An object believed to be drifting in from the Oort Cloud could
give astronomers a rare chance to examine a leftover artefact from
the birth of the Solar System.
The Oort Cloud is the hypothesised region beyond the Kuiper Belt
containing the remnants of our Solar System's origin. As the inner
and outer planets, the dwarf planets and asteroids coalesced out of
interstellar matter, the gravitational dance span some of them into
weird eccentric orbits at angles at odds to the rest of the
gravitational plane. Oort Cloud object 2010 WG9 appears to be
one of these.
Observations made of it between December 2010 and December 2012
by the Cerro Tololo Inter-American Observatory and the European
Southern Observatory (both in Chile) have revealed "peculiar
properties" as detailed by a paper submitted to arXiv. It's a mass with a
diameter of roughly 100km, with an extremely eccentric orbit --
it's coming in at 70 degrees relative to the Earth's orbital plane -- and,
observations show, peculiar properties that could make it a relic
of the Solar System's formation.
Beyond the orbit of Neptune, certain objects never come into the
Solar System proper. Planetesimals like Pluto and Eris are in the
Kuiper Belt beyond the orbit of Neptune (although they occasionally
come inside of it), each of them too small to have formed into one
large planet. But beyond there lie objects that have extremely
Theoretically, the Oort Cloud has billions of small icy
fragments, which the larger gas giants will have gravitationally
tossed away during the Solar System's formation. They could even
have been disturbed by other stars which formed close to our own.
There they remain, until something disturbs them again and sends
them back inside -- like comets.
"Observations of 2010 WG9 present a unique opportunity to
measure the physical properties of a likely returning member of the
Oort Cloud before it has reached the inner solar system," the study
-- headed by Yale astrophysicist David Rabinowitz --
notes. "Unlike long-period and Halley-family comets, Damocloids
[asteroids with comet-like orbits], and other high-inclination
objects with perihelia interior to Saturn's orbit, 2010 WG9 has
likely never approached the Sun closer than Uranus since its
ejection into the Oort Cloud early in the Solar System's
That orbit is key to why 2010 WG9 is interesting. We can see
comets because of their tails, which are of course the melted
surface ice being lost to space. Comets -- and asteroids like the
that have come into the Solar System and been heated by the Sun
after becoming trapped by the gas giants -- no longer reflect the
early Solar System because they've lost those key surface materials
"The body has likely spent most of the age of the Solar System
at distances larger than ~1000 AU, where volatile loss rates due to
solar heating are negligible," the study notes. "In the Kuiper
Belt, on the other hand, it is expected that all but the largest
bodies have lost their surface volatiles owing to solar heating. It
is therefore possible that 2010 WG9 retains a pristine surface
composition compared to most trans-Neptunian objects and to
previously observed bodies of likely Oort Cloud origin."
Because it hasn't ever -- and won't ever -- sail closer to the
Sun than the orbit of Uranus, 2010 WG9 might offer a unique
opportunity to study the Oort Cloud without actually having to go
to the Oort Cloud. Voyager 1, our furthest-travelled probe, is
around 125 AU from the Earth after decades of travel -- objects
like 2010 WG9 can be ten times that distance from the Sun. Waiting
for one of these objects to tumble in towards us and catching it
before it melts seems a better bet than sending something a light
year away from Earth in the hope it might catch a dark,
hard-to-spot dirty block of rock and ice.
The study analysed 2010 WG9 using blue, visible, red and
infrared light wavelengths to ascertain certain properties. Its
surface showed high variations in colour while rotating, which
indicates that the surface is "dominated by carbonaceous material
with some fraction of polymerised organics". Whether this colour
variation is caused by the sublimation of a methane-rich surface,
as with Pluto, isn't confirmed yet.
It also has an extremely slow rotation for a trans-Neptunian
object -- rather than a "day" of a few hours, it's estimated to
have one in the order of 11 Earth days. The most plausible
explanation for this is that it's actually a closely linked and
tidally-locked binary system where each body keeps the other from
spinning too fast -- just like with Pluto and Charon.
We can't know until it gets closer, though. Sending a probe to
investigate 2010 WG9 isn't going to be easy either, its high
orbital approach making it difficult to envision slingshotting
something into its orbit. Remote telescopic observation with
something like the Hubble telescope is probably more viable.