Puffy Planets

Hot air balloons over Malqata, Egypt, site of the jubilee palace of Amenhotep III (ca. 1350 BC)

Recent work by Eric Lopez and Jonathan Fortney brings welcome clarity to the bewildering array of Kepler transit data on small planets. These esteemed planetologists just circulated a preprint entitled “Understanding the mass-radius relation for sub-Neptunes: Radius as a proxy for composition” (hereafter LF13). It addresses questions that have surfaced several times in this blog: How can we distinguish Super Earths from Neptune-like exoplanets? And what is the likely composition of each species? That topic is explored here, here, here, here, and most recently here.

puff threshold

Using the results from a large ensemble of thermal evolution models, LF13 draw a physically motivated boundary between true Super Earths (which are scaled-up versions of Earth, consisting only of heavy elements) and planets like Uranus and Neptune (which maintain at least a small percentage of their mass in a hydrogen/helium (H/He) envelope).

As Lopez and Fortney conclude, “for most of Kepler’s Neptune and sub-Neptune sized planets, radius is quite independent of planet mass and is instead a direct measure of bulk H/He envelope fraction.” They offer a method to estimate the likely composition of low-mass transiting planets simply on the basis of radius. This is an extremely useful contribution, because for the vast majority of Kepler candidates, radius and orbital period are the only reliable information we have.

LF13 find that the maximum size for a typical rocky planet is 1.75 Earth radii (Rea). Although some planets more massive than 5 Earth masses (Mea) may have radii larger than 2 Rea because they consist primarily of ices, with no appreciable contribution from H/He, the vast majority of planets with a substantial rocky component must be smaller than 1.75 Rea. It follows that most planets larger than 2 Rea must have some percentage of H/He in their atmospheres. A well-known example is Kepler-22b, announced in 2011 and sometimes described as a “habitable Super Earth.” According to LF13, however, this object would be a miniature version of Uranus, given its radius of 2.2 Rea.

LF13 describe a hypothetical planet with a radius of 2 Rea and an orbit within its system’s classical habitable zone. Assuming a rocky core and an overall mass of 5 Mea, 0.5% of the planet’s total mass must be H/He. This tiny fraction creates an atmospheric pressure of about 20 kilobars of H/He, which as LF13 say is 20 times higher than the pressure at the bottom of the Marianas Trench, the deepest fissure in Earth’s global ocean. In the habitable zone, such an atmosphere will produce surface temperatures around 3000 K, more than 10 times higher than the mean surface temperature on Earth (288 K). This environment is hostile to liquid water and carbon-based life.

Figure 1. Puffy Planets. Selected transiting planets between 3 and 7 Earth radii, with estimated fractions of hydrogen/helium (H/He). All values except for Neptune are taken from Lopez & Fortney 2013. The approximate H/He fraction for Neptune is inferred from their Table 3. 

The astronomer Robin Wordsworth has presented a sceario in which a rocky planet of 5 Mea with a dissipating hydrogen atmosphere, orbiting a Sun-like star at a distance of 2.5 AU (i.e., well outside the classical habitable zone, near the system ice line), will experience a transient but potentially significant epoch during which it might sustain liquid water. However, this optimistic model seems to suffer from fine-tuning. As even Wordsworth concludes, when it comes to hydrogen, “usually, either far too much or too little of it is present” (Wordsworth 2012). For Kepler planets larger than 2 Rea, it’s “far too much.”

degeneracy on ice

A few years ago, virtually everyone assumed that some subset – maybe the majority – of Super Earths would be “Water Planets” or “Ocean Planets.” The consensus has changed dramatically since the discovery of the Kepler-11 system in 2011. Now investigators routinely model rocky planets with H/He envelopes, while the likelihood of Water Planets is questioned. Nevertheless, LF13 accept the possibility of rock/ice planets, noting that a planet of 10 Mea consisting of 20% rock/metal and 80% water would have a radius of 2.7 Rea. They concede that their model suffers from the same “degeneracy” as previous efforts when it comes to distinguishing icy planets from gassy planets. In other words, the same radius could correspond to very different compositions.

For example, 55 Cancri e has a measured mass and radius of 8.3 Mea and 1.99 Rea, respectively, and is sometimes proposed as a Water Planet (Dragomir et al. 2013). Since its temperature is about 2000 K and its age is around twice that of the Solar System, Diana Dragomir and colleagues argue that a trace hydrogen envelope would have dissipated eons ago, while a substantial fraction of ices might remain.

Without providing a rationale, LF13 model 55 Cancri e as a rocky object instead, proposing a modest H/He envelope amounting to only 0.14% of its bulk composition. However, they leave open the possibility that other planets between 2 and 3 Rea might qualify as Water Planets (and thus Super Earths), as long as they have a large enough fraction of ices in their composition.

If they exist, planets that contain a significant fraction of water or high-pressure ices, but lack a H/He envelope, are of great scientific interest. Nevertheless, they appear to have little relevance for astrobiology. A growing consensus holds that, in order to maintain bodies of liquid water, a rocky planet requires both plate tectonics and a carbon cycle. Neither is possible on the Water Planets proposed by contemporary theorists. An object of 5 Mea whose bulk composition is 25% water will have a differentiated structure, with a rock/metal core surrounded by a layer of high-pressure ice thousands of kilometers thick. This layer would block chemical interactions between atmospheric gases and heavy elements in deeper strata, preventing the development of a carbon cycle (Lammer et al. 2010, Alibert 2013). Yann Alibert finds that a planet of Earth mass and composition can maintain both a global ocean and a carbon cycle only if its water content is 2% or less, with the maximum percentage decreasing with increasing mass (Alibert 2013).

In addition, many astrobiologists assume that plate tectonics requires liquid water rather than ice to lubricate continental plates (Korenaga 2010, Lammer et al. 2010). Also widely endorsed is the argument that solid objects above a few times the mass of the Earth, even if they somehow retained thin envelopes of water, cannot support plate tectonics at all (O’Neill & Lenardic 2007, Morard et al. 2011, Stein et al. 2013). Ocean Planets look less and less attractive than they used to.

low-mass cousins

To illustrate their model, LF13 present a table summarizing all confirmed exoplanets smaller than Saturn with reliably measured masses and radii. Although this sample is too small to yield robust statistics, suggestive trends are evident. The six planets with radii between 1 and 2 Rea (where LF13 place the transition from Super Earths to Neptune-like planets) range in mass from 1.9 to 8.45 Mea, with a mean of 5.8 Mea and a median of 6 Mea. The corresponding mass values are remarkably similar for the six planets between 2 and 3 Rea, which LF13 call “sub-Neptunes” and which others have shown to represent the most numerous population of Kepler planets (Petigura et al. 2013). For these planets, the range in mass is 2 to 7.86 Mea, the mean is 5.6 Mea, and the median is 6.7 Mea. According to LF13, objects in this group have mass fractions of H/He ranging from 0.31% to 5%, substantially smaller than the fractions for Uranus and Neptune.

For the 12 planets between 3 and 7 Rea, which by all agreement have H/He envelopes, masses range widely, rising from 7.3 Mea to 68.6 Mea, with a mean of 20.6 Mea and a median of 16.8 Mea. However, confirmed exoplanets are sparse between 25 and 60 Mea; LF13 offer only three examples, none of them between 27 and 40 Mea. Such a gap in the distribution justifies removal of the most obvious outlier, CoRoT-8b. With a mass of almost 70 Mea, CoRoT-8b resembles a gas giant, but its unexpectedly small radius of 6.38 Rea implies a composition dominated by heavy elements. LH13 list its bulk composition as only one-third H/He. If we eliminate CoRoT-8b from the sample of Neptune-like objects, the mass range becomes 7.3 to 26.2 Mea and the mean becomes 14.9 Mea, while the median remains almost the same at 16.3 Mea. A representative sample of this group is illustrated above in Figure 1.

inner system regulars

The latest Kepler data show that puffy planets of 3 to 7 Rea are common within 1 astronomical unit (AU) of Sun-like stars. Objects in this range of radii, as we just saw, have a median mass very similar to Neptune’s. A number of recent studies have calculated the distribution of these planets and their smaller, lower-mass siblings.

Andrew Youdin found that 3 Rea marks a clear divide in the distribution of low-mass planets on short-period orbits (< 50 days) around Sun-like stars (Youdin 2011). Planets of this approximate radius are extremely rare on periods shorter than 7 days, whereas both smaller and larger planets are plentiful in the same orbital space. Youdin interpreted this distribution as a probable result of the thermal evolution of H/He envelopes around low-mass planets that migrate to the immediate vicinity of their host stars. Planets of the lowest mass (less than about 8 Mea) will lose their envelopes completely, leaving bare rocky spheres smaller than 2 Rea. More massive planets with more substantial cores can sustain lightweight atmospheres against stripping, resulting in final radii larger than 3 Rea.

In a more recent analysis using a larger set of Kepler data, Subo Dong and Zhaohuan Zhu characterize the planet population within 0.75 AU of Sun-like stars. This boundary corresponds to an orbit of about 250 days, similar to the orbit of Venus (225 days). They find that planets smaller than 4 Rea have a relatively flat distribution at periods longer than 10 days, whereas planets of 4-8 Rea have a steadily increasing distribution at longer periods (Dong & Zhu 2013). In general, they observe, the relative fraction of “big planets” (those of 3 Rea or more) increases with increasing period, but the increase is most pronounced for planets smaller than 10 Rea (i.e., Saturn-size or less).

Dong and Zhu provide an illuminating overview of the cumulative frequencies of planets of all sizes in the Kepler sample. Within 0.75 AU, planets of 1-2 Rea (which they call “Earth-size”) have a frequency of 28%; those of 2-4 Rea (“super-Earth-size”) have a very similar frequency, at 25%; those of 4-8 Rea (“Neptune-size”) are far less common, at 7%; while those larger than 8 Rea (“Jupiter-size”) are the least common of all, at only 3%.

The relative abundance of dwarfs and rarity of giants has been widely observed. It is rapidly becoming a cornerstone of exoplanetary science, analogous to the abundance of lower-mass stars (M dwarfs) versus their higher-mass (O and B-type) siblings.

REFERENCES

Alibert Y. (2013) On the radius of habitable planets. Astronomy & Astrophysics, in press.

Dragomir D, Matthews JM, Winn JN, Rowe JF, MOST Science Team (2013) New MOST photometry of the 55 Cancri system. In press. Abstract: http://adsabs.harvard.edu/abs/2013arXiv1302.3321D

Dong S & Zhu Z. (2013) Fast rise of “Neptune-size” planets (4-8 Rearth) from P ∼10 to ∼250 days: Statistics of Kepler planet candidates up to ∼0.75 AU. Astrophysical Journal 778, 53.

Korenaga J. (2010) On the likelihood of plate tectonics on Super-Earths: Does size matter? Astrophysical Journal Letters 725, L43-L46.

Lammer H, Selsis F, Chassefiere E, Breuer D, Griessmeier J-M, Kulikov YN, et al. (2010) Geophysical and atmospheric evolution of habitable planets. Astrobiology 10, 45-68.

Lopez ED & Fortney JJ. (2013) Understanding the mass-radius relation for sub-Neptunes: Radius as a proxy for composition. In press. Abstract: http://adsabs.harvard.edu/abs/2013arXiv1311.0329L

O’Neill C & Lenardic A. (2007) Geological consequences of super-sized Earths. Geophysical Research Letters 34.

Morard G, Bouchet F, Valencia D, Mazevet S, Guyot F. (2011) The melting curve of iron at extreme pressures: Implications for planetary cores. High Energy Density Physics 7, 141-144.

Petigura EA, Geoffrey MW, Howard AW. (2013) A plateau in the planet population below twice the size of Earth. Astrophysical Journal 770, 69. Abstract: http://adsabs.harvard.edu/abs/2013ApJ...770...69P

Stein C, Lowman JP, Hansen U. (2013). The influence of mantle internal heating on lithospheric mobility: Implications for super-Earths. Earth and Planetary Science Letters 361, 448-459.

Wordsworth R. (2012) Transient conditions for biogenesis on low-mass exoplanets with escaping hydrogen atmospheres. Icarus 219, 267-273.

Youdin AN. (2013) The exoplanet census: A general method applied to Kepler. Astrophysical Journal 742, 38. Abstract: http://adsabs.harvard.edu/abs/2011ApJ...742...38Y

1010 Extrasolar Planets

Figure 1. View from an imaginary exomoon of an imaginary exoplanet orbiting an imaginary binary star by Luis Calcada, reigning master of astro-art.

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The ones and zeroes in my headline are not a binary expression of the number more commonly known as diez, ash’ra, dus, desyat’, or shr. No, those digits are in good old base ten, and they’re telling us that the census maintained by the Extrasolar Planets Encyclopaedia has reached vertiginous heights: as of yesterday, one thousand and ten planets have been detected around stars other than our Sun. More than a hundred were reported just in the past three months in a big statistical surge.

So now we can point to 1,010 different alien worlds where it’s night right now. Given the frequency of tight orbits and tidally locked rotation, most of those nights will last forever.

Since I did a descriptive survey of the full extrasolar population in July, I’ll hold off on an update until the end of the year. I just want to observe this moment in history and share it with whoever is reading.

But I can’t resist a few geeky observations. Of the hundred-odd objects of planetary mass reported between July 1, when the census reached 900, and October 22, when it jumped from 999 to 1010, only 16 were discovered by radial velocity (RV) observations. Four others were identified by microlensing, and two were imaged (not counting several brown dwarfs that were also photographed). All the rest – 83 planets – were discovered in transit, and more than three-quarters of those were captured by a single instrument: our beloved but now lamented Kepler spaceborne telescope.

Considering this imbalance in returns, RV has apparently assumed minority status among planet-seeking methods, while transit surveys have become the principal way to find new worlds. Just five years ago, when the exoplanet census broke 300, transit detections accounted for 17% of all detections, while RV was at 78%. Now those numbers are 39% vs. 53%.

Most of the recent bounty of transiting planets comes to us courtesy of Kepler. As mission scientists predicted when data collection ended catastrophically this past May, so many observations were already in the pipeline that analyses could go on for years to come, with new exoplanets continuing to emerge from the light curves just as they have over the past several months. Those predictions have been amply confirmed.

The influx of new transiting objects is bound to slow down eventually, since the data are finite. But it’s hard to see how RV can pick up the slack, unless a space-based RV observatory of unprecedented precision and longevity is lofted into orbit soon. Sadly, I don’t know of any such mission in the works. The James Webb Space Telescope, in development for almost 20 years, may finally launch in five more years - but that launch date isn't guaranteed, and RV observations are likely to be a small part of the JWST agenda, even if it does become a reality someday.

Au-delà de neuf cents exoplanètes

Figure 1. Just as the exoplanet census passed 900, a new study reported that HD 189733 b, the nearest transiting Hot Jupiter, is a deep blue world where molten glass rains through seething hydrogen.

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On Monday, July 1, the census ticker at the venerable Extrasolar Planets Encyclopaedia (EPE) turned over a new planetary century: 900 confirmed exoplanets, including the results of all current detection methods. Since then the count has moved on, hovering at 919 planets today. All these worlds are the harvest of 25 years of searching.

What planetary flavors does this numerical abundance offer? How much variety do we find in the Milky Way? How many Earth-like planets are out there?

To answer these questions, we need to take into account the fact that different search methods find different kinds of planets. The strengths and limitations of each method introduce enormous bias into the exoplanetary census.

search methods & planet populations

Most (58%) of the known exoplanets have been discovered by the radial velocity method. It returns data on the masses and orbital periods of specific kinds of planets: gas giants like Jupiter orbiting within about 5 AU of their host stars, and low-mass planets (Super Earth to Neptune-size) orbiting within about 1 or 2 AU. Under optimal conditions, it can also detect Earth-mass planets orbiting within a few tenths of an AU. This method is sensitive to multiplanet systems of many kinds, as well as to single planets. However, it isn’t very good at finding systems like our own or planets like Earth.

The next largest share of exoplanets (34%) have been observed in transit across the face of their host stars, so that we can calculate their radii and orbital periods, but not their masses. The transit method is sensitive only to planets that happen to orbit their stars precisely along the line of sight from Earth. Its effectiveness in ground-based surveys is limited, since 97% of transiting objects detected from the ground are Hot Jupiters (i.e., gas giants with periods shorter than 10 days – a monotonous selection indeed). Fortunately, detections by space-based programs now outnumber those from the ground, and the vast majority of those detections represent planets of Neptune size or less. Space-based observations have also been remarkably successful in finding low-mass multiplanet systems similar to our radial velocity neighbors, HD 40307 and HD 69830. Until other search methods improve, they remain our only hope of detecting Earth-size planets with habitable temperatures. Meanwhile, all the planets on which we have enough data to establish that they are rocky (like Earth) and smaller than two Earth radii (2 Rea) are Hellworlds, orbiting their stars in periods of a few days or less.

Figure 2. Small Planets with Measured Masses & Radii

The major drawback of transit data is the absence of constraints on an object’s mass, except in rare cases where a planet’s orbital period is modified by the influence of an adjacent transiting planet (a phenomenon known as transit timing variations or TTV). The major strength of transit data is that, when they can be supplemented by radial velocity data, the resulting synergy provides our clearest understanding of exoplanetary environments. Figure 2 shows artist’s views of half a dozen small, well-constrained planets (“Super Earths” and “Mini Neptunes”) with masses (M) and radii (R) expressed in Earth units. The three Kepler planets are located at distances of 173 to 613 parsecs (565-2000 light years away); the other three are next-door neighbors of our Sun, located within 22 parsecs (72 light years). Similar line-ups of low-mass planets are available here and here.

Three other search methods are also in use, but they offer a modest contribution to the total census (collectively, less than 10%, even allowing for overlap between transiting and radial velocity detections). Microlensing, which depends on chance alignments of distant stars , is sensitive primarily to planets separated from their host stars by about 1.5 AU to 5 AU (Bennett 2008). It has detected 20 planets so far (2% of the total), ranging from Super Earths to gas giants; none are tightly constrained. Pulsation timing has reported 15 candidate planets that EPE tags as confirmed, although several have been challenged as unphysical. The most reliable detections in this group accompany neutron stars. Imaging, the old-fashioned way of doing astronomy, has returned a much larger yield – 34 purported planets (almost 4% of the total census) – but except for a small subset, this group is dubious. Most candidates are located too far from their proposed host stars to have formed in a circumstellar disk, and many have estimated masses consistent with brown dwarfs instead of planets.

All five methods have been in use for more than a decade, each contributing its own piece of the picture. In the past few years, a new approach to detection – reanalysis of existing radial velocity and transit data, using Bayesian inference – has also proved fruitful. In fact, the exoplanetary census just surged to 900 and beyond partly as a result of two brand-new reanalyses of radial velocity data, discussed here and here.

two kinds of planets

Given the limitations of the available search methods, only radial velocity and transit data are useful for making generalizations about the known exoplanets. It seems clear that this population has two major divisions, with the dividing line at about 8 Rea and 40 Earth masses (Mea). Below this boundary are the low-mass planets, which include two subspecies: 1) rocky planets like Earth (“terrestrial” or “telluric” planets), without any substantial amount of hydrogen in their bulk composition; and 2) planets with rocky (or perhaps rocky/icy) cores surrounded by hydrogen envelopes constituting at least a few percent, but less than half, of their bulk composition (variously “gas dwarfs,” “Mini Neptunes,” “ice giants,” or “exo-Neptunes”). It remains moot whether this group also includes a third subspecies, consisting of relatively massive objects (1 Mea or more) that contain a substantial amount of volatiles (water, ammonia, methane) along with rock, without any significant hydrogen envelope. Such planets, if they exist, would be scaled-up versions of Ganymede and Titan in our Solar System.

Figure 3. Radii of 298 Transiting Planets < 18 Earth Radii: July 2013

Above the boundary are the gas giants, defined as objects whose bulk composition is dominated by hydrogen. These planets have radii larger than 8 Rea and masses in excess of 40 Mea. Although gas giants constitute a majority (72%) of exoplanets detected by radial velocity and transit searches, they are a minority among all planets in our Galaxy (Mann et al. 2010).

Figure 3 illustrates the double-peaked distribution of exoplanet radii. The letters E, U, S, and J indicate where the radii of Earth, Uranus, Saturn, and Jupiter fall on the same scale. Recent analyses argue that the population peak between 2 and 3 Rea is real, and not an artifact of detection bias (Petigura et al. 2013). In other words, the population of low-mass planets does not continue to increase at radii smaller than 2 Rea. At best, the actual number of Earth-size planets – admittedly harder to detect than larger planets – may rival that of their siblings between 2 and 3 Rea, but it probably does not exceed that population. Some well-known examples of the latter are illustrated in Figure 2.

Current radial velocity data indicate that the median mass of all extrasolar gas giants is 1.5 Jupiter masses (Mjup). This figure has remained fairly consistent over the past five years, even as the number of exoplanets has tripled. Ninety percent of gas giants have minimum masses of 7 Mjup or less. Unlike the case in lightweight planets, we see little correspondence between mass and radius for the more massive gas giants observed in transit. Instead, temperature plays the defining role. With a single exception, the objects in Figure 3 with radii above 12 Rea are Hot Jupiters, which have very high equilibrium temperatures and usually inflated atmospheres (see Figure 1 for an example). 

The fraction of Hot Jupiters is another relatively consistent feature of the overall sample. They constitute 35% of all known gas giants and 25% of all well-constrained exoplanets of any kind. However, several studies have demonstrated that the true occurrence rate for these planets is much lower: Hot Jupiters orbit far fewer than 1% of the stars in the Milky Way.

More than half (53%) of all exoplanets with well-defined orbits have semimajor axes smaller than Mercury’s (0.49 AU), while only 23% have semimajor axes larger than that of Mars (1.52. AU). All the cooler planets are gas giants, and rather massive ones at that. Just four objects on cool orbits fall in the low-mass tail of the gas giant range, with minimum masses between 45 and 60 Mea. The median mass for all cool giants (i.e., trans-Martians) is 2 Mjup, illustrating the tendency of more massive planets to travel on wider orbits.

Among low-mass planets, hot orbits predominate. Although the range of orbital periods for this group extends from 8.5 hours to 600 days, almost 40% have periods shorter than 10 days, and only 5% have periods longer than 225 days (the period of Venus around our Sun). However, this distribution is certainly the result of detection bias; a hidden population of small planets on longer-period orbits still awaits discovery. Among low-mass planets for which masses can be estimated, the median is 10 Mea, corresponding to an object more reminiscent of Uranus than of Venus. Again, this figure might be the result of detection biases instead of a reflection of reality.

three kinds of systems

One welcome trend is the increasing share of multiplanet systems within the full census of well-constrained exoplanets. Currently, just 60% of planets occur in systems with only one planet, and within this subpopulation, 92% are gas giants (among which almost half are Hot Jupiters). Another 23% of the full census are found in systems with two planets. Like the singleton systems, these are dominated by gas giants (60% of all planets in two-planet configurations). However, only 5% of planets in these systems are Hot Jupiters. The two-planet sample provides an interesting illustration of the adage that birds of a feather flock together, since 84% of systems consist of either of two gas giants or two low-mass planets, while only 16% have one of each. Even among the odd couples, the vast majority of giant companions are less massive than 0.5 Mjup.

Finally, 17% of the total appear in systems with three or more planets; 73% of planets in these systems are low-mass, and as in the two-planet configurations, only 5% of systems contain Hot Jupiters.

This breakdown of systems by planeticity (an indispensable coinage that hasn’t yet found its way into standard dictionaries) tells us something significant about the character of the two exoplanet species. The little folk are gregarious and abundant, while the heavyweights are hostile and aloof. The former play well with others, while the latter prefer solitude, or at most the company of a few representatives of their own kind. When the two species manage to co-exist in the same system, something special sometimes happens, such as life.

The study of highly planetic systems – i.e., those with three or more planets – opens a unique window on extrasolar system architectures. In a previous post I proposed three basic architectural types, defined by their planet populations: low-mass, mixed-mass, and high-mass. I continue to find these categories useful for conceptualizing the exoplanet census.

Low-mass systems contain closely-spaced planets of similar size traveling on circular orbits near their host stars. They are often extremely compact. For example, Kepler-33 has five planets (three of them larger than Neptune) within a semimajor axis of 0.26 AU, while Kepler-11 has six planets within a semimajor axis of 0.5 AU.

Mixed-mass systems, such as the Solar System, 55 Cancri, and HD 10180, contain a broader range of planet masses, with at least one, sometimes two, and rarely three gas giants. Orbits in these systems tend to be more expansive than those of low-mass systems, while their giants are typically much less massive than the median for the full sample. None have Hot Jupiters. Overall, the mixed-mass systems look more like low-mass systems than like high-mass systems

High-mass systems containing three or more gas giants, but no low-mass planets, are quite rare. (Nevertheless, one of the best-studied systems detected by direct imaging, HR 8799, seems to host at least four giants on wide orbits). The components of these systems tend to be more massive than the median for all gas giants, and their orbits are wider and more eccentric than those in the other two architectural types. Half have Hot Jupiters.

In general, gas giants are intolerant of other gas giants, especially in close proximity. Among all extrasolar systems detected to date, only six (less than 1%) contain three or more giants. Two are mixed-mass systems; the rest constitute the high-mass sample shown in the table below. The more massive the giant, the less likely it is have neighbors, in particular diminutive neighbors.

Table 1. 38 systems with three or more planets, binned by architectural category

Early in 2012, after a few months of unusually rapid growth in the census at EPE, I made a prediction about where the numbers were headed: “At this rate, we can expect the census to reach the magic number of 1000 before the end of 2013.” Given the much slower growth over the past year, my prediction may be on shaky ground. It will be interesting to see how things pan out.

REFERENCES

Bennett DP. (2008) Detection of extrasolar planets by gravitational microlensing. In Exoplanets, ed. John Mason. Berlin: Springer, 2008. Abstract. 
Mann AW, Gaidos E, Gaudi BS. The invisible majority? Evolution and detection of outer planetary systems without gas giants. (2010) Astrophysical Journal 719, 1454–1469. Abstract: http://adsabs.harvard.edu/abs/2010ApJ...719.1454M 
Petigura EA, Geoffrey MW, Howard AW. (2013) A plateau in the planet population below twice the size of Earth. In press.