How the second known interstellar visitor makes ‘Oumuamua seem even odder

New observations show that 2I/Borisov looks like a comet, just what astronomers expected


The second mysterious interstellar object found zipping through the solar system, 2I/Borisov, looks like a run-of-the-mill comet. That stands in stark contrast to the first known interstellar object, ‘Oumuamua (illustrated).

Stuart Rankin/Flickr (CC BY-NC 2.0)

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 

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
‘Oumuamua 2I/Borisov
Constellation of originLyraCassiopeia
Halo and tailNoYes
Width400 meters~2 kilometers
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/ 2019, A.M. Hein et al/Acta Astronautica 2019

Previously the staff writer for physical sciences at Science News, Maria Temming is the assistant managing editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

More Stories from Science News on Space