A newly discovered comet has astronomers excited. Formally
named C/2019 Q4 (Borisov), the object appears to have come from outside our
solar system. If confirmed, that would make it the second known interstellar
visitor to our solar system, after the space rock ʻOumuamua.
The potential interstellar comet,
which so far is only a fuzzy blip, was first seen on August 30 by observer
Gennady Borisov. Ever since then, astronomers
around the world have been tracking it and mapping its trajectory to determine
its origin. The preliminary data suggests that the object could have originated
outside our solar system.
wasn’t seen until it was on its way out of the solar system, C/2019 Q4 is still on its way in — and should
remain visible for a year. With current orbit calculations, C/2019 Q4 is expected to reach its closest
approach to the Sun in December 2019, providing astronomers with time to
observe it more closely. The comet will likely come within 2 AU of the Sun,
which is outside the orbit of Mars. Though currently around a brightness magnitude
around 18 —too dim to be seen with a small telescope — the comet is expected to brighten to around
In order to determine if an object started its life outside
of the solar system, astronomers plot its orbit to find the arc and speed at
which the object travels. In order to get the most accurate trajectory, astronomers
need to have multiple days, ideally weeks, between measurements. Then, using
mathematical models, astronomers fit the measurements to an orbit, which lets them
see where the object came from.
Early calculations suggest that C/2019 Q4 has a hyperbolic
orbit — meaning one that that has the ability to escape the Sun’s gravitational
pull. The measurements so far suggest C/2019 Q4 is interstellar, but not all objects
confirmed to have a hyperbolic orbit are interstellar visitors. There are over
hundreds of known comets with hyperbolic orbits that have originated in the
Oort Cloud, an icy group of leftover material on the outer edge of the solar
system. Occasionally, objects from the cloud are dislodged and sent on a
one-time trip into the inner solar system where they are sling-shotted by the
Sun and out into interplanetary space.
Initially, scientists thought C/2019 Q4 could be one of
these objects, but with more observations, it’s looking more and more like
C/2019 Q4 is interstellar.
“Last Sunday it was pretty on the fence whether it was just going to turn out to be an Oort Cloud comet,” said Michele Bannister, an astronomer at Queen’s University Belfast in Ireland. “I think at this point … we’re pretty confident. I think it’s okay to say it’s interstellar.”
The first interstellar visitor, now called ‘Oumuamua, was
discovered in 2017 when it streaked past Earth as it started to fly out of our solar
system. Astronomers had long anticipated finding interstellar comets.
But ‘Oumuamua had a
uniquely enlongated cigar shape that had never been seen before, which
indicated that perhaps the star around which it originated had different
conditions to the Sun’s. Similariliy, observations of C/2019 Q4, if intestellar, could help
astronomers continue to study other stellar systems. And since it’s still
inbound, there will be plenty of opportunity to study it in the months to come.
“So the question
we’ll be wanting to answer is: what’s the composition of the coma — the fuzzy
bit around the comet core. Is it like comets that we see in the solar system,
or is it completely different?” Bannister said.
While observations of interstellar objects are incredibly rare, it is estimated that several such objects pass within Earth’s orbit every year undetected. Another 10,000 likely pass within Neptune’s orbit annually.