars that formerly belonged to classes G, F, A, or B. This finding conforms with the theory of planet formation by accretion, insofar as more massive stars typically sustain more massive protoplanetary disks, and more massive disks are likely to yield more massive planets.

Nevertheless, the host stars of Megajovians do not exhibit any particular enhancement in metal content. Their metallicity values run the gamut from -0.68 to +0.46, with a median of +0.08. This is virtually identical to the median metallicity of +0.12 for all exoplanet host stars.

Apart from mass, the most striking feature of this population is its preponderance of wide, eccentric orbits. The median semimajor axis for Megajovians is about 1.80 AU, compared to a median of 0.75 AU for all exoplanets. Fewer than 10% have semimajor axes smaller than 0.1 AU, compared to 30% for all exoplanets, while 70% orbit beyond 1 AU (the semimajor axis of Earth), compared to 40% for all exoplanets. Evidently the mechanisms that deliver gas giants to small orbits – e.g., Type II migration and planet-planet scattering – are less effective for objects of 5 MJUP or more.

Wide orbits are generally more eccentric than tight orbits, but even at the same range of semimajor axes, the orbits of Megajovians are notably more elliptical than those of less massive exoplanets. For semimajor axes of 0.1 AU or more, the median eccentricity among Megajovians is 0.36, while the median for less massive planets is 0.24. Two-thirds of Megajovians in this orbital space have substantial eccentricities of 0.3 or more, while 10% have extreme values of 0.7 or more. Only 39% of less massive planets orbiting at 0.1 AU and beyond have eccentricities of 0.3 or more, and only about 4% have eccentricities of 0.7 or more.

A key unanswered question is whether these distinctive characteristics – large planet mass, large host star mass, large semimajor axis and eccentricity, absence of enhanced metallicity – imply a special evolutionary path. Fortunately, three Megajovians have so far been detected by photometric transit surveys (HD 147506 b, XO-3 b, and WASP-14 b), so the outlook for eventual resolution of this problem is good.

A transit detection enables the determination of a planet’s radius and true mass, facilitating estimates of its internal composition and providing clues to its formation. Among the trio of transiting Megajovians, WASP-14 b (7.34 MJUP) seems to fit prevailing theoretical models for a massive, extremely dense gas giant containing a large heavy element core (Joshi et al. 2009). The status of XO-3 b (11.8 MJUP) is less certain, since its radius appears to be slightly inflated, and a recent study could not decide whether it is metal-rich or metal-poor (Winn et al. 2008). HD 147506 b (also known as HAT-P-2b) is ambiguous for the opposite reason, because its mass of 8.78 MJUP and unusually small radius of 0.95 Rjup imply a density more appropriate for a brown dwarf than for a gas giant (Loeillet et al. 2008).

These ambiguities have prompted speculation that the Megajovians represent a unique class unto themselves. As Loeillet and colleagues suggest, “Supermassive planets like HAT-P-2b may constitute a new class of stellar companions, in between Hot-Jupiters and low-mass stars and near the brown dwarf population. . . . . This could suggest a different formation process” (Loeillet et al. 2008). In a study of transiting planets, Torres and colleagues agreed on the oddball status of HD 147506 b/HAT-P-2b, observing that this object “may belong to a different category of planet” (Torres et al. 2008).

Yet in many respects the Megajovians behave like less massive planets. They have been found in “hot” orbits smaller than 0.1 AU and “cool” orbits larger than 3 AU; they occur in systems containing one or more additional planets (HD 68988, HD 74156, HD 38529, HD 202206, HD 168443), on interior as well as exterior orbits; and they have been detected in systems that harbor massive debris belts (70 Virginis, HD 38529). Since the core accretion theory has been successful in explaining all of these system architectures, it may also prove adequate to explain the formation of Megajovians.

cool giants and jupiter twins

Once a rarity, cooler gas giants with semimajor axes of 0.5 AU or more are now regularly reported. They currently constitute 49% of all detected exoplanets. Most of this population travel on orbits far more eccentric than those of the Solar planets. Thus only 40% of gas giants with semimajor axes of 0.5 or more have eccentricities smaller than 0.2; altogether they amount to about 20% of the full sample of exoplanets.

These longer-period planets also tend to be more massive than the ones orbiting closer to their parent star. Only 7 exoplanets beyond 0.5 AU have minimum masses less than 0.5 MJUP, whereas more than three dozen planets orbiting in this space have masses between 5 and 13 MJUP, the nominal cutoff for brown dwarfs. It is uncertain whether the deficit of lower-mass planets on wide orbits is real or simply a reflection of the limitations of the radial velocity method.

Looking at a volume-limited sample may provide a less biased picture. Of the 64 exoplanets detected by radial velocity searches within 20 parsecs (as of November 2009), more than half (55%) have semimajor axes smaller than 0.5 AU. Most of these also have orbital eccentricities smaller than 0.2. Of the planets on wider orbits within this sample, just under half have eccentricities in the same range. Within the longer-period group, regardless of eccentricity, three out of 29 (10%) have minimum masses less than 0.5 MJUP, while only two (14 Herculis b and Pi Mensae b) have masses larger than 5 MJUP.

Thus the general shortage of lightweight planets on longer-period orbits seems to be a selection effect rather than an accurate depiction of the planets in our region of the Milky Way Galaxy. Just 20% of the exoplanets within 20 parsecs (13/63) qualify as “cool eccentric giants,” understood as gas giants with semimajor axes of 0.5 AU or more and eccentricities of 0.2 or more, while a similar proportion are cool giants of more moderate habits.

Again, within the full sample, which now includes more than 400 exoplanets, just one well-constrained planet can plausibly be described as a Jupiter twin. This would be a gas giant with [1] a semimajor axis of 3.5 AU or more, [2] an orbital eccentricity smaller than 0.1, and [3] no interior giant companions. The unique object in question is HD 154345 b, the only planet of a nearby G8 star. It has a minimum mass identical to Jupiter’s, a semimajor axis of 4.2 AU, and an eccentricity of 0.04 or less. The discovery team noted that over 10 years of observing this system, no evidence of any additional giant planets has emerged. Thus HD 154345 is by far the best candidate for hosting Earth-mass planets in the system’s habitable zone (Wright et al. 2008).

Honorable mention in the Jupiter lookalike contest goes to HD 117207 b, a gas giant of at least 2 Jupiter masses orbiting a mature, metal-enriched G-type star at a semimajor axis of 3.78 AU with an eccentricity of 0.16. This relatively nearby star is also a candidate host for terrestrial planets with bodies of liquid water.

The theories of Sudarsky and colleagues provide an intriguing preview of the varied group of longer-period gas giants (Sudarsky et al. 2003). They suggest two alternatives: colorful methane-ammonia clouds like those of Jupiter and Saturn for the coldest planets (probably beyond 2 AU for a G-type star), and water-ice clouds like those of Earth for planets in smaller orbits (but still cooler than 300 K). The latter type, which they term “Water Class II,” is especially interesting, because such planets may look deceptively like giant versions of our own homeworld. Moreover, as Sudarsky and colleagues point out, even at large orbital separations the most massive planets will maintain high core temperatures and thus present the appearance of water-cloud giants or – if young and massive enough – clear blue spheres like the “Gaseous Class III” model (Sudarsky et al. 2003).

Unless otherwise noted, statistical information on this page refers to a sample of 369 well-constrained radial velocity planets (and subsets thereof) posted at http://exoplanet.eu as of 19 October 2009.

Last revised November 2009