The
First Interstellar Object Ever Observed in our Solar System
Updated
2017 December 1
On
October 19th the Pan-STARRS survey made a remarkable discovery. It
was a faint (magnitude 20) object that at first appeared little
different from the haul of asteroids that were discovered that
night. But as more observations came in, it became apparent that
the orbit was very eccentric (elongated). Assuming that it was a
comet, the Minor Planet Center placed it on the Possible Comet
Conformation Page to attract more observations. As the
observations came in, the true nature of the orbit began to be
revealed. This object appeared to be from outside of our solar
system, which made it unlike any other ever observed.
So
how do we know it came from outside of our solar system? Imagine
two identical hills. A track runs down from the top of one hill,
and then back up the other. If you start a ball rolling down one
hill it will pick up speed until it reaches the bottom, then as it
climbs the other hill it will begin to slow. If there is no friction,
the ball will climb back up to the same height that it started
from. Think of the top of the hills as the Oort cloud that
surrounds our Solar System. This is a cloud of comets that slowly
orbit far from the sun. Every once in a while the orbit of one of
these comets is disturbed, and it begins to fall toward the sun,
like the ball rolling down the hill. The height of the hill
determines how much potential energy the ball has, which
determines how fast it falls, and ultimately how far it will roll
to the top of the other hill. In the same way, the potential
energy of the comet coming from the Oort cloud is determined by
the distance that the comet falls from. Assuming the comet does
not hit the sun, or pass close to Jupiter, it will swing around
the sun and head back out toward the Oort cloud, slowing until it
reaches the distance from which it started. This is how we know
the Oort cloud exists, because comets regularly fall toward the
sun, but none has ever fallen with enough speed to continue beyond
the Oort cloud and escape the solar system. That is, until the
recent Pan-STARRS discovery.
Analysis
of the orbit reveals that the object entered our solar system with
a velocity of 26 km/sec. This is fast enough that it can only have
come from beyond, and it will keep going right out of our solar
system, never to return.
Several
amateurs contributed to the astrometry of the object, helping to
better define its orbit. Observations were made using iTelescopes
(iTelescope.com) from their
Siding Spring Australia and Mayhill New Mexico sites, and by David Rankin (Big
Water Observatory), the Pistoia Mountains Astronomical
Observatory, and the Great
Shefford Observatory. As of October 30, it was nearing 23rd
magnitude.
NASA/JPL
The
Minor Planet Center assigned the provisional cometary designation
of C/2017 U1 (PANSTARRS) to the discovery on October 25. But
later observations revealed no cometary coma or tail. The rule is
that an object is considered to be an asteroid unless a coma or
tail is observed, so the designation was changed to A/2017 U1
and thus considered a minor planet rather than a comet.
This created a naming problem. If the current minor planet rules are applied to this object, it
is unlikely to be given a formal name because it will not be
observed at more than one opposition. But as a unique and
interesting object, people naturally wish to name it. So on
October 6, the Minor Planet Center announced that the IAU had
created a new naming convention for interstellar objects. As a
result it is now designated 1I/2017 U1 ('Oumuamua). The I
is for Interstellar and 1 denotes the first interstellar object
(1I). The name 'Oumuamua was chosen by the Pan-STARRS team.
According to the announcement in MPEC
2017-V17, it is of "Hawaiian
origin and reflects the way this object is like a scout or messenger sent from the distant past to reach out to us."
It is apparently pronounced "Oh-owe moo-ah moo-ah."
The
object passed on the other side of the sun from us, too faint to
be detected. But after it swung around the sun (passing within
0.25 AU), it passed close enough to the earth (0.16 AU) to be
detected. We were lucky that this happened when the object would
appear opposite the sun and with no interference from the moon or
we would have missed it. At this point it was already heading out of the solar
system and fading fast.
Astronomers
around the world began scrambling for observations. On October 25,
spectra were obtained (below) by Alan Fitzsimmons with the William
Herschel 4.2m Telescope that confirmed the earlier lower
resolution spectrum obtained by Joseph Masiero with the 200-inch
at Palomar.
Science
Results So Far
The
spectrum is featureless, rising on the red end. This is consistent
with the spectra of distant Kuiper Belt objects in our own solar
system, such as Pluto. This suggest an origin far from its parent
star.
 On
the other hand, A/2017 U1 passed relatively close to the sun, yet
showed no sign of a coma or tail. This means that it either formed
close enough to a star that the volatiles, such as water, were not
present, or passed repeatedly near to a star to lose its volatiles.
Together, these two observations suggest that this object has had
an interesting history.
Eric
Mamajek of JPL has submitted a paper
regarding the kinematics of A/2017 U1. He calculates that it is not
likely to have originated from the Oort cloud associated with the
nearby Alpha Cen star system or any of the other dozen close
stars. He argues that its velocity is more consistent with a
distant extrasolar origin beyond the nearest stars.
A
paper submitted by Knight et al. presents a partial light curve
for 'Oumuamua that sets limits on its size and shape. They confirm
that the object has no coma and determine that the rotational
period is more than 3 hr, and likely longer than 5 hr. Their
analysis infers a diameter of 90 - 180 km, and a lower limit to
its elongation of 3:1. They find that its properties are typical
of minor planets from our solar system.
Another
paper, published in Nature by by Meech et al. suggested an
elongation of 10:1, which inferred a rigid metallic body in order
to remain intact. These finding were released through ESO and NASA
press release channels with a questionable artist rendering and
much hyperbole. I and others question their result because it is
not consistent with what others have found and did not take into
account light curve data other than their own. Sadly, their artist
rendering has become the most common image of the object.
Fraser
et al. recently submitted a paper that interprets the light curve
in terms of tumbling, and looked at timescales for tumbling in the
absence of the forces normally felt in a star system that are due
to radiation from the star. They note that "No single rotation period can explain the
exhibited brightness variations. Rather, 1I/‘Oumuamua appears to be in an excited rotational
state undergoing Non-Principal Axis (NPA) rotation, or tumbling."
They find an elongation of 5:1. They also find that the time to
dampen the tumbling is at least a billion years, which constrains
the age of the object.
Other
papers have speculated about the star systems that the object
could have originated from (UCAC4 535-0
65571 or GJ 876), assuming that it originated nearby. Interesting,
buy it is important to keep in mind that this is an assumption
with no real basis.
Yet
another paper speculated about the possibility that the object was
composed of macroscopic dark matter. If so, they predict small but
measurable deviations in the orbits of Earth, Moon, and Mercury.
They intend to look for such an effect. Of course, there is
currently no reason to suspect it is anything other than a
rock.
Greg Crinklaw —
Astronomer and Developer of
SkyTools
Want
to be among the first to have SkyTools 4? For a limited time we
are offering a free copy of SkyTools 4 to everyone who purchases SkyTools
3.
SkyTools
3, because the astronomy matters.
|
