Jupiter's asteroid BZ is an interstellar visitor orbiting near the planet
since 4.5 billion years.
BZ’s orbit is retrograde, unlike most of the bodies in our solar system. It is
not orbiting Jupiter as a moon would, but only crossing Jupiter’s orbit while
orbiting the sun. Bz’s orbit also has the same duration as that of Jupiter (one
year of Jupiter). THis is this retrograde orbit that reveals it has come from outside our solar
system at the time it was forming , that is, 4.5 billion years ago.
Neptune's
moon Triton (below) also has a retrograde orbit; it may have been captured from the
Kuiper belt.
Neptune's
moon Triton (credit BBC)
'Permanent' interstellar visitor found (21 May 2018)
An asteroid in Jupiter's orbit may have come from outside our
Solar System, according to a new study. Unlike
'Oumuamua, the interstellar object which
briefly visited the Solar System earlier this year, 2015 BZ509 (affectionately
known as BZ) seems to have been here for 4.5 billion years.
This
makes it the first known interstellar asteroid to have taken up residence
orbiting the Sun. It
is not yet known where the object came from."That's
what we need to figure out next," laughs Dr
Fathi Namouni from the Universite Cote d'Azur,
one of the study's authors.
"Because
'Oumuamua was just passing by… it's not that difficult to go back and pinpoint
where it came from," he told BBC News. "BZ reached the Solar System
when it was forming, when the planets themselves were not exactly where they
are now. So it's a little more tricky to figure out where it came from."
Prof
Sara Russell from the Natural History Museum
points out that the Solar System is actually littered with small bodies.
"There
used to be this idea that the asteroids were stuck in a belt between Mars and
Jupiter, and the Kuiper Belt was full of icy objects. Now we know that's
completely untrue, everything's kind of jumbled up," she explains.
BZ
itself was discovered in a wide-sky survey of such objects in 2015, but it took
some time for its unusual behaviour to be understood.
Going against the flow
BZ
is in a retrograde orbit, moving around the Sun in the opposite direction to
the eight planets, and the majority of other objects in the Solar System.
This
immediately makes it stick out, as almost everything which formed from the disk
of gas and debris around the Sun follows the direction of the star's rotation.While
this backwards habit doesn't necessarily mean that a body has been captured
into its current orbit, it is certainly a strong hint.
Thinking
that BZ might have been drawn in from another group of small bodies in the
Solar System, Namouni and colleague Helena Morais from the Universidade Estadual
Paulista in Brazil set out to model the object's orbital history.
Long time neighbour
"We
had this simulation which uses intense computing... to actually trace back in
time to where this asteroid was when the planets finished forming," Dr Namouni
explains.
The
results suggested that BZ's only possible origin was outside our Solar System. "Honestly
we didn't have any idea what we were going to find. The last thing we
expected... was that BZ has been in its current position most of the
time."
Previously,
it was thought that life among the giant planets was a turbulent one for small
objects. The gravitational pull of the larger bodies would make asteroid
behaviour unpredictable over long time spans, with their orbits changing after
mere millions of years.
BZ's
orbital longevity may be a result of its pattern - which matches, or is in
"resonance" with Jupiter's.
One
year in the asteroid's life lasts as long as a year on Jupiter, even though
they travel in opposite directions around the Sun.
Prof Russell, who was not involved in the study,
felt that its "implications were amazing." "From a sense of our understanding of how the
Solar System formed, it's quite plausible. There could have been some crossover
of young stars' material," she told BBC News.
There is much left to learn about BZ's origin and
composition, but it may also have something to tell us about other planets in
the early history of the Solar System.
Working back through the asteroid's entry into
Jupiter's orbit may help to determine what the giant planet's motion was during
its early history - a key question for models of Solar System formation.
"We can work back and constrain the position
of the different planets. Now that we have this object we can hope to
discriminate between the various models... but it's going to be
difficult," commented Dr Namouni.
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