THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
(207) 780-4249 usm.maine.edu/planet
43.6667° N 70.2667° W
Founded January 1970
2022-2023: II
Sunrise: 6:03 a.m.
Sunset: 7:20 p.m.
Civil twilight ends: 7:50 p.m.
Sun's host constellation: Leo the Lion
Moon phase: Waxing crescent (12% illuminated)
Julian date: 2459820.16
“There’s as many atoms in a single molecule of your DNA as there are stars
in the typical galaxy. We are, each of us, a little universe.”
-Neil deGrasse
Tyson
THE DAILY ASTRONOMER
Tuesday, August 30, 2022 Rogue Swarm
So, here we are together: standing on a frozen lake at 1:30 a.m. about a
week before the winter solstice. A perfectly pristine planetarium sky:
innumerable stars adorning the unbounded black dome extending in all
directions toward horizons that blend seamlessly with the distant landforms
from which not even the slightest bump protrudes. Of course, this firmament
looms much larger than that which hangs above any planetarium, particularly
our own. We require such a broad view in order to address today's topic.
Before we proceed, however, we advise you to envelop yourself in your
warmest garments. Having coffee, tea or any other hot beverage nearby
wouldn't come amiss, either.*
Throughout the preceding centuries, humans often wondered while observing
these incandescent light points if any of them harbored planets. Well,
these contemplations truly began once humanity realized that our Sun,
though splendidly radiant and quite large from our perspective, is just the
closest star. Within the last thirty years, astronomers have found more
than 5,000 planets around other stars. Based on these findings, most have
concluded that almost all stars must have at least one planet, as planet
formation is a natural consequence of star formation. When regarding the
stars we can now be confident that they all cast a portion of their light
onto their own attendant worlds. According to the recent estimations, the
number of planets within the Milky Way might be two to four times the
stellar population.
However, not all of those planets are gravitationally bound to stars. In
fact, researchers at the University of Leiden in the Netherlands recently
developed a simulation suggesting that approximately fifty billion rogue
planets, those not contained within a star system, might at this very
moment be careening through the dark and vast fields of interstellar space.
Fifty billion! An immense number that is difficult to comprehend. As an
illustration, fifty billion seconds equals 1,600 years!
[image: Trapezium-Cluster.jpg]
Trapezium Cluster in M42. Image: NASA; K.L. Luhman (Harvard-Smithsonian
Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G.
Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory,
University of Arizona, Tucson, Ariz.)
While this number does astound us, one must wonder how this team could
have arrived at such a value, especially since astronomers have confirmed
the existence of only a handful of such rogue planets.** They based this
estimate on a simulation of the Trapezium Cluster, embedded with the Orion
Nebula. They assumed that five hundred stars -about one third of all the
stars-within the cluster each contained between 4 - 6 planets, a somewhat
conservative estimation. After millions of years, about 14% of those
planets had become dislodged from their parent stars due to the
gravitational interactions with other stars. No longer constrained, these
rogue planets moved along independent paths through the cluster and, in
some instances, out of it.
[image: image_4849e-simp0136.jpg]
Almost all stars, including the Sun, form inside star clusters. Globular
clusters, the largest and oldest types generally found within the spherical
halo surrounding the galactic nucleus, tend to retain nearly all of their
stars for billions of years. However, galactic or open clusters, such as
the one that once contained the Sun, dissipate generally over millions of
years. Consequently, stars and their attendant worlds likely experience
the same gravitational disturbances that are present in the Trapezium
Cluster. The predicted number of rogue planets within the galaxy is
based on the presumption that the percentage of detached planets within all
open clusters is comparable to that within the Trapezium simulation. Such
estimations will naturally vary as information pertaining to other models
becomes available. Perhaps the fifty billion value constitutes either a
wild exaggeration or a mere fraction of the total rogue planet population.
And, that is why we're here now (or, in effect, later) at the tail end of
this year's astronomical autumn on a frozen lake. For a moment, we'll
imagine standing on such a rogue world: one of billions swarming through
the galaxy. Displaced from the stellar campfire, the notion of diurnal
transition between day, night and twilight is meaningless. Rogue planets
most assuredly still rotate as a consequence of the formation process which
induces rotational motion onto the coalescing proto-planet. Yet, one
would only notice the rotation by observing the rising and setting of
constellations. Without the parent star's obscuration, all the
constellations visible at a given latitude would be seen during each
rotation. Deprived of a star''s heat, a rogue planet's surface would
become extremely cold: even colder than the surface of our ice-coated
lake. In fact, whatever atmospheric gases were present when the planet
was still part of a stellar system, would soon solidify and settle onto the
surface. The gases that might have once sustained life would all become
permafrost. Any liquid body, be it lake, river or ocean, would also
become supercooled ice: solid from base to surface.
Year after year; millennium after millennium, the rogue planet travels
through interstellar space. Provided that it doesn't wander close enough
to another star to be captured (a highly improbable occurrence), the
planet would remain as an airless, barren world thoroughly frozen, apart
from whatever heat energy contained in and around the center.***
We Earthlings are accustomed to marking time with days and seasons. For
instance, now we are preparing for autumn's imminent arrival. Along a
rogue world, there are no sunrises, no seasonal transitions, no harvests,
winters or summers.
It is highly likely that during the Sun's first billion years, when it was
still a part of an open star cluster, it drew close to other stars and
consequently propelled many of its attendant worlds out into space.
During the intervening four billion years, these planets have scattered
all around the galaxy and are at this moment careening in the deepest gulfs
of space. Before we leave this frozen lake, let's take a moment to
regard the darkness between the stars. Well concealed within this darkness
-and tens of thousands of light years away- lurk Earth's rogue siblings:
lifeless, frozen worlds ticking away countless millennia in the eternal
night.
Let's return to August now. I'm in the mood for warm sunshine.
*Why are we here? Heavens above, August hasn't even ended, yet. Well,
starting any new school year is akin to standing at the nether edge of a
large continent that we will all slowly traverse. We'll proceed through a
sultry late summer, an aestival early autumn followed by a cooler mid-fall
before encountering the late autumnal frost, festive December snows, deep
hiemal cold, the early spring thaw, tenacious March-April chill concluding
in May's warmth and verdancy. For just a bit, we decided to leap ahead and
spend a few moments in the distant winter. No worries. We'll return to our
proper time frame soon enough.
**Astronomers have compiled a list of about two dozen rogue planet
candidates, but only about four have been confirmed.
***Radioactive activity would generate this heat. There might also be
residual heat stemming from the planet's formation. However, this energy
would likely remain far away from the surface.
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