A brief visit from a red and extremely elongated interstellar asteroid

Abstract

None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space. The high predicted number density of icy interstellar objects (2.4 × 10 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of the Solar System today, but there has been no way of telling whether the Solar System is typical of planetary systems. Here we report observations and analysis of the object 1I/2017 U1 ('Oumuamua) that demonstrate its extrasolar trajectory, and that thus enable comparisons to be made between material from another planetary system and from our own. Our observations during the brief visit by the object to the inner Solar System reveal it to be asteroidal, with no hint of cometary activity despite an approach within 0.25 astronomical units of the Sun. Spectroscopic measurements show that the surface of the object is spectrally red, consistent with comets or organic-rich asteroids that reside within the Solar System. Light-curve observations indicate that the object has an extremely oblong shape, with a length about ten times its width, and a mean radius of about 102 metres assuming an albedo of 0.04. No known objects in the Solar System have such extreme dimensions. The presence of 'Oumuamua in the Solar System suggests that previous estimates of the number density of interstellar objects, based on the assumption that all such objects were cometary, were pessimistically low. Planned upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to result in the detection of more interstellar objects in the coming years.

1I/‘Oumuamua as a Tidal Disruption Fragment from a Binary Star System

“…Against expectations based on comets, 'Oumuamua does not show any activity and has a very elongated figure (Meech et al 2017), and also exhibits undamped rotational tumbling . In contrast, 'Oumuamua's trajectory indicates that it was moving with the local stars, as expected from a low-velocity ejection from a relatively nearby system (Mamajek 2017).…”

“…Therefore, one cannot say if 'Oumuamua's spectral similarity to certain outer Solar System bodies is meaningful or a coincidence. The most puzzling feature of 'Oumuamua is its very elongated shape, with aspect ratio of 5:1 to 10:1 (Bolin et al 2017;Meech et al 2017). This is an extreme value for Solar System bodies of similar size, and may indicate that 'Oumuamua has internal strength .…”

“…Much of Oumuamua's velocity relative to the Solar System is a reflection of the Sun's own velocity relative to the local standard of rest (Mamajek 2017), making 'Oumuamua's trajectory close to our expectations for an interstellar interloper. However, 'Oumuamua's physical characteristics were unexpected, starting with a complete lack of cometary activity (Knight et al 2017;Meech et al 2017) or associated meteoroids (Ye et al 2017). The observed spectrum of 'Oumuamua is featureless and somewhat red (Masiero 2017;Bannister et al 2017), similar to a number of outer Solar System objects.…”

“…1I/'Oumuamua is the first known interstellar object, that was discovered in October 2017 after it has already passed perihelion (Bacci et al 2017;Meech et al 2017). 'Oumuamua is clearly extrasolar in origin, having a velocity at infinity of 26 km/s and an eccentricity of 1.2.…”

OSSOS. VII. 800+ Trans-Neptunian Objects—The Complete Data Release

“…An upper limit for the retrograde population could be created, which would be highly dependent on the assumed distribution of a, q, and H, and we defer that to future work. OSSOS has less sensitivity to unbound interstellar interlopers like 1I/'Oumuamua (Meech et al 2017). Interstellar objects have very large heliocentric velocity vectors; cf.…”