ALMA 1.3 Millimeter Map of the HD 95086 System

ALMA 1.3 Millimeter Map of the HD 95086 System

Authors:

Su et al

Abstract:

Planets and minor bodies such as asteroids, Kuiper-belt objects and comets are integral components of a planetary system. Interactions among them leave clues about the formation process of a planetary system. The signature of such interactions is most prominent through observations of its debris disk at millimeter wavelengths where emission is dominated by the population of large grains that stay close to their parent bodies. Here we present ALMA 1.3 mm observations of HD 95086, a young early-type star that hosts a directly imaged giant planet b and a massive debris disk with both asteroid- and Kuiper-belt analogs. The location of the Kuiper-belt analog is resolved for the first time. The system can be depicted as a broad (ΔR/R∼0.84), inclined (30\arcdeg±3\arcdeg) ring with millimeter emission peaked at 200±6 au from the star. The 1.3 mm disk emission is consistent with a broad disk with sharp boundaries from 106±6 to 320±20 au with a surface density distribution described by a power law with an index of --0.5±0.2. Our deep ALMA map also reveals a bright source located near the edge of the ring, whose brightness at 1.3 mm and potential spectral energy distribution are consistent with it being a luminous star-forming galaxy at high redshift. We set constraints on the orbital properties of planet b assuming co-planarity with the observed disk.

Proxima Centauri Appears to Have a Kuiper Belt

Our nearest neighboring star just got a whole lot richer as a system—and a whole lot weirder.

In research published today in the Astrophysical Journal Letters, researchers from the European Southern Observatory announced … quite a few things, really. The biggest and brightest—literally—of their discoveries is a ring of icy dust around our nearest star, Proxima Centauri, that’s sort of like that system’s version of the Kuiper Belt.

The Kuiper Belt is a circumstellar disc of material that envelopes the solar system’s planets. The belt contains rocks and ices left over from the formation of the solar system’s planets, and also includes dwarf planets like Pluto. But Proxima Centauri is a star that’s much smaller than the Sun, so its dust belt sits much closer, about 1 to 4 times the distance of the Earth to the Sun (called an astronomical unit or AU in space parlance). And when you spot these kinds of belts around stars, it’s often a strong indication of planetary formation.

“[I]n my opinion what we found in Proxima Centauri suggests an elaborate system that might be harboring several planets,” Mayra Osorio of the Instituto de Astrofísica de Andalucía and a coauthor on the paper says.

link.

Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets

Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets 

Authors:

Matrà et al

Abstract:
Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4σ detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived (0.65−42×10−7 M⊕) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO2 mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that (49±27)% of the detected flux originates from a region near the eccentric belt's pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.

Exozodiacal Dust Clouds Could be Formed by Exoplanets in Resonant, Eccentric Orbits

Inner mean-motion resonances with eccentric planets: A possible origin for exozodiacal dust clouds

Authors:

Faramaz et al

Abstract:

High levels of dust have been detected in the immediate vicinity of many stars, both young and old. A promising scenario to explain the presence of this short-lived dust is that these analogues to the Zodiacal cloud (or exozodis) are refilled in situ through cometary activity and sublimation. As the reservoir of comets is not expected to be replenished, the presence of these exozodis in old systems has yet to be adequately explained.

It was recently suggested that mean-motion resonances (MMR) with exterior planets on moderately eccentric (ep≳0.1) orbits could scatter planetesimals on to cometary orbits with delays of the order of several 100 Myr. Theoretically, this mechanism is also expected to sustain continuous production of active comets once it has started, potentially over Gyr-timescales.

We aim here to investigate the ability of this mechanism to generate scattering on to cometary orbits compatible with the production of an exozodi on long timescales. We combine analytical predictions and complementary numerical N-body simulations to study its characteristics.

We show, using order of magnitude estimates, that via this mechanism, low mass discs comparable to the Kuiper Belt could sustain comet scattering at rates compatible with the presence of the exozodis which are detected around Solar-type stars, and on Gyr timescales. We also find that the levels of dust detected around Vega could be sustained via our proposed mechanism if an eccentric Jupiter-like planet were present exterior to the system's cold debris disc.

Incidence of debris discs around FGK stars in the solar neighbourhood

Incidence of debris discs around FGK stars in the solar neighbourhood

Authors:

Montesinos et al

Abstract:

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. The full sample of 177 FGK stars with d less than 20 pc proposed for the DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE photometry, were obtained. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the DEBRIS consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age.

The subsample of 105 stars with d less than 15 pc containing 23 F, 33 G and 49 K stars, is complete for F stars, almost complete for G stars and contains a substantial number of K stars to draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type 0.26 (6 objects with excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49 K stars), the fraction for all three spectral types together being 0.22 (23 out of 105 stars). Uncertainties corresponding to a 95% confidence level are given in the text for all these numbers. The medians of the upper limits of L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K); the lowest values being around 4.0E-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.

Detecting Micro Lensing of Free Floating Planets

Microlensing by Kuiper, Oort, and Free-Floating Planets

Author:

Gould

Abstract:

Microlensing is generally thought to probe planetary systems only out to a few Einstein radii. Microlensing events generated by bound planets beyond about 10 Einstein radii generally do not yield any trace of their hosts, and so would be classified as free floating planets (FFPs). I show that it is already possible, using adaptive optics (AO), to constrain the presence of potential hosts to FFP candidates at separations comparable to the Oort Cloud. With next-generation telescopes, planets at Kuiper-Belt separations can be probed. Next generation telescopes will also permit routine vetting for all FFP candidates, simply by obtaining second epochs 4-8 years after the event. At present, the search for such hosts is restricted to within the "confusion limit" of theta_confus ~ 250 mas, but future WFIRST observations will allow one to probe beyond this confusion limit as well.

How to Tell if the Proposed Planet Nine is Really a Captured Exoplanet

Is there an exoplanet in the Solar System?

Authors:

Mustill et al

Abstract:

We investigate the prospects for the capture of the proposed Planet 9 from other stars in the Sun's birth cluster. Any capture scenario must satisfy three conditions: the encounter must be more distant than ~150 au to avoid perturbing the Kuiper belt; the other star must have a wide-orbit planet (a>~100au); the planet must be captured onto an appropriate orbit to sculpt the orbital distribution of wide-orbit Solar System bodies. Here we use N-body simulations to show that these criteria may be simultaneously satisfied. In a few percent of slow close encounters in a cluster, bodies are captured onto heliocentric, Planet 9-like orbits. During the ~100 Myr cluster phase, many stars are likely to host planets on highly-eccentric orbits with apastron distances beyond 100 au if Neptune-sized planets are common and susceptible to planet--planet scattering. While the existence of Planet 9 remains unproven, we consider capture from one of the Sun's young brethren a plausible route to explain such an object's orbit. Capture appears to predict a large population of Trans-Neptunian Objects (TNOs) whose orbits are aligned with the captured planet, and we propose that different formation mechanisms will be distinguishable based on their imprint on the distribution of TNOs.

How to Detect Exosolar Kuiper Belt Objects Through Intermediate-Luminosity Optical Transients

Using Intermediate-Luminosity Optical Transients (ILOTs) to reveal extended exo-solar Kuiper belt objects

Authors:

Bear et al

Abstract:

We suggest that in the rare case of an Intermediate-Luminosity Optical Transient (ILOTs) event, evaporation of exo-solar Kuiper belt objects (ExoKBOs) at distances of d~500 - 10000AU from the ILOT can be detected. If the ILOT lasts for 1 month to a few years, enough dust might be ejected from the ExoKBOs for the IR emission to be detected. Because of the large distance of the ExoKBOs, tens of years will pass before the ILOT wind disperses the dust. We suggest that after an ILOT outburst there is a period of months to several years during which IR excess emission might hint at the existence of a Kuiper belt analog (ExoK-Belt).

The Initial Mass and Size Distribution of Planetesimals

The Initial Mass and Size Distribution of Planetesimals. I. The Effect of Resolution, Gravity, and Initial Conditions in Streaming Instability Calculations

Authors:

Simon et al

Abstract:

We study the formation of planetesimals in protoplanetary disks from the gravitational collapse of solid over-densities generated via the streaming instability. To carry out these studies, we implement and test a particle-mesh self-gravity module for the Athena code that enables the simulation of aerodynamically coupled systems of gas and collisionless self-gravitating solid particles. Upon employment of our algorithm to planetesimal formation simulations, we find that (when a direct comparison is possible) the Athena simulations yield predicted planetesimal properties that agree well with those found in prior work using different numerical techniques. In particular, the gravitational collapse of streaming-initiated clumps leads to an initial planetesimal mass function that is well-represented by a power-law, dN/dM ~ M^(-p),with p = 1.6 +/- 0.1. We find no significant trends with resolution from a convergence study of up to 512^3 grid zones and N_par ~ 1.5x10^8 particles. Likewise, the power-law slope appears indifferent to changes in the relative strength of self-gravity and tidal shear, and to the time when (for reasons of numerical economy) self-gravity is turned on, though the strength of these claims is limited by small number statistics. For a typically assumed radial distribution of minimum mass solar nebula solids (assumed here to have dimensionless stopping time {\tau} = 0.3), our results support the hypothesis that bodies on the scale of large asteroids or Kuiper Belt Objects could have formed as the high-mass tail of a primordial planetesimal population.

A TNO, SuperEarth or Cool Brown Dwarf Tracking With α Centauri System

A new submm source within a few arcseconds of α Centauri: ALMA discovers the most distant object of the solar system

Authors:

Liseau et al

Abstract:

The understanding of the formation of stellar and planetary systems requires the understanding of the structure and dynamics of their outmost regions, where large bodies are not expected to form. Serendipitous searches for Sedna-like objects allows the observation of regions that are normally not surveyed. The Atacama Large Millimeter/submillimeter Array (ALMA) is particularly sensitive to point sources and it presents currently the only means to detect Sedna-like objects far beyond their perihelia. ALMA observations 10 months apart revealed a new blackbody point source that is apparently comoving with α Cen B. We exclude that source to be a sub-/stellar member of the α Centauri system, but argue that it is either an extreme TNO, a Super-Earth or a very cool brown dwarf in the outer realm of the solar system.

Gna? Or Something else in the Far Outer Solar System?

The serendipitous discovery of a possible new solar system object with ALMA

Authors:

Vlemmings et al

Abstract:

The unprecedented sensitivity of the Atacama Large millimeter/submillimeter array (ALMA) is providing many new discoveries. Several of these are serendipitous to the original goal of the observations. We report the discovery of previously unknown continuum sources, or a single fast moving new source, in our ALMA observations. Here we aim to determine the nature of the detections. The detections, at greater than 5.8σ in the image plane and greater than 14σ in the (u,v)−plane, were made in two epochs of ALMA observations of a 25 arc second region around the asymptotic giant branch star W Aql in the continuum around 345 GHz. At a third epoch, covering 50x50 arcseconds, the source(s) were not seen. We have investigated if the detections could be spurious, if they could constitute a population of variable background sources, or if the observations revealed a fast moving single object. Based on our analysis, we conclude that a single object (with a flux of ∼3.0 mJy) exhibiting a large proper motion (∼87 arcsec/yr) is the most likely explanation. Until the nature of the source becomes clear, we have named it Gna. Unless there are yet unknown, but significant, issues with ALMA observations, we have detected a previously unknown objects in our solar system. Based on proper motion analysis we find that, if it is gravitationally bound, Gna is currently located at 12−25 AU distance and has a size of ∼220−880 km. Alternatively it is a much larger, planet-sized, object, gravitationally unbound, and located within ∼4000 AU, or beyond (out to ∼0.3~pc) if it is strongly variable. Our observations highlight the power of ALMA in detecting possible solar system objects, but also show how multiple epoch observations are crucial to identify what are otherwise probably assumed to be extra-galactic sources.

Forming the Cold Classical Kuiper Belt in a Light Protoplanetary Disk

Forming the Cold Classical Kuiper Belt in a light Disk

Authors:

Shannon et al

Abstract:

Large Kuiper Belt Objects are conventionally thought to have formed out of a massive planetesimal belt that is a few thousand times its current mass. Such a picture, however, is incompatible with multiple lines of evidence. Here, we present a new model for the conglomeration of Cold Classical Kuiper belt objects, out of a solid belt only a few times its current mass, or a few percent of the solid density in a Minimum Mass Solar Nebula. This is made possible by depositing most of the primordial mass in grains of size centimetre or smaller. These grains collide frequently and maintain a dynamically cold belt out of which large bodies grow efficiently: an order-unity fraction of the solid mass can be converted into large bodies, in contrast to the ~0.1% efficiency in conventional models. Such a light belt may represent the true outer edge of the Solar system, and it may have effectively halted the outward migration of Neptune. In addition to the high efficiency, our model can also produce a mass spectrum that peaks at an intermediate size, similar to the observed Cold Classicals, if one includes the effect of cratering collisions. In particular, the observed power-law break observed at ~30 km for Cold Classicals, one that has been interpreted as a result of collisional erosion, may be primordial in origin.

A new Inclination Instability Reshapes Keplerian Disks Into Cones

A new inclination instability reshapes Keplerian disks into cones: application to the outer Solar System

Authors:

Madigan et al

Abstract:

Disks of bodies orbiting a much more massive central object are extremely common in astrophysics. When the orbits comprising such disks are eccentric, we show they are susceptible to a new dynamical instability. Gravitational forces between bodies in the disk drive exponential growth of their orbital inclinations and clustering in their angles of pericenter, expanding an initially thin disk into a conical shape by giving each orbit an identical 'tilt' with respect to the disk plane. This new instability dynamically produces the unusual distribution of orbits observed for minor planets beyond Neptune, suggesting that the instability has shaped the outer Solar System. It also implies a large initial disk mass (1-10 Earth masses) of scattered bodies at hundreds of AU; we predict increasing numbers of detections of minor planets clustered in their angles of pericenter with high inclinations.

How Dwarf Planets can Heat Kuiper Belts

Dynamical Heating Induced by Dwarf Planets on Cold Kuiper Belt-like Debris Disks

Authors:

Muñoz-Gutiérrez et al

Abstract:

With the use of long-term numerical simulations, we study the evolution and orbital behavior of cometary nuclei in cold Kuiper belt-like debris disks under the gravitational influence of dwarf planets (DPs); we carry out these simulations with and without the presence of a Neptune-like giant planet. This exploratory study shows that in the absence of a giant planet, 10 DPs are enough to induce strong radial and vertical heating on the orbits of belt particles. On the other hand, the presence of a giant planet close to the debris disk, acts as a stability agent reducing the radial and vertical heating. With enough DPs, even in the presence of a Neptune-like giant planet some radial heating remains; this heating grows steadily, re-filling resonances otherwise empty of cometary nuclei. Specifically for the solar system, this secular process seems to be able to provide material that, through resonant chaotic diffusion, increase the rate of new comets spiraling into the inner planetary system, but only if more than the ∼10 known DP sized objects exist in the trans-Neptunian region.

Sedna: The Exoplanet in our own Solar System

How Sedna and family were captured in a close encounter with a solar sibling

Authors:

Jilkova et al

Abstract:

The discovery of 2012VP113 initiated the debate on the origin of the Sedna family of planetesimals in orbit around the Sun. Sednitos roam the outer regions of the Solar System between the Egeworth--Kuiper belt and the Oort cloud, in extraordinary wide (a greater than 150au) orbits with a large perihelion distance of q greater than 30au compared to the Earth's (a=1au and eccentricity e=(1-q/a) ~ 0.0167 or q=1au). This population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i=10--30deg. They also have similar argument of perihelion omega=340+/-55deg. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disk of another star.Assuming the orbital elements of Sednitos have not changed since they acquired their orbits, we reconstruct the encounter that led to their capture. We conclude that they might have been captured in a near miss with a 1.8MSun star that impacted the Sun at ~340au at an inclination with respect to the ecliptic of 17--34deg with a relative velocity at infinity of ~4.3km/s. We predict that the Sednitos-region is populated by 930 planetesimals and the inner Oort cloud acquired ~440 planetesimals through the same encounter.

HD 11560's Kuiper Belt Directly Imaged, Slightly Blue to Grey Color, Probably Water Ice

Direct Imaging and Spectroscopy of a Young Extrasolar Kuiper Belt in the Nearest OB Association

Authors:

Currie et al

Abstract:

We describe the discovery of a bright, young Kuiper belt-like debris disk around HD 115600, a ∼ 1.4--1.5 M⊙, ∼ 15 Myr old member of the Sco-Cen OB Association. Our H-band coronagraphy/integral field spectroscopy from the \textit{Gemini Planet Imager} shows the ring has a (luminosity scaled) semi major axis of (∼ 22 AU) ∼ 48 AU, similar to the current Kuiper belt. The disk appears to have neutral scattering dust, is eccentric (e ∼ 0.1--0.2), and could be sculpted by analogues to the outer solar system planets. Spectroscopy of the disk ansae reveal a slightly blue to gray disk color, consistent with major Kuiper belt chemical constituents, where water-ice is a very plausible dominant constituent. Besides being the first object discovered with the next generation of extreme adaptive optics systems (i.e. SCExAO, GPI, SPHERE), HD 115600's debris ring and planetary system provides a key reference point for the early evolution of the solar system, the structure and composition of the Kuiper belt, and the interaction between debris disks and planets.

Kuiper Belts Around Systems With SuperEarth Sized Exoplanets

Kuiper belt structure around nearby super-Earth host stars

Authors:

Kennedy et al

Abstract:

We present new observations of the Kuiper belt analogues around HD 38858 and HD 20794, hosts of super-Earth mass planets within 1 au. As two of the four nearby G-type stars (with HD 69830 and 61 Vir) that form the basis of a possible correlation between low-mass planets and debris disc brightness, these systems are of particular interest. The disc around HD 38858 is well resolved with Herschel and we constrain the disc geometry and radial structure. We also present a probable JCMT sub-mm continuum detection of the disc and a CO J=2-1 upper limit. The disc around HD 20794 is much fainter and appears marginally resolved with Herschel, and is constrained to be less extended than the discs around 61 Vir and HD 38858. We also set limits on the radial location of hot dust recently detected around HD 20794 with near-IR interferometry. We present HARPS upper limits on unseen planets in these four systems, ruling out additional super-Earths within a few au, and Saturn-mass planets within 10 au. We consider the disc structure in the three systems with Kuiper belt analogues (HD 69830 has only a warm dust detection), concluding that 61 Vir and HD 38858 have greater radial disc extent than HD 20794. We speculate that the greater width is related to the greater minimum planet masses (10-20 M⊕ vs. 3-5 M⊕), arising from an eccentric planetesimal population analogous to the Solar System's scattered disc. We discuss alternative scenarios and possible means to distinguish among them.

Dust in Exo Kupier Belts

Warm exo-Zodi from cool exo-Kuiper belts: the significance of P-R drag and the inference of intervening planets

Authors:

Kennedy et al

Abstract:

Poynting-Robertson drag has been considered an ineffective mechanism for delivering dust to regions interior to the cool Kuiper belt analogues seen around other Sun-like stars. This conclusion is however based on the very large contrast in dust optical depth between the parent belt and the interior regions that results from the dominance of collisions over drag in systems with detectable cool belts. Here, we show that the levels of habitable zone dust arising from detectable Kuiper belt analogues can be tens to a few hundreds of times greater than the optical depth in the Solar Zodiacal cloud. Dust enhancements of more than a few tens of `zodi' are expected to hinder future Earth-imaging missions, but relatively few undetectable Kuiper belts result in such levels, particularly around stars older than a few Gyr. Thus, current mid to far-IR photometric surveys have already identified most of the 20-25% of nearby stars where P-R drag from outer belts could seriously impact Earth-imaging. The LBTI should easily detect such warm dust around many nearby stars with outer belts, and will provide insight into currently unclear details of the competition between P-R drag and collisions. Given sufficient confidence in future models, the inevitability of P-R drag means that the non-detection of warm dust where detectable levels were expected could be used to infer additional dust removal process, the most likely being the presence of intervening planets.

Does the Presence of Exoplanets Affect the Formation of Kuiper Belts?

Does the presence of planets affect the frequency and properties of extrasolar Kuiper Belts? Results from the Herschel DEBRIS and DUNES surveys

Authors:

Moro-Martin et al

Abstract:

The study of the planet-debris disk connection can shed light on the formation and evolution of planetary systems, and may help predict the presence of planets around stars with certain disk characteristics. In preliminary analyses of the Herschel DEBRIS and DUNES surveys, Wyatt et al. (2012) and Marshall et al. (2014) identified a tentative correlation between debris and low-mass planets. Here we use the cleanest possible sample out these surveys to assess the presence of such a correlation, discarding stars without known ages, with ages less than 1 Gyr and with binary companions less than 100 AU, to rule out possible correlations due to effects other than planet presence. In our sample of 204 FGK stars, we do not find evidence that debris disks are more common or more dusty around stars harboring high-mass or low-mass planets compared to a control sample without identified planets, nor that debris disks are more or less common (or more or less dusty) around stars harboring multiple planets compared to single-planet systems. Diverse dynamical histories may account for the lack of correlations. The data show the correlation between the presence of high-mass planets and stellar metallicity, but no correlation between the presence of low-mass planets or debris and stellar metallicity. Comparing the observed cumulative distribution of fractional luminosity to those expected from a Gaussian distribution, we find that a distribution centered on the Solar system's value fits well the data, while one centered at 10 times this value can be rejected. This is of interest in the context of future terrestrial planet characterization because it indicates that there are good prospects for finding a large number of debris disk systems (i.e. with evidence of harboring the building blocks of planets) with exozodiacal emission low enough to be appropriate targets for an ATLAST-type mission to search for biosignatures.

How the Kuiper Belt & Exoplanetary Kuiper Belts Form

Conglomeration of kilometre-sized planetesimals

Authors:

Shannon et al

Abstract:

We study the efficiency of forming large bodies, starting from a sea of equal-sized planetesimals. This is likely one of the earlier steps of planet formation and relevant for the formation of the asteroid belt, the Kuiper belt and extra-solar debris disks. Here we consider the case that the seed planetesimals do not collide frequently enough for dynamical collisional to be important (the collisionless limit), using a newly constructed conglomeration code, and by carefully comparing numerical results with analytical scalings. In the absence of collisional cooling, as large bodies grow by accreting small bodies, the velocity dispersion of the small bodies (u) is increasingly excited. Growth passes from the well-known run-away stage (when u is higher than the big bodies' hill velocity) to the newly discovered trans-hill stage (when u and big bodies both grow, but u remains at the big bodies' hill velocity). We find, concurring with the analytical understandings developed in Lithwick (2014), as well as previous numerical studies, that a size spectrum dn/dR∝R−4 results, and that the formation efficiency, defined as mass fraction in bodies much larger than the initial size, is ∼afew×R⊙/a, or ∼10−3 at the distance of the Kuiper belt. We argue that this extreme inefficiency invalidates the conventional conglomeration model for the formation of both our Kuiper belt and extra-solar debris disks. New theories, possibly involving direct gravitational collapse, or strong collisional cooling of small planetesimals, are required.