Humanity’s exploration of the stars, science provides the answer

In the movie 'Project Hail Mary', humanity sends an interstellar spaceship to the Tau Ceti star system to save the planet from alien life threatening Earth. But what are the chances that life actually exists on Tau Ceti? Among the thousands of exoplanets, how do scientists decide where to look for life?

A recent study published in Monthly Notices of the Royal Astronomical Society answers this question. A research team led by Dr. Lisa Kaltenegger, an astrophysicist at Cornell University, selected 45 exoplanets with the highest potential for life and proposed search techniques for additional discoveries.

How is the search for exoplanets accomplished?

There are two main ways astronomers find exoplanets. The first is the 'transit method', which measures how dark the star's light becomes when a planet passes in front of a star. The larger the planet, the more pronounced the photoreduction phenomenon. The second is a method of measuring the ‘wobble’ of a star. This is a phenomenon that occurs because a planet pulls on a star with its gravity, and Dr. Kaltenegger likened it to “a dog straining on a leash.” The closer the planet and the smaller the star, the more evident the shaking is.

There are more than 6,000 exoplanets discovered to date, but most of them have environments that are difficult for life to survive. Many of the discovered planets are gas giants called 'hot Jupiters', which orbit extremely close to their stars. The fact that so many of these planets have been discovered may not be because they are actually common, but because they are easy to detect. The research team suggested that finding rocky planets around smaller, cooler stars would be advantageous in the search for life.

Conditions for existence of life: rocky surface and habitable zone

To be considered a candidate for life, a planet must meet two conditions. It must have a rocky surface and be located in the 'habitable zone', the orbital distance at which liquid water can exist.

Interestingly, after the publication of the novel 'Project Hail Mary' in 2021, scientists determined that there was a high possibility that Tau Ceti had no planets in the habitable zone. Dr. Kaltenegger joked, “This is good news for humanity because it means that astrophage, which eats the sun, could not have evolved there.”

Where to go: Potential candidate planets

The first strong candidate selected by the research team is the TRAPPIST-1 star system. Discovered in 1999, this small red dwarf is home to as many as seven rocky planets within its habitable zone. It is currently the main observation target of the James Webb Space Telescope.

A lesser known candidate is TOI-715. Around this red dwarf, TOI-715 b, a 'super-Earth' with a mass three times that of Earth, orbits in the habitable zone. However, it is 139 light years away, making it difficult to reach even with future interstellar spacecraft.

The closest candidate is Proxima Centauri. This star system, located just 4.25 light-years away from Earth, also has a terrestrial planet within its habitable zone.

Planets on the Edge: Life without water?

In addition to the 45 planets in the habitable zone, the research team selected 24 additional planets on the border. “These planets are the most interesting,” said Dr. Kaltenegger. It is an orbit where liquid water is difficult to exist, but that does not mean that there is no life. This is because, like the home planet of the alien ‘Rocky’ in the movie ‘Project Hail Mary’, there may be life forms that have found a way to survive without water.

The future of the search for extraterrestrial life [AI analysis]

This research is significant in that it redefines the direction of the search for extraterrestrial life. As the observation capabilities of the James Webb Space Telescope are in full swing, it is becoming possible to analyze the atmospheric composition of rocky planets within the habitable zone. There is a high possibility that 'biosignatures', signs of the existence of life, will be discovered on these planets within the next 10 years.

Additionally, stellar systems with multiple rocky planets, such as TRAPPIST-1, provide an ideal environment for comparative studies. By comparing multiple planets with different conditions around the same star, understanding of the conditions for the development of life is expected to deepen.

Proxima Centauri is likely to emerge as the top destination for future interstellar exploration plans, given that it is the closest candidate. If a micro probe project like Breakthrough Starshot becomes a reality, it could be the first time in human history that we directly explore another star system.