The TRAPPIST-1 System and its Extraordinary Exoplanets

by Samantha Grimes

As There Are Stars in the Sky

The ongoing quest to find life beyond Earth and recent advances in observational techniques has propelled the search for exoplanets in recent years. The term “exoplanet” denotes a planet orbiting a star other than the Sun, and the first exoplanet was not detected until 1988 by a Canadian research team. However, their results were not validated until 2002 because the planet—known as Tadmor and found around Gamma Cephei, a star approximately 45 light years away—had an orbit smaller than scientists believed was possible at the time. The first two confirmed exoplanets were discovered around a pulsar star named Lich in 1992. Pulsar stars, as the name suggest, emit a beam of electromagnetic radiation when pointed toward the Earth (much like a rotating light house), making it easier to detect exoplanets when they come between the star and Earth, which temporarily blocks the emission. However, it was not until 1995 that an exoplanet, 51 Pegasi b, was confirmed around a main-sequence (or Sun-like) star.

Since that time, various observatories and telescopes have confirmed a total of 3,586 exoplanets as of March 1st, 2017. By cataloging these exoplanets scientists hope to determine the relationship between star type and the resulting planetary system, but so far, it seems that planetary systems can contain rocky, gaseous and icy planets just as our solar system does. Unlike our solar system, however, most planets are as large as Jupiter, or even larger, and no star besides our Sun has had eight or more planets confirmed in its orbit. Though these numbers are likely limited by our observational abilities, even a conservative estimate results in an average of one planet per star. Thus there may be as many, and likely many more, unique planets as there are stars in the sky.

Seven New Worlds

In May 2016, researchers monitoring the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the La Silla Observatory in Chile saw a blip. Michaël Gillon and his team, hailing from Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, had directed the telescope at an ultra-cool dwarf star 39 light years from Earth. The team had hopes of discovering Earth-sized exoplanets, and this dimming seemed to be a confirmation of such a planet. Now known as TRAPPIST-1a, this ultra-cool dwarf star harbors seven terrestrial planets-TRAPPIST-1b, c, d, e, f, g, and h- three of which fit the European Organization for Astronomical Research in the Southern Hemisphere’s (ESO) criteria for potential life. These rocky planets, TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g, all have similar masses to that of the Earth (a feature important so that a planet can hold its atmosphere), and are found in the famed “Goldilocks zone,” where the temperature is such that liquid water can exist in its surface.

Besides these tantalizing hints toward amicable conditions for life, there are other aspects of the TRAPPIST system that make it a fascinating study. Its star, TRAPPIST-1a, is only about 84 times more massive than Jupiter and is both young and slow-burning; at only 500 million years old, it will likely live to the ripe old-age of 12 billion years. Further, the TRAPPIST-1 system is the largest exoplanetary system discovered to date, and the only system around an ultra-cool red dwarf. These seven planets are also exceptionally close to their star and have faster orbits than any planet in our own solar system. In fact, TRAPPIST-1b completes its orbit in a mere 1.5 Earth days, and TRAPPIST-1h, the furthest from the star, only takes 20 Earth days.

Because these exoplanets are so close to their star, it seems likely that all seven planets are tidally locked, so that the same face of the planet is always toward the star. However, this might not necessarily impede life, so long as the planets have atmospheres than can dissipate heat over the planet’s surface. Though the two innermost planets have been confirmed as lacking a hydrogen atmosphere, which would have been promising in the hunt for extraterrestrial life, the atmospheres of the other planets are still undetermined. The process of characterizing these atmospheres is an ongoing task, which will be aided significantly by the launch of the James Webb Space Telescope in 2018. This telescope will analyze the system with infrared wavelengths in order to determine the atmospheric composition. Certain combinations of carbon dioxide, oxygen and methane might indicate the presence of life, but will scientists have a way to feasibly ascertain if life truly exists in this system?

A Long Way from Home

For now, the only analyses we can realistically expect to perform on the TRAPPIST system must be done through telescopes and observatories, because our current technology prohibits us from sending a spacecraft 39 light years in any reasonable amount of time. Since the TRAPPIST system is approximately 2.29 x 1014 miles away, it would take 726,607.85 years to travel there in a spacecraft traveling at the current record speed of 36,000 miles per hour, which Horizons reached on its journey to image Pluto and the Kuiper Belt. Even if novel and experimental technologies are considered, such as Ion Thrusters, which can temporarily reach speeds up to 201,324 miles per hour, it would still take 129,929.11 years to arrive. If we hoped to send a spacecraft, manned or otherwise, which would arrive in 1,000 years, the craft would need to travel at 26,157,882 miles per hour, which is approximately 4 per cent of the speed of light. Nonetheless, space travel has progressed at an incredible rate in the last 60 years, and we can only hope that engineers and physicists will continue to delight us with their ingenuity.   

The discovery of the TRAPPIST system has been part of a global return to a fascination with space exploration, and our enchantment with exoplanets like those in the TRAPPIST system stems from the fact that they offer promising results, but also unmatched challenges. Gillon, the lead scientist of the Belgian team that discovered the TRAPPIST system, explained in a 2016 statement through the ESO that “systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”