One of the running jokes in astronomy has been that one object is a paper on a new phenomenon, two to five is the start of a new field of investigation, and at six examples you get to start doing statistics. This has started to change with large scale surveys of the sky. There is still a problem with there being so many things out there that have only been seen a handful of times, that it is easy to rediscover things by accident. The time and energy that goes into the whole “we have discovered” process prior to realizing you haven’t actually discovered something, well, it’s a lot. To try and make it easier to know what’s already out there, folks have been making and sharing catalogues for hundreds of years. Charles Messier’s catalog of bright extended objects was originally intended to help people know when they had found an actual comet and when they had just rediscovered a globular cluster for the Nth time.

Today, this “let’s make a catalogue” tradition is carried forward by the Breakthrough Listen project, which is taking more of a guidebook approach to listing out what can be found among the stars. They list out more than seven hundred distinct radio targets with the goal of describing one of every kind of naturally occurring radio signal. Like a nature guide, it doesn’t just list what prototypes for each class of objects should look like, it also lists out the extremes that can be seen. There are also objects that are anomalies, and here I’m going to quote what that means directly from the press release: Anomalies are enigmatic targets whose behavior is currently not satisfactorily explained. For instance, the famous “Tabby’s Star” with its bizarre dimming behavior; ‘Oumuamua — the interstellar object that passed near Earth in 2017; unexplained optical pulses that last mere nanoseconds; and stars with excess infrared radiation that could conceivably be explained as waste heat from alien megastructures. 

Yes, folks, I said alien megastructures. Now they aren’t trying to say there are aliens; they are trying to say there could be aliens and to make it as easy as possible to identify them against the background noise of our universe. Just as the Messier catalogue made it easier to discern the real comets against the background of clusters, nebulae, and galaxies that litter the sky, it is hoped that the Breakthrough Listen project’s catalogue will make it easier identify possible signs of extraterrestrial life against the background of pulsars, quasars, and young stars that all emit their own radio signals naturally. The catalogue also discusses what different technosignatures of advanced civilizations could look like. This catalogue also contains control samples that shouldn’t produce signals and can be used to calibrate instruments, as well as a discussion on how to classify both the included objects and future new kinds of radio sources.  While the authors’ goal is to make finding life easier, this catalog will benefit the entire field. As put by lead author Brian Lacki: The catalog is not just limited to SETI, though. My hope is that any program with a new capability may use the Exotica catalog as a shakedown cruise around the universe.

Ultimately, to understand our universe, we want to find more than one of everything. Heck, we really want to find more than six or even six hundred. We want complete populations of objects so we can understand every facet of an object in every possible combination of circumstances, but sometimes, just finding one is hard enough.

More Information

Breakthrough Initiatives press release 

“One of Everything: The Breakthrough Listen Exotica Catalog,” Brian C. Lacki et al., 2020 [preprint pdf]