Just months after the discovery of our solar system’s first known interstellar visitor, it turns out there’s another alien asteroid residing in plain view.
Scientists reported that the interstellar resident is an asteroid sharing Jupiter’s orbit, but circling in the opposite direction.
They say the asteroid, known as 2015 BZ509, has been in a peculiar backward orbit around the sun ever since getting sucked into our solar system.
About two miles across, it joined our cosmic club in the first moments after our solar system was formed 4.5 billion years ago.
French and Brazilian researchers based their findings on extensive computer simulations showing BZ always has orbited around the sun in reverse and thus hearkens back to the beginning of our solar system.
The finding, published in the Royal Astronomical Society journal, comes months after the discovery of our first known interstellar visitor, a cigar-shaped asteroid that zoomed by last autumn.
That passer-by was named Oumuamua, Hawaiian for “messenger from afar arriving first”, or “scout”.
If we may have asteroids that pass by, then we should also expect asteroids that come to stay,Helena Morais
Lead author Fathi Namouni, of the University of Cote d’Azur in Nice, said: “Oumuamua is of interstellar origin but it is also only a tourist passing by our solar system.
“BZ is not. It is a bona fide immigrant and the notion of immigration is a hot topic nowadays all over the world.”
Mr Namouni said stars were closer back when our solar system was forming, and asteroids were zipping around between star systems. It is extremely unlikely – “practically zero” – that BZ came from the same star system as Oumuamua, he noted.
He expects there are lots more interstellar immigrants in our backyard.
“There is no reason why there shouldn’t be more masquerading as solar system asteroids like BZ did so far,” he wrote.
Mr Namoui added that the area just beyond Neptune, the farthest planet in our solar system, might be teeming with extrasolar asteroids – or exo asteroids – like BZ.
Co-author Helena Morais of Sao Paulo State University in Brazil said she was surprised by the finding, but noted “that’s part of the fun” of science.
“If we may have asteroids that pass by, then we should also expect asteroids that come to stay,” she said.
By identifying more immigrant asteroids, Mr Namouni said, scientists can determine their composition.
If BZ contains water, for example, researchers can compare it with Earth’s water and, perhaps, better understand how water originated here on our planet.