#borisov

Sergei Borisov

Goodbye 'Oumuamua, Hello Borisov; This Is What Two Interstellar Interlopers Can Teach Us

“The most interesting aspect of what we know so far about interstellar objects is how different the first two, 'Oumuamua and Borisov, truly are from one another. There are a number of ways to form interstellar bodies: from failed star systems in star-forming regions, from ejected asteroids, from comets, and from collisional debris. We do not know how common or rare objects of all these different types are, nor how to definitively classify the ones we've seen so far, but hope is on the way. Beginning in the 2020s, the Large Synoptic Survey Telescope will come online, expected to uncover dozens of such objects by 2030.

What are the size and frequency distributions of the interstellar population? How old and/or weathered are they? Are they comet-like or asteroid like; volatile-rich or volatile-free, and do different classes of object originate from different regions in the sky? Are most small objects inactive while most large ones are active? With the tip of the interstellar iceberg uncovered, the answers to these questions and more are at last within our reach.”

In our solar system, asteroids, centaurs, comets, Kuiper belt and Oort cloud objects all have their own unique story, but possess many qualities common to each separate class. Occasionally, one of these objects will get ejected, where it will wander the Milky Way indefinitely, until it encounters another object.

Back in our Solar System, we’ve begun discovering and identifying objects in our neighborhood that originated from interstellar space. In 2017, ‘Oumuamua became the first, and it was extremely odd: small, elongated, and already on its way out. Our picture is now changing dramatically, as a second interstellar object, 2I/Borisov, was just discovered.

An object that seems to be a comet from around another star is speeding through the solar system. This comet, dubbed C/2019 Q4 (Borisov), marks the second time that astronomers have seen an interstellar visitor on its way past the sun.

Ukrainian amateur astronomer Gennady Borisov spotted the comet on August 30. In the days that followed, other astronomers took enough observations of the comet to show that its orbit is not bound to the sun, a telltale sign that the object is from interstellar space, says astrophysicist Matthew Holman. He is the interim director of the International Astronomical Union’s Minor Planet Center in Cambridge, Mass., which reported C/2019 Q4’s trajectorySeptember 11 in a public bulletin.

“There have been three or four different groups that have checked the orbit and poked at it from different directions, and everybody reaches the same conclusion,” Holman says. But he cautions that the comet is close to the sun and low on the horizon, which make it difficult to observe.

“This is an unusual thing. You always want to be extra careful when you get an answer that’s not typical,” he says. “That said, I think it will hold up.”

The first known interstellar object, named ‘Oumuamua, was spotted in October 2017 as it sped away from the solar system (SN: 10/27/17). Astronomers could only grab fleeting glimpses of ‘Oumuamua on its way out the door. That left many theories for what the object was — an asteroid, a fluffy ice fractal, a skeleton of a comet, even an alien spaceship. But little data existed to back up any of those ideas (SN: 2/27/19).

The possible interstellar comet C/2019 Q4 (Borisov)’s trajectory will take the object between the orbits of Jupiter and Mars. CREDIT: TONY873004/WIKIMEDIA COMMONS (CC BY-SA 4.0)

C/2019 Q4, on the other hand, is just past the solar system’s doorstep, on a trajectory that will take it between the orbits of Jupiter and Mars. It will make its closest approach to Earth (at about twice the distance between Earth and the sun) on December 29.

“It’s going to be a nice Christmas comet,” says astronomer Michele Bannister of Queen’s University Belfast in Northern Ireland. The object will remain in view for several months after that.

In total, “we’ll be able to observe it for probably a year, which is really different from ‘Oumuamua,” Holman says. ‘Oumuamua was tracked for a short 2.5 months before it became too faint to see.

Already, C/2019 Q4 looks different from its predecessor. ‘Oumuamua resembled an asteroid more than a comet, with no obvious cloud of gases. C/2019 Q4 is surrounded by a bright cometlike halo, indicating that it’s sloughing off gas and dust.

That cometlike appearance is more like what astronomers expected from the first interstellar object than ‘Oumuamua was, Bannister says. Comets are a type of leftover planetary building block, called planetesimals, and most planetesimals form far from their stars and are therefore icy. Most stars end up losing their small planetesimals, either while planets are forming or after the star dies. “Almost all the planetesimals that each planetary system makes end up being scattered out to go wander the galaxy,” Banister says.