arXiv subjects

Amir Siraj

Publications and source records attributed to Amir Siraj.

At least 19 recordsLinked to original sources

arXiv2026

Measuring Apsidal Clustering

The decade-long debate over the existence of apsidal clustering in the outer solar system is poised for reignition given the plethora of distant trans-Neptunian object (TNO) discoveries expected from the forthcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). Here, we present a new conditional-likelihood method to measure apsidal clustering that is insensitive to uneven survey footprints. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the known sample of relevant objects from 21 to 25. We apply our new method to this up-to-date sample, showing that the significance of the apsidal clustering in the outer solar system has fallen from $2.7σ$ to $1.9σ$, and that the direction of clustering is not well constrained. This new method is suitable for application to the growing sample of known TNOs, and the results will reveal whether the evidence for a hypothetical Planet X from apsidal clustering is real or spurious.

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arXiv2025

The Inner Kernel of the Classical Kuiper Belt

The `kernel' of the classical Kuiper belt was discovered by Petit et al. (2011) as a visual overdensity of objects with low ecliptic inclinations and eccentricities at semimajor axes near 44 AU. This raises the question - are there other structures present in the classical Kuiper belt? If there are, clustering algorithms applied to orbits transformed into free elements may yield the best chance of discovery. Here, we derive barycentric free orbital elements for objects in the classical Kuiper belt, and use the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm to identify a new structure, which we dub the inner kernel, located at $a \sim 43 \mathrm{\; AU}$ just inward of the kernel ($a \sim 44 \mathrm{\; AU}$), which we also recover. It is yet unclear whether the inner kernel is an extension of the kernel or a distinct structure. Forthcoming observations, including those by the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) may provide further evidence for the existence of this structure, and perhaps resolve the question of whether there are two distinct structures.

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arXiv2025

Limits on Stellar Flybys in the Solar Birth Cluster

The orbits of small bodies in the outer solar system are particularly sensitive to gravitational perturbations, including stellar flybys. Stellar clusters, with low velocity dispersions and high number densities, can be the source of strong and frequent flybys. As a result, we can infer what properties of the solar birth environment would be incompatible with the structure of the outer solar system observed today. Here, we explore with $n-$body simulations the implications of the low inclinations ($i < 20^{\circ}$) of the distant sednoids (objects with perihelia $q > 40 \mathrm{\; AU}$ and semimajor axes $a > 400 \mathrm{\; AU}$) for the properties of the solar birth cluster. We find that the existence of these orbits, if they were in place in the Sun's birth cluster phase, would limit the product of the stellar number density and the Sun's residence time in the birth cluster to $\lesssim 5 \times 10^3 \mathrm{\; Myr \; pc^{-3}}$, as compared to the weaker limit $\lesssim 5 \times 10^4 \mathrm{\; Myr \; pc^{-3}}$ implied by the low inclinations of the cold classical Kuiper belt.

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arXiv2025

Measuring the Mean Plane of the Distant Kuiper Belt

In the absence of any unseen planetary-mass bodies in the outer solar system, the mean plane of the distant Kuiper belt should be the same as the plane orthogonal to the angular momentum vector of the solar system -- the invariable plane. Here, we measure the mean plane of the non-resonant Kuiper belt across semimajor axes 50 - 400 AU. We introduce a new method to measure the mean plane that we demonstrate to be independent of observational bias. In particular, our results are not biased by surveys that look only at limited areas on the celestial sphere. We find a warp relative to the invariable plane at semimajor axes of 80 - 400 AU (98% confidence) and 80 - 200 AU (96% confidence), but not at 50 - 80 AU or 200 - 400 AU. If it is not spurious, a possible explanation for this warp is an unseen planet in the outer solar system. With $n$-body simulations, we find that a planet with mass between that of Mercury and the Earth, semimajor axis in the range 100 - 200 AU, and inclination $\gtrsim 10^{\circ}$ to be the most likely cause of the warp; however, parameters outside of these ranges are still possible. Such a body is distinct in both mass and semimajor axis from the various versions of an unseen planet invoked to explain apsidal clustering in the outer solar system. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is expected to confirm or deny the existence of the warp reported here, and might detect the planet that may produce it.

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arXiv2024

Orbit of a Possible Planet X

The plausibility of an unseen planet in the outer solar system, and the expected orbit and mass of such a planet, have long been a topic of inquiry and debate. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the sample of objects that would carry dynamical information about a hypothetical unseen planet in the solar system. Using this expanded sample, we find statistically significant clustering at the $\sim 3 σ$ level for TNOs with semimajor axes $>170$ AU, in longitude of perihelion ($\varpi$), but not in inclination ($i$), argument of perihelion ($ω$) or longitude of node ($Ω$). Since a natural explanation for clustering in $\varpi$ is an unseen planet, we run 300 $n$-body simulations with the giant planets, a disk of test particles representing Kuiper belt objects, and an additional planet with varied initial conditions for its mass, semimajor axis, eccentricity, and inclination. Based on the distribution of test particles after $1-2$ Gyr, we compute relative likelihoods given the actual distribution of $\varpi$ as a function of semimajor axis for distant TNOs on stable orbits using a significantly larger sample than previous work. We find the best-fit unseen planet parameters to be: mass $m_p = 4.4\pm1.1\mathrm{\;M_{\oplus}}$, semimajor axis $a_p=290\pm30\mathrm{\;AU}$, eccentricity $e_p=0.29\pm0.13$, and inclination $i_p=6.8\pm5.0^{\circ}$. Only $0.06\%$ of the Brown & Batygin (2021) Planet Nine reference population produce probabilities within $1σ$ of the maximum within our quadrivariate model, indicating that our work identifies a distinct preferred region of parameter space for an unseen planet in the solar system.

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arXiv2023

Are There Terrestrial Planets Lurking in the Outer Solar System?

Motivated by recent measurements of the free-floating planet mass function at terrestrial masses, we consider the possibility that the solar system may have captured a terrestrial planet early in its history. We show that $\sim 1.2$ captured free-floating planets with mass strictly greater than that of Mars may exist in the outer solar system, with a median predicted distance of $\sim 1400 \mathrm{\; AU}$. If we consider a logarithmic bin centered on the mass of Mars, rather than a cutoff, we find that $\sim 2.7$ captured free-floating planets with mass comparable to Mars may exist in the outer solar system. We derive an expectation value of $\sim 0.9$ for the number of captured free-floating planets with mass comparable to that of Mars ($\sim 1.4$ for mass comparable to that of Mercury) that are currently brighter than the 10-year co-added point source detection limits of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). Blind shift-and-stack searches could potentially enable the detection of such a planet if it is currently in the Southern sky. The theoretical argument presented here does not rely on the existence of posited patterns in the orbital elements of small bodies in and beyond the Kuiper belt, in contrast with other hypothetical outer-solar-system planets motivated in recent years.

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arXiv2023

Discovery of Spherules of Likely Extrasolar Composition in the Pacific Ocean Site of the CNEOS 2014-01-08 (IM1) Bolide

We have conducted an extensive towed-magnetic-sled survey during the period 14-28 June, 2023, over the seafloor centered around the calculated path of the bolide CNEOS 2014-01-08 (IM1) about 85 km north of Manus Island, Papua New Guinea. We found about 700 spherules of diameter 0.05-1.3 millimeters in our samples, of which 57 were analyzed so far. The spherules were significantly concentrated along the expected meteor path. Mass spectrometry of 47 spherules near the high-yield regions along IM1's path reveals a distinct extra-solar abundance pattern for 5 of them, while background spherules have abundances consistent with a solar system origin. The unique spherules show an excess of Be, La and U, by up to three orders of magnitude relative to the solar system standard of CI chondrites. These "BeLaU"-type spherules, never seen before, also have very low refractory siderophile elements such as Re. Volatile elements, such as Mn, Zn, Pb, are depleted as expected from evaporation losses during a meteor's airburst. In addition, the mass-dependent variations in $^{57}$Fe/$^{54}$Fe and $^{56}$Fe/$^{54}$Fe are also consistent with evaporative loss of the light isotopes during the spherules' travel in the atmosphere. The "BeLaU" abundance pattern is not found in control regions outside of IM1's path and does not match commonly manufactured alloys or natural meteorites in the solar system. This evidence points towards an association of "BeLaU"-type spherules with IM1, supporting its interstellar origin independently of the high velocity and unusual material strength implied from the CNEOS data. We suggest that the "BeLaU" abundance pattern could have originated from a highly differentiated magma ocean of a planet with an iron core outside the solar system or from more exotic sources.

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arXiv2023

Close Encounters of the Interstellar Kind: Examining the Capture of Interstellar Objects in Near Earth Orbit

Recent observations and detections of interstellar objects (ISOs) passing through the solar system have sparked a wave of interest into these objects. Although rare, these ISOs can be captured into bound orbits around the Sun. In this study, we investigate the novel idea of capture of ISOs into near-Earth orbits and find that a steady population of ISOs exists among the current population of Near Earth Objects (NEOs). Using numerical simulations, we find that the capture of ISOs into near-Earth orbits is dominated by Jupiter which is $10^4\times$ more efficient in capturing ISOs compared to Earth. Captured ISOs are more likely to be in orbits with high eccentricities and low inclinations. We also investigate the stability of captured ISOs and find that they are generally unstable and have an average survival life time of $\sim 1$ Myr, consistent with lifetime of NEOs originating from outer asteroid belt, and are ejected from the solar system due to interactions with other planets or the Sun. Our results have important implications for understanding the population of ISOs in the solar system and possible future detection. We find that about one to a few $50-70$ m sized captured ISOs among NEOs would be detectable by LSST over its lifetime. By detecting and studying captured interstellar objects, we can learn about the properties and origins of such objects, and the formation and evolution of exoplanetary systems and even our solar system.

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arXiv2023

Localizing The First Interstellar Meteor With Seismometer Data

The first meter-scale interstellar meteor (IM1) was detected by US government sensors in 2014, identified as an interstellar object candidate in 2019, and confirmed by the Department of Defense (DoD) in 2022. We use data from a nearby seismometer to localize the fireball to a $\sim 16 \mathrm{\; km^2}$ region within the $\sim 120 \mathrm{\; km^2}$ zone allowed by the precision of the DoD-provided coordinates. The improved localization is of great importance for a forthcoming expedition to retrieve the meteor fragments.

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arXiv2023

Expected Fragment Distribution from the First Interstellar Meteor CNEOS 2014-01-08

In 2014, the fireball of the first interstellar meteor CNEOS 2014-01-08 (IM1) (Siraj & Loeb 2019), was detected off the northern coast of Papua New Guinea. A recently announced ocean expedition will retrieve any extant fragments by towing a magnetic sled across a 10 km x 10 km area of ocean floor approximately 300 km north of Manus Island (Siraj, Loeb, & Gallaudet 2022). We formulate a model that includes both the probabilistic mass distribution of meteor fragments immediately after the fragmentation event, the ablation of the fragments, and the geographic distribution of post-ablation fragments along the ground track trajectory of the bulk fragment cloud. We apply this model to IM1 to provide a heuristic estimate of the impactor's post-ablation fragment mass distribution, constructed through a Monte Carlo simulation. We find between ~8% and ~21% of fragments are expected to survive ablation with a mass $\geq$ .001 g, depending on the impactor's empirical yield strength. We also provide an estimation for the geographic distribution of post-ablation fragments.

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arXiv2022

Interstellar Meteors are Outliers in Material Strength

The first interstellar meteor larger than dust was detected by US government sensors in 2014, identified as an interstellar object candidate in 2019, and confirmed by the Department of Defense in 2022. Here, we describe an additional interstellar object candidate in the CNEOS fireball catalog, and compare the implied material strength of the two objects, referred to here as IM1 and IM2, respectively. IM1 and IM2 are ranked 1 and 3 in terms of material strength out of all 273 fireballs in the CNEOS catalog. Fitting a log-normal distribution to material strengths of objects in the CNEOS catalog, IM1 and IM2 are outliers at the levels of $3.5 σ$ and $2.6 σ$, respectively. The random sampling and Gaussian probabilities, respectively, of picking two objects with such high material strength from the CNEOS catalog, are $\sim 10^{-4}$ and $\sim 10^{-6}$. If IM2 is confirmed, this implies that interstellar meteors come from a population with material strength characteristically higher than meteors originating from within the solar system. Additionally, we find that if the two objects are representative of a background population on random trajectories, their combined detections imply that $\sim 40\%$ of all refractory elements are locked in meter-scale interstellar objects. Such a high abundance seemingly defies a planetary system origin.

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arXiv2022

Physical Considerations for an Intercept Mission to a 1I/'Oumuamua-like Interstellar Object

In this paper, we review some of the extant literature on the study of interstellar objects (ISOs). With the forthcoming Vera C. Rubin Telescope and Legacy Survey of Space and Time (LSST), we find that $0.38 - 84$ `Oumuamua-like interstellar objects are expected to be detected in the next 10 years, with 95\% confidence. The feasibility of a rendezvous trajectory has been demonstrated in previous work. In this paper, we investigate the requirements for a rendezvous mission with the primary objective of producing a resolved image of an interstellar object. We outline the rendezvous distances necessary as a function of resolution elements and object size. We expand upon current population synthesis models to account for the size dependency on the detection rates for reachable interstellar objects. We assess the trade-off between object diameter and occurrence rate, and conclude that objects with the size range between a third of the size and the size of `Oumuamua will be optimal targets for an imaging rendezvous. We also discuss expectations for surface properties and spectral features of interstellar objects, as well as the benefits of various spacecraft storage locations.

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arXiv2022

The 2019 Discovery of a Meteor of Interstellar Origin

The earliest confirmed interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the abundance of interstellar objects of its size $\sim 100\;$ m. This was followed by the discovery of Borisov, which allowed for a similar calibration of its size $\sim 0.4 - 1 \mathrm{\; km}$. One would expect a much higher abundance of significantly smaller interstellar objects, with some of them colliding with Earth frequently enough to be noticeable. Based on the CNEOS catalog of bolide events, we identify the $\sim 0.45$m meteor detected at 2014-01-08 17:05:34 UTC as originating from an unbound hyperbolic orbit with 99.999\% confidence. The U.S. Department of Defense has since verified that "the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory." We infer that the meteor had an asymptotic speed of $v_{\infty} \sim 42.1 \pm 5.5\; \mathrm{km \; s^{-1}}$ outside of the solar system. Its origin is approximately towards R.A. $49.4 \pm 4.1^{\circ}$ and declination $11.2 \pm 1.8^{\circ}$, implying that its initial velocity vector was $58\pm6\; \mathrm{km\;s^{-1}}$ away from the velocity of the Local Standard of Rest (LSR). Its high LSR speed implies a possible origin from the deep interior of a planetary system or a star in the thick disk of the Milky Way galaxy. The local number density of its population is $10^{6{^{+0.75}_{-1.5}}} \; \mathrm{AU^{-3}}$ or $9 \times 10^{21{^{+0.75}_{-1.5}}} \; \mathrm{pc^{-3}}$ (necessitating 0.2 -- 20 Earth masses of material to be ejected per local star). We show that the detections of CNEOS 2014-01-08, `Oumuamua, and Borisov collectively imply that the differential size distribution in good agreement with a collisional distribution, with a power-law slope is $q \sim 3.6 \pm 0.5$, where the quoted uncertainty corresponds to $2 σ$.

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arXiv2022

An Ocean Expedition by the Galileo Project to Retrieve Fragments of the First Large Interstellar Meteor CNEOS 2014-01-08

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arXiv2022

New Constraints on the Composition and Initial Speed of CNEOS 2014-01-08

We study the newly released light curve from the fireball of the first interstellar meteor CNEOS 2014-01-08. The measured velocity and three observed flares down to an altitude of $18.7 \mathrm{\; km}$ imply ambient ram pressure in the range of $113-194$ MPa when the meteor disintegrated. The required yield strength is $\gtrsim 20$ times higher than stony meteorites and $\gtrsim 2$ times larger than iron meteorites. The implied slowdown in the atmosphere suggests an initial speed of about $66.5 \; {\rm km~s^{-1}}$, strengthening the case for an interstellar origin of this meteor and making it an outlier relative to the velocity dispersion of local stars.

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arXiv2021

The New Astronomical Frontier of Interstellar Objects

The upcoming commencement of the Vera C. Rubin Observatory's Legacy Survey of Space of Time (LSST) will greatly enhance the discovery rate of interstellar objects (ISOs). `Oumuamua and Borisov were the first two ISOs confirmed in the Solar system, although the first interstellar meteor may have been discovered earlier. We discuss the properties of `Oumuamua and Borisov and explore the expected abundance of ISOs as a function of size in the solar neighborhood. We compare the expected abundance of ISOs to that of objects in the Oort cloud, and draw conclusions about the mass budget per star that is required to produce ISOs. We also investigate the possibility of ISOs being captured into bound orbits within the solar system, both from its birth star cluster and in the field. We examine the potential for ISOs to transport prebiotic or biotic material between planetary systems. We consider signatures of ISOs colliding with the Earth, the Moon, and neutron stars, as well as the possibility of differentiating ISOs from solar system objects in stellar occultation surveys. Finally, we discuss advantages that the imminent advent of LSST will afford the field of ISO studies, including large-number statistics that will reveal the origins of ISOs and discoveries of rare ISOs providing insights into exotic phenomena. One of the two branches of the newly established Galileo Project seeks to learn more about the nature of ISOs like `Oumuamua by performing new searches and designing follow-up observations.

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arXiv2021

The Mass Budget Necessary to Explain `Oumuamua as a Nitrogen Iceberg

Recently, a nitrogen iceberg was proposed as a possible origin for the first interstellar object, 1I/2017 U1, also known as `Oumuamua. Here, we show that the mass budget in exo-Pluto planets necessary to explain the detection of `Oumuamua as a nitrogen iceberg chipped off from a planetary surface requires a mass of heavy elements exceeding the total quantity locked in stars with 95\% confidence, making the scenario untenable because only a small fraction of the mass in stars ends in exo-Plutos.

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arXiv2021

Preliminary Evidence That Protoplanetary Disks Eject More Mass Than They Retain

If interstellar objects originate in protoplanetary disks, they can be used to calibrate the fraction of mass that such disks eject. The discoveries of interstellar objects 1I/`Oumuamua and 2I/Borisov, taken together with rogue planets statistics, allow for the calibration of mass locked in interstellar objects in size range $\sim 10^{4} - 10^{9} \; \mathrm{cm}$. Here, we show that at least $\sim 10\%$ of stellar mass is required to produce the observed population of interstellar objects, with a 95\% confidence interval spanning $\sim 2\% - 50\%$. We call this quantity the Minimum Ejection Fraction (MEF), representing a new constraint on planetary system formation that necessitates an order of magnitude more mass to be processed per star than in the Minimum Mass Solar Nebula (MMSN) model. Future discoveries of interstellar objects with LSST on the Vera C. Rubin Observatory will provide a test of our predictions and improve the statistics.

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