An international team of astronomers has discovered three Earth-like exoplanets orbiting an ultra-cool dwarf star—the smallest and dimmest stars in the Galaxy—now known as TRAPPIST-1. The discovery, made with the TRAPPIST telescope at ESO's La Silla Observatory, is significant not only because the three planets have similar properties to Earth, suggesting they could harbor life, but also because they are relatively close (just 40 light years away) and they are the first planets ever discovered orbiting such a dim star. A detailing the teams findings was published today in the journal Nature.
"What is super exciting is that for the first time, we have extrasolar worlds similar in size and temperature to Earth—planets that could thus, in theory, harbor liquid water and host life on at least a part of their surfaces—for which the atmospheric composition can be studied in detail with current technology," lead researcher Michaël Gillon of the University of Liège in Belgium said in an email to Seniorhelpline.
It is more difficult to detect exoplanets orbiting small, dim stars such as TRAPPIST-1 because the changes in brightness when a planet passes in front of the star are harder to detect on dim stars than bright stars. But now that we know where the planets are, it will be easier to study their atmospheres for signs of life, since there's less light from the host star impacting telescope observations.
"The atmospheric composition [of the three planets] can be studied in detail with current technology, including the possible existence of chemical disequilibria of biological origins," says Gillon. "For all other potentially habitable exoplanets found so far, detailed atmospheric studies were not possible with our current technology, because of their larger and brighter host stars that would swamp any atmospheric signals from the planets."
Spectroscopy can be used to measure the abundance of certain elements and molecules in the atmosphere, and chemical imbalances would suggest the presence of life. For example, the amount of oxygen molecules on Earth is much higher than chemical equilibrium would allow for without the influence of plants engaging in photosynthesis.
The fact that the planets are orbiting an ultra-cool dwarf star is significant for other reasons. Stars like TRAPPIST-1 have incredibly long lifetimes—longer than the current age of the universe—giving life plenty of time to take root. TRAPPIST-1 is roughly estimated to be between 1 and 10 billion years old, according to Gillon, and the star's life will continue for tens of billions of years.
The discovery of multiple exoplanets orbiting a small dwarf star also suggests that there could be a large number of undiscovered planets orbiting similar stars.
"This is not one but three planets that have been detected," says Gillon. "This indicates that the formation of Earth-sized planets around these downplayed tiny stellar objects—which are much more frequent than Sun-like stars in the Galaxy—is very efficient. This is a brand new planetary population that is revealed, and it could well dominate the total number of planets in the Milky Way."
One potential problem for life on these newly discovered planets is that the two inner planets are tidally locked with their small star, meaning one half of the planet is always receiving light while the other half experiences constant darkness. Constant day on one side of the planet and constant night on the other would prevent a unique challenge for life.
The two inner planets are slightly closer to the star than the habitable zone where liquid water can form, and the third is either in the habitable zone or just outside it. However, tidal forces could provide energy to melt ice and create constant liquid water on the planets. More observations are needed to determine if the planets could harbor life, but Gillon says, "in theory, all three planets have some habitability potential."
The planets orbit their host star in just two to six days, meaning that we will be able to gather more information about them quickly as they transit their star so frequently. The research team set up a to follow the exploration progress of TRAPPIST-1 and its three orbiting planets.
It's still hard to say for sure if these planets are our best bet, within the solar system or without, to find extraterrestrial life. But unlike the potentially life-harboring moons in our solar system—like Europa and Enceldadus—the rich atmospheres on the newly discovered planets mean that we just might be able to detect life using nothing more than existing telescope technology.
"Europa and Enceladus do not have a significant atmosphere that we can probe with our telescopes," Gillon says. "If life exists there, it is hidden kilometers below the surface, and to find it we have to design robotic probes able to study these moons in situ. For TRAPPIST-1, remote studies with big telescopes could be enough."