Sky & Telescope - March 2023 - 11

'OUMUAMUA: © WILLIAM K. HARTMANN; L1527: NASA /
ESA / CSA / STSCI / J. DEPASQUALE (STSCI)
SOLAR SYSTEM
Was 'Oumuamua a Chunk of an Exo-Pluto?
THE INTERSTELLAR OBJECT 'Oumuamua
(1I/2017 U1) was an odd duck
(S&T: Oct. 2018, p. 20). Its strong
brightness variations suggested a
pancake-like aspect ratio of 6:6:1. It also
slowed more than expected - 10 times
more than it would if it were a typical
comet - as it exited the solar system.
Yet it exhibited no visible coma, nor a
tail of either dust or gas.
At the Exoplanets in Our Backyard 2
workshop held in November in Albuquerque,
New Mexico, Steve Desch
(Arizona State University) presented a
plausible scenario that accounts for all
of these aspects.
Desch and team member Alan Jackson
(also Arizona State) investigated
the sublimation behavior of various ices
common in the outer solar system. They
found that nitrogen ice could provide
the right reflectivity, size, and mass
to reproduce 'Oumuamua's cometary
trajectory without producing any of the
hallmarks of comets.
A Star Is Born
pStudies have shown 'Oumuamua to have a
pancake-like 6:6:1 aspect ratio.
Pure nitrogen-ice would solve other
puzzles, too, such as how the body survived
passage within 0.26 astronomical
unit (a.u.) of the Sun. According to
Desch and Jackson's model, 'Oumuamua
would have lost 95% - most but
not all - of its mass by the time it
exited the inner solar system (when it
was discovered). Such extreme mass loss
would also explain its shape: If you add
20 times the present mass in concentric
layers around the present pancake,
reversing its evaporation by the Sun, the
original body would have had a much
more normal 2:1 aspect ratio.
Desch suggests that the chunk
came from a Pluto-like object around
a low-mass star (M8 or smaller). Such
stars are more common than the Sun,
and they also provide more favorable
environments for nitrogen-ice-covered
worlds. While the solar system might
have ejected some 100 trillion ice fragments
in its lifetime, that's not enough
to explain the statistics of finding
'Oumuamua. However, more abundant
M stars could each have ejected
40 times more fragments, making
'Oumuamua's discovery more likely.
'Oumuamua's appearance in our
backyard is therefore a probable accident,
Desch argues - and a recent one,
too. Based on its trajectory, which lies
in the galactic plane and has a relatively
low speed for an interstellar interloper,
'Oumuamua exited its parent system up
to a few hundred million years ago.
¢EMILY LAKDAWALLA
At visible wavelengths, the Taurus Molecular Cloud
appears as a dark nebula of dust and gas. But longer
wavelengths have pierced the veil to reveal a bustling
stellar nursery. At some 450 light-years away, it's one
of the nearest such regions, home to at least 250 infant
stars. The James Webb Space Telescope has turned
its infrared eyes on one of these, designated L1527.
This protostar is a hot blob of gas that has only 20% to
40% the Sun's mass. At only about 100,000 years old,
it hasn't yet begun fusion in its core. It's still feeding on
the cloud around it, which funnels gas and dust into
a disk that's visible as a dark line across the middle
of the image. That line is a few hundred astronomical
units long, about the size of our solar system. As
it feeds, this infant star also ejects mass, clearing out
space above and below the disk. The star's light suffuses
these emptied regions. Dust still veils our view,
just some regions less than others: Blue areas have
less dust in front of them, while light that must pass
through more dust appears orange. This view represents
a look back in time at a star very like our Sun just
after it was born.
¢ MONICA YOUNG
Read details at https://is.gd/protostar.
sk yand tele scope .o r g * MARCH 2023 11
https://www.is.gd/protostar
Sky & Telescope - March 2023

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