In October 2017, a fast moving, cigar-shaped comet of extrasolar origin was discovered close to the Earth with the Pan-STARRS1 telescope in Hawai’i. Officially named 1I/2017 U1 (‘Oumuamua), the comet was presumably ejected from a forming planetary system, probably by a gravitational kick from a giant exoplanet. Now, using data from ESA’s star-mapping Gaia satellite, Dr. Coryn Bailer-Jones of the Max Planck Institute for Astronomy and co-authors have identified four stars as possible homes of ‘Oumuamua.
This artist’s impression shows the first interstellar comet ‘Oumuamua. Image credit: M. Kornmesser / ESO.
Comets are leftovers of the formation of planetary systems, and it is possible that ‘Oumuamua was ejected from its home star’s realm while planets were still taking shape there.
To look for its home, astronomers had to trace back in time not only the trajectory of the interstellar comet, but also of a selection of stars that might have crossed paths with this object in the past few million years.
To this aim, Dr. Bailer-Jones and his colleagues dived into the data from Gaia’s second release.
The Gaia data contain positions, distance indicators and motions on the sky for more than a billion stars in our Milky Way Galaxy; most importantly, the data set includes radial velocities — how fast they are moving towards or away from us — for a subset of seven million, enabling a full reconstruction of their trajectories.
The astronomers looked at these seven million stars, complemented with an extra 220,000 for which radial velocities are available from the astronomical literature.
As a result, they identified four stars whose orbits had come within a couple of light years of ‘Oumuamua in the near past, and with relative velocities low enough to be compatible with likely ejection mechanisms.
All four had their ‘close’ encounter with ‘Oumuamua between one and seven million years ago.
However, none of these stars is known to either harbor planets or to be part of a binary stellar system; a giant planet or companion star would be the preferred mechanism to have ejected the small body.
“All four of them are dwarf stars,” Dr. Bailer-Jones and co-authors said.
“The one that came closest to ‘Oumuamua, at least about one million year ago, is HIP 3757, a reddish dwarf star some 81 light-years away. It approached within about 1.96 light-years.”
“Given the uncertainties unaccounted for in this reconstruction, that is close enough for ‘Oumuamua to have originated from its planetary system. However, the comparatively large relative speed (around 56,000 mph, or 25 km/s) makes it less probable for this to be ‘Oumuamua’s home.”
“The next candidate, HD 292249, is similar to our Sun and is approximately 135 light-years away. It was a little bit less close to the object’s trajectory 3.8 million years ago, but with a smaller relative speed of 22,400 mph, or 10 km/s.”
“The two additional candidates met ‘Oumuamua 1.1 and 6.3 million years ago, respectively, at intermediate speeds and distances.”
While future observations of these four stars might shed new light on their properties and potential to be the home system of ‘Oumuamua, the astronomers are also looking forward to future releases of Gaia data.
The team’s paper will be published in the Astronomical Journal.
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C.A.L. Bailer-Jones et al. 2018. Plausible home stars of the interstellar object ‘Oumuamua found in Gaia DR2. AJ, in press; arXiv: 1809.09009
