An Interstellar Object’s Masquerade

Some of the smaller inhabitants of our own Solar System are naughty little objects that teasingly refuse to be designated as one thing or another. But, what is true for our Sun and its family is apparently also true for the children of other stars, and a recent invader from a stellar system beyond our own has made this very clear. Oumuamua is the first known visitor from a distant stellar system to travel through our own Solar System, and it is certainly a mischievous little refugee from interstellar space. First assumed to be a comet, it was then determined to be an interstellar asteroid because it did not display gas emission or a dusty environment in observations–and without these hallmarks of cometary behavior, it was reclassified as the first interstellar asteroid known. But this strange story has a delightful twist, because on June 27, 2018, a team of astronomers reported that “Oumuamua is an inactive comet, and not an asteroid as previously thought.”

Formally designated iI/2017 U1, Oumuamua was discovered by Dr. Robert Weryk on October 19, 2017. Dr. Weryk. who is of the University of Hawaii, used the Pan-STARRS telescope located at Haleakala Observatory in Hawaii to make his discovery 40 days after Oumuamua had made its closest approach to our Sun. When this mysterious object was first spotted, it was approximately 21,000,000 miles–or about 0.22 astronomical units (AU) from Earth–or 85 times as far away as the Moon. One AU is the equivalent of Earth’s average distance from our Star, which is about 93,000,000 miles. At the time of its discovery, Oumuamua was already zipping away from our Sun. On November 6, 2017, Oumuamua was designated as the first of a new class of interstellar objects.

Oumuamua is of a dark red hue, which makes it similar in color to objects in the outer regions of our Solar System. It is also small, being only approximately 800 feet x 100 feet in size. Because Oumuamua displayed no signs of a thrashing comet tail, despite its close approach to the heat of our fiery, brilliant Star, and because it also displayed both a significant elongation and rotation rate, it was originally considered to be a metal-rich rock with a relatively high density. It also tumbled around chaotically, instead of smoothly rotating. In addition, it was traveling so swiftly relative to our Sun that there was no possibility it had been born in our own Solar System. This swift speed also indicates that Oumuamua cannot be snared into an orbit around our Sun. This means that it will eventually flee from our Solar System and again become a traveling denizen of the space between stars. Oumuamua’s system of origin, and the amount of time it has spent traveling through interstellar space, remain mesmerizing mysteries.

Oumuamua is the word for “scout from the distant past” in Hawaiian. Alas, what this zippy little refugee from the family of an alien star actually is–an asteroid or a comet–has presented a complicated problem, and has proven to be difficult to determine. Soon after Oumuamua was first discovered, astronomers from all over the world attempted to discover its true identity.

The first tantalizing clue, revealing Oumuamua’s real nature, has to do with its trajectory. Follow-up observations conducted by astronomers using the Canada-France-Hawaii Telescope (CFH), the European Space Agency’s (ESA’s) Optical Ground Station telescope in Tenerife, Canary Islands, and other telescopes around the world, have helped astronomers determine the mysterious past and elusive identity of this weird wanderer. Unlike any asteroid or comet ever seen before, this new and baffling object zipped past the Sun, approaching from “above” the plane of the planets on a highly inclined orbit. It was also certainly moving fast enough at the impressive clip of 70,800 miles per hour (as of June 1, 2018) to escape the gravitational grip of our Star.

Comets, Asteroids, And The Mysterious Identity of Oumuamua

In our own Solar System, comets are the lingering remnants of the multitude of icy planetesimals that built up the quartet of giant, gaseous planets inhabiting the outer regions of our Sun’s domain–Jupiter, Saturn, Uranus and Neptune. The ancient planetesimals were the “seeds” from which the planets ultimately emerged. Asteroids, that are mostly found in the Main Asteroid Belt between Mars and Jupiter, are the rocky and metallic objects that built up the four inner, solid planets–Mercury, Venus, Earth and Mars. Because comets dwell in our Solar System’s deep freeze, in a remote twilight region where our Sun’s golden light and warmth can barely reach, they preserve in their frozen hearts the original elements that gave birth to our Solar System about 4.56 billion years ago.

Comets come screeching inward towards our Star from three regions in our Solar System’s outer limits: the Kuiper Belt, Scattered Disk, and Oort Cloud. Recent studies from the mid-1990s have shown that the Kuiper Belt is dynamically stable, and that comets from this region actually originate in the Scattered Disk. The Scattered Disk is a dynamically active domain that probably formed as the result of the outward migration of Neptune, in the early days of our Solar System. The icy objects that bounce around within the Kuiper Belt, along with the frozen occupants of the Scattered Disk, are collectively called trans-Neptunian objects.

The very remote Oort Cloud is a thousand times farther away than the Kuiper Belt and, unlike the Kuiper Belt, it is not flat. The Oort Cloud is actually an enormous shell composed of icy objects that surrounds our entire Solar System–and it reaches halfway to the nearest star beyond our Sun.

Small and dark, Oumuamua is the first known interstellar object to have invaded our Solar System, and it appears to have come from about the same direction as the star Vega in the constellation Lyra. The direction of the incoming motion of Oumuamua indicates that it comes from the most likely direction that alien objects would take when entering our Solar System from interstellar space. Soon after its discovery, Oumuamua was playfully compared to the fictional alien spacecraft Rama because of its interstellar origin. Both the real and the fictional objects are unusually elongated and limited in size. However, Oumuamua‘s reddish color and fluctuating brightness at first suggested that it is an asteroid.

On October 26, 2017, two earlier observations of Oumuamua, derived from the Catalina Sky Survey, were found that were dated October 14th and 17th 2017. A two week observation verified a strongly hyperbolic trajectory. Indeed, Oumuamua has a hyperbolic excess velocity of about 58,900 miles–its speed relative to our Sun when in interstellar space. The Catalina Sky Survey‘s purpose is to discover comets and asteroids. It is conducted at the Steward Observatory’s Catalina Station located near Tucson, Arizona.

By the middle of November 2017, astronomers were convinced that Oumuamua was a migrating interstellar object. Based on observations made over a period of 34 days, Oumuamua’s orbital eccentricity of 1.20 was determined–the highest ever seen. An eccentricity higher than 1.0 indicates that an object exceeds our Sun’s escape velocity, and is therefore not bound to our Solar System. Indeed, Oumuamua’s eccentricity is so high that it could not have been caused by an encounter with any of our Solar System’s planets, either known or as yet undiscovered. This is because even undiscovered planets–if any exist beyond Neptune–could not explain Oumuamua’s trajectory. Any hypothetical undiscovered planet would have to be located very far from our Star and, therefore, would have to be traveling very slowly, according to Kepler’s law of planetary motion. Encounters with such an undiscovered planet could not speed up Oumuamua’s movement to the value observed. This strongly indicates that Oumuamua can only be an interstellar vagabond, entering our Solar System from the space between stars.

Oumuamua migrated into our Solar System from above the plane of the ecliptic, and the powerful pull of our Star’s gravity caused it to speed up until it had attained a maximum speed of 196,200 miles per hour as it traveled below the ecliptic on September 6, 2017. It then made a sharp upward turn when it was at its closest approach to our Sun (perihelion). On September 9, 2017, Oumuamua’s was closest to our Sun at a distance of 23,700,000 miles–or approximately 17% closer than Mercury’s closest approach to our Star. The mysterious little vagabond is now screeching away from our Sun–in the direction of Pegasus–at an angle of 66 degrees from the direction of its approach.

Oumuamua, on the outward leg of its long and treacherous travels through our Solar System, passed below Earth on October 14, 2017 at a distance of about 15,020,000 miles from our planet, and then shot back above the ecliptic on October 16, 2017, passing above the orbit of Mars on November 1, 2017. It passed above Jupiter’s orbit in May 2018, and is currently scheduled to pass above Saturn’s orbit in January 2019, and Neptune’s orbit in 2022. As it escapes from the gravitational grip of our Solar System, Oumuamua will be approximately right ascension 23h51m and declination +24degrees 45, in Pegassus. It will continue to slow down until it has reached a speed of 26.33 kilometers relative to our Sun. This is the same speed it had before its approach to our Solar System. It will take this little wanderer about 20,000 years to free itself from our Solar System’s gravitational grasp entirely.

Oumuamua’s place of origin seems to be from the star Vega. Vega (Alpha Lyrae) is the brightest star in the constellation Lyra, which is the 5th brightest star in the sky, as well as the second brightest star in the northern celestial hemisphere after Arturus. Vega is relatively close to our Solar System, at a mere 25 light-years from our Sun. It is only about 1/10th the size of the Sun, but it is 2.1 times as massive. Both our Star and Vega are considered to be approaching stellar mid-life. Because Vega is more massive than our Sun, even though it is younger, it has a shorter “life” span, and so reaches it’s stellar mid-life at a younger age. The more massive the star, the shorter its “life” on the hydrogen burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.

Taking into account Vega’s proper motion, it would have taken Oumuamua 600,000 years to reach our Solar System from its original birthplace in the family of Vega. However, as a nearby star, Vega was not in the same part of the sky at that time. For this reason, astronomers have calculated that one hundred years ago, this interstellar vagabond was about 561 AU from our Star and traveling through space at a speed that is very close to the mean motion of material in our Milky Way Galaxy in the general neighborhood of our Star. This is also known as the local standard of rest. This particular velocity profile also suggests an extrasolar origin. However, it also apparently rules out the closest dozen stars. Indeed, the strong correlation between Oumuamua’s velocity and the local standard of rest may suggest that it has circulated our Galaxy several times and therefore may have actually been born in an entirely different part of the Milky Way.

For this reason, it is not known how long this mysterious little traveler has been speeding its way through the space between stars. Some astronomers propose that our Solar System is probably the first star system that Oumuamua has visited during its long journey, after being unceremoniously evicted from its parent-star’s birth system long ago. Indeed, this event potentially occurred several billion years ago. In addition, astronomers have suggested that this bewitching, bewildering little object may have been hurled out of a stellar system located in one of the local kinematic associations of bright young stars, such as Carina or Columba specifically, within a range of approximately 100 parsecs. Both the Carina and Columba associations are now very far away in the sky from the Lyra Constellation, the direction Oumuamua took when it first migrated into our Solar System. In addition, there is another interesting possibility, proposed by other astronomers, suggesting that Oumuamua was hurled out from a white dwarf system and that its constituent volatiles were lost when its star evolved into a swollen red giant.

A white dwarf is a dense little stellar corpse left behind by a sun-like star after it has run out of nuclear fuel, and has perished as a result. A white dwarf is really the progenitor star’s very dense core, and it is usually surrounded by a breathtakingly beautiful planetary nebula–the candy-colored shimmering gases that were once the outer layers of the erstwhile small progenitor star. Before a small star like our Sun perishes to evolve into a white dwarf, it swells to monstrous proportions, and sports a red hue. This type of swollen red star is called a red giant, and our own Sun is destined to evolve into just such an enormous dying star. When our Sun goes red giant, it will first incinerate Mercury, then Venus and, after that, possibly Earth. As our dying Star continues to balloon in size, its heat will also move outwards. In the end, before our dying Sun–in its red giant phase–leaves its relic core behind in the form of a white dwarf, it will convert Pluto, its large moon Charon, and other currently frozen denizens of the distant Kuiper Belt into tropical havens.

Approximately 1.3 million years ago, Oumuamua may have wandered within a distance of 0.52 light-years of the nearby star dubbed TYC 4742-1027-1. However, its velocity is too high to have been born from that star system, and it likely just passed through the system’s own Oort Cloud, composed of icy comet nuclei, at a swift speed of 230,000 miles-per-hour.

One especially interesting theory suggests that Oumuamua may be a fragment from a tidally disrupted planet. This particular scenario explains very well its elongated shape and “refractory” composition. Oumuamua probably contains nickel-iron, as well as other metals. This makes little Oumuamua a rare treasure of an unusual object, much less abundant than other extrasolar bodies that have been characterized as either “dusty snowballs” or asteroids.

An Interstellar Masquerade

At first, astronomers assumed Oumuamua was a comet. This is because current understanding of planet formation predicts that interstellar comets are much more abundant than interstellar asteroids. However, because initially astronomers detected no evidence of gas emission or a dusty environment–characteristic of comets–Oumuamua was determined to be an interstellar asteroid. Without these tattle-tale characteristics of comets, astronomers concluded that it could not be a comet, and had to be an asteroid.

But this weird tale of a mysterious object, dancing through the space between stars, has a strange twist. This is because, after the initial discovery observations with Pan-STARRS, a team of astronomers led by Dr. Marco Micheli of the ESA’s SSA-NEO Coordination Centre, and Dr. Karen Meech of the University of Hawaii’s Institute of Astronomy continued to make high precision measurements of the object and its position using a number of ground-based facilities like CFHT, as well as the Hubble Space Telescope (HST). The final images were obtained using HST in January 2018. This was before Oumuamua had grown too faint to be observed as it streaked away from Earth on its way out of our Solar System.

But, contrary to their expectations, the team of astronomers found that Oumuamua was not following the trajectory predicted if only our Sun’s gravity and the planets were influencing its path. “Unexpectedly, we found that Oumuamua was not slowing down as much as it should have due to just gravitational forces,” Dr. Micheli noted in a June 27, 2018 University of Hawaii Press Release. Dr. Micheli is the lead author of the paper reporting the team’s findings, published in the June 27, 2018 issue of the journal Nature.

So, what could be causing the curious behavior of this weird wanderer from interstellar space?

Careful analysis eliminated a range of possible influences–for example, radiation pressure or thermal effects from our Sun, or even interaction with our Sun’s solar wind. Less likely scenarios include a collision with another body, or the possibility that Oumuamua is really a duo of separate objects, loosely bound together.

Comets contain ices that can sublimate. This means that they can experience a sea change from a solid to a gas when warmed by our Sun. This process drags out dust from the comet’s surface, forming a fuzzy “atmosphere”, and sometimes a tail. The release of gas pressure at both different times and locations can push the comet slightly off course compared with the expected path it would take if only gravitational influences were playing a role.

“Thanks to the high quality of the observations we were able to characterize the direction and magnitude of the non-gravitational perturbation, which behaves the same way as comet outgassing,” commented Dr. Davide Farnocchia in the June 27, 2018 University of Hawaii Press Release. Dr. Famocchia is of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

The astronomers have not as yet spotted dusty material or chemical signatures that would normally indicate a comet, even in the deepest images obtained from the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT), HST, and the Gemini South Telescope. “Oumuamua is a small object–no more than a mile long–and it could have been releasing a small amount of relatively large dust for it to have escaped detection. To really understand Oumuamua we need to send a space probe to it. This is actually possible, but it would be very expensive and take a long time to get there, so it isn’t practical this time. We just have to be ready for the next one,” commented Dr. Meech in the same University of Hawaii Press Release.

“It was relatively surprising that Oumuamua first appeared as an asteroid–given that we expect interstellar comets should be far more abundant, so we have at least solved that particular puzzle. It is still a tiny and weird object that is not behaving like a typical comet, but our results certainly lean towards it being a comet and not an asteroid after all,” explained Dr. Oliver Hainaut to the press on June 27, 2018. Dr. Hainaut is of the ESO.

Because Oumuamua is so small and faint, today’s observations do not provide all the information astronomers need to determine important aspects of the comet’s surface. Dr. Ken Chambers from Pan-STARRS noted that “When Oumuamua was discovered the astronomy community gathered as much data as possible, but ultimately the object was just not visible long enough to answer all our questions. With Pan-STARRS monitoring the skies, we hope to discover more Oumuamua-like objects in the future and begin to answer the really interesting questions about this class of objects.”