Shmuel Bialy and Abraham Loeb report that ‘Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, Micheli, et al. (2018) reported that ‘Oumuamua showed deviations from a Keplerian orbit at a high statistical significance. The observed trajectory is best explained by an excess radial acceleration ∆a ∝ r−2, where r is the distance of ‘Oumuamua from the Sun.
Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that ‘Oumuamua is not an active comet. They explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is (m/A) ≈ 0.1 g cm−2. For a thin sheet this requires a width of ≈ 0.3−0.9 mm. They found that although extremely thin, such an object would survive an interstellar travel over Galactic distances
of ∼ 5 kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. They discuss the possible origins of such an object including the possibility that it might be a lightsail of artificial origin. Their general results apply to any light probes designed for interstellar travel.
They have shown that the observed non-gravitational acceleration of ‘Oumuamua, may be explained by Solar radiation pressure. This requires a small mass-to-area ratio for
‘Oumuamua of (m/A) ≈ 0.1 g cm−3. For a planar geometry and typical mass densities of 1–3 g cm−3 this gives an effective width of only 0.9–0.3 mm, respectively. For a material with lower mass density, the inferred effective width is proportionally larger. They found that although very thin, such an object can travel over galactic distances, maintaining its momentum and withstanding collisional destruction by dust grains and gas, as well as centrifugal and tidal forces. For ‘Oumuamua, the limiting factor is the slow-down by accumulated ISM mass, which limits its maximal travel distance to ∼ 10 kpc (for a mean ISM particle density of ∼ 1 cm−3).
Their inferred thin geometry is consistent with studies of its tumbling motion. In particular, Belton et al. (2018) inferred that ‘Oumuamua is likely to be an extremely oblate spheroid (pancake) assuming that it is excited by external torques to its highest energy state. While their scenario may naturally explains the peculiar acceleration of ‘Oumuamua, it opens up the question what kind of object might have such a small mass-to-area ratio? The observations are not sufficiently sensitive to provide a resolved
image of ‘Oumuamua, and one can only speculate on its possible geometry and nature. Although periodic variations in the apparent magnitude are observed, there are still too many degrees of freedom (e.g., observing angle, non-uniform reflectively, etc.) to definitely constrain the geometry. The geometry should not necessarily be that of a planar sheet, but may acquire other shapes, e.g., involving a curved sheet, a hollow cone or ellipsoidal, etc. Depending on the geometry our estimated value for the mass-to-area ratio will change (through CR in Eq. 5), but the correction is typically of order unity. Known Solar System objects, like asteroids and comets have mass-to-area ratios orders of magnitude larger than our estimate for ‘Oumuamua. If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally,through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin.
Considering an artificial origin, one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment (Loeb 2018). Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative2. The lightsail technology might be abundantly used for transportation of cargos between planets (Guillochon & Loeb 2015) or between stars (Lingam & Loeb 2017). In the former case, dynamical ejection from a planetary System could result in space debris of equipment that is not operational any more 3 (Loeb 2018), and is floating at the characteristic speed of stars relative to each other in the Solar neighborhood. This would account for the various anomalies of ‘Oumuamua, such as the unusual geometry inferred from its lightcurve (Meech et al. 2017; Fraser et al. 2018; Drahus et al. 2018; Belton et al. 2018), its low thermal emission, suggesting high reflectivity (Trilling et al. 2018), and its deviation from a Keplerian orbit (Micheli et al. 2018) without any sign of a cometary tail (Meech et al. 2017; Knight et al. 2017; Jewitt et al. 2017;Ye et al. 2017; Fitzsimmons et al. 2017) or spin-up torques (Rafikov 2018). Although ‘Oumuamua has a red surface color, similar to organic-rich surfaces of Solar-System comets
and D-type asteroids (Meech et al. 2017), this does not contradict the artificial scenario, since irrespective of the object’s composition, as it travels through the ISM its surface will be covered by a layer of interstellar dust, which is itself composed of organic-rich materials (Draine 2003).
Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization. Based on the PAN-STARRS1 survey characteristics, and assuming natural origins following random trajectories, Do et al. (2018) derived that the interstellar number density of ‘Oumuamua-like objects should be extremely high, ∼ 2×1015 pc−3, equivalent to ∼ 1015 ejected planetesimals per star, and a factor of 100 to 108 larger than predicted by theoretical models (Moro-Martin et al. 2009). This discrepancy is readily solved if ‘Oumuamua does not follow a random trajectory but is rather a targeted probe. Interestingly,
‘Oumuamua’s entry velocity is found to be extremely close to the velocity of the Local Standard of Rest, in a kinematic region that is occupied by less than 1 to 500 stars
(Mamajek 2017).
Since it is too late to image ‘Oumuamua with existing telescopes or chase it with chemical rockets (Seligman & Laughlin 2018), its likely origin and mechanical properties could only be deciphered by searching for other objects of its type in the future. In addition to the vast unbound population, thousands of interstellar ‘Oumuamua-like space-debris are expected to be trapped at any given time in the Solar System through gravitational interaction with Jupiter and the Sun (Lingam & Loeb 2018). Deep wide-area surveys of the type expected with the Large Synoptic Survey Telescope (LSST)4 will be particularly powerful in searching for additional members of ‘Oumuamua’s population of objects.
A survey for lightsails as technosiganures in the Solar System is warranted, irrespective of whether ‘Oumuamua is one of them.
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