Scientists Narrow the Best Stars for an Interstellar Life Hunt
Picking a real destination for an interstellar probe is no longer just a science-fiction exercise. With thousands of exoplanets now on the books, astronomers are increasingly sorting them not by novelty, but by which ones are actually worth the decades, technology, and risk that a mission beyond the solar system would demand.
A recent study identified 45 known exoplanets as particularly strong candidates in the search for life. Another 24 worlds were relegated to the fringes. The logic is more selective than the early days of exoplanet discovery. In the past, the goal was simply to find these distant worlds. Today, however, the search is focused on the smaller, cooler stars. Rocky planets orbiting those stars are easier to detect with the two most common methods of detecting exoplanets: the transit method, in which the planet passes in front of the star it orbits, and radial velocity, in which the star wobbles in the presence of the planet. The search for new worlds needs more than just a catchy name. It needs a planet with a surface that might harbor liquid water and an orbit that keeps it safe from the star it orbits. It needs a star that does not strip away any atmosphere that the planet might be able to build.
This last filter is increasingly difficult to ignore. A 3-D chart of nearby stars, using Chandra X information, further emphasized that the habitable zone is not as reliable as it once was. A planet in that coveted zone may still be exposed to severe levels of high-energy radiation that will damage its atmosphere and interfere with life’s chemistry. According to Breanna Binder, Without characterizing X-rays from its host star, we would be missing a key element on whether a planet is truly habitable or not. This means that some famous fictional decisions now seem less logical in light of current data.
Tau Ceti, for instance, used in “Project Hail Mary,” is now seen as not as promising as it once was. The stronger contenders are TRAPPIST-1, with its cluster of small worlds in a tight configuration, and TOI-715, with its super-Earth in its host’s habitable zone. For an actual spacecraft, however, distance will be the key factor in making any decision. TOI-715 is 139 light-years away, which is beyond any first-generation mission. Proxima Centauri is different. It is only 4.24 light-years from Earth and has a planet in its habitable zone. That is why it is the focus of Breakthrough Starshot’s proposed mission. This is a flyby mission that is part of a project that involves gram-mass spacecraft propelled by a laser array to significant fractions of the speed of light. At speeds that are part of this project, it will take 20 to 30 years for the spacecraft to reach Proxima Centauri. The signal will then take an additional four years to be sent back.
This is very demanding architecture. However, it is also true that the concept of searching for planets using this method has inspired the mission planners to understand that the first star mission is likely to be a flyby of the first accessible potential habitable world, not necessarily the most interesting. There is also a reason that astronomers have been on the lookout for stranger candidates. Lisa Kaltenegger’s team has discovered 24 “planets on the edge,” which are planets that do not fit into the category of liquid water but could still be interesting. We call them ‘planets on the edge,’ Kaltenegger said. They’re the most interesting ones. The point is not necessarily to eliminate the more interesting candidates. It is not necessarily to make the interstellar travel approach so inflexible that it ignores what life could possibly be. This is what is at issue in the field today: finding planets that are accessible but not necessarily too accessible.
