Red dwarfs, such as TRAPPIST-1, make for great stars to search because they are already dim, making any change in brightness easier to detect. Planets orbiting dwarf stars generally have short orbital periods, making an entire trip around in weeks or even days, so there is plenty of opportunity to catch them moving across the face of the star. (The TRAPPIST-1 planets orbit their star in anywhere from 1.5 to about 20 days).
Red dwarfs are also the most common and longest-living stars in the galaxy. An estimated 85 percent of the 100 billion stars in the Milky Way are red dwarfs. TRAPPIST-1 will continue slowly burning hydrogen for another 10 trillion years, 700 times longer than the entire history of the universe—plenty of time for life to take root and evolve.
These worlds, as exciting as they are, have a few properties that call their potential habitability into question. For one thing, the planets are likely tidally locked with TRAPPIST-1, meaning the same face of the planet is always facing the star, just like one half of the moon always faces the Earth. If this is the case, one side of each planets experiences perpetual daylight while the opposite side is stuck in an endless night.
Another concern is that red dwarfs can be particularly active stars with stellar eruptions, flares, and coronal mass ejections bombarding the nearby planets with radiation. A recent study casts doubt on the habitability of the closest exoplanet to Earth, Proxima-b, for this reason. That said, TRAPPIST-1 is a cooler star than Proxima Centauri, and it might not rain down high-energy particles on its planets so relentlessly.
"To the best of our knowledge, TRAPPIST-1 appears particularly quiet," says de Wit.
It may be that TRAPPIST-1 is a gentle, life-giving star after all. But it's almost 40 light-years away, particularly small and exceedingly dim. How do we find out for sure?
PROBING THE ATMOSPHERES OF EXOPLANETS
There is a developing plan to launch small nanoprobes to the Alpha Centauri system, the closest stars to us, known as Breakthrough Starshot. With a network of large lasers on Earth, it just might be possible to accelerate a probe to roughly 20 percent the speed of light by constantly hitting a reflective surface with the concentrated beams of light. This technology, known as photonic propulsion, could allow us to reach Alpha Centauri in 20 or 30 years.
But we're not going to TRAPPIST-1 anytime soon. It's about eight times farther away than Alpha Centauri. Even if we could launch a probe at relativistic speeds, it would take two centuries to get there, the immense distance makes it unlikely that it would arrive at all. Even if it did, detecting a signal from a small nanoprobe 40 light-years away would be darn near impossible. For now, we need to study the TRAPPIST system from home.
We can do more with our telescopes than you might think, especially new ones expected to go online in the 2020s. By imaging the light that passes through an exoplanet's atmosphere, we can look for gaps in the electromagnetic spectrum—wavelengths of light that are absorbed by the presence of specific elements. This technique, called absorption spectroscopy, can tell us the composition of a planet's atmosphere, even 40 light-years away.
"There is a lot to be done with what we already have and what we are about to have!" says de Wit in regard to telescopes. "With observations of this system taken by Hubble last May, we have already ruled out the presence of puffy, hydrogen-dominated atmospheres around the two innermost planets, which means that they are not 'mini-Neptunes' that would be uninhabitable, but are terrestrial like Mercury, Venus, Earth and Mars."
The launch of the James Webb Space Telescope (JWST) next year and the completion of the Giant Magellan Telescope (GMT) in the high Atacama Desert of Chile will give us a clearer picture of these exoplanets than ever before. James Webb's 18, gold-plated mirrors will detect objects 16 times fainter than Hubble, and the gargantuan GMT, with seven 15-ton mirrors, should be capable of imaging exoplanets directly rather than looking for dips in a star's brightness. These two scopes, along with others such as the European Extremely Large Telescope, will tell us exactly what kind of atmospheres the TRAPPIST-1 planets have.
"Over the next two years, we are hoping to leverage Hubble's capabilities to search for the presence of water- or methane-dominated atmospheres," says de Wit. "In the future, upcoming observatories like the James Webb Space Telescope will help us constrain the planets' atmospheric composition, temperature, and pressure profiles—all essential information for determining the surface conditions possible over their globes."
This information will not provide clear-cut proof, but it could provide substantial visual evidence if one of these planets does in fact support life. Astronomers will be searching for "biosignatures," such as high levels of oxygen that could be the result of photosynthesis. Abundant oxygen isn't enough to determine the presence of life, though.
"We need more than just O2," says de Wit. "Biosignatures can appear in many forms from complex molecules like CFCs [chlorofluorocarbons] or mixes of molecules such as H2O, O2/O3, CO2 or CH4. JWST may not provide sufficient evidence to prove the presence of a biomass by itself, but it will inform us on the habitability of the planets."
Though the TRAPPIST-1 system is too far away for a probe, it is wonderfully close on a galactic scale. With the planets orbiting their sun every few days, we will have hundreds of opportunities to observe these tantalizing worlds with our ever-improving telescopes. What's more, many astronomers believe that TRAPPIST-1 might not be an anomaly at all, but that thousands of nearby red dwarfs and other stars have extensive planetary systems.
It's hard not to think that somewhere, one of those planets harbors lush alien gardens, and with continued science like this, we are going to find it.