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!

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.

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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