Comet Visits from Outside Our Solar System

The Cosmic Companion
Sep 14 · 5 min read

Astronomers at the Gemini Observatory recently examined a comet from another solar system passing through our family of planets. What do we know about this mysterious visitor?

For just the second time, astronomers have found an object visiting our family of planets from beyond the solar system. This object, C/2019 Q4 (Borisov), is also the first interstellar comet ever known to visit our family of planets. The comet was discovered by Russian amateur astronomer Gennady Borisov on August 30, 2019.

Detailed examination of this mysterious comet was carried out on the night of September 9–10, using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North Telescope, located on Maunakea in Hawaii.

A pair of telescope domes, as one fires a laser into the sky.
A pair of telescope domes, as one fires a laser into the sky.
A montage of the twin telescopes — Gemini South on the summit of Cerro Pachón in Chile (left) and Gemini North on the summit of Mauna Kea in Hawai’i (right). Image credit: Gemini/NSF/AURA

“This image was possible because of Gemini’s ability to rapidly adjust observations and observe objects like this, which have very short windows of visibility. However, we really had to scramble for this one since we got the final details at 3:00 am and were observing it by 4:45!” said Andrew Stephens of Gemini Observatory.

The interstellar comet was photographed in two color bands, revealing a tail pointed away from the Sun. This feature, caused by ice melting as a result of solar heating, defines the nature of comets.

Interstellar Snowball Fight!

A fuzzy comet and star streaks.
A fuzzy comet and star streaks.
This two-color composite showing the coma (head) of the comet C/2019 Q4. The streaks are stars appearing to move due to the rotation of Earth, as astronomers track the comet over time. Image credit: Gemini Observatory/NSF/AURA. Composite image by Travis Rector.

Comets can be compared to dirty snowballs — frozen chunks of water ice and dry ice, ammonia, dust, and debris. The comets with which we are familiar originate in the depths of the Solar System, and head in toward the Sun. As they approach our parent star, the frozen body warms, and forms a tail which can sometimes be seen from Earth.

As time goes on, C/2019 Q4 will draw nearer to the Sun and Earth, causing the tail to grow and to brighten. Although this comet is heading toward the Sun, amateur astronomers will never directly views of the visitor, as it will remain further from Earth than we are from the orbit of Mars. This interstellar visitor will never grow bright enough to see from the Earth without a professional telescope. At its closest, C/2019 Q4 will come within 300 million kilometers (190 million miles) from our home world.

“We spread our sleeping bags on the snow and crawled inside. The vantage point was dizzying. It was impossible to tell whether the comet was above us or we were above the comet; we were all falling through space, missing the stars by inches.”
― Anne Fadiman, At Large and at Small: Familiar Essays

This object is approaching the sun in a hyperbolic orbit, revealing its origin from outside the Solar System. It is currently on nearly the opposite side of the Sun as the Earth, making observations of the comet difficult.

“The comet’s current velocity is high, about 93,000 mph [150,000 kph], which is well above the typical velocities of objects orbiting the Sun at that distance. The high velocity indicates not only that the object likely originated from outside our solar system, but also that it will leave and head back to interstellar space,” Davide Farnocchia of NASA’s Center for Near-Earth Object Studies at Jet Propulsion Laboratory (JPL) states.

The Buzz About the Second Alien Visitor

A long, thin, rock-looking thing.
A long, thin, rock-looking thing.
An artist’s concept of Oumuamua, the first known interstellar visitor, passing through our solar system. Image Credit: European Southern Observatory / M. Kornmesser

When first spotted in October 2017, the first interstellar object ever detected, named ‘Oumuamua, did not show any tail as it passed through our local planetary system. Although initially classified as a comet, ‘Oumuamua was reclassified as the object failed to develop a tail, even at it made its closest approach to the Sun at 314,300 kilometers (196,000 miles) per hour.

Analysis of the path of ‘Oumuamua suggests that object may have wandered alone through the Milky Way galaxy for hundreds of millions of years before encountering our solar system. Astronomers have long believed that rogue planets, asteroids, and comets wander in the distances between solar systems, but we are only now developing the instruments to detect such interstellar interlopers.

“For decades we’ve theorized that such interstellar objects are out there, and now―for the first time―we have direct evidence they exist,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, in November 2017.

The discovery of C/2019 Q4 in August 2019 adds wandering comets to the litany of unusual bodies known to make their way through the galaxy and — occasionally — pay us a visit.

Two Eyes are Better than One!

The Gemini collaboration consists of a pair of 8.1-meter telescopes — a northern observatory located in Hawaii and a southern facility on Cerro Pachón in central Chile. Using both telescopes, the team is able to image the entire sky, over both hemispheres.

Sky trails over a telescope dome.
Sky trails over a telescope dome.
A time lapse photo of the dome surrounding the 8.1-meter telescope at Gemini North telescope. Image credit: Gemini Observatory image by Joy Pollard

The primary mirrors in the observatories are able to collect both visible light and near-infrared energies from distant targets.

Telescopes this size require active optics, altering the positions of large mirrors to compensate for changes in temperature, mechanical stresses, and other movements within the internal structure of the instrument. These active optics, like those found in the Gemini Observatories, should not be confused with adaptive optics, which operate on smaller secondary mirrors, correctting for atmospheric disturbances.

This international project is a facility of the National Science Foundation (NSF) and is operated by the Association of Universities for Research in Astronomy, Inc. (AURA).

This discovery by Gennady Borisov is another example showing how astronomy is one of the few branches of science where amateur scientists can still make significant contributions to our global storehouse of knowledge.

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The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

The Cosmic Companion

Written by

James Maynard is the author of two books, and thousands of articles about space and science. E-mail:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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