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Oort cloud comets are currently believed to have formed in the Sun's protoplanetary disk and to have been ejected to large heliocentric orbits by the giant planets. Detailed models of this process fail to reproduce all of the available observational constraints, however. In particular, the Oort cloud appears to be substantially more populous than the models predict. Here we present numerical simulations that show that the Sun captured comets from other stars while it was in its birth cluster. Our results imply that a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary disks of other stars.

The motion of planetesimals initially located in the feeding zone of the planet Proxima Centauri c , at distances of 500 AU from the star to the star’s Hill sphere radius of 1200 AU was considered. In the analyzed non-gaseous model, the primary ejection of planetesimals from most of the feeding zone of an almost formed planet c to distances greater than 500 AU from the star occurred during the first 10 million years. Only for planetesimals originally located at the edges of the planet’s feeding zone, the fraction of planetesimals that first reached 500 AU over the time greater than 10 million years was more than half. Some planetesimals could reach the outer part of the star’s Hill sphere over hundreds of millions of years. Approximately 90% of the planetesimals that first reached 500 AU from Proxima Centauri first reached 1200 AU from the star in less than 1 million years, given the current mass of the planet c . No more than 2% of planetesimals with aphelion orbital distances between 500 and 1200 AU followed such orbits for more than 10 million years (but less than a few tens of millions of years). With a planet mass equal to half the mass of the planet c , approximately 70–80% of planetesimals increased their maximum distances from the star from 500 to 1200 AU in less than 1 million years. For planetesimals that first reached 500 AU from the star under the current mass of the planet c , the fraction of planetesimals with orbital eccentricities greater than 1 was 0.05 and 0.1 for the initial eccentricities of their orbits e o = 0.02 and e o = 0.15, respectively. Among the planetesimals that first reached 1200 AU from the star, this fraction was approximately 0.3 for both e o values. The minimum eccentricity values for planetesimals that have reached 500 and 1200 AU from the star were 0.992 and 0.995, respectively. In the considered model, the disk of planetesimals in the outer part of the star’s Hill sphere was rather flat. Inclinations i of the orbits for more than 80% of the planetesimals that first reached 500 or 1200 AU from the star did not exceed 10°. With the current mass of the planet c , the percentage of such planetesimals with i > 20° did not exceed 1% in all calculation variants. The results may be of interest for understanding the motion of bodies in other exoplanetary systems, especially those with a single dominant planet. They can be used to provide the initial data for models of the evolution of the disk of bodies in the outer part of Proxima Centauri’s Hill sphere, which take into account gravitational interactions and collisions between bodies, as well as the influence of other stars. The strongly inclined orbits of bodies in the outer part of Proxima Centauri’s Hill sphere can primarily result from bodies that entered the Hill sphere from outside. The radius of Proxima Centauri’s Hill sphere is an order of magnitude smaller than the radius of the outer boundary of the Hills cloud in the Solar System and two orders of magnitude smaller than the radius of the Sun’s Hill sphere. Therefore, it is difficult to expect the existence of a similarly massive cloud around this star as the Oort cloud around the Sun.

Comets have three known reservoirs: the roughly spherical Oort Cloud (for long-period comets), the flattened Kuiper Belt (for ecliptic comets), and, surprisingly, the asteroid belt (for main-belt comets). Comets in the Oort Cloud were thought to have formed in the region of the giant planets and then placed in quasi-stable orbits at distances of thousands or tens of thousands of AU through the gravitational effects of the planets and the Galaxy. The planets were long assumed to have formed in place. However, the giant planets may have undergone two episodes of migration. The first would have taken place in the first few million years of the Solar System, during or shortly after the formation of the giant planets, when gas was still present in the protoplanetary disk around the Sun. The Grand Tack (Walsh et al. in Nature 475:206–209, 2011 ) models how this stage of migration could explain the low mass of Mars and deplete, then repopulate the asteroid belt, with outer-belt asteroids originating between, and outside of, the orbits of the giant planets. The second stage of migration would have occurred later (possibly hundreds of millions of years later) due to interactions with a remnant disk of planetesimals, i.e., a massive ancestor of the Kuiper Belt. Safronov (Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets, 1969 ) and Fernández and Ip (Icarus 58:109–120, 1984 ) proposed that the giant planets would have migrated as they interacted with leftover planetesimals; Jupiter would have moved slightly inward, while Saturn and (especially) Uranus and Neptune would have moved outward from the Sun. Malhotra (Nature 365:819–821, 1993 ) showed that Pluto’s orbit in the 3:2 resonance with Neptune was a natural outcome if Neptune captured Pluto into resonance while it migrated outward. Building on this work, Tsiganis et al. (Nature 435:459–461, 2005 ) proposed the Nice model, in which the giant planets formed closer together than they are now, and underwent a dynamical instability that led to a flood of comets and asteroids throughout the Solar System (Gomes et al. in Nature 435:466–469, 2005b ). In this scenario, it is somewhat a matter of luck whether an icy planetesimal ends up in the Kuiper Belt or Oort Cloud (Brasser and Morbidelli in Icarus 225:40–49, 2013 ), as a Trojan asteroid (Morbidelli et al. in Nature 435:462–465, 2005 ; Nesvorný and Vokrouhlický in Astron. J. 137:5003–5011, 2009 ; Nesvorný et al. in Astrophys. J. 768:45, 2013 ), or as a distant “irregular” satellite of a giant planet (Nesvorný et al. in Astron. J. 133:1962–1976, 2007 ). Comets could even have been captured into the asteroid belt (Levison et al. in Nature 460:364–366, 2009 ). The remarkable finding of two “inner Oort Cloud” bodies, Sedna and 2012 VP 113 , with perihelion distances of 76 and 81 AU, respectively (Brown et al. in Astrophys. J. 617:645–649, 2004 ; Trujillo and Sheppard in Nature 507:471–474, 2014 ), along with the discovery of other likely inner Oort Cloud bodies (Chen et al. in Astrophys. J. Lett. 775:8, 2013 ; Brasser and Schwamb in Mon. Not. R. Astron. Soc. 446:3788–3796, 2015 ), suggests that the Sun formed in a denser environment, i.e., in a star cluster (Brasser et al. in Icarus 184:59–82, 2006 , 191:413–433, 2007 , 217:1–19, 2012b ; Kaib and Quinn in Icarus 197:221–238, 2008 ). The Sun may have orbited closer or further from the center of the Galaxy than it does now, with implications for the structure of the Oort Cloud (Kaib et al. in Icarus 215:491–507, 2011 ). We focus on the formation of cometary nuclei; the orbital properties of the cometary reservoirs; physical properties of comets; planetary migration; the formation of the Oort Cloud in various environments; the formation and evolution of the Kuiper Belt and Scattered Disk; and the populations and size distributions of the cometary reservoirs. We close with a brief discussion of cometary analogs around other stars and a summary.

Simulations show that the inner Oort Cloud is the source of many more long-period comets than expected. A few times a year, new long-period comets (LPCs) on elongated orbits come to within ∼ 1 astronomical unit (AU) of the Sun (1 AU is the distance between the Earth and the Sun), where they draw our attention by releasing majestic tails of dust and ions from their frozen surfaces. The original orbits typically trace back out to distances of at least 20,000 AU. Since Jan Oort's classic 1950 paper ( 1 ), it was believed that the reservoir for the LPCs is a roughly spherical “Oort Cloud” of approximately 1 trillion comets, which extends from 20,000 to 100,000 AU (about halfway to the nearest stars). The bodies in the Oort Cloud are thought to be the surviving population of unincorporated remnants of planetary building blocks that were gravitationally scattered outward by the growing planets. On page 1234 of this issue, Kaib and Quinn ( 2 ) present results of a simulation that suggest that a substantial fraction of LPCs (perhaps the majority) are stored in an “inner” Oort Cloud reservoir considerably closer to the Sun than expected, at distances of around 3000 to 10,000 AU.

Ever since Pluto lost its status as one of the main planets of our solar system and was demoted to just another frozen denizen of the Kuiper belt, we have had to make do with eight, albeit in a pleasing symmetry, with four rocky ones this side of the asteroid belt and four giants on the far side. Now it looks like number nine is back on the slate: the existence of a large planet, about ten times as massive as Earth and hundreds of times more distant from the Sun than Earth itself, has been postulated to explain the curiously bunched-up orbits of several small celestial bodies, far beyond the orbit of Neptune. To date, we have only \"proof by simulation\" and we are yet to observe this massive planet in the backyard of our solar system by more direct means. However, powerful new telescopes should provide visual evidence within the next few decades.

In this work, a new equation derived to obtain some physical properties like radius and luminosity of comets using distances between the comet, the Earth and the Sun that represent the geocentric distance and heliocentric distance at applying it to a some of comets in two different regions, namely the Kuiper belt and the Oort cloud. This relation gave very good results when compared. From the results, it can form important relation between the radius with luminosity, when the radius increased the luminosity increased too this relation proportional. And the calculation has been made for the phase angle. The relation between luminosity as a function of radius have been found.

A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .

We describe the selection of the James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-domain Field (TDF), a 14′ diameter field located within JWST's northern continuous viewing zone (CVZ) and centered at (R.A., decl.)J2000 = (17:22:47.896, +65:49:21.54). We demonstrate that this is the only region in the sky where JWST can observe a clean (i.e., free of bright foreground stars and with low Galactic foreground extinction) extragalactic deep survey field of this size at arbitrary cadence or at arbitrary orientation, and without a penalty in terms of a raised zodiacal background. This will crucially enable a wide range of new and exciting time-domain science, including high-redshift transient searches and monitoring (e.g., SNe), variability studies from active galactic nuclei (AGNs) to brown dwarf atmospheres, as well as proper motions of possibly extreme scattered Kuiper Belt and Inner Oort Cloud Objects, and of nearby Galactic brown dwarfs, low-mass stars, and ultracool white dwarfs. A JWST/NIRCam+NIRISS GTO program will provide an initial 0.8-5.0 m spectrophotometric characterization to m AB ∼ 28.8 0.3 mag of four orthogonal \"spokes\" within this field. The multi-wavelength (radio through X-ray) context of the field is in hand (ground-based near-UV-visible-near-IR), in progress (VLA 3 GHz, VLBA 5 GHz, HST UV-visible, Chandra X-ray, and IRAM 30 m 1.3 and 2 mm), or scheduled (JCMT 850 m). We welcome and encourage ground- and space-based follow-up of the initial GTO observations and ancillary data, to realize its potential as an ideal JWST time-domain community field.

A bulk acoustic wave cavity as high frequency gravitational wave antenna has recently detected two rare events at 5.5MHz. Assuming that the detected events are due to gravitational waves, their characteristic strain amplitude lies at about hc≈2.5×10-16. While a cosmological signal is out of the picture due to the large energy carried by the high frequency waves, the signal could be due to the merging of two planet mass primordial black holes (≈4×10-4M⊙) inside the Oort cloud at roughly 0.025 pc (5300 AU) away. In this short note, we show that the probability of one such event to occur within this volume per year is around 1:1024, if such Saturn-like mass primordial black holes are 1% of the dark matter. Thus, the detected signal is very unlikely to be due the merger of planet mass primordial black holes. Nevertheless, the stochastic background of saturn mass primordial black holes binaries might be seen by next generation gravitational wave detectors, such as DECIGO and BBO.