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| This artist's impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on Oct. 19, 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO's Very Large Telescope in Chile and other observatories around the world show that it was travelling through space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red highly-elongated metallic or rocky object, about 400 metres long, and is unlike anything normally found in the Solar System. Credit: ESO/M. Kornmesser |
For the first time ever astronomers have studied an asteroid that has entered the Solar System from interstellar space. Observations from ESO's Very Large Telescope in Chile and other observatories around the world show that this unique object was traveling through space for millions of years before its chance encounter with our star system.
It appears to be a dark,
reddish, highly-elongated rocky or high-metal-content object. The new results
appear in the journal Nature on 20 November 2017.
On Oct. 19, 2017, the
Pan-STARRS 1 telescope in Hawai'i picked up a faint point of light moving
across the sky. It initially looked like a typical fast-moving small asteroid,
but additional observations over the next couple of days allowed its orbit to
be computed fairly accurately.
The orbit calculations revealed beyond any doubt
that this body did not originate from inside the Solar System, like all other
asteroids or comets ever observed, but instead had come from interstellar
space. Although originally classified as a comet, observations from ESO and
elsewhere revealed no signs of cometary activity after it passed closest to the
Sun in September 2017. The object was reclassified as an interstellar asteroid
and named 1I/2017 U1 ('Oumuamua).
"We are continuing to observe this unique
object," concludes Olivier Hainaut, "and we hope to more accurately
pin down where it came from and where it is going next on its tour of the
galaxy.
"We had to act
quickly," explains team member Olivier Hainaut from ESO in Garching,
Germany. "'Oumuamua had already passed its closest point to the Sun and
was heading back into interstellar space."
ESO's Very Large Telescope
was immediately called into action to measure the object's orbit, brightness
and colour more accurately than smaller telescopes could achieve. Speed was
vital as 'Oumuamua was rapidly fading as it headed away from the Sun and past
the Earth's orbit, on its way out of the Solar System. There were more
surprises to come.
Combining the images from
the FORS instrument on the VLT using four different filters with those of other
large telescopes, the team of astronomers led by Karen Meech (Institute for
Astronomy, Hawai`i, USA) found that 'Oumuamua varies dramatically in brightness
by a factor of ten as it spins on its axis every 7.3 hours.
Karen Meech explains the
significance: "This unusually large variation in brightness means that the
object is highly elongated: about ten times as long as it is wide, with a
complex, convoluted shape. We also found that it has a dark red colour, similar
to objects in the outer Solar System, and confirmed that it is completely
inert, without the faintest hint of dust around it."
These properties suggest
that `Oumuamua is dense, possibly rocky or with high metal content, lacks
significant amounts of water or ice, and that its surface is now dark and
reddened due to the effects of irradiation from cosmic rays over millions of
years. It is estimated to be at least 400 metres long.
Preliminary
orbital calculations suggested that the object had come from the approximate
direction of the bright star Vega, in the northern constellation of Lyra.
However, even travelling at
a breakneck speed of about 95 000 kilometres/hour, it took so long for the
interstellar object to make the journey to our Solar System that Vega was not
near that position when the asteroid was there about 300 000 years ago.
'Oumuamua may well have been wandering through the Milky Way, unattached to any
star system, for hundreds of millions of years before its chance encounter with
the Solar System.
Astronomers estimate that an
interstellar asteroid similar to 'Oumuamua passes through the inner Solar
System about once per year, but they are faint and hard to spot so have been
missed until now. It is only recently that survey telescopes, such as
Pan-STARRS, are powerful enough to have a chance to discover them.
And now that we have found
the first interstellar rock, we are getting ready for the next ones!"
Via Phys.org