Searching for habitable planets around ultra-cool dwarfs has long been considered a waste of time. Even as astronomers found that exoplanetary systems are generally different from the solar system, old attitudes lingered. The Earth and Sun appear so normal and hospitable to our eyes that we get blinded by their attributes. Major programs are therefore directed at finding an Earth twin: a planet the mass and size of our own, orbiting a star just like the Sun, at the same Earth-Sun distance. The detection of such a world remains decades away.
In the effort to answer the question “Is there life elsewhere?” the focus on Earth twins is perceived as a safe path, since we can expect that similar conditions will lead to similar results (at least part of the time). However, we argue that this is far too conservative a goal, considering the huge number and diversity of available planets. That is part of the message of TRAPPIST-1. Research should be about finding what we don’t already know. Identifying a life-bearing Earth twin would be a resounding scientific success, but it would teach little about the overall emergence of biology in the Universe.
Our ambition is wider. Instead, we seek an answer to “How frequently is life found elsewhere?” This simple change of words means that we should also be investigating planetary
systems unlike the solar system. It would be disappointing and surprising if Earth were the only template for habitability in the Universe. Sun-like stars represent just 15 percent of all stars in the Milky Way. More than half of those, in turn, exist in binary star systems that have also been disregarded as being too different from the conditions present in the solar system. The search for Earth twins therefore covers a nearly insignificant fraction of all the outcomes in nature.
Once we reset the goal to measuring the total frequency of biology, ultra-cool dwarfs become an obvious target. Half the stars in the Milky Way have masses less than one-quarter of the Sun’s. Our preliminary results suggest that rocky worlds are common orbiting low-mass stars, including ultra-cool dwarf systems, possibly more so than in orbit around Sun-like stars. Ultra-cool dwarfs also open a much easier route to detecting and studying temperate, Earth-like planets.
The scientific advantages of ultra-cool dwarfs come from their stellar properties, from how we identify exoplanets, and from how we expect to investigate their atmospheres. The TRAPPIST-1 planets were found as they passed in front of their star, events known as transits. When the planet transits, it casts a shadow whose depth tells us how much of the stellar surface is being hidden by the planet; the bigger the planet, the deeper the shadow. Because ultra-cool dwarfs are so small, the transit of an Earth-sized planet in front of TRAPPIST-1A is approximately 80 times as prominent as an equivalent transit against a much larger, Sun-like star.