‘Oumuamua’s Origin: A Work in Progress

The much discussed interstellar wanderer called ‘Oumuamua made but a brief pass through our Solar System, and was only discovered on the way out in October of last year. Since then, the question of where the intriguing interloper comes from has been the object of considerable study. This is, after all, the first object known to be from another star observed in our system. Today we learn that a team of astronomers led by Coryn Bailer-Jones (Max Planck Institute for Astronomy) has been able to put Gaia data and other resources to work on the problem.

The result: Four candidate stars identified as possible home systems for ‘Oumuamua. None of these identifications is remotely conclusive, as the researchers make clear. The significance of the work is in the process, which will be expanded as still more data become available from the Gaia mission. So in a way this is a preview of a much larger search to come.

What we are dealing with is the reconstruction of ‘Oumuamua’s motion before it encountered our Solar System, and here the backtracking become tangled with the object’s trajectory once we actually observed it. Its passage through the system as well as stars it encountered before it reached us all factor into determining its origin.

What the Bailer-Jones teams brings to the table is something missing in earlier attempts to solve the riddle of ‘Oumuamua’s home. We learned in June of 2018 that ‘Oumuamua’s orbit was not solely the result of gravitational influences, but that a tiny additional acceleration had been added when the object was close to the Sun. That brought comets into the discussion: Was ‘Oumuamua laden with ice that, sufficiently heated, produced gases that accelerated it?

The problem with that idea was that no such outgassing was visible on images of the object, the way it would be with comets imaged close to the Sun. Whatever the source of the exceedingly weak acceleration, though, it had to be factored into any attempt to extrapolate the object’s previous trajectory. Bailer-Jones and team manage to do this, offering a more precise idea of the direction from which the object came.

Image: This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that it was travelling through space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red object, either elongated, as in this image, or else shaped like a pancake. Credit: ESO/M. Kornmesser.

At the heart of this work are the abundant data being gathered by the Gaia mission, whose Data Release 2 (DR2) includes position, on-sky motion and parallax information on 1.3 billion stars. As this MPIA news release explains, we also have radial velocity data — motion directly away from or towards the Sun — of 7 million of these Gaia stars. The researchers then added in Simbad data on an additional 220,000 stars to retrieve further radial velocity information.

To say this gets complicated is a serious understatement. 4500 stars turn up as potential homes for ‘Oumuamua, assuming both the object and the stars under consideration all moved along straight lines and at constant speeds. Then the researchers had to take into consideration the gravitational influence of all the matter in the galaxy. The likelihood is that ‘Oumuamua was ejected from a planetary system during the era of planet formation, and that it would have been sent on its journey by gravitational interactions with giant planets in the infant system.

Calculating its trajectory, then, could lead us back to ‘Oumuamua’s home star, or at least to a place close to it. Another assumption is that the relative speed of ‘Oumuamua and its parent star is comparatively slow, because objects are not typically ejected from planetary systems at high speed. Given all this, Bailer-Jones and team come down from 4500 candidates to four that they consider the best possibilities. None of these stars is currently known to have planets at all, much less giant planets, but none has been seriously examined for planets to this point.

Let’s pause on this issue, because it’s an interesting one. Digging around in the paper, I learned that unstable gas giants would be more likely to eject planetesimals than systems with stable giant planets, a consequence of the eccentric orbits of multiple gas giants during an early phase of system instability. It also turns out that there are ways to achieve higher ejection velocities. Does ‘Oumuamua come from a binary star? Let me quote from the paper on this:

Higher ejection velocities can occur for planetesimals scattered in a binary star system. To demonstrate this, we performed a simple dynamical experiment on a system comprising a 0.1 M ⊙ star in a 10 au circular orbit about a 1.0 M ⊙ star. (This is just an illustration; a full parameter study is beyond the scope of this work.) Planetesimals were randomly placed between 3 au and 20 au from the primary, enveloping the orbit of the secondary… Once again most (80%) of the ejections occur at velocities lower than 10 km s−1, but a small fraction is ejected at higher velocities in the range of those we observe (and even exceeding 100 km s−1).

So keep this in mind in evaluating the candidate stars. One of these is the M-dwarf HIP 3757, which can serve as an example of how much remains to be done before we can claim to know ‘Oumuamua’s origin. Approximately 77 light years from Earth, the star as considered by these methods would have been within 1.96 light years of ‘Oumuamua about 1 million years ago. This is close enough to make the star a candidate given how much play there is in the numbers.

But the authors are reluctant to claim HIP 3757 as ‘Oumuamua’s home star because the relative speed between the object and the star is about 25 kilometers per second, making ejection by a giant planet in the home system less likely. More plausible on these grounds is HD 292249, which would have been within a slightly larger distance some 3.8 million years ago. Here we get a relative speed of 10 kilometers per second. Two other stars also fit the bill, one with an encounter 1.1 million years ago, the other at its closest 6.3 million years ago. Both are in the DR2 dataset and have been catalogued by previous surveys, but little is known about them.

Now note another point: None of the candidate stars in the paper are known to have giant planets, but higher speed ejections can still be managed in a binary star system, or for that matter in a system undergoing a close pass by another star. None of the candidates is known to be a binary. Thus the very mechanism of ejection remains unknown, and the authors are quick to add that they are working at this point with no more than a small percentage of the stars that could have been ‘Oumuamua’s home system.

Given that the 7 million stars in Gaia DR2 with 6D phase space information is just a small fraction of all stars for which we can eventually reconstruct orbits, it is a priori unlikely that our current search would find ‘Oumuamua’s home star system.

Yes, and bear in mind too that ‘Oumuamua is expected to pass within 1 parsec of about 20 stars and brown dwarfs every million years. Given all of this, the paper serves as a valuable tightening of our methods in light of the latest data we have about ‘Oumuamua, and points the way toward future work. The third Gaia data release is to occur in 2021, offering a sample of stars with radial velocity data ten times larger than DR2 [see the comments for a correction on this]. No one is claiming that ‘Oumuamua’s home star has been identified, but the process for making this identification is advancing, an effort that will doubtless pay off as we begin to catalog future interstellar objects making their way into our system.

The paper is Bailer-Jones et al., “Plausible home stars of the interstellar object ‘Oumuamua found in Gaia DR2,” accepted for publication in The Astrophysical Journal (preprint).