About 40 light-years away from Earth, the alien worlds in the TRAPPIST-1 system are believed to be the most likely place for biological life beyond our solar system. TRAPPIST-1 is a dwarf star with at least seven exoplanets orbiting around it. The full system was discovered in 2017, and it marked a monumental milestone for astrophysics, because at the time, it was the greatest number of Earth-like planets ever found around a single star.
Astronomers first discovered the TRAPPIST-1 star in 1999. They classified it as an ultra-cool dwarf star, which means its temperature is low enough that liquid water could likely survive on planets orbiting very close to it. Over a decade later, scientists discovered the star was host to three exoplanets using the Transiting Planets and Planetesimals Small Telescope - TRAPPIST. And then, after about a year, NASA’s Spitzer Space Telescope helped reveal even more exoplanets, bringing the total to seven planets. Scientists realized that the TRAPPIST-1 system also set a new record for the greatest number of planets - possibly four - in the so-called habitable zone.
Experts say that almost every normal star has a habitable zone, which is the range of distance from its star where temperatures are right for water to remain liquid. Four of the seven TRAPPIST-1 exoplanets are in this zone. Well, technically the jury’s still out on where planet d lies, which is an indication of just how new this discovery is. The Hubble Space Telescope conducted a spectroscopic survey to learn more about the habitability of the planets, which revealed that planets d, e and f don’t seem to have puffy, hydrogen-rich atmospheres like a gas giant. This is good news in the search for life because hydrogen is a greenhouse gas and can make planets orbiting close to their parent star too hot and thus inhospitable. Planets d, e and f instead appear to have atmospheres similar to terrestrial or rocky planets. So far, scientists consider planet e to be the most Earth-like in terms of its estimated radius, mass and the amount of radiation that it receives from its host star, and they believe, of all the planets, it’s the most likely to have liquid water on its surface.
Astronomers are looking to the upcoming James Webb Space Telescope to help better understand these alien worlds. Unlike Spitzer, Hubble and Kepler, the Webb telescope has extended wavelength coverage that will be able to detect atmospheric properties of the planets with greater precision. But the new telescope won’t begin its cosmic journey until the 2020s, so in the meantime, astronomers will have to get creative to uncover more about TRAPPIST-1’s life harboring potential.