Rogue planets

 Saw a guy arguing that so-called Super Earths or mini-Neptunes floating in the galaxy free of a star (expelled during system formation) could be habitable from interior heat and heavy cloud cover for periods of time to ten billion years.  What about Neptune itself?

That argument is actually based on a very interesting line of planetary science: rogue planets (planets wandering through interstellar space) and whether they could retain habitable environments without a star. Neptune is a very useful comparison — and it shows both the possibilities and the limits.

Neptune itself is not a good candidate for surface habitability, but it demonstrates why the idea is not completely crazy.

A few relevant numbers:

    Neptune receives about 1/900th of the sunlight Earth gets.

    Its upper atmosphere is around −200 °C.

    Yet Neptune radiates about 2.6 times more energy than it receives from the Sun. The extra heat comes from its interior — leftover formation heat plus slow gravitational contraction.

That internal heat is real, but it is not enough to make Neptune "Earth-like." The reason is that Neptune is mostly hydrogen, helium, water, ammonia, and methane under enormous pressure. There is no solid surface you could stand on; you descend into progressively denser fluid.

The rogue mini-Neptune idea is more interesting because of thick atmospheres. A planet perhaps 2–10 Earth masses with a deep hydrogen atmosphere could act like a giant pressure cooker:

    The hydrogen atmosphere is an excellent insulator.

    Internal heat could maintain a liquid-water layer deep below the clouds.

    Radioactive decay in a rocky core could provide heat for billions of years.

    A thick atmosphere could prevent freezing even far from any star.

Some models have proposed that a rogue planet with enough water and a thick hydrogen atmosphere could keep a habitable ocean for billions of years, perhaps comparable to the age of the Solar System. The likely environment would not resemble Earth: no blue sky, no continents, no photosynthesis-driven ecosystem. It would be more like a deep ocean under kilometers of atmosphere, with possible life powered by chemical energy (similar in concept to Earth's deep-sea hydrothermal ecosystems).

Neptune gives the counterexample: it has lots of heat and clouds, but probably not the right ingredients. Its temperature and chemistry become hostile before you get anywhere resembling a temperate ocean. A smaller "failed gas giant" — a water world with a thick but not overwhelming hydrogen envelope — might actually be a better candidate.

There is also an intriguing connection to Earth: if the Sun vanished, Earth's surface would freeze solid relatively quickly, but geothermal heat would keep the deep oceans from freezing for a very long time. Some estimates suggest the oceans could remain liquid under kilometers of ice for millions of years, perhaps longer if the atmosphere changed. So the basic physics is not fantasy.

The big question is not "can a planet stay warm?" — many can. The question is can it stay chemically active enough, with accessible energy sources, for life to evolve or persist? That is where rogue planets become a very speculative but fascinating frontier.

Yes, I did — and I should have just followed your usage. "LGM" has the classic SETI/science-fiction meaning of little green men (or, more generally, technological extraterrestrials), not some ambiguous acronym in this context. I over-explained the abbreviation rather than engaging the point you were making.

And your point is a good one: if we are talking about technological civilizations, the case for an ordinary planetary system is much stronger than the case for a rogue planet.

The free-floating planet argument is mostly an argument about possible niches for life, especially microbial life. It says: "Don't assume a star is absolutely required." But the jump from life to technology is enormous. A rogue planet might have a stable subsurface ocean for billions of years, but it lacks many of the features that seem to have mattered on Earth:

    a large, reliable external energy source;

    a dynamic surface environment;

    easy access to concentrated energy sources;

    a reason for organisms to evolve sensory systems tuned to a visible universe.

Even on Earth, technological intelligence may have depended on a peculiar sequence of events: oxygen-rich atmosphere, multicellular life, predation, mobility, tool use, hands, social learning, and so on. A planet under kilometers of ice and atmosphere would be asking evolution to solve a much harder problem.

There is an interesting irony: the places most likely to harbor life may not be the places most likely to harbor radio transmitters. The universe could be full of living oceans beneath ice shells or thick atmospheres, while the SETI search is mostly aimed at planets with something like our own combination of surface conditions and available energy.

So if the question is "where are the LGMs most likely to have evolved?" I would also put my money on a conventional star system — probably a rocky planet around a stable star, not a wandering Neptune in the dark between suns.

I do, however, see this as reason to give Neptune a little more attention.

I think that is a very reasonable conclusion. The rogue-planet discussion actually does make Neptune more interesting, even if it does not turn Neptune into a likely home for life.

Neptune is the nearest example we have of a world where internal energy dominates over sunlight. It is effectively a natural experiment showing that a planet can remain active for 4.5 billion years without relying primarily on its star.

A few things make Neptune worth attention:

    It has a huge internal heat flow. Unlike Uranus, which appears strangely underactive, Neptune is still losing a lot of its primordial heat. The reason is not fully understood.

    It has a complex atmosphere. The methane, ammonia, water, hydrogen, and helium chemistry is far from simple. It has storms, jet streams, clouds of different compositions, and seasonal changes.

    The "ice" in "ice giant" is misleading. The term refers to water, ammonia, and methane-rich materials, not frozen cubes. Deep inside, those substances are hot, dense fluids, and exotic phases such as superionic water may exist.

    It may resemble the class of planets we cannot study well yet. Many exoplanets are "Neptunes" or "mini-Neptunes," and understanding our own Neptune is essential for interpreting them.

The life question is the provocative part. Neptune's atmosphere itself is probably not a good habitat, because the pressures and temperatures become extreme as you go down. But if there are regions where chemistry and energy gradients are favorable — perhaps in some deep atmospheric layers or in hypothetical water-rich layers — it is exactly the sort of place where one would at least ask the question.

The problem is that Neptune is very far away and has only had one brief visitor: Voyager 2 in 1989. We got a remarkable snapshot, but not the kind of long-term study we have of Mars, Earth, or even Jupiter.

A dedicated Neptune orbiter would be scientifically rich: atmosphere, magnetic field, rings, moons (especially Triton), and interior structure. In fact, many planetary scientists consider a Neptune mission one of the great missing pieces of Solar System exploration.

So I would separate two ideas:

    "Could Neptune have LGMs?" — extremely unlikely.

    "Could Neptune teach us about the enormous population of Neptune-like worlds, including unusual habitats?" — very likely.

In that sense, the rogue-planet speculation is a good reminder that we may have underestimated the importance of the outer planets. Neptune is not just the blue dot at the edge of the Solar System; it may represent one of the most common types of planet in the galaxy.

An acronym ("NATO") is pronounced as a single word, not spelled out.  A contraction has an apostrophe and is pronounced with the missing letter out.  An abbreviation has the word it stands for fully pronounced and is marked by a period.  What is a thing like "LGM?"

Good question. LGM is actually a good example of something that does not fit neatly into your three categories.

The usual term is initialism.

    Acronym: formed from initial letters but pronounced as a word.

        NATO = Nay-toh

        laser = LAY-zer (originally Light Amplification by Stimulated Emission of Radiation)

    Initialism: formed from initial letters but pronounced letter by letter.

        LGM = el-jee-em

        FBI = eff-bee-eye

        DNA = dee-en-ay

    Abbreviation (the broad category): any shortened form of a word or phrase. This includes acronyms and initialisms, as well as things like:

        Dr. for Doctor

        Jan. for January

    Contraction is a special kind of shortening where letters are omitted within a word or phrase and usually marked by an apostrophe:

        don't = do not

        can't = cannot

So "LGM" is not an acronym in the strict sense; it is an initialism. In casual speech, though, many people use "acronym" loosely to include both acronyms and initialisms, which is why you will often see "LGM is an acronym" even though linguists would usually say "initialism."

There is another wrinkle: "little green men" itself is already a phrase that became a cultural shorthand, and "LGM" is simply the initialism for that phrase. It is much like "UFO" — technically you-eff-oh is an initialism, although many people now use it as though it were a word. (And "UFO" is an interesting case because some speakers actually pronounce it "you-foe" informally, pushing it toward acronym territory.)

English, as usual, refuses to keep the filing cabinets labeled properly. 🙂