The Sky Above You, December 2025

by Duncan Lunan

Although the Moon will spoil the spectacle, timings for the occultations of the bright stars are on p.53 of the December issue of Astronomy Now.  The Moon is near Uranus on the 3^rd and near Jupiter, Castor and Pollux on the night of the 7^th.   As it nears Third Quarter it will approach the ‘Sickle’ of Leo, the head of the Lion, and on the morning of the 10^th it will occult Regulus, the brightest star in Leo, disappearing behind the left-hand limb of the Moon at 7.17 a.m., as seen from Scotland, an hour before sunrise.  It will be to the right of Mercury, low in the morning sky on the 17^th, and New Moon is on December 20^th, after which the waxing crescent is right of Saturn on the 26^th.  Meanwhile the midwinter solstice is on December 21^st.

The planet Mercury rises at 6 a.m. in the first half of the month, at its furthest from the Sun on the 7^th, and is passed by the Moon on the 17^th, in the head of Scorpius and very low in the morning sky.  Nevertheless this is the best chance to see Mercury in the morning sky this year, in crescent phase on the 1^st and gibbous by the 20^th, when it will be bright but very low down.

Venus is not visible in December, in one of the few years in which it can’t be mistaken for the Star of Bethlehem or a UFO at Christmas.

Mars is still out of sight behind the Sun this month.  

Jupiter in Gemini now rises around 6 p.m., below Castor and Pollux, and is brilliant for the rest of the night, passed by the Moon on the 7^th, with chances to see the moons and their shadows getting better as the planet approaches opposition on January 10^th.

Saturn in Aquarius sets about midnight, with the Moon nearby on the 26^th.  The largest moon Titan transits the planet several times in December  (times are on p.53 of Astronomy Now), and emerges from an occultation by the planet on 16^th December at 21:56 hrs..

Uranus in Taurus, 4 degrees below the Pleiades, sets about 6 a.m., south of the Moon on December 4^th and 31^st.

Neptune in Pisces sets around 00.30 a.m., near the Moon on the 27^th, and is at its ‘stationary point’ on the 11^th, after passing opposition in September.

The Geminid meteor shower from the asteroid Phaethon peaks on the nights of the 13^th-15^th, and will be spoiled by the waning Moon after it rises at 1 a.m., 3 a.m. on the 15^th.  The Ursid meteor shower peaks on the 22^nd, when the Moon will not be a problem.  The shower is usually inconspicuous, but not always – see ‘Green Green It’s Green They Say’, ON, 21^st January 2024.

A year ago, after giving the positions of Uranus and Neptune in ‘The Sky Above You’  (ON, 1^st December 2024), I added, “One of the surprises from the Voyager 2 flyby of Uranus in 1986 and Neptune in 1989 was that neither planet had a dipole magnetic field, unlike Earth, Jupiter and Saturn.  One of the possible explanations was that Uranus had a huge liquid water ocean under its visible cloud layers, as Jupiter was believed to have  (probably frozen)  but turned out not to, in the spacecraft flybys.  (Byron Preiss, ed., The Planets, Vintage, 1985;  Pioneering the Space Frontier, The Report of the National Commission on Space, Bantam, 1986…

“Burkhard Militzer, a planetary scientist at the University of California, Berkeley, has spent 10 years experimenting with mixtures of carbon, oxygen, nitrogen and hydrogen, in the proportions found in the early Solar System, and found that in certain conditions of high temperature and pressure, they can form separate layers, with water above, and heavier nitrogen and hydrocarbons below, preventing interior convection and the formation of a dipole magnetic field.  So far, these simulations apply only to the interior of Uranus, and more information on the composition of Neptune is needed to test the process there.  The resulting model predicts that the gaseous outer layer of Uranus is 3000 miles thick, above a liquid water layer 5000 miles deep, and a similar hydrocarbon layer below that, overlying a rock and metal core the size of Mercury, which was detected in the Voyager 2 flyby… In a water ocean of that depth, water would be compressed into forms unknown on Earth  (Fig. 1), and its properties in those conditions are largely unknown, because they can be generated in the laboratory only in minute quantities.”  

It’s possible that these exist within some of the exoplanets which have been discovered, either on ‘super-Earths’ much larger than ours  (Figs. 2 & 3), or in ‘water worlds’ whose low density suggests that they are almost entirely composed of it  (Fig. 4). 

In September, the whole concept of ‘Hycean worlds’ was challenged by researchers at Zurich, Heidelberg and Los Angeles, arguing that such ‘sub-Neptunes’ would go through a prolonged phase with surface oceans not of water but of magma  (Fig. 5), which would react with water vapour in the atmosphere and remove it.  (Stephane Baum, Robert Egan, ETH Zurich, ‘Sub-Neptune exoplanets unlikely to be water-rich ocean worlds, researchers say’, Phys.org, September 18^th, 2025.) 

The conclusion was very quickly challenged, with other findings that solid crusts on sub-Neptunes were entirely possible  (Figs. 6 & 7:  Evan Gough, ‘It Looks Like All Mini-Neptunes Aren’t Magma Oceans After All’, Universe Today, 6^th November 2025).

The study concentrated on planets orbiting dwarf stars like the Sun, and it’s not clear whether the magma ocean would still form in other circumstances.  The idea that there could be ‘stray planets’ orbiting freely in interstellar space was first advocated by Harlow Shapley. 

Some, he suggested, might be large enough to have molten interiors and solid crusts  (Fig. 8), perhaps even supporting life  (Fig. 9).  (‘Crusted Stars and Self-heating Planets’, Matematica y Fisica Teorica Serie A 14, 1962).  The first possible example of a stray planet was found in 2016;  possible ones were indicated by gravitational lensing within the Kepler space observatory’s study field  (Fig. 10);  more have been found by the PanSTAARS observatory on Hawaii, and the one in Fig. 11 featured in NASA’s series of retro space tourism posters  (Fig. 12). 

In 2023 Wikipedia listed 33 possible ones, 4 of them doubtful but quite a few confirmed, but the accompanying article suggests that there could be far more, possibly hundreds of thousand times as many in the Galaxy as there are stars, and many of them in the range between Jupiter and the smallest brown dwarf and red dwarf stars.

Shapley thought that planet-sized bodies could form in interstellar space by the same processes which formed stars and solar systems.  Other origins for stray planets have been suggested since;  expulsion during turbulent phases in the formation of planetary systems, or at the other end of stellar evolution, expulsion from around giant stars when they explode as supernovae  (Fig. 13).  Those giant planets could have moons, perhaps as large as Earth, with deep oceans formed by tidal heating, like Europa, Ganymede and possibly Io and Callisto in the Jupiter system  (Fig. 14).  A new study suggests that moons like those could escape with their host planets, to become strays with habitable interiors.  (Evan Gough, ‘Never Mind Rogue Planets. Their Rogue Moons Could Support Life’, Universe Today, November 10^th, 2025.). 

On November 16^th I took part in an interesting online discussion about this idea, which centred on the question, could such planets of such worlds sustain life, even if they held liquid water?  The surfaces wouldn’t be completely dark, unlike the clouded one of Adrian Tchaikovsky’s novel Shroud, which I hope to  review here shortly.  Humans can see by starlight, once fully dark-adapted, and Freeman Dyson calculated that if genetically modified trees on comets grew to a height of 10 miles, their foliage would capture enough of it to keep them alive.  (See ‘Comets and Spacecraft’. ON. January 2^nd, 2022.)  But although the surface of a planet-sized moon would gather a lot of energy from starlight, it would be spread over a very large area.  Orbiting a gas giant planet like Jupiter it would get some heat from the planet’s continuing contraction, at least on the side facing it.  But we’re still talking about very little overall, relative to the sunlight the moon previously gathered.  Admittedly there are ‘extremophile’ bacteria on Earth which have adapted to harness very low light levels, for instance in the mud at the bottom of ponds;  but the point is that they have evolved to do that, not had the ability from the outset.

On Europa, it’s thought that there’s a constant infall of sulphur compounds from the volcanoes on Io  (Fig. 14 background), and that subduction round the plumes may conduct those into the depths below.  One might imagine living organisms in Europa’s ice crust migrating along the cracks in it, as they become opaque to sunlight through time and  become translucent to sunlight elsewhere.  Where two or more such moons orbited a ‘stray’ gas giant, the chemical input would continue, but would there be enough starlight to harness it? 

Another possibility, even a likelihood, is that on the floor of Europa’s tidally heated ocean there may well be volcanic vents, supporting living organisms by chemical energy alone, like the ‘black smokers’ on Earth  (see ‘Going under the Ice’, ON, 21^st September 2025), perhaps coming into being there and evolving to spread upwards as it may have done on Earth.  The problem there is that Europa’s ocean may be 200 miles deep, and at that depth, pressures and temperatures may form allotropic forms of ice as in Fig.1, and if heavier than water ice as we know it, they may seal off the vents and prevent the origin of life there, much less hypothetical forms of its evolution.  

But the possibility which seems to have escaped consideration is that those moons might be as large as Earth, as I mentioned above.  If they were, they would be internally heated by radioactive decay, and might not lose their water due to interaction with a long-lasting magma ocean, as above.  The Moon formed one in its early history, during the Final Bombardment 4.4 to 3.9 billion years ago, but it’s by no means clear that the Earth did – evidence from zircon crystals 4.2 billion years old shows that there was open water at least on parts of Earth when the bombardment was at its height.  Cooling faster, stable oceans might form on earthlike moons of giant planets even by the time of the star’s supernova a billion years or so later, and a separation of continents rising above sea level might occur within another billion years due to internal processes, as seems to have been the case on Earth  (Anon, ‘When Earth’s First Continents Rose above Its Oceans’, EarthSky, 19^th July 2015).  So we could imagine an earth-sized exomoon of a stray planet, with relatively shallow oceans, and chemical input to sustain evolving life due to glacial action and longer geological processes  (Fig. 15).  So I’m not giving up on Shapley’s original concept, just yet.  There may be places out there, between the stars, where much stranger things go on than we suppose.

You can download a copy of the star map for December here:

Duncan Lunan’s recent books are available through Amazon;  details are on Duncan’s website, www.duncanlunan.com.

Space Notes, December 2025

See more