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The earliest mentions of the star appeared in 1999 when it was cataloged under 2MASS J23062928-0502285, referencing its discovery through the Two Micron All-Sky Survey (2MASS) which classified stars and celestial objects based on infrared data. More recently, the star was a candidate for detecting transiting planets at the La Silla Observatory in Chile. Ultracool dwarves are particularly suited for this type of detection because they are dim and not very large, allowing even small planets block a significant portion of the light across their surface. The team initially monitored a small sample of ultracool dwarves – however, early success detecting three planets around TRAPPIST-1 led them to focus more attention on this particular star and eventually the count of orbiting planets rose to seven.

The planets that make up the TRAPPIST-1 system (1b to h) are configured in such a way that the six inner planets form an orbital resonant chain – meaning their orbital periods (time it takes to complete a trip around the host) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. Their orbital resonance means they exert regular, periodic gravitational influence on each other. To date, this system’s resonance chain is the longest ever discovered.

One of the first questions planetary scientists consider is whether the newly discovered planet(s) are within the habitable (or Goldilocks) zone. According to the usual definition, a planet resides in the circumstellar habitable zone (HZ) if, being a terrestrial-mass planet with a CO2-H2O-N2 atmosphere, it can sustain liquid water on its surface. Planets 1b, c, and d are too hot to contain liquid water and are likely stripped of their atmosphere from the intense electric currents caused by the rapid speed and close proximity at which they orbit their host star. However, 1e, f and g do not experience these issues. While being able to retain an atmosphere, they are also more temperate – and with 1e having the highest density it is suspected to consist of a rock/ice/ocean terrain mix; the latter two are estimated to be icy. Even though the planets closest to the host may be unable to harbor liquid water, it could still exist as atmospheric water vapor.

This leaves us with seven planets comprised of attributes that suggest compatibility with water in various states, and a chance of the possibility of life. This could mean certain types of primordial life or even dense vegetation. The unique characteristics of 1e given it’s central position among the planets have lead experts to believe it is the candidate most suited to life in any form. Even if there is no life to be found, there are still plenty of reasons to be interested in this system.

There are many things we don’t know about the TRAPPIST-1 system and plenty of questions remain unanswered, what researchers have discovered so far is significant. In less than a decade and from 229 trillion miles away, we have managed to measure properties from the transit configuration including the longest known resonance chain to precise densities of each planet. This culmination of these findings is what makes TRAPPIST-1 the most thoroughly known planetary system apart from our own.