The vagabond child of a mysterious parent-star beyond our own Sun, ‘Oumuamua sings a haunting sirens’ song to curious astronomers who are trying to find its elusive stellar parent. ‘Oumuamua was discovered in late 2017, and this alien visitor from far, far away is currently the only known escapee from the family of a distant star to have been found wandering through our own Solar System. Unfortunately, like many runaway children, ‘Oumuamua‘s parentage is a well-kept secret. However, in September 2018, a team of astronomers led by Dr. Coryn Bailer-Jones of the Max Planck Institute for Astronomy in Germany, announced that they have managed to track ‘Oumuamua back to several possible parent-stars. The astronomers found four separate candidate stars where ‘Oumuamua may have originated before it embarked on its solitary, long and treacherous journey through the space between stars to finally reach a temporary home in the family of our Sun.
‘Oumuamua displays a dark red hue, which makes it similar in color to icy objects that trip the light fantastic in the frozen outer limits of our own Solar System, where our Sun is so far away that it appears to be just another star–albeit a large one–swimming through a celestial ocean along with countless other glimmering stars in a remote region of perpetual twilight.
In September 2018, Dr. Bailer-Jones and his team announced that they have managed to track ‘Oumuamua back to several possible stellar-parents. In order to accomplish this difficult task, the astronomers used data gathered from the European Space Agency’s (ESA’s) astrometry satellite Gaia to find the four candidate stellar parents of this wandering alien object.
The discovery of ‘Oumuamua in October 2017 was a true stellar opening performance. For the first time, astronomers were able to observe up close and personal an invading interstellar object screeching through our own Solar System. Unfortunately, this runaway refugee from afar was spotted just as it was about to depart. Nevertheless, astronomers were still able to use both Earth-bound and space-telescopes in order to measure ‘Oumuamua’s movements.
The new research, conducted by Dr. Bailer-Jones and colleagues, succeeded in tracking ‘Oumuamua’s travels in order to find its four potential parent-stars. Previous observations had tried similar reconstructions, but were unable to succeed. This is because the previous studies were missing some critical information. Indeed, it was not until June 2018 that Dr. Marco Micheli (ESA) and his colleagues were able to show that ‘Oumuamua’s orbit within our own Solar System is not that of an object in free fall. This indicates that ‘Oumuamua is not traveling solely under the influence of gravity, but has also experienced additional acceleration as it wandered towards our own Sun. Currently, the most likely explanation for this behavior is that ‘Oumuamua is an alien comet. Comets are encased within shells of ice, and when they approach the melting heat of our Sun, they form a gas that accelerates them in a way similar to that of an extremely weak rocket engine. Even though ‘Oumuamua’s acceleration was weak, it was nonetheless too large to be ignored when astronomers tried to track ‘Oumuamua’s orbit back in time. The outgassing could not be seen on images, as it is with comets that are situated closer to our searing-hot Sun.
Dr. Bailer-Jones and his team took into consideration how ‘Oumuamua’s orbit evolved as it traveled closer and closer to our Sun. This new information provided the astronomers with an exact estimate of the direction from which ‘Oumuamua had come, as well as its speed when it entered our Solar System.
Mysterious Stranger From Afar
‘Oumuamua is classified as a mildly active comet. It is formally designated 1/2017 U1. This fascinating object was discovered by Dr. Robert Weryk on October 19, 2017 using the PanSTARRS telescope at Haleakala Observatory in Hawaii. This occurred a mere 40 days after it had made its closest approach to our Sun. When it was first discovered, it was about 21,000,000–or 0.22 astronomical units (AU)–from our planet. One AU is equivalent to the average distance between our planet and the Sun, which is 93,000,000 miles. This indicates that at that time ‘Oumuamua was approximately 65 times farther away than Earth’s own Moon, and it was already in the process of flying away from our Star.
“Oumuamua is a small object, and even though its dark crimson color is similar to that of certain inhabitants of the outer Solar System, at first glance it revealed no signs of the streaming tail that comets are so famous for–despite its close proximity to our searing-hot, roiling Star. However, since its original detection, ‘Oumuamua has experienced that non-gravitational acceleration akin to comet outgassing. It also displays both a significant elongation and rotation rate. This suggests to many astronomers that it is metal-rich with a relatively high density. In addition, ‘Oumuamua is tumbling around rather than smoothly rotating, and it is whizzing by so fast relative to our Sun that it cannot have been born in our own Solar System. This observation also indicates that ‘Oumuamua is not a permanent resident of our Star’s family, and it can exit from it just as freely as it entered it in the first place. ‘Oumuamua is destined to eventually leave our Solar System, and again go on its lost and lonely journey through interstellar space. However, certain nagging questions remain: where is ‘Oumuamua’s original stellar system, and how long has it been roaming through the space between stars?
Because it is the first known object of its invasive type, ‘Oumuamua proved to be a challenge for the International Astronomical Union (IAU), which is responsible for classifying celestial objects. Initially,‘Oumuamua was designated as comet C/2017 U1, but it was later reclassified as asteroid A/2017 U1. This heavenly confusion occurred because ‘Oumuamua did not show a defining characteristic of a comet–a coma. When it was finally determined to be a refugee from beyond our Sun’s family, an entirely new classification was created for it–1I/2017/ U1 (‘Oumuamua). The name ‘Oumuamua itself comes from the Hawaiian word ‘oumuamua, which means “scout”.
The first tantalizing hint that ‘Oumuamua is a wandering comet from the alien family of a distant star has to do with its trajectory. Follow-up studies were conducted by astronomers using the Canada-France-Hawaii Telescope (CFH), the ESA’s Optical Ground Station Telescope located in Tenerife, Canary Islands, as well as an assortment of other telescopes all over the globe. These follow-up observations helped curious astronomers determine both the history and mysterious nature of this weird vagabond from far away. ‘Oumuamua had apparently zipped past our Sun, approaching from “above” the plane of the planets on a highly inclined orbit–and this was unlike the trajectory of any comet or asteroid ever observed before. In addition, ‘Oumuamua was flying fast enough at the breathtaking speed of 70,800 miles per hour (as of June 2018) to enable it to free itself from the gravitational ties that bind coming from our Sun.
Astronomers realized that ‘Oumuamua had invaded our Solar System from approximately the same direction as the star Vega (Lyra Constellation). The path that ‘Oumuamua had taken indicated that it arrived from the direction expected for objects that had come from the space between stars. Soon after ‘Oumuamua’s detection, astronomers compared it to the fictional alien spacecraft dubbed Rama. This is because of its interstellar origin. Both the fictional and real objects are extremely elongated and limited in size. However, adding to the general confusion, ‘Oumuamua’s dark crimson hue and fluctuating brightness at first suggested to astronomers that it was the asteroid that it is not.
In October 2017, two earlier detections of ‘Oumuamua were found that had been obtained from the Catalina Sky Survey. The observations were dated October 14th and 17th 2017. Observations that continued on for two weeks showed a strongly hyperbolic trajectory. Indeed, ‘Oumuamua has a hyperbolic excess velocity of about 58,900 miles–this means its speed relative to our Star when it was still in interstellar space. The Catalina Sky Survey’s purpose is to detect asteroids and comets. The survey is conducted at the Steward Observatory Catalina Station that is located near Tucson, Arizona.
By the second week of November 2017, the astronomers were convinced that ‘Oumuamua was truly a refugee arriving from the space between stars. As the result of observations obtained over a span of 34 days, ‘Oumuamua‘s orbital eccentricity was calculated to be 1.20–the highest eccentricity ever observed. Indeed, its orbital eccentricity is so high that it could not have resulted from an encounter with any of the planets in our Sun’s family–either known or undiscovered.
Searching For ‘Oumuamua’s Mysterious Parent-Star
But what about the stars that ‘Oumuamua met up with while it was enroute to our Solar System? Could their combined gravity influence this little wanderer’s trajectory? In order to answer these important questions in their reconstruction, Dr. Bailer-Jones used a treasure chest filled with data obtained from ESA’s Gaia Mission that had been released in April 2018–Gaia’s Data Release 2 (DR2). Dr. Bailer-Jones, as a leader of one of the teams in charge of preparing data obtained from Gaia, is certainly familiar with this particular set of data. DR2 includes very exact information about on-sky motion, parallax (as a measure of distance), and positions for 1.3 billion stars. For about one million of those stars, there is even data pertaining to their radial velocity, which is the motion directly towards or away from us. Dr. Bailer-Jones and his team used the astronomical data base dubbed Simbad, that added 220,000 more stars to their study. The radial velocity of those additional stars is aavailable only from the Simbad data base.
Next, the team of astronomers devised an approximate track back in time. This was a simplified model in which both ‘Oumuamua and all of the stars move along straight lines at constant speeds. Using this model, the astronomers then picked out approximately 4500 stars that were strong candidates for a closer encounter with the wandering ‘Oumuamua. Next came the third step of tracing the earlier movements of those candidate stars, as well as for ‘Oumuamua, using a smoothed out version of the gravitational influence of all the matter contained within our own Milky Way Galaxy. This is termed the smoothed out Galactic potential.
Dr. Bailer-Jones and his team then found a quartet of stars that, when taken individually, might possibly be the stellar parent of the wandering ‘Oumuamua. All four of the candidates are dwarf stars. About one million years ago, the closest of the four stars to ‘Oumuamua was a small red star dubbed HIP 3757. At that time, HIP 3757 was located close enough to ‘Oumuamua for it to have been born in its system–that is, if it has one. Nonetheless, its comparatively great relative speed of approximately 25 kilometers per second makes HIP 3757 a somewhat weaker candidate for being the long-lost stellar parent of ‘Oumuamua.
The second candidate star, HD 292249, is similar to our own Star, the Sun. HD292249 was a little farther from ‘Oumuamua’s trajectory than HIP 3757 was 3.8 million years ago, but it travels at a slightly slower relative speed of 10 kilometers per second.
The two remaining parent-star candidates encountered ‘Oumuamua approximately 1.1 and 6.3 million years ago, respectively–at intermediate distances and speeds. However, little is known about the two stars even though they had already been catalogued in previous surveys.
While all four of these stars can potentially be ‘Oumuamua’s stellar parent, the smoking gun is missing for each of them. In order to toss ‘Oumuamua out into interstellar space at the observed speeds, the original planetary system would have needed to host a giant planet that could play the important role of slingshot. So far, no gigantic culprit has been found orbiting any of the candidate stars. However, since none of the four stars has been studied closely enough for astronomers to spot a guilty giant planet, that could easily change at a future date.
This study is further limited by the small number of radial velocities that are included in Gaia’s second data release. Gaia’s third release, currently scheduled for 2021, should provide important data for a sample of stars ten times greater in number. This potentially could result in the identification of more stellar-parent candidates.
The hunt for ‘Oumuamua’s nursery continues. This recent study presents a quartet of interesting possibilities, but our alien visitor’s stellar parent has yet to be found beyond a shadow of a doubt.