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how-the-second-known-interstellar-visitor-makes-ae˜oumuamua-seem-even-odder

The second space rock seen visiting our
solar system from another star is proving just how bizarre the first known
interstellar object, ‘Oumuamua, really was.

‘Oumuamua raised eyebrows when it appeared
in October 2017
looking more like a rocky asteroid than an
icy comet (SN: 10/27/17). Because
comets form farther from their host stars than asteroids, it should be easier
for comets to escape their star’s gravity to wander the galaxy. So astronomers
expect the vast majority of interstellar vagabonds to be icy bodies. But
‘Oumuamua didn’t sport the gaseous halo or tail that forms when sunlight
vaporizes a comet’s ice.

Now, new telescope images confirm that a
second interstellar object called 2I/Borisov (originally dubbed C/2019 Q4
(Borisov)) looks
like a garden-variety comet
, researchers report online October 14 in Nature Astronomy. The cometlike
appearance of this object, first
glimpsed August 30
, suggests that ‘Oumuamua’s weirdness was a
one-off, and that astronomers’ models of planetary systems are on the right
track (SN: 9/12/19).

Astronomers observed 2I/Borisov on two
nights in September with the William Herschel Telescope in the Spanish Canary
Islands and the Gemini North Telescope in Hawaii. Those images reveal that,
like comets native to our solar system, 2I/Borisov’s core is shrouded in a
gaseous halo trailed by a faint, broad stream of gas and dust.

‘Oumuamua and 2I/Borisov
The first known interstellar object, ‘Oumuamua, appeared to be a naked space rock (left, center). But 2I/Borisov (right, center) bears the usual characteristics of a comet: It’s cloaked in a halo of vaporizing gas and trailing a gaseous tail in its wake.Gemini Observatory/NSF/AURA; Alan Fitzsimmons/ARC/Queen’s University Belfast, Isaac Newton Group, NASA

“It’s kind of relieving that finally we
have something that meets our expectations,” says study coauthor Michał Drahus,
an astronomer at Jagiellonian University in Kraków, Poland. “Now we really can
be absolutely sure that ‘Oumuamua was one weird object.”

Whereas ‘Oumuamua vanished within weeks of
its discovery, astronomers have several months to take a closer look at
2I/Borisov. Higher-resolution telescope images may tease out the exact size and
shape of its core, and inspecting the specific wavelengths of light emanating
from the comet could help astronomers flesh out its chemical composition.

Preliminary wavelength observations have already hinted that 2I/Borisov contains cyanogen gas (made of carbon and nitrogen atoms), which is relatively common in comets native to the solar system. Astronomer Alan Fitzsimmons of Queen’s University Belfast in Northern Ireland and colleagues report these findings October 2 at arXiv.org. 


An odd couple

The second known interstellar object, a comet currently cruising through our solar system called 2I/Borisov, looks fairly different from the first discovered interstellar visitor — an asteroid-like object called ‘Oumuamua that skirted by Earth in 2017. Here are some of the key distinctions between this pair of galaxy-trotting space rocks.

Differences between the first and second discovered interstellar objects
‘Oumuamua2I/Borisov
Constellation of originLyraCassiopeia
Halo and tailNoYes
Width400 meters~2 kilometers
ShapeOblongUnknown
Closest distance to Earth0.16 au (astronomical units)1.9 au
Closest approach to EarthOctober 14, 2017December 28, 2019
Time observableWeeksMonths

Sources: P. Guzik et al/Nature Astronomy 2019, ‘Oumuamua ISSI team/arXiv.org 2019, A.M. Hein et al/Acta Astronautica 2019


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