After years of searching and hypothesizing, we have finally discovered a macroscopic object passing through our solar system that came from interstellar space! An elongated rocky object with approximate dimensions 755 × 115 × 115 ft. entered the solar system from the direction of the constellation Lyra at a velocity (v∞) of 26 km/s (16 mi/s or 58,000 mph), and will exit the solar system at essentially the same speed in the direction of the constellation Pegasus, within the Great Square.
This interstellar object (ISO) is called 1I/2017U1 ‘Oumuamua. What’s in a name? A lot! Let’s separate the three different parts of this designation, discussing each in turn.
1I – “I” stands for “interstellar” and “1” indicates that it is the first interstellar solar system visitor discovered.
2017U1 – indicates that it was the first object discovered during the half-month October 16-31 in the year 2017.
Here’s a brief timeline of the encounter.
September 9, 2017 – Closest approach to the Sun (0.26 AU)
October 14, 2017 – Closest approach to the Earth (0.16 AU)
October 19, 2017 – Discovered by Robert Weryk with Pan-STARRS
It is very difficult for us to discover objects coming towards us from the inner solar system and the glare of the Sun, so it is not surprising that ‘Oumuamua was discovered after it had passed by the Earth on its way out of the solar system.
Rob Weryk, a post-doc at the University of Hawaii Institute for Astronomy, discovered ‘Oumuamua in images taken by the Pan-STARRS1 1.8-meter Ritchey–Chrétien telescope at the summit of the dormant volcano Haleakalā on the island of Maui. Pan-STARRS is an acronym for “Panoramic Survey Telescope and Rapid Response System” and is primarily used to search for Near Earth Objects (NEOs). It has been estimated that Pan-STARRS should be able to detect an interstellar object like ‘Oumuamua passing through our solar system about once every 5 years.
But the 8.4-meter Large Synoptic Survey Telescope (LSST) in Chile, which will see first light in 2019, is expected to be able to detect at least one interstellar object passing through our solar system each year.
While we don’t know ‘Oumuamua’s place of origin, we do know that it originated outside our solar system, and that is exciting. Was it ejected from a binary system? Or through a chance encounter with a giant planet in its outer solar system? Is it an “extinct” interstellar comet? Perhaps it is a former asteroid of a dying star. Even our own Sun, which is expected to reach a peak luminosity of 5200 L☉ as a red giant star in a few billion years, will lose mass and transition to a white dwarf, causing a dynamical reshuffling that will eject a large number of asteroids, trans-Neptunian objects, and comets from our solar system (Seligman & Laughlin 2018). Perhaps ‘Oumuamua long ago suffered a similar fate.
A detailed astrometric study (ground-based and HST) of ‘Oumuamua’s trajectory through the inner solar system has revealed a small non-gravitational acceleration component directed radially away from the Sun (Micheli et al. 2018). After ruling out other known gravitational and non-gravitational accelerators, the authors conclude that the most probable explanation is cometlike outgassing, though ‘Oumuamua displayed no detectable coma during its all-too-brief apparition. Astronomers expect that only a small fraction of interstellar objects should be asteroidal, and this study bolsters—but does not prove—the notion that ‘Oumuamua is an interstellar comet.
However, no change in the rotational state of ‘Oumuamua occurred during the month-long interval over which it was observed. If the anomalous acceleration away from the Sun was caused by cometary activity, a measurable effect on ‘Oumuamua’s rotation should have been seen (Rafikov 2018). This argues against the idea that ‘Oumuamua is an interstellar comet.
What, then, is the cause of the anomalous acceleration? It is worth considering that ‘Oumuamua might be of artificial origin (Bialy & Loeb 2018) . It could be a lightsail that long ago was ejected from its solar system of origin, and this interstellar debris just happened to encounter our solar system. Or, perhaps, it is (or was) an operational space probe purposefully directed towards Earth’s vicinity by an alien civilization. Incidentally, no radio emissions were detected from ‘Oumuamua (yes, we looked).
There may yet be some other explanation for the acceleration ‘Oumuamua experienced during its journey through our solar system. Our experience with the Pioneer anomaly (now explained), or the still unexplained flyby anomaly, might lead us towards new insights. The likelihood that ‘Oumuamua is a highly elongated and/or flattened object only adds to the mystery.
Bialy, S., Loeb, A. 2018, ApJL, in press (arXiv: 1810.11490)
McNeill, A., Trilling, D. E., Mommert, M. 2018, ApJL, 857, L1 (arXiv:1803.09864)
Micheli, M., Farnocchia, D., Meech, K.J., et al. 2018, Nature,
Rafikov, R. R. 2018, arXiv preprint 1809.06389
Seligman, D. & Laughlin, G. 2018, AJ, in press (arXiv:1803.07022)