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.

One remarkable difference between the reported population of exoplanets and the more familiar worlds of our Solar System is the exoplanets' trend toward high mass. Currently, the median mass for known extrasolar gas giants is about 1.7 MJUP, which exceeds the aggregate mass of all eight planets in the Solar System. The heaviest 10% of exoplanets have masses in excess of 7 MJUP, meaning that each one contains the equivalent of four or more Solar Systems' worth of planets (EPE).

Further, the commonly cited parameters for exoplanets actually understate their likely masses, because for most of them we have only minimum values rather than true values. Radial velocity data cannot yield precise exoplanetary masses unless the orbital inclination of the object is also known. Minimum masses are calculated on the assumption that we view the planet's orbit precisely edge-on.

For example, radial velocity data alone indicated minimum masses of 1.96 and 4.33 MJUP, respectively, for the two outer planets of the Upsilon Andromedae system (often known as Ups And c and Ups And d). However, long-term photometric observations of this system have recently enabled their actual masses to be calculated at 13.98 MJUP for Ups And c and 10.25 MJUP for Ups And d. Evidently, the true mass of some exoplanets may exceed the minimum estimate by several hundred percent.

Another striking difference between Solar and extrasolar planets appears in the shapes and sizes of their orbits. 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 0.10. 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. Almost 45% have semimajor axes smaller than Mercury's, and 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 about 40% have e > 0.30.

A small group – less than 5% of the total sample – have extreme eccentricities of 0.70 or higher, like the orbits of comets rather than those of planets as we usually imagine them.

Data on masses and orbits can tell us a great deal about the existing population of extrasolar planets, but only if they are interpreted in the context of the masses, luminosities, spectral types, and evolutionary histories of their parent stars. 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

The first exoplanet detected in orbit around a main-sequence star was 51 Pegasi b, an object more massive than Saturn that completes its orbital period in less than 5 days. Similarly massive objects on short-period orbits have been reported by search programs ever since, making them the first exotic planetary species to be formally identified. These objects orbit so close to their host stars that they are substantially hotter than Venus, the planet with the highest surface temperature in the Solar System. As a result they are usually known in English as Hot Jupiters. In French they may be called pegasides (Pegasids) in honor of 51 Pegasi b, but this designation also applies to a well-known meteor swarm.

Hot Jupiters are gas giants, meaning that they are at least 20% as massive as Jupiter, with a bulk composition that is typically at least 80% hydrogen. They have semimajor axes smaller than 0.1 AU and orbital periods of 10 days or less.

Hot Jupiters currently comprise almost 25% of all planets detected by radial velocity observations, given that method's stong bias toward finding massive objects with short periods. This bias becomes more pronounced with distance from the Solar System. Only about 10% of exoplanets observed at 100 parsecs (326 light years) or less are Hot Jupiters, whereas the proportion rises above 70% for planets at greater distances.

evolutionary history

Despite their current “torch” orbits, astronomical consensus favors a cooler and more remote 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 after the formation of the central star, encouraging newborn gas giants to capture massive atmospheres of hydrogen. In a significant fraction of extrasolar systems, one or more of these giants eventually travel inward to the close vicinity of their parent star.

The most likely mechanism behind such a journey is Type II migration, a product of tidal interactions between the forming gas giant and its surrounding protoplanetary nebula. This interaction 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. Two additional pathways to short-period orbits have also been identified: planet-planet scattering, the result of gravitational perturbation between two or more gas giants, and Kozai cycles, the result of perturbation by a massive outer object such as a binary companion star, a brown dwarf, or even a planet on a wide orbit (Nagasawa et al. 2008, Narita et al. 2010). Each of these three processes can operate alone or in conjunction with one or both of the others (Schlaufman 2010).

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 their atmospheres to expand and become increasingly tenuous, resulting in the inflated radii measured by 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).

Tides rather than heat pose the greater threat to a Hot Jupiter's long-term survival. Recent theoretical studies argue that planets with very short periods (e.g., less than 3 days) are likely to undergo orbital decay through tidal drag from their parent stars, eventually to be engulfed by the stellar photosphere, often well before these stars leave the main sequence (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 similar to that of the full sample, ranging from -0.4 to +0.5. Nevertheless, the known population of Hot Jupiters is slightly biased in favor of high mass and enrichment in metals, given a median host star mass of 1.11 Msol and a median metallicity of +0.11. At the same time, 30% 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 origin and fate, Hot Jupiters still manage to survive on billion-year timescales. 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 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. As noted above, this pile-up of short-period orbits implies that many Hot Jupiters are eventually consumed by their parent stars.

A second distinguishing feature of this population is mass. Although the complete sample covers the full mass range of extrasolar gas giants (0.21-13.75 MJUP), numbers peak around the mass of Jupiter. The median Hot Jupiter mass is 0.95 MJUP, in contrast to the median of 1.7 MJUP for all known gas giants. Eighty percent are lighter than 2 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 June 2010, more than 80 transiting exoplanets had been characterized through a combination of radial velocity and photometric studies (Torres et al. 2008, Extrasolar Planets Encyclopaedia). Among them, almost 95% are gas giants, and all but 4 of these giants have semimajor axes smaller than 0.1 AU. Thus the great majority of Hot Jupiters now known have been observed in transit.

The median mass of 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 0.95 MJUP, quoted above, which was derived from combining data on both populations of hot giants. Both analyses agree that the typical Hot Jupiter is virtually 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

A small but growing subset of the exoplanetary population consists of ice giants corresponding to Uranus and Neptune in our Solar System. These objects are heavy enough to retain substantial atmospheres of hydrogen and helium, but not so heavy that their bulk composition is dominated by lightweight elements. Their mass range begins at about 10 MEA and may rise to the vicinity of 60 MEA, although most candidates detected so far are under 35 MEA. Thus they are somewhat heavyweight cousins to the ice giants in our Solar System. Since their orbits are typically smaller than Mercury’s, they are often nicknamed Hot Neptunes.

Of all extrasolar species, these exo-Neptunes have benefited the most from complementary observing techniques. Radial velocity observations have established that a significant population follows short-period orbits around stars in the immediate Solar neighborhood, even though this method is not yet sensitive enough to detect ice giants at wider semimajor axes (Mann et al. 2010). Transit searches have discovered 3 exo-Neptunes to date (GJ 436 b, HAT-P-11b, Kepler-4b), all with periods shorter than 5 days and internal structures dominated by rock, metal, and ice (Gillon et al. 2007, Figueira et al. 2009, Bakos et al. 2010, Borucki et al. 2010). Microlensing programs have identified 2 more, at radial separations of 2-4 AU, thereby enlarging our knowledge of their potential orbital environments (Extrasolar Planets Encyclopaedia). Finally, planetary astronomy within the Solar System itself provides a convenient sample of ice giants against which we can compare these exo-Neptunes. We have a similar control group for extrasolar gas giants, in the form of Jupiter and Saturn, whereas we lack any home-system analogies for another exoplanetary species of interest, the Super Earths.

Despite these synergies, the observed sample of extrasolar ice giants suffers from the biases of the 2 most successful detection methods, radial velocimetry and transit observations, both of which typically find exo-Neptunes on tight orbits around low-mass stars.

orbital environment

It remains uncertain whether current data on the orbital environments of extrasolar ice giants reveal anything more than the shortcomings of our detection methods. If we define a minimum mass of 45 Mea (0.126 Mjup) as the upper limit for exo-Neptunes (contra Mayor & Udry 2008, who suggest 23 Mea, and Lovis et al. 2009, who suggest 30 Mea), we obtain a sample of 28 candidates characterized by radial velocimetry. Ninety percent have periods shorter than 50 days and semimajor axes smaller than 0.30 AU; none have been found outside 0.65 AU. The most eccentric exo-Neptune has e = 0.35, while all the rest have eccentricities smaller than 0.3.

The known extrasolar gas giants actually tend to have more circular orbits than the exo-Neptunes, since 85% of gas giants with periods shorter than 50 days have e < 0.2. The two species differ, however, with regard to the so-called “3-day pile-up,” which refers to the statistical tendency for the orbital periods of Hot Jupiters to cluster around 3 days (Wu et al. 2007, Wright et al. 2009). Instead, the Hot and Warm Neptunes are more evenly dispersed over a range of semimajor axes. For periods shorter than 50 days, the median for exo-Neptunes is about 9 days, while the median for gas giants is currently 3.7 days.

A slight majority of exo-Neptunes are solitary, residing in systems that lack any other exoplanet detections. Nevertheless, about 45% are found in systems with one or more additional planets, while only 20% of gas giants occur in such environments. Among the exo-Neptunes with companions, about half (25% of the complete sample) are found in systems where the most massive planet is an ice giant, while the rest (~20% of the complete sample) are accompanied by gas giants. The only notable exception to the companionability of exo-Neptunes is their observed absence from systems containing a Hot Jupiter. This ability to play well with others is undoubtedly related to their low masses (since more massive planets would be likely to eject other objects from their vicinities) and moderate eccentricities. More extreme eccentricities are typically understood as the outcome of planet-planet scattering, suggesting that native environments of the known extrasolar Neptunes were relatively placid.

Like Super Earths, exo-Neptunes tend to be found around lower-mass stars. More than two-thirds of extrasolar ice giants accompany stars less massive than our Sun. The most heavyweight host star in this sample is HD 219828, a yellow subgiant of 1.24 Msol. However, the absence of exo-Neptunes around more massive stars may simply be a result of detection biases. With regard to spectral type, the current breakdown for the host stars of exo-Neptunes is M dwarfs, 14%; K dwarfs, 36%; and G-type dwarfs and subgiants, 50%.

composition & structure

Detailed simulations of the formation of GJ 436 b suggest that Hot Neptunes – those with orbital periods of 10 days or less – 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, Borucki et al. 2010). 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 planet HAT-P-12b, a transiting giant 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 vs. ice

A few years ago, theorists also predicted a “planet desert” between then-known exo-Neptunes 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 June 2010, the Extrasolar Planets Encyclopaedia lists 23 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 4 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). Finally, an additional transiting Super Earth has been observed around a very dim and lightweight M dwarf, GJ 1214, which is fortuitously located in the immediate Solar neighborhood. GJ 1214 b has a mass of 6.6 MEA and a radius of 2.7 REA, intermediate between the diameters of Earth and Uranus.

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 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).

GJ 1214 b is most likely an icy Super Earth with a fairly thick atmosphere, although current data cannot rule out a scenario in which it is a primarily rocky planet that nevertheless accreted a dense hydrogen-helium atmosphere (Miller-Ricci & Fortney 2010, Rogers & Seager 2010). Other candidate Super Earths, such as Gliese 581 c, d and HD 40307 b, c, d, are also 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 1.7 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 more than half of all detected exoplanets. Most of this population travel on orbits far more eccentric than those of the Solar planets. Only 40% of gas giants with semimajor axes of 0.5 or more have eccentricities smaller than 0.2.

Longer-period planets also tend to be more massive than the ones orbiting closer to their parent star. Only 11 exoplanets beyond 0.5 AU have minimum masses less than 0.5 MJUP, whereas more than 40 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, which is most sensitive to massive planets on short-period orbits.

Looking at a volume-limited sample may provide a less biased picture. Of the 73 exoplanets detected by radial velocity searches within 20 parsecs (as of June 2010), 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 32 have minimum masses smaller than 0.5 MJUP, while four out of 32 (14 Herculis b, Pi Mensae b, Upsilon Andromedae c & d) have masses larger than 5 MJUP.

On this evidence, the 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. Further, about 20% of the exoplanets within 20 parsecs 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 two well-constrained planets can plausibly be described as Jupiter twins. These would be gas giants with [1] semimajor axes of 3.5 AU or more, [2] orbital eccentricities smaller than 0.1, and [3] no interior giant companions.

The unique objects in question are HD 154345 b, which orbits a nearby G8 star, and HD 13931 b, which orbits a more distant G0 star. HD 154345 b 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) The system of HD 13931 has received less intensive study, since its giant was announced only in 2010 (Howard et al. 2010). This planet has a minimum mass of 1.88 Mjup, a semimajor axis virtually identical to Jupiter's, and an eccentricity calculated at only 0.02. Notably, its discovery team described the parent star as a "near Solar twin." Both HD 154345 b and HD 13931 b actually have orbits more circular than Jupiter's, whose eccentricity is 0.048.

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. Its parent 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 417 well-constrained radial velocity planets (and subsets thereof) posted at http://exoplanet.eu as of 6 June 2010.

Last revised July 2010