We are on the cusp of a new epoch in the search for life beyond Earth. Sun-like stars represent just 15 percent of all stars in the Milky Way. And nearly half of those have binary star companions that suppressed the formation of planets. The search for Earth analogs around single, solar-type stars therefore covers a nearly insignificant fraction of all the outcomes in nature.
Searching for Planetary Systems Unlike Our Solar System
“How frequently is life found elsewhere?” asked the research teams at the University of Cambridge and the University of Liège in Belgium. This simple change of words means that we should also be investigating planetary systems unlike the solar system. It would be disappointing and surprising if Earth were the only template for habitability in the Universe.
“No matter what we find by studying planets orbiting ultra-cool dwarfs, we cannot lose. We can only learn,” said the researchers. “If we manage to identify the presence of life on a planet similar to those in the TRAPPIST-1 system, then we can start measuring how frequently biology emerges in the universe. We could have the first clues of extraterrestrial biology in a decade!”
In March of 2017, the team reported that a nearby star, called TRAPPIST-1, is orbited by seven planets similar in size and mass to Earth. The TRAPPIST-1 star is a very cool red M-dwarf that can barely fuse hydrogen in its core; it has 9% the mass, 12% the radius, and only 0.06% the luminosity of our yellow Sun. The seven planets orbit TRAPPIST-1 with very short periods, ranging from 1.5 – 18.8 days. Nonetheless, because TRAPPIST-1 is so cool and faint, all seven planets orbiting TRAPPIST-1 are temperate, meaning that under the right atmospheric and geologic conditions, they could sustain liquid water. Three of the planets show particular potential for habitability, receiving about as much energy from their star as the Earth receives from the Sun.
“That the planets are so similar to Earth bodes well for the search for life elsewhere,” says University of Birmingham astronomer Amaury Triaud.
Targeting Ultra-cool Dwarf Systems
Once we reset the goal to measuring the total frequency of biology, ultra-cool dwarfs become an obvious target. Half the stars in the Milky Way have masses less than one-quarter of the Sun’s. The preliminary results suggest that rocky worlds are commonly orbiting low-mass stars, including ultra-cool dwarf systems, possibly more so than in orbit around Sun-like stars. Ultra-cool dwarfs also open a much easier route to detecting and studying temperate, Earth-like planets.
The scientific advantages of ultra-cool dwarfs come from their stellar properties, from how we identify exoplanets, and from how we expect to investigate their atmospheres. The TRAPPIST-1 planets were found as they passed in front of their star, events known as transits. When the planet transits, it casts a shadow whose depth tells us how much of the stellar surface is being hidden by the planet; the bigger the planet, the deeper the shadow. Because ultra-cool dwarfs are so small, the transit of an Earth-sized planet in front of TRAPPIST-1 is approximately 80 times as prominent as an equivalent transit against a much larger, Sun-like star.
During a transit, any gasses in the planet’s atmosphere change the appearance of starlight streaming through. Around ultra-cool dwarfs, the atmospheric signature is boosted by about a factor of 80. The atmospheric composition of the TRAPPIST-1 planets will be detectable using current and upcoming facilities, such as the James Webb Space Telescope that just launched this past week, unlike the decades of technological development needed to study the atmosphere of an Earth analog.
Extracting a Reliable Atmospheric Signal
Extracting a reliable atmospheric signal requires observing dozens of transits. Here, too, systems such as TRAPPIST-1 have huge advantages. Around tiny ultra-cool dwarfs, transits of temperate planets happen once every few days to every couple of weeks, instead of once a year for a planet exactly like Earth.
Signs of Biologically Produced Gases
Astronomers, the authors wrote, have already begun investigating the compositions of giant planets around other stars, detecting molecules such as water, carbon monoxide, methane, and hydrogen cyanide. With the discovery of the TRAPPIST-1 system, we can extend those explorations to Earth-sized planets. Their first efforts will be to characterize the greenhouse gas content of the atmosphere, and assess whether the surface conditions are conducive for liquid water. Then we will seek out signs of biologically produced gasses, analogous to ways that living organisms have transformed the composition of Earth’s atmosphere.
Claiming a discovery of life will be hard. We cannot rely on the detection of a single gas but instead will need to detect several, and will need to measure their relative abundances. In addition, we will have to be extremely wary of false positives. For instance, repeated stellar flares could build up oxygen in an atmosphere without the presence of life.
Weeding out False Positives
The richness of the TRAPPIST-1 system is an important asset, because we can compare its planets to one another. All seven planets originated from the same nebular chemistry; they share a similar history of receiving flares and meteoritic impacts. Weeding out false positives will be much easier here than in planetary systems containing only one or two temperate, potentially Earth-like worlds.
More importantly, TRAPPIST-1 is not a one-off discovery. Ultra-cool dwarf stars are so common that there could be numerous other similar systems close to us in the galaxy. The TRAPPIST (Transiting Planets and Planetesimals Small Telescopes) facility they used to find the TRAPPIST-1 planets was just the prototype of a more ambitious planet survey called SPECULOOS (Search for habitable Planets Eclipsing Ultra-Cool Stars).
We expect to find many more Earth-sized, rocky planets around dwarf stars the team noted. With this sample in hand, they will explore the many climates of such worlds. The solar system contains two: Venus and Earth. How many different types of environments will we discover?
Using SPECULOOS, the team will begin to address the many objections scientists have raised about the habitability of planets around ultra-cool dwarfs. One argument is that such planets will be tidally locked, meaning that they have permanent day and night sides. Planets orbiting in close proximity around small stars could excite each other’s orbits, leading to major instabilities. Ultra-cool dwarf stars frequently flare up, emitting ultraviolet and X-rays that might vaporize a planet’s oceans into space.
The Last Word –“TRAPPIST-1 is the Ultimate James Webb Space Telescope Target”
In an email to The Daly Galaxy co-author, astronomer Michael Gillon at University of Liège wrote: “TRAPPIST-1 remains the only known planetary system around an ultracool dwarf star (UCDS). TESS has found a few systems of rocky planets around very-low-mass M-dwarfs, but still a bit too big and hot to fall in the UCDS category. This was to be expected: TESS’ telescopes are too small to efficiently explore nearby UCDS for rocky planets. Such exploration is the goal of our ground-based transit search SPECULOOS, which has been operational since 2019. It has just produced its first discovery, a system containing at least 2 planets slightly larger than Earth around an UCD slightly more massive and larger than TRAPPIST-1 (Delrez et al, in prep.). One of these planets lies in the star’s habitable zone. But TRAPPIST-1 is much more interesting from a detailed characterization point of view. In fact, for reasons described in this paper, TRAPPIST-1 will certainly remain THE ultimate JWST target for the study of potentially habitable Earth-sized planets, and by far.
“We are of course thrilled by the successful launch of JWST!,” Gillon continued in his email. “The deployment of the telescope has also started with success, while it is still on its way to its final orbit. It should be fully operational in April or May. About 200hr of telescope time has been dedicated to the observation of TRAPPIST-1 for the first JWST cycle (up to mid-2023). We should have the first data to analyze in the second part of 2022. We are looking forward to analyzing them and using them to search for traces of atmospheres around the seven planets.
“We still consider it possible to discover chemical traces of biological activity on some TRAPPIST-1 planets with JWST,” Gillon concludes. “But one step at a time. We have first to determine if these planets have managed to keep a significant atmosphere or not. We should learn it within the first two years of JWST, before 2024. If such atmospheres are detected, then we will do all we can to intensify the observation of TRAPPIST-1 with JWST, to determine the chemical compositions of these atmospheres, and possibly to detect a strong chemical disequilibrium of biological origin. We’ll see! In any case, the next few years are going to be extremely exciting!”
“Rocky planets transiting ultra cool dwarfs are our fastest route towards exploring and understanding alien climates, and initiate the search for evidence of biological activity beyond the Solar system,” Amaury Triaud told The Daily Galaxy. “They might also represent the largest population of rocky planets in the Universe. The TRAPPIST-1 system was the first and so far remains the only such ultra-cool dwarf system available for investigation by the Webb telescope. However many others are suspected and our six SPECULOOS telescopes are hard at work to discover new transiting ultra-cool dwarf systems to turn the Webb to.”
Image credit top of page: Artist’s view at top of page of planets transiting a red dwarf star in the TRAPPIST-1 system. Credit: NASA, ESA, and STScI