When we think about whether or not aliens exist, we generally imagine them on a vaguely Earth-like planet circling a distant star. We do not normally think of them living out in space itself.

But maybe that is not such a ridiculous idea. In April 2016, researchers reported that some of the key building blocks of life can be produced from simple substances under harsh conditions mimicking those of interstellar space.

Cornelia Meinert at the University of Nice, France and colleagues showed that a mixture of frozen water, methanol and ammonia – all compounds known to exist in the vast "molecular clouds" from which stars form – can be transformed into a wide range of sugar molecules when exposed to ultraviolet rays, which pervade space. The sugars included ribose, which is a part of the DNA-like molecule RNA.

This suggests that the fundamental molecules of life might be formed in outer space, and then delivered to planets like Earth by icy comets and meteorites.

The finding is actually not surprising. We have known for decades that other building blocks of life can be formed from chemical reactions like this, before being incorporated into comets, asteroids and planets.

However, there is a more intriguing possibility. Life itself might not have needed a warm and comfortable planet bathed in sunlight to get going. If the raw ingredients were already out there in interplanetary limbo, might life have started there too?

Ideas about the origins of life do not often consider this scenario. It is hard enough to figure out how life could have begun on the early Earth, let alone at temperatures close to absolute zero and the near vacuum of interstellar space.

There is no fundamental reason why life might not arise far from any star

Making the basic building blocks of life, like sugars and amino acids, is the easy part. There are lots of chemically-

plausible ways to do that, starting with the simple molecules found in young solar systems.

The hard bit is persuading these complicated molecules to assemble into something capable of life-sustaining processes like replication and metabolism. Nobody has ever done this, or come up with a completely plausible way it might happen, in the nurturing environment of a warm, rocky planet – let alone in space.

Still, there is no fundamental reason why life might not arise far from any star, in what is often regarded as the barren desert of interstellar space. Here is how it might happen.

First, we had better agree on what counts as "life". It does not necessarily have to look like anything familiar.

The broad basis of life on Earth – that it is carbon-based and requires water – "reflects a universal norm"

As an extreme case, we can imagine something like the Black Cloud in astronomer Fred Hoyle's classic 1959 science-fiction novel of that name: a kind of sentient gas that floats around in interstellar space, and is surprised to discover life on a planet.

But Hoyle could not offer a plausible explanation for how a gas, with an unspecified chemical make-up, could become intelligent. We probably need to imagine something literally a bit more solid.

While we cannot be sure that all life is carbon-based, as it is on Earth, there are good reasons to think that it is likely. Carbon is much more versatile as a building block for complex molecules than, say, silicon, the favourite element for speculations about alternative alien biochemistries.

Astrobiologist Charles Cockell at the University of Edinburgh in the UK thinks that the broad basis of life on Earth – that it is carbon-based and requires water – "reflects a universal norm". He concedes that "I have a quite conservative view, which science generally proves is misguided." But still, for now let's stick with carbon-based life. How could it be generated in outer space?

The basic chemistry is not a problem. As well as sugars, life on Earth needed amino acids, the building blocks of proteins. But we know that these can be formed in outer space too, because they have been found in "primitive" meteorites that have never seen a planetary surface.

Some chemicals could react even when chilled to just four degrees above absolute zero

They might be made on icy grains from some variation of a chemical reaction called the Strecker synthesis, after the 19th-Century German chemist who discovered it. The reaction involves simple organic molecules called ketones or aldehydes, which combine with hydrogen cyanide and ammonia. Alternatively, light-driven chemistry triggered by ultraviolet light will do the job.

It looks at first as though these reactions should not take place in deepest space, without a source of heat or light to drive them. Molecules encountering one another in frigid, dark conditions do not have enough energy to get a chemical reaction started. It is as if they run into a barrier that is too high for them to jump over.

However, in the 1970s the Soviet chemist Vitali Goldanski showed otherwise. Some chemicals could react even when chilled to just four degrees above absolute zero, which is about as cold as space gets. They just needed a bit of help from high-energy radiation such as gamma-rays or electron beams – like the cosmic rays that whizz through all of space.

Under these conditions, Goldanski found that the carbon-based molecule formaldehyde, which is common in molecular clouds, could link up into polymer chains several hundred molecules long.

Goldanski believed that such space-based reactions might have helped the molecular building blocks of life assemble from simple ingredients like hydrogen cyanide, ammonia and water.

Generally I think these environments aren't very conducive to very complex molecules

But it is far more difficult to coax such molecules to combine into more complex forms. The high-energy radiation that might help get the first reactions started then becomes a problem.

Ultraviolet and other forms of radiation can induce reactions like those Meinert demonstrated. But Cockell says they are just as likely to smash molecules as they are to form them. Potential biomolecules – progenitors of proteins and RNA, say – would be broken apart faster than they were being produced.

"Ultimately the question is whether other completely alien environments would give rise to self-replicating chemical systems that can evolve," says Cockell. "I don't see why that wouldn't happen in very cold environments or on the surfaces of ice grains, but generally I think these environments aren't very conducive to very complex molecules."

Planets offer two much gentler energy sources: heat and light. Life on Earth is largely powered by sunlight, and it is a fair bet that life on "exoplanets" around other stars would draw on the energy reserves of their own suns.

Galaxies might be full of "rogue planets" floating beyond the outermost reaches of a stellar neighbourhood

Vital heat can also come from elsewhere. Some scientists believe that the first life on Earth was not powered by sunlight, but by volcanic energy released from the planet's interior at hot vents in the deep sea. Even today, these vents still spew out a warm, mineral-rich brew.

There is also heat in Jupiter's major moons. This comes from the huge tidal forces exerted by the giant planet, which squeeze the interiors of the moons and heat them up through friction. This tidal energy keeps the sub-surfaces of the icy moons Europa and Ganymede melted into oceans, and makes Io's surface fiery and volcanic.

It is hard to see how molecules clinging to icy grains in interstellar space could find any such nurturing energy. But that might not be the only option out there.

In 1999, planetary scientist David Stevenson of the California Institute of Technology proposed that galaxies might be full of "rogue planets" floating beyond the outermost reaches of a stellar neighbourhood, too far from their "parent" star to feel its gravity, heat or light.

These rogue planets might be "the most common sites of life in the Universe"

These worlds, Stevenson said, could have formed like any other regular planet, close to a nascent star and within its surrounding nebula of gas and dust.

But then the gravitational tug of large planets, like our own Jupiter and Saturn, could sling some planets on "escape trajectories", propelling them off beyond their solar system into the empty space between stars.

That might seem to consign them to a cold and barren future. Yet Stevenson argued that, on the contrary, these rogue planets might be "the most common sites of life in the Universe" – because they might stay warm enough to support liquid water under, as it were, their own steam.

All of the rocky planets in the inner solar system come with two internal heat sources.

First, each planet has a fiery core still hot from the primordial fury of its formation. On top of that, they contain radioactive elements. These warm up the interior of the planet with their decay, just as a lump of uranium is warm to the touch. On Earth, radioactive decay inside the mantle contributes about half of the total heating.

Rogue planets would not be plagued by giant meteorite impacts

Primordial heat and radioactive decay inside rocky rogue planets could warm them for billions of years – perhaps enough to keep the planets volcanically active and provide the energy for life to start.

Rogue planets could also have dense, heat-retaining atmospheres. Compared with gas giants like Jupiter and Saturn, Earth's atmosphere is thin and tenuous, because the Sun's heat and light have stripped away lighter gases like hydrogen. Mercury is so close to the Sun that it barely has any atmosphere at all.

Yet on an Earth-sized rogue planet, far beyond its parent star's influence, much of the original atmosphere might remain in place. Stevenson estimated that the resulting temperature and pressure could be enough to sustain liquid water at the surface, even without any sunlight.

What's more, rogue planets would not be plagued by giant meteorite impacts, as Earth has been. They might even be ejected from their native solar system with moons in tow, giving them the benefit of some heating by tidal forces.

Even if a rogue planet did not have a thick atmosphere, it could still be habitable.

Any life in this strange habitat would exist like a lone wolf wandering the galactic steppe

In 2011, planetary scientist Dorian Abbot and astrophysicist Eric Switzer at the University of Chicago calculated that planets about three and a half times the size of the Earth could become covered over with a thick layer of ice. This would insulate an ocean of liquid water many kilometres below the surface, heated by its interior.

"The total biological activity would be lower than on a planet like Earth, but you should still be able to have something," says Abbot.

He hopes that when space probes investigate the subsurface oceans of Jupiter's icy moons in the coming decades, we will learn more about the possibilities of life on iced-over rogue planets.

Abbot and Switzer called these orphaned worlds "Steppenwolf planets", because, they say, "any life in this strange habitat would exist like a lone wolf wandering the galactic steppe". The habitable lifetime of such a planet could be up to ten billion years or so, similar to that of Earth, says Abbot.

If these ideas are right, then outside our solar system rogue planets in interstellar space could be the closest places where extraterrestrial life exists.

Imagine bathing in warm volcanic springs under perpetual night, like a winter vacation in Iceland

They would be very hard to spot at such a distance, being dark and relatively tiny.

But with luck, say Abbot and Switzer, such a planet passing within about a thousand times the Earth-Sun distance could just about be discerned from the small amount of sunlight it would reflect and the infrared radiation of its own warmth. We might hope to see it with the telescopes currently used to look for exoplanets around other stars.

If life can originate and survive on an interstellar Steppenwolf planet, say Abbot and Switzer, there is a profound implication: life "must be truly ubiquitous in the Universe".

It would be a strange kind of life on these interstellar worlds. Imagine bathing in warm volcanic springs under perpetual night, like a winter vacation in Iceland. But if that is all you had ever known, it would seem like home.

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