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The First Interstellar Object Ever Observed in our Solar System

Updated 2017 November 7

On October 19th the Pan-STARRS survey made a remarkable discovery. It was a faint (magnitude 20) object that at first appeared little different from the haul of asteroids that were discovered that night. But as more observations came in, it became apparent that the orbit was very eccentric (elongated). Assuming that it was a comet, the Minor Planet Center placed it on the Possible Comet Conformation Page to attract more observations. As the observations came in, the true nature of the orbit began to be revealed. This object appeared to be from outside of our solar system, which made it unlike any other ever observed. 

So how do we know it came from outside of our solar system? Imagine two identical hills. A track runs down from the top of one hill, and then back up the other. If you start a ball rolling down one hill it will pick up speed until it reaches the bottom, then as it climbs the other hill it will begin to slow. If there is no friction, the ball will climb back up to the same height that it started from. Think of the top of the hills as the Oort cloud that surrounds our Solar System. This is a cloud of comets that slowly orbit far from the sun. Every once in a while the orbit of one of these comets is disturbed, and it begins to fall toward the sun, like the ball rolling down the hill. The height of the hill determines how much potential energy the ball has, which determines how fast it falls, and ultimately how far it will roll to the top of the other hill. In the same way, the potential energy of the comet coming from the Oort cloud is determined by the distance that the comet falls from. Assuming the comet does not hit the sun, or pass close to Jupiter, it will swing around the sun and head back out toward the Oort cloud, slowing until it reaches the distance from which it started. This is how we know the Oort cloud exists, because comets regularly fall toward the sun, but none has ever fallen with enough speed to continue beyond the Oort cloud and escape the solar system. That is, until the recent Pan-STARRS discovery. 

Analysis of the orbit reveals that the object entered our solar system with a velocity of 26 km/sec. This is fast enough that it can only have come from beyond, and it will keep going right out of our solar system, never to return. 

Several amateurs contributed to the astrometry of the object, helping to better define its orbit. Observations were made using iTelescopes (iTelescope.com) from their Siding Spring Australia and Mayhill New Mexico sites, and by David Rankin (Big Water Observatory),  the Pistoia Mountains Astronomical Observatory, and the Great Shefford Observatory. As of October 30, it was nearing 23rd magnitude. 


The Minor Planet Center assigned the provisional cometary designation of  C/2017 U1 (PANSTARRS) to the discovery on October 25. But later observations revealed no cometary coma or tail. The rule is that an object is considered to be an asteroid unless a coma or tail is observed, so the designation was changed to A/2017 U1 and thus considered a minor planet rather than a comet. This created a naming problem. If the current minor planet rules are applied to this object, it is unlikely to be given a formal name because it will not be observed at more than one opposition. But as a unique and interesting object, people naturally wish to name it. So on October 6, the Minor Planet Center announced that the IAU had created a new naming convention for interstellar objects. As a result it is now designated 1I/2017 U1 ('Oumuamua). The I is for Interstellar and 1 denotes the first interstellar object (1I). The name 'Oumuamua  was chosen by the Pan-STARRS team. According to the announcement in MPEC 2017-V17, it is of "Hawaiian origin and reflects the way this object is like a scout or messenger sent from the distant past to reach out to us." It is apparently pronounced "Oh-owe moo-ah moo-ah."

The object passed on the other side of the sun from us, too faint to be detected. But after it swung around the sun (passing within 0.25 AU), it passed close enough to the earth (0.16 AU) to be detected. We were lucky that this happened when the object would appear opposite the sun and with no interference from the moon or we would have missed it. At this point it was already heading out of the solar system and fading fast.

Astronomers around the world began scrambling for observations. On October 25, spectra were obtained (below) by Alan Fitzsimmons with the William Herschel 4.2m Telescope that confirmed the earlier lower resolution spectrum obtained by Joseph Masiero with the 200-inch at Palomar.


Science Results So Far

The spectrum is featureless, rising on the red end. This is consistent with the spectra of distant Kuiper Belt objects in our own solar system, such as Pluto. This suggest an origin far from its parent star.

On the other hand, A/2017 U1 passed relatively close to the sun, yet showed no sign of a coma or tail. This means that it either formed close enough to a star that the volatiles, such as water, were not present, or passed repeatedly near to a star to lose its volatiles. Together, these two observations suggest that this object has had an interesting history.

Eric Mamajek of JPL has submitted a paper regarding the kinematics of A/2017 U1. He calculates that it is not likely to have originated from the Oort cloud associated with the nearby Alpha Cen star system or any of the other dozen close stars. He argues that its velocity is more consistent with a distant extrasolar origin beyond the nearest stars.

A paper submitted by Knight et al. presents a partial light curve for 'Oumuamua that sets limits on its size and shape. They confirm that the object has no coma and determine that the rotational period is more than 3 hr, and likely longer than 5 hr. Their analysis infers a diameter of 90 - 180 km, and a lower limit to its elongation of 3:1. They find that its properties are typical of minor planets from our solar system. 

Greg Crinklaw Astronomer and Developer of SkyTools 

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