SEWANEE, Tenn. — Something strange is sailing toward us. Something small and cold and extraordinarily fast. No one knows where it came from, or where it is going. But it’s not from around here.
This is an interstellar comet — an ancient ball of ice and gas and dust, formed on the frozen outskirts of a distant star, which some lucky quirk of gravity has tossed into our path.
To astronomers, the comet is a care package from the cosmos, a piece of a place they will never be able to visit, a key to worlds they cannot directly observe.
It is only the second interstellar interloper scientists have seen in our solar system. And it’s the first one they’ve been able to get a good look at. By tracking the comet’s movement, measuring its composition and monitoring its behavior, researchers are seeking clues about the place it came from and the space it crossed to get here.
They have already found a carbon-based molecule and possibly water — two familiar chemicals in such an alien object.
As the sun sinks behind the Tennessee mountains, and stars wink into view, astronomer Doug Durig climbs onto the rooftop of his observatory, powers up his three telescopes, and angles them skyward.
Every night, the comet grows bigger and brighter in the sky, expelling streams of gas and dust that may offer up clues to its history. On Dec. 8, it will make its nearest approach to Earth, offering researchers an up-close glimpse before it zooms back into the freezing, featureless void.
Far below in the darkness, Durig will be waiting.
Each star in the night sky represents a possible solar system. Every light in the universe is, more likely than not, some alien planet’s sun.
This is the chief lesson of two decades of studying exoplanets. Scientists have identified thousands of worlds beyond our solar system: gas giants and tiny rocky spheres, worlds lit by dim red suns and ones that orbit the spinning remains of collapsed stars. There are even planets circling medium-size yellow suns like ours — though nothing found so far can match the breathable atmosphere and deep, blue oceans of Earth.
Yet even when viewed through the most powerful telescopes, exoplanets are not discernible as anything more than specks of light. And no human alive has a hope of traveling to another star; merely approaching the nearest one would take 40,000 years.
Scientists’ best hope for closely examining another solar system was to wait for a piece of one to come to us.
It was Aug. 30, in the quiet moments before dawn, when a self-taught astronomer in a Crimean mountain village spotted a faint smudge low on the horizon.
Gennady Borisov submitted his observations to the Minor Planet Center, the astronomers’ clearinghouse for information about small bodies in the solar system, so other scientists could take a look.
One night later, halfway across the planet, the strange report caught Durig’s eye.
‘‘I was the second person to observe it,’’ Durig said. ‘‘That confirmed the comet was real.’’
Within a couple of weeks, scientists had collected enough observations to calculate the comet’s orbit. But they did not find the oval path that comets typically make around the sun. Instead, the orbit was hyperbolic — it did not close in on itself. The object was also traveling at the blistering speed of 93,000 miles per hour, far faster than any comets, asteroids, or planets orbiting our sun.
‘‘Wow,’’ said Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory, who was among the first people to determine that the comet came from another star. ‘‘I was not expecting to see anything like that.’’
There has been only one other interstellar object spotted in our solar system: a cigar-shaped rock named ‘Oumuamua, a Hawaiian word that translates to ‘‘messenger from afar.’’
But ‘Oumuamua was already on its way out of the system when it was discovered in October 2017, and it was so faint that scientists were never able to view it as more than a single pixel of light.
So researchers were thrilled when, less than two years later, another interstellar traveler arrived.
The new comet, which has been named 2I/Borisov (indicating its discoverer and its status as the second known interstellar object), is expected to be within reach of telescopes until fall 2020. At its closest approach, next month, it will be twice as far from Earth as Earth is from the sun.
Though it entered the solar system from the direction of the constellation Cassiopeia, scientists do not know yet where 2I/Borisov came from, or how long it has drifted through the desolation of interstellar space. Given its current speed, it has certainly been traveling for millions, if not billions, of years.
As the object gets closer to the sun’s warmth, ices on its surface turn into gas. This creates the characteristic halo-like ‘‘coma,’’ which scientists can scrutinize to determine what the comet is made of.
Already, 2I/Borisov has been observed more than 2,000 times.
‘‘That’s going to be fun, in terms of looking at this object . . . as it comes in from the deep freeze for the very first time,’’ said Michele Bannister, an astronomer at Queen’s University Belfast. ‘‘Let’s open it up and see what we have with this particular present from another star.’’