that the object is exceptionally elon-
gated in shape. “What we found was
a rapidly rotating object, at least the
size of a football field, that changed in
brightness quite dramatically,” said Kar-
en Meech of the University of Hawai‘i’s
Institute for Astronomy, and the leader
of the discovery team that first obtained
Gemini DDT observations of the object.
“This change in brightness hints that
‘Oumuamua could be ten times lon-
ger than it is wide — something which
has never been seen in our own Solar
System.” The best current estimates are
that the object has the dimensions,
roughly speaking, of five football fields
laid end to end (omitting endzones).
The research led by Meech combines
observations from the Gemini South
and Very Large Telescopes in Chile, as
well as from Pan-STARRS, Keck 2, Can-
ada-France-Hawai‘i, and the United
Kingdom Infrared Telescope in Hawai‘i.
The study was published in the Novem-
ber 20th online issue of Nature.
Although the shape of ‘Oumuamua is
like nothing seen in our Solar System,
its color is more conventional. “Our
first interstellar planetesimal is just slightly
redder than reflected sunlight,” said Michele
Bannister, an astronomer at the Astrophys-
ics Research Centre of Queen’s University in
Belfast, and the leader of another team that
obtained Gemini DDT observations of ‘Ou-
muamua. “This is fascinating, as we might
have expected it would be deep red from
spending a long time travelling between
stars, where cosmic rays would alter organic
molecules on its surface. Instead, its colour
looks a lot like those of tiny minor planets in
our own Solar System that orbit in Jupiter’s
Trojan clouds, or some that orbit beyond
Neptune.”
Bannister adds, “Gemini’s ability to observe
near-simultaneously in the optical and near-
January 2018 / 2017 Year in Review
infrared with rapid instrument-switching
was ideal, as ‘Oumuamua turned out to be
strongly variable in brightness, and we had
to quantify that to properly measure its co-
lours.” A paper presenting the results from
Bannister’s team has been accepted by The
Astrophysical Journal Letters. A preprint is
available online.
An additional surprise from ‘Oumuamua
was highlighted in a study by the third team
that obtained Gemini observations during
the event. The team, led by Piotr Guzik and
Michal Drahus of the Astronomical Obser-
vatory of Jagiellonian University in Kraków,
obtained 442 individual exposures all in the
same red filter. “Thanks to the long, continu-
ous observations at Gemini, we found that
GeminiFocus
Figure 2.
Variations in the
brightness (top) and
magnitude (bottom) of
‘Oumuamua, as measured
in the r-band filter with
GMOS on successive
nights in late October.
The rotation period is
determined with high
precision to be 7.5483
hours, but the light curve
does not repeat exactly
from one rotation to
the next. This indicates
the object is “tumbling.”
Figure reproduced from
Drahus et al., https://arxiv.
org/abs/1712.00437.
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