In the previous tutorial we talked about dwarf planets, which are rather large objects that
don’t quite fulfill the criteria for planetary status.
But while dwarf planets like Pluto and Eris are large enough to crush themselves into
spheres by their own gravity, even smaller objects are unable to do this, which is why
they have irregular shapes.
Examples of such objects are asteroids and comets, and we discussed these earlier in
the series as well.
These kinds of objects can be categorized by how far away from the sun they form and
orbit.
Asteroids are very rocky and orbit relatively close to the sun, like the million or so asteroids
that comprise the asteroid belt between Mars and Jupiter.
They formed along with the rest of the solar system, and tend to have nearly circular orbits
just like the planets.
Comets, on the other hand, are made of lots of ice and dust, as well as rock.
This is because although they formed along with the rest of the solar system, just like
asteroids, comets formed far, far away from the sun, in the Kuiper belt, the scattered
disc, and beyond, perhaps as far as the still hypothetical Oort cloud.
Unlike asteroids, comets have highly elliptical orbits, having been jostled from their positions
by gravitational influence, which sends them hurtling towards the inner solar system, only
to slingshot around the sun and zoom back out to distances of tens of thousands of astronomical
units from the sun.
We’ve been learning quite a bit more about comets in recent years, owing largely to the
European Space Agency’s Rosetta mission, which successfully landed a spacecraft on
comet 67p in 2014.
This fascinating object consists of two lobes each a few kilometers across, connected by
a narrow neck, the result of a gentle collision some time in their past.
As fascinating as comet 67p may be, it is still an object from our own solar system.
However in 2017, an interstellar object passing through our solar system was detected for
the first time.
This object, named ‘Oumuamua, was spotted moving away from the sun, only a fifth of
an astronomical unit away from Earth.
It was determined to be rather small, and possessing a peculiar shape, with a length
approaching a kilometer, while being only about 100 meters thick.
This elongated shape and strange trajectory was taken at first to be evidence that the
object might even be an alien spacecraft.
But further observation showed that this was a natural object, with a rocky body quite
similar to any other comet.
However, ‘Oumuamua did not have a coma, like the comets in our solar system, because
it is not of our solar system.
Analysis of the object was performed, focusing on its tumbling motion, in stark contrast
with the smoothly rotating objects we are used to observing.
In addition, its speed and trajectory are quite telling.
It’s orbital eccentricity in approaching from above the ecliptic was the highest of
any observed object, meaning that in slingshotting around the sun roughly near Mercury’s orbit,
it gained enough speed to exceed the escape velocity of the sun.
So after having rounded the sun, ‘Oumuamua was on its way out of the solar system, back
into interstellar space.
These observations are what led to the conclusion that ‘Oumuamua is a rogue comet, also referred
to as an exocomet.
This is an object that originated in some other plantary system, and was jostled from
that system, presumably due to gravitational perturbations, and was sent through interstellar
space for some unknown duration of time, only to encounter our own solar system just a few
years ago.
Which system did ‘Oumuamua come from?
We aren’t sure, and although analysis has identified several stars that it may have
passed by in the past few million years, given the incredible amount of time that is required
for such an object to get from one star to another, it seems like it could have come
from almost any system in the Milky Way, having traveled for millions or even billions of
years before this brief encounter.
Interestingly enough, as much as it would seem like ‘Oumuamua was a once in a millennium
find, another interstellar object has already been discovered.
An exocomet named Borisov was detected in 2019, arriving from the direction of Cassiopeia
in a hyperbolic trajectory.
Who knows what other such objects we will find in the years to come?
Perhaps we will one day even be able to spot one on its way in and send a craft to intercept
it, landing on it the way we were able to do with comet 67p.
It would be the first touch down on an object of extrasolar origin, and there’s no telling
what it would help us understand about
the universe.