The interstellar comet had been drifting through interstellar space for unknown millions of years, but by back-tracking its trajectory through the solar system, astronomers could get a general idea as to where the object was traveling from in our galaxy and which stars it may have encountered along its way. For help they turned to the rich array of information from Gaia's Data Release 2 (GDR2), which was made available to the scientific community in April.
"GDR2 provides us with the 3-D positions and 3-D velocities for 7 million stars," astronomer Coryn Bailer-Jones, who works at the Max Planck Institute for Astronomy in Heidelberg, Germany, explains. "Without such information it would be impossible to trace the orbits of the stars back in time."
Before Gaia, the best data set we had came from the European Hipparcos satellite, which was in operation until 1993 and the first mission to carry out precision astrometry (an astronomical method to measure the positions and movements of stars) and complemented by other ground-based surveys. This database contains 2.5 million stars. Launched in 2013, Gaia is far more advanced and contains precise information about the positions, motions and distance of 1.3 billion stars. Seven million of those also contain information about their radial velocity (i.e., the speed at which the star is moving either toward or away from us). A further 220,000 stars were added to the analysis for which their radial velocities were known.
Knowing the area of sky that 'Oumuamua originated from, Bailer-Jones and his team were able to select 4,500 stars from GDR2 that the interstellar traveler may have encountered on its long journey. But to narrow this number down even further, they had to turn back time and trace out the motions of these stars and see if 'Oumuamua's path through the galaxy would have crossed, or come near to, their historic motions.
This analysis may seem like an impossible task. After all, there are many stars creating a helter-skelter-like gravitational landscape through which 'Oumuamua would have traveled. How did the researchers calculate its path?
"In principle, we would need to know the path and mass of every star in the galaxy, as well as the dark matter, in order to trace orbits back in time," explains Bailer-Jones. "In practice, this is not necessary; we can approximate it using a smooth gravitational potential model, which takes into account both the visible matter and the dark matter [in our galaxy]."
There's a problem with making this approximation, however. This model doesn't evolve with time, and it loses precision over longer time frames. "Consequently, while we can be reasonably confident about tracing orbits back in time for say a few tens of millions of years, beyond that the uncertainties build up too much for us to say anything with good confidence," he says.