The star is only about the size of Jupiter and much colder and redder than the sun. Its luminosity is much less than 1 percent of our star’s — so dim that even though the “ultracool” dwarf star called TRAPPIST-1 is less than 40 light-years from Earth, it can only be seen with relatively powerful telescopes.
Still, it’s a star worth seeking out. Astronomers using a 60 cm telescope specially designed to study such stars and all the planets around them have found that this system contains some of the most habitable exoplanets yet discovered. When European astronomers looked at TRAPPIST-1 from September to December last year, they found a slight, periodic eclipse indicating the presence of three worlds that lie close to or within the system’s habitable zone. They all have a radius between 1.05 and 1.17 that of the Earth’s radius.
This is evident from observations published in the journal on Monday Nature, the two inner planets orbit the star every 1.51 days and 2.42 days. The innermost planet, TRAPPIST-1b, likely receives about four times the solar radiation from its star than Earth, and astronomers estimate its surface temperature is likely closer to the upper end of a range between 11 degrees and 127 degrees Celsius. The next planet, TRAPPIST-1c, receives slightly more than twice as much solar radiation as Earth and has a surface temperature likely between -30 degrees and 69 degrees Celsius. The researchers speculate that these worlds are likely tidally locked and therefore, even if they have extremely average temperatures, they may have habitable regions along the terminator or poles.
A third planet, TRAPPIST-1d, is even more intriguing. Although astronomers have fewer confirmed observations of this world, they estimate its orbital period to be between 4 and 70 days, and it is quite a bit further away, perhaps 0.146 astronomical units (the Earth-Sun distance) from its star. Nevertheless, between the star’s warmth and the likely presence of internal tidal heating, they speculate that this world probably lies within or just outside the star’s habitable zone.
However, much uncertainty remains about the nature of these three worlds, as only so much information can be gleaned from the star’s light. A big question concerns the masses of the three planets, which cannot be determined from existing observations. An analysis of data from Kepler spacecraft found that most Earth-sized worlds in close orbit around Sun-like stars are rocky. However, much less is known about early conditions of a system that forms around ultracool dwarfs, and so it is not clear whether these are icy, rocky or gassy planets.
Nevertheless, the finding is significant for at least a number of reasons. First, it provides some observational support for the theory that small, cool stars may be reservoirs of planets. Astronomers estimate that about 15 percent of the “neighborhood” stars around the sun are these ultracool dwarf stars. Second, the proximity of the TRAPPIST system opens the door to observations with existing large telescopes.
“Why are we trying to detect Earth-like planets around the smallest and coolest stars near the Sun? The reason is simple: systems around these tiny stars are the only places where we can test life on an Earth-sized exoplanet with our power.” technology, said Michaël Gillon, lead author of the Nature paper. “So if we want to find life elsewhere in the universe, we have to start looking here.”
Indeed, observations with the Hubble Space Telescope could provide some initial constraints on the atmospheres of these three worlds. Then the James Webb Space Telescope, scheduled to launch in late 2018, could provide crucial information about the abundance of molecules in the atmosphere, including the biologically interesting water, carbon dioxide, methane and ozone. This information would also allow astronomers to place constraints on the surface temperatures of these worlds. Other much larger ground-based telescopes, such as the Giant Magellan Telescope, which will come online in the 2020s, will provide more detail.
Nature2016. DOI: 10.1038/nature17448 (About DOIs).