Alien Asteroids Are Here. Get Used to Them.

Astronomers said Monday that they had identified another invasive asteroid.

Unlike Oumuamua, the cigar-shaped rock that caused a sensation when it cruised through the inner solar system and right back out toward interstellar space last winter, however, this asteroid has taken up permanent residence among us, according to a new study published Monday in Monthly Notices of the Royal Astronomical Society: Letters.

The asteroid, known as 2015 BZ509 — “BZ” for short — was discovered in 2014 sharing orbital space with Jupiter, making a circuit of the sun about every 11.6 years. But it goes around the sun in the opposite direction of Jupiter and the other planets — in a so-called retrograde orbit. The only reason it can avoid banging into Jupiter is that its orbit is egg-shaped and so the rock slips inside and then outside of the giant planet’s orbit as it goes around.

Fathi Namouni, of the Observatoire de la Côte d’Azur in France, and his colleague Helena Morais, of the Universidade Estadual Paulista in Brazil, were trying to figure out how the asteroid got that way, when they discovered that it couldn’t have gotten to where it is, at least according to the history of the solar system as we understand it. BZ had to have been an outsider from the start.

Their curiosity was piqued, Namouni said in an email, by the further discovery last year that the planet and asteroid are locked together in an orbital and gravitational resonance: The asteroid completes an orbit in the same time it takes Jupiter, and so the pair would regularly tug on each other. That meant the asteroid’s orbit should be stable.

“But at the beginning of this investigation, we did not suspect BZ to be of interstellar origin,” he explained. “We developed a new method that allows us to follow the asteroid back in time to see which part of the solar system it came from.”

But as far back as they could trace the jigsaw parts of the solar system — rocks and planets swirling and mixing — all the way to its birth, 4.5 billion years ago, BZ kept going around the same wrong way. Which meant the asteroid could not have been part of the swirling cloud of dust and gas from which the sun and planets originally condensed so long ago.

In other words, it is an immigrant. The results, they say, suggest that there are plenty more “extrasolar” immigrants out there, most likely in orbits that take them over the poles of the sun. Theories of the formation of the solar system, Namouni and Morais write, may need to be revised to account for “interstellar contamination.”