Interstellar comet soars through solar system
Astronomers examine a chunk of deep space
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’s 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 all the 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 at which they’ve been able to get a good look. 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 carbonbased molecule and possibly water — two familiar chemicals in such an alien object.
As the sun sinks behind the Tennessee mountains, astronomer Doug Durig climbs onto the rooftop of his observatory and powers up his three telescopes.
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, Mr. 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 remains of collapsed stars. There are even planets circling medium-size yellow suns like ours — though nothing found so far can match our breathable atmosphere and blue oceans.
Even when viewed through the most powerful telescopes, exoplanets are not discernible as anything but 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, barely distinguishable against the glittering background of stars.
Gennady Borisov sent 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 Mr. Durig’s eye.
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 mph — far faster than any comets, asteroids or planets orbiting our sun.
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. They were not quite sure what they had seen, and they were unsure whether the detection was just a lucky fluke or a harbinger of things to come.
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.
Identifying the comet’s parent star would be a tremendous feat, the astronomical equivalent of tracing a message in a bottle back to the person who sent it millions of years ago from billions of miles away. It may not turn out to be possible, most scientists acknowledge.
But maybe that’s OK, they say. Because the comet will have already revealed so much else. It will have told us something about the birth of solar systems. It will have connected our home to the workings of the wider galaxy. And now that we have seen it, it is easier to believe that more are out there, waiting to be found.