Can life exist on a rogue planet?

This article appeared in the September / October 2021 issue of Discover. Become a subscriber for unlimited access to the Discover archives.


Even as a child, before devoting his life to the search for extraterrestrial life, Frank Drake wondered if the Earth was the only one able to shelter life. He was not the first nor the only one to ask the question. There’s a reason so many people are fascinated by the question: its answer helps reveal humanity’s place in the cosmos.

Drake’s reflections inspired him to pursue astronomy, as director of the Arecibo Observatory in Puerto Rico and president of the SETI Institute – which, as the acronym suggests, is devoted to research. alien intelligence, and explores the possibilities of life elsewhere in the universe. Drake is perhaps most famous for his eponymous equation – an estimate of the number of alien civilizations that could exist in our galaxy. Introduced in 1961, the equation is widely regarded as the start of a new era of extraterrestrial intelligence research.

But decades after the invention of this famous equation, Drake admitted that his estimates were too conservative. Among the overly moderate assumptions was that another potentially inhabited world must have been orbiting a star – neglecting the possibility of life on rogue planets.

Sometimes called “nomads of the galaxy” or “orphan planets”, these cold and dark worlds crisscross space without a house, without a solar system, without an orbiting sun. A long time ago they were formed around a star, but were thrown away, abandoned by their parents. There are billions of rogue planets – astronomers estimate there could be at least one for every star – roaming the galaxy.

It may seem futile to seek life in such cold and desolate environments, but over the past two decades astronomers have come up with a number of possible scenarios that would make life possible on a starless planet.

Beyond Goldilocks

While there are many ingredients necessary for the proper development of life, two of the most essential are a liquid solvent, which helps transport minerals and other vital substances between cells, and a source of energy that organisms can harvest. to continue to live and grow. .

A planet like ours – close enough to a star to have oceans of liquid water (meeting solvent needs) and an abundant source of visible light for photosynthesis (meeting energy needs) – is a paradise for organisms. living. If we were closer to the sun, the light would be too intense, the heat boiling the oceans. If we were further away, the oceans would freeze over and plants would have a hard time getting enough light to convert into chemical energy. As it stands, the Earth is perfectly located. Neither too hot nor too cold, but square in what astronomers call the living area or “Goldilocks”.

Planets like ours, in the habitable zone around a sun, were at the center of most searches for life. But this research may have been too limiting. (I credit: Roen Kelly / Discover)

The search for extraterrestrial life has been dominated by the search in the skies for Goldilocks planets like ours. But many astronomers believe this research strategy betrays a lack of imagination that unnecessarily limits our research. Who Said Life Should Look Exactly As It Is On Earth?

Sara Seager, an MIT professor and exoplanet research expert, is among those who think we should broaden our notions of habitability. “Everywhere we look, we see ingredients for life. And the idea is that it has come together on our planet, so if the ingredients for life are on other planets, life should be able to be born somehow, ”she says. For Seager, that means looking for exoplanets orbiting stars that might be outside the traditional habitable zone, or that might have unique biochemical signatures unlike those we find on Earth.

But Florida Institute of Technology professor Manasvi Lingam took the idea even further. Why should the planet orbit a sun? Can such a rogue planet still contain the ingredients of life?

Thief of life

Building on other work started in 1999, Lingam and his colleague Avi Loeb explore this question. In an article published in the International Journal of Astrobiology in 2019, they are studying how life could survive on a rogue planet in the oceans under an outer layer of ice. This layer of ice would form if an Earth-like planet were to be thrown out of its sun’s orbit. The cold of interstellar space would be too strong for the oceans to remain entirely liquid. But the ice sheet could also help isolate the inner part of the planet from the frigid environment, acting as a blanket. And deep in the planet’s core, radioactive elements would continue to produce heat, warming it from the inside out.

This combination of exterior insulation and interior heating could protect the oceans from freezing throughout. If Earth were to be ejected now, the amount of radioactivity probably wouldn’t be enough to keep its oceans from freezing, and we probably wouldn’t have enough water or geothermal activity to support life. But Lingam and Loeb believe that if he had been ejected soon after its formation, when there was much more geothermal activity in the core, then the oceans might have survived.

Lingam’s proposal is just one of the few situations where a rogue planet might still be able to maintain a temperature that allows liquid water to exist. If the rogue planet had a dense atmosphere of hydrogen, that would also act as a blanket, even allowing liquid to exist on the surface rather than under a thick layer of ice. Or if the planet were to be ejected from the orbit of a star with a moon still attached to it, then the moon could remain hot due to the friction caused by the tidal forces of the planet.

So the solvent requirement could be met on a rogue planet. “But there is still one factor missing, namely: you may have liquid water, but where does the energy for organisms to metabolize come from? [come from]? Where will they find the energy to meet their needs? Lingam said.

Finding the right balance

For this, Lingam considered an unlikely source. Some galaxies are considered active, emitting intense radiation when matter falls into a supermassive black hole at its center. And if the rogue planet is close to that center – the so-called active galactic nucleus – then it might be able to use this radiation to fuel photosynthesis.

Lingam has discovered that there is a balance between the usefulness of this light in photosynthesis and the damage caused by extreme ultraviolet light on cells. But a number of shields, such as the ground or the first few meters of seawater, could also protect life in underground seas from ultraviolet radiation, while still allowing enough visible light to pass through to promote photosynthesis. Lingam determined that an active galactic nucleus might be able to support life on a rogue planet that is less than about 1,000 light years from the center of the galaxy (for comparison, Earth is 25,000 years away). -light of the center of the Milky Way).

(Credit: Grossinger / Shutterstock)

Even with these conditions met, the question of whether we can detect any signs of life in the near future remains open. This is especially difficult in the case of rogue planets, as we probably couldn’t use light from a parent star as a signal, as we do in typical studies of exoplanets. According to Seager, “The research is for future generations. … It will be very difficult.

What could life be like on such a planet? “You can certainly think of having something bigger than germs,” Lingam says. “Although it’s not as complex as the more complex things we see here [on Earth]. We may not see dolphins, ”or any similar developed species.

But whether or not we can find it anytime soon, and whether or not intelligent life may or may not thrive, whether life is possible on these planets may indicate a hidden abundance and diversity of organisms in the universe. Even on Earth, we have marveled at the abilities of extremophiles to survive in the most inhospitable corners of our planet. Why not also in the dark cold of interstellar space, on a rogue planet?

Maybe life isn’t just a happy accident that happens under perfect conditions on a Goldilocks planet – maybe it is inevitable.


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Arline J. Mercier

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