The exoplanet K2-18b hopes for life beyond Earth

Traces of dimethyl sulfide on K2-18b: possible sign of life on an ocean exoplanet 120 light-years from Earth. The most promising discovery ever?

The centuries-old search for life beyond Earth’s boundaries has led scientists through fascinating cosmic scenarios, from Martian methane emissions to the enigmatic phosphine clouds of Venus. Yet, until today, humanity seemed to contemplate a silently empty universe. Now a team of researchers presents what it defines as the most convincing evidence of extraterrestrial existence, not in our planetary neighborhood, but on a gas giant planet 120 light-years away, known as K2-18b, on which scientists’ attention has been focused, truth be told, for some years now.

A further in-depth analysis of this exoplanet’s atmosphere suggests the abundant presence of a molecule that, on Earth, has only one established origin: living organisms such as marine algae.

It would be the first time

“It is in no one’s interest to prematurely declare the discovery of life,” specified Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the study. However, he added that the most plausible explanation for his group’s observations is that K2-18b is enveloped by a vast, warm ocean, potentially teeming with life. “This is a crucial moment,” Madhusudhan emphasized. “It is the first time humanity has glimpsed potential biosignatures on a planet located in the habitable zone of its star.” The research, published in The Astrophysical Journal Letters, has been welcomed by other scientists as an exciting first step in deciphering the nature of K2-18b. However, the scientific community remains cautious about drawing definitive conclusions.  

In any case, patience will be needed

“It’s not definitive proof,” stressed Stephen Schmidt, a planetary scientist at Johns Hopkins University. “It’s a significant clue, but we cannot yet state with certainty that the planet is habitable.”  

If extraterrestrial life were indeed to exist on K2-18b, or elsewhere in the cosmos, its revelation will require patience. “Unless we run into an alien waving at us, irrefutable proof will take time,” said Christopher Glein, a planetary scientist at the Southwest Research Institute in San Antonio, Texas.

The history of the exoplanet K2-18b

K2-18b was discovered in 2017 by Canadian astronomers, thanks to observations conducted with ground-based telescopes in Chile. It belongs to a class of planets common outside our solar system, the so-called sub-Neptunes, which have no analogues in our immediate vicinity. These worlds are significantly larger than the inner rocky planets but smaller than Neptune and the other outer gas giants.  

Already in 2021, Madhusudhan and his collaborators had hypothesized that sub-Neptunes could be covered by warm water oceans and enveloped by atmospheres rich in hydrogen, methane, and other carbon compounds. To describe these unusual planets, they coined the term Hycean, a fusion of the words “hydrogen” and “ocean”.  

The help of the James Webb Space Telescope

The advent of the James Webb Space Telescope in December 2021 offered astronomers an unprecedented tool to closely scrutinize sub-Neptunes and other remote planets. When an exoplanet transits in front of its parent star, its atmosphere, if present, is illuminated. The gases that compose it alter the color of the starlight reaching the Webb telescope. By analyzing these variations in wavelengths, scientists can deduce the chemical composition of the atmosphere.

Traces of dimethyl sulfide (which smells like the sea!)

In 2023, Dr. Madhusudhan’s team reported faint traces of a potentially important molecule: dimethyl sulfide, a compound made of sulfur, carbon, and hydrogen. On Earth, life is the only known source of dimethyl sulfide. In the oceans, some species of algae produce this compound, which diffuses into the air, contributing to the characteristic scent of the sea. This happens when these organisms die or are consumed by bacteria or zooplankton: in that case, the dimethylsulfoniopropionate produced by the algae degrades and releases dimethyl sulfide.  

Astrobiologists had already hypothesized that dimethyl sulfide could represent a biological signature on other worlds. Last year, Madhusudhan’s group had a second opportunity to search for dimethyl sulfide. As K2-18b transited again in front of its star, they used a different instrument on the Webb telescope to analyze the starlight filtered through the planet’s atmosphere. This time, they observed an even stronger dimethyl sulfide signal, accompanied by a similar molecule, dimethyl disulfide. “It’s mind-blowing,” commented Madhusudhan. “We spent a huge amount of time just trying to rule out the signal.”  

And what if it’s not a Hycean?

Regardless of the analyses conducted, the signal remained robust. The researchers concluded that K2-18b might indeed host a significant amount of dimethyl sulfide in its atmosphere, thousands of times higher than the levels found on Earth. This would suggest that its Hycean seas could be rich in life.  

However, other scientists emphasize the need for further research. An open question concerns the actual habitability of K2-18b as a Hycean world. Glein and his colleagues have proposed that K2-18b could be a huge rocky body with a magma ocean and a dense, scorching hydrogen atmosphere, a scenario unfavorable to life as we know it. Laboratory experiments will also be necessary to interpret the new data, for example, by recreating the possible conditions on sub-Neptune planets and verifying if dimethyl sulfide behaves similarly to how it is observed on Earth.

How Long Would It Take to Reach Exoplanet K2-18b?

Exoplanets capture our imagination, offering glimpses into worlds beyond our solar system. K2-18b, located approximately 124 light-years away in the constellation Leo, is one such intriguing target. But how feasible is a journey to this distant world? Let’s break down the travel time based on current technology and explore potential future advancements.

A Reality Check with the Parker Solar Probe

To understand the scale of the challenge, we need a benchmark. Currently, the fastest human-made object is NASA’s Parker Solar Probe.

• Record Speed: During its closest approaches to the Sun, the Parker Solar Probe has reached speeds of approximately 700,000 kilometers per hour (about 430,000 miles per hour).
• Cosmic Perspective: While incredibly fast by terrestrial standards, this speed is only about 0.00064c, or 0.064% of the speed of light.

Calculating the Journey Time:

1. Distance: K2-18b is 124 light-years away. One light-year is the distance light travels in a year, roughly 9.46 trillion kilometers. So, the distance to K2-18b is approximately 1.17×1015 kilometers (124×9.46×1012 km).
2. Time Calculation: Using the simple formula Time = Distance / Speed:
   • Time = (1.17×1015 km)/(700,000 km/h)
   • Time ≈1.67×109 hours
3. Converting to Years: Since there are 8,760 hours in a standard year:
   • Time ≈(1.67×109 hours)/(8,760 hours/year)
   • Time ≈190,500 years

Using our fastest current technology, a one-way trip to K2-18b would take nearly 200,000 years. This starkly illustrates the immense distances involved in interstellar travel and the profound limitations of our existing propulsion systems for such journeys.

Future and Theoretical Propulsion Concepts

Clearly, reaching K2-18b within human lifetimes requires revolutionary breakthroughs in propulsion. Here are some alternative and theoretical concepts being explored:

1. Nuclear Fusion Propulsion (e.g., Project Daedalus / Icarus)

• Top Speed: Potentially ~10% the speed of light (0.1c).
• Estimated Travel Time to K2-18b: ~1,240 years.
• Status: Theoretical; large-scale fusion propulsion hasn’t been built.
• Concept: Harnesses the immense energy released from nuclear fusion reactions (like those powering stars) to generate thrust. Projects like Daedalus (1970s) and its successor Icarus explored designs for interstellar probes.

2. Antimatter Propulsion

• Top Speed: Theoretically up to ~50% the speed of light (0.5c).
• Estimated Travel Time to K2-18b: ~250 years.
• Status: Highly theoretical; producing and safely storing significant amounts of antimatter is an enormous challenge.
• Concept: Utilizes the complete energy conversion when matter and antimatter annihilate each other, potentially offering the highest energy density of any known reaction.

3. Light Sail Propulsion (e.g., Breakthrough Starshot)

• Top Speed: Potentially ~20% the speed of light (0.2c) for very small probes.
• Estimated Travel Time to K2-18b: ~620 years.
• Status: Early-stage development and research.
• Concept: Uses the pressure exerted by photons from powerful ground-based or space-based lasers reflecting off vast, ultra-thin sails to accelerate tiny nanocraft to relativistic speeds.

4. Alcubierre Warp Drive (Faster-than-Light Travel)

• Top Speed: Theoretically allows effective faster-than-light travel (>1c).
• Estimated Travel Time to K2-18b: Potentially months or years, depending on the effective speed.
• Status: Purely theoretical; relies on manipulating spacetime in ways that require exotic matter or “negative energy,” which may not exist or be controllable.
• Concept: Contracts spacetime ahead of the spacecraft and expands it behind, creating a “warp bubble” that moves space itself around the stationary ship.

5. Generation Ships

• Speed: Likely comparable to advanced conventional rockets or slightly better (e.g., 0.001c to 0.01c).
• Estimated Travel Time to K2-18b: Tens of thousands to over a hundred thousand years.
• Status: Theoretical concept; requires immense advancements in closed-loop life support, long-term reliability, and societal structures.
• Concept: Vast, self-sustaining spacecraft designed for voyages so long that the original crew’s descendants would be the ones to arrive. It addresses the time problem by extending the human lifespan across the journey.

Quick Comparison: Reaching K2-18b (124 light-years)

Propulsion Type	Potential Speed (fraction of c)	Estimated Time to K2-18b	Status
Current Tech (Parker)	0.00064c	~190,500 years	Existing
Nuclear Fusion	0.1c	~1,240 years	Theoretical
Light Sail (Nanocraft)	0.2c	~620 years	Early Research
Antimatter	0.5c	~250 years	Highly Theoretical
Generation Ship	0.001c−0.01c	~12,000 – 124,000 years	Theoretical
Alcubierre Warp Drive	>1c (effective)	< 124 years (potentially much less)	Purely Theoretical