A Strange Wanderer From The Space Between Stars

Interstellar comets and asteroids are alien wanderers that sometimes enter our Solar System after they have been ejected from the families of distant stars beyond our Sun. These orphaned objects are not bound to a parent-star, and they travel through the wilderness of the space between stars, lost and alone. Unlike the known asteroids and comets that inhabit the remote outer limits of our own Solar System, an interstellar object can only be spotted if it wanders through our own Sun’s region of gravitational influence, and an interstellar comet can only be distinguished from one of our own comets, flying inward from the Oort cloud, by its strongly hyperbolic trajectory that indicates it is not bound by gravity to our own Star. In October 2017, astronomers announced that they have spotted an asteroid–or comet–that appears to have been born beyond our Solar System. This alien visitor is thought to have traveled here from somewhere else in our Milky Way Galaxy–and, if so, it would be the first “interstellar object” to be observed and confirmed by astronomers.

This bewitching, bewildering, and bizarre object–currently dubbed A/2017 U1–is extremely small, less than a quarter-mile in diameter. It is also traveling remarkably fast. Astronomers are currently working to point telescopes, both around the world and in space, at this very unusual object. Once the necessary data is obtained and analyzed, astronomers may come to understand more about the origin, and possibly the composition, of this intriguing object.

A/2017 U1 was discovered on October 19, 2017, by the University of Hawaii’s Pan-STARRS 1 telescope on Haleakala during the course of its routine nightly hunt for Near-Earth Objects (NEOs) for NASA. Dr. Rob Weryk, a postdoctoral researcher at the University of Hawaii Institute for Astronomy (IfA), was the first to detect the traveling object and submit it to the International Astronomical Union’s (IAU’s) Minor Planet Center. Dr. Weryk then went on to sift through the Pan-STARR’s image archive–where he found the mysterious object in images taken the previous night. However, it was not initially identified by the moving object processing.

A/2017 U1

A/2017 U1 has the distinction of being both the first discovered, as well as the only verified, interstellar object to date. This strange wandering worldlet was at first designated C/2017 U1 because it was assumed to be a comet. However, the astronomical community came to the realization that, because it showed no cometary activity, that its true identity was questionable. The names given to the little wanderer are provisional because it has not been verified as either a comet or an asteroid, visiting our Solar System, after forming in a stellar family far beyond our Sun. This is because, as the first object of its type ever to be discovered, the IAU has yet to determine rules for naming this type of wandering worldlet.

Models devised to show how our Solar System’s remote Oort cloud formed predict that more comets are unceremoniously evicted into the space between stars than are retained within this cloud, which is populated by a multitude of icy comet nuclei. Other supercomputer simulations indicate that approximately 90 to 99% of comets are booted out of the Oort cloud into the cold wilderness of interstellar space, the orphaned children of our Sun. There is also evidence showing that comets have formed in the distant families of other stars, and there is no reason to think that the comets belonging to other star systems would not also be similarly scattered into space.

Indeed, if interstellar comets really do exist, they must, every-now-and-then, wander into the warm and well-lit region of our inner Solar System, where Earth–as well as the major planets Mercury, Venus, and Mars–are situated in their balmy orbits close to our Star. These interstellar travelers would approach our Solar System with random velocities, and come primarily from the region of the constellation Hercules. This is because our Sun and its family is moving in that particular direction. However, the fact that no comet, with a speed exceeding our Star’s escape velocity, has ever been detected places upper limits to their density in interstellar space. Indeed, some research suggests that the density of interstellar comets should be no more than about 10 trillion comets per cubic parsec. Other research, derived from LINEAR, sets the upper limit at 1 trillion comets per cubic parsec.

On rare occasions, an interstellar comet can be snared into a heliocentric orbit, as it travels through our Solar System. Supercomputer simulations show that our Solar System’s banded behemoth, Jupiter, is the only planet sufficiently massive to be able to capture one, and that this can be expected to occur only about once every sixty million years. The comets Macholz 1 and Hyakutake C/1996 B2 may be examples of such comets. This is because both objects possess unusual chemical compositions that are unlike those of comets born in our Sun’s own family.

A/2017 U1 was determined to be completely asteroidal in nature on October 25, 2017. The non-cometary characteristics of this wanderer indicate that it was born in the inner regions of whatever stellar system it was evicted from–or that it had spent eons making perihelion passes close to its parent-star and has, as a result, undergone a sea-change into an extinct comet. A/2017 U1 displays an eccentricity of 1.193, which is the highest eccentricity ever seen for any Solar System object–by a very wide margin.

Of Comets, Asteroids, And Baby Star Systems

Our Sun, like other stars, was born encircled by a whirling, swirling disk composed of gas and dust. These disks, termed protoplanetary accretion disks, surround baby stars and they contain the precious and necessary ingredients from which a family of planets and smaller objects can form. Astronomers have observed a number of such protoplanetary accretion disks surrounding bright, distant, and fiery baby stars inhabiting our Galaxy. These disks form at about the same time that the neonatal star is born (protostar), that is tucked within an obscuring, blanketing cloud, that serves as the stellar infant’s cradle.

Protoplanetary accretion disks contain large amounts of nutritious gas and dust that feed hungry and growing protoplanets. Our own Solar System, like many other planetary systems, emerged when a relatively small, extremely dense blob, embedded within the billowing folds of a giant, cold, and dark molecular cloud, collapsed gravitationally under the squeeze of its own merciless gravity. These enormous, ghostly, swirling clouds float around our Galaxy in large numbers, and they serve as the frigid and dark nurseries of brilliant newborn stars. Most of the collapsing gas contained within the blob collects at the center, and ultimately ignites as a result of the process of nuclear fusion–lighting the stellar fires of the new star. The remaining gas and dust, that did not contribute material to the baby star, ultimately evolves into the protoplanetary accretion disk that gives rise to planets, moons, asteroids, and comets that will orbit their stellar-parent. At first, the accretion disks are both searing-hot and very massive, and they can orbit their young star for as long as ten million years.

By the time a searing-hot and dazzling baby star has finally reached what is called the T Tauri stage of its development, the massive surrounding accretion disk has grown both cooler and thinner. A T Tauri star is a mere toddler inhabiting the stellar zoo. It is a very young, variable star, similar to the way our Sun once was billions of years ago in its youth, and it is very active at the age of a mere 10 million years. These fiery tots sport huge diameters that are several times the diameter currently possessed by our own middle-aged Sun. However, T Tauri tots are still in the act of shrinking. Unlike human babies, stars like our Sun shrink as they grow up. By the time the new star has reached the T Tauri phase of its development, less volatile materials have started to condense close to the center of the encircling disk. At this point, very fine and sticky dust motes have started to form, and these fragile, delicate particles contain crystalline silicates.

The tiny, sticky motes of dust collide and merge in the dense environment of the protoplanetary accretion disk. As a result, ever larger and larger objects start to grow–from pebble size, to boulder size, to mountain size, to moon size, to planet size. These growing bodies eventually evolve into planetesimals–the primordial building-blocks of planets. Planetesimals can attain impressive sizes of 1 kilometer across–or even larger–and they represent an extremely heavy population inhabiting this primordial disk circling the stellar toddler. They can also linger long enough for some of them to still be present billions of years after a fully formed, mature planetary system has developed. In our own Solar System, the asteroids are similar to the relic rocky and metallic planetesimals that served as the building blocks of the quartet of solid, and relatively small, inner planets: Mercury, Venus, our Earth and Mars. Conversely, the icy, dusty comets are the lingering leftovers of the frozen planetesimals that built up the quartet of gaseous, giant planets of our Solar System’s outer region: Jupiter, Saturn, Uranus and Neptune.

A Strange Wanderer From The Space Between Stars

Dr. Weryk immediately understood that A/2017 U1 was an unusual object. “Its motion could not be explained using either a normal Solar System asteroid or comet orbit,” he explained in an October 26, 2017 IfA Press Release. Dr. Weryk then contacted IfA graduate Marco Micheli, who had come to the same realization using his own follow-up images obtained from the European Space Agency’s telescope on Tenerife in the Canary Islands.

When the data was combined, a picture began to emerge that made sense. “This object came from outside our Solar System,” Dr. Weryk commented in the IfA Press Release.

“This is the most extreme orbit I have ever seen. It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the Solar System and not coming back,” noted Dr. Davide Farnocchia in the IfA Press Release. Dr, Farnocchia is a scientist at NASA’s Center for Near-Earth Object Studies (CNEOS) at the agency’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

The astronomers then went on to plot the strange object’s current trajectory and even took a peek into what the future holds in store for it. A/2017 U1 came from the direction of the constellation Lyra, soaring through the space between stars at the speedy pace of 15.8 miles per second.

The mysterious visitor from interstellar space approached our Solar System from almost directly “above” the ecliptic. The ecliptic is the plane in space close to where the planets and most asteroids orbit our Star, so it did not have any jostling close encounters with the eight major planets during its hazardous plunge in the direction of our fiery, roiling Sun. On September 2, 2017, the small body crossed beneath the ecliptic just inside of Mercury’s orbit. It then made its closest approach to our Sun on September 6, 2017. Tugged on by our Star’s relentless gravitational pull, the little object made an intricate turn under our Solar System, passing below Earth’s orbit on October 14, 2017, at a distance of approximately 15 million miles. This amounts to about 60 times the distance to Earth’s Moon. A/2017 U1 has now zipped back up above the plane of the planets and is currently traveling at the breathtaking pace of 27 miles per second, with respect to the Sun. The little alien asteroid is now speeding toward the constellation Pegasus.

“We have long suspected that these objects should exist because during the process of planet formation a lot of material should be ejected from planetary systems. What’s most surprising is that we’ve never seen interstellar objects pass through before,” commented Dr. Karen Meech in the October 26, 2017 IfA Press Release. Dr. Meech is an astronomer at the IfA specializing in small bodies and their relationship to solar system formation.

The little object has been assigned its temporary designation of A/2017 U1 by the Minor Planet Center (MPC) located in Cambridge, Massachusetts. All observations of small bodies in our Solar System–and now those that are merely passing through–are collected by the MPC.

“This kind of discovery demonstrates the great scientific value of continual wide-field surveys of the sky, coupled with intensive follow-up observations, to find things we wouldn’t otherwise know are there,” noted MPC Director Dr. Matt Holman in the October 26, 2017 IfA Press Release.

Because little A/2017 U1 is the first object of its type ever discovered, rules for naming this type of visitor from interstellar space will need to be established by the IAU.

As CNEOS Manager Dr. Paul Chodas commented:

“We have been waiting for this day for decades. It’s long been theorized that such objects exist–asteroids or comets moving around between the stars and occasionally passing through our Solar System–but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it.”