THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N, 70.2667° W
Founded January 1970
"It is then unnecessary to investigate whether there be beyond
the
heaven Space, Void or Time. For there is a single general space, a
single vast immensity which we may freely call Void; in it are
innumerable globes like this one on which we live and grow. This space
we declare to be infinite, since neither reason, convenience,
possibility, sense-perception nor nature assign to it a limit. In it are
an infinity of worlds of the same kind as our own."
-Giordano Bruno (1548-1600)
THE DAILY ASTRONOMER
Thursday, March 24, 2016
If You Love Astronomy II: Samples of Other Star Systems
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.