Let's maneuver ourselves through this pesky little computer screen and
find, on the observe side of the electron lattice, a typical night sky.
We'll position ourselves along some remote point, be it promontory or
precipice is your choice. Our focus is above: the star adorned
tapestry/firmament (you may also choose whichever worn out poetic image
you prefer.) We wonder, as sky admirers often do, when will we humans will
travel amongst the bright cool points dotting our sky. The notion of
such intergalactic meanderings have captivated us ever since we realized
our solar system wasn't enclosed in an impermeable crystalline sphere.
Our impulse to explore other star systems and mingle with alien
creatures has been somewhat gratified by the myriad science fiction
adventures enabling us to live vicariously through characters who aren't
as Earth-tethered as we are.
Nevertheless, we wonder: when we will be there? What will we find? How
much do we have to explore? Well, we can at least address the final
question: for we know that our galaxy spans 100,000 light years,
contains more than 250 billion stars, and, to give you a scale model, if
the Milky Way Galaxy were the size of North America, our entire solar
system would fit inside a coffee cup.*
And, these stars are not exactly traveling together like Tokyo
commuters: a great deal of space separates "adjacent stars." Our
Sun's closest neighbor, Proxima Centauri, is 4. 2 light years** away.
This distance equals 5.8 trillion miles (9.3 trillion kilometers.) To
employ another scale model, if the Sun were an orange in Portland,
Maine, Proxima Centauri would be a small apple in Florida. Viewing
the matter three-dimensionally, we see that the Sun has 309 cubic light
years to itself. Obviously, when we start our trudge through the
galaxy, we'll have to employ technology not yet developed. After all,
Voyager I, the most distant spacecraft we've ever launched, is
more than ten billion miles from our shore, a distance equal to 0.19% of
a light year. By stellar standards, it just lumbered out onto the
porch and it's been traveling for more than three decades.
The question of "when will we be there?" is impossible to answer. We
have a long way to go as a species before we'll become star-faring.
Then again, in an exponentially advancing world such as ours, such time
frames might be shorter than contemporary reckoning would suggest.
(Who would have thought that merely 66 years would have separated the
Wright brothers first flight and the Apollo 11 Moon landing?)
The other question, "What will we find?" is also unknown. Astronomers
have detected more than two thousand exo-planets (planets around other star
systems) and will likely discover thousands more. Of course, merely
finding these planets is not the same as exploring them. We are still
in the process of learning about our own solar system worlds, including
Earth, and we grew up here. What we'll actually see in other solar
systems -smoldering caldera fields; iridescent fern jungles, Daleks- is
anybody's guess. We have to send humans, or at least probes, to these
other star systems to rummage around and, perhaps, convey samples back
to us just as the Apollo astronauts delivered lunar rocks to Earth more
than forty years ago. One could imagine that centuries will elapse
before we'll ever see such samples of other star systems. The
Smithsonian needn't allot space for such a sample anytime soon. And,
moreover, we will never see it.
Of course, during the time you've been outside admiring the heavens, you
might have already seen such a sample: only, you didn't notice.
While we've watched the sky, ruminated about humanity's future and
lamented the realization that we'd never see pieces of other star
systems, we've periodically noticed meteors flashing by here and there.
Meteor sightings are not restricted to meteor showers, but are visible
on any given night. We call these isolated light flicks "sporadic
meteors." It is possible that some of those could have originated from
another star system. So, even though it's unlikely that any of us will
venture to another solar system, some samples of other star systems
might well have come to us. They might infiltrate our atmosphere to
either burn up in the sky or settle down on our ground as "meteorites."
One might wonder: how is it possible that pieces of such remote star
systems could be sitting on our ground at this very moment? Such a
notion seems a bit far fetched, considering the vast distances
separating star systems. Understanding how these samples arrived here
requires us to venture out the solar system's outermost region: a
rarefied realm called "The Oort Cloud." This "cloud" is a
hypothesized** spherical region of cometary nuclei consisting of two
"shells," an inner shell extending between 2,500 - 20,000 Au and an
outer shell encompassing a region between 20,000 -50,000 AU from the
Sun. (An "AU," or Astronomical Unit, equals the average Earth-Sun
distance of 93 million or 150 million kilometers.) While the inner
shell, called the "Hills Cloud," is close enough to Sol to keep its
particles within the solar system, the outer shell nuclei are more
tenuously bound. (50,000 AU is almost equal to one light year.)
Recent NASA research suggests that these cloud nuclei, which number in
the billions, formed within the inner solar system. Gravitational
interactions with the outer planets then propelled them far into the
great black yonder they now occupy. Also, we know that the Sun did not
form alone, but in a cluster: so as it developed,the Sun interacted
with other forming stars. As these stars were so close, they likely
exchanged material with one another. So, even in its infancy, the Sun
received extra-solar material as well as imparting its own matter onto
proximate systems. Therefore, many Oort cloud comets originated
around other stars,
Also, as the outer Oort Cloud particles are still so far from the Sun,
nearby stars can dislodge them from their orbits, sending them toward a
long interstellar journey into another star system. Again, as we assume
that other stars would have spherical comet clouds similar to our own,
the Sun could also ensnare comets from close stars.
These cometary nuclei could then move toward the Sun if other bodies,
such as large planets, disturb them. When they venture close to the
Sun, they develop two tails: an ion tail consisting of charged particles
repelled by the solar wind and a curving dust tail composed of dust
dislodged from the sublimating*** ices. We term these particles
suspended in space "meteoroids." Meteoroids produce meteors (light
streaks) when they descend through the atmosphere. So, some of those
meteors might be created by meteoroids detached from comets that
originated in another star system. We cannot differentiate
between home grown comets and exo-planets (even Kuiper Belt comets might
have once been in the Oort Cloud).
It is likely that you've seen samples of other star systems if you've
spent any time watching meteors. So, the assumptions we introduced in
the preamble about never seeing such things in our lifetimes turned out
to be so much flotsam.
*Just to maintain the scale model integrity: here, we're referring to a
regular coffee cup: not the bath pools with handles that you'll be
offered in coffee shops so that the addiction will really stick after
just the first cup.
**Dutch astronomer Jan Oort (1900-1992) proposed that long period comets
(those with periods greater than 125 years) originate from a spherical
distribution of cometary nuclei far beyond the Kuiper Belt, the
provenance region of the short period comets. Dr. Oort hypothesized
the existence of this cloud as he noticed that the longer periods comets
were isotropic, i.e. they entered the inner solar system from any
direction, as opposed to the short period comets that seemed restricted
to the ecliptic: the disc-thick plane along which the planets orbit.
***Sublimating: the process by which a solid directly transforms into a gas without first liquefying.