An international team of astronomers has announced the discovery of seven Earth-sized exoplanets in orbit around a nearby ultracool red dwarf star. Whilst all the exoplanets discovered around the red dwarf, known as TRAPPIST-1, are capable of hosting liquid water on their surfaces, three are in orbit in what is known as a star's habitable zone, making them an attractive prospect for scientists searching for life outside of our solar system.
TRAPPIST-1 is only a little larger than the planet Jupiter, with a mass of only eight percent of our Sun. The eight exoplanets were confirmed following an intensive imaging campaign undertaken by a host of observatories, including the European Southern Observatory's Very Large Telescope, located at the Paranal Observatory in Chile, and NASA's orbital Spitzer Space Telescope.
The team was able to deduce the number of exoplanets, as well as certain characteristics of these alien worlds such as their size, orbit, and composition, by identifying dips in the perceived light output of TRAPPIST-1 as the planets passed between Earth and the star, blocking a portion of its light.
Due to its diminutive size, TRAPPIST-1 appears very dim in the sky, despite lying only 40 light-years distant from Earth – relatively close in astronomical terms.
"The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun," comments Study co-author Amaury Triaud. "Planets would need to be in far closer orbits than we see in the Solar System if there is to be surface water. Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1!"
The planets have been named TRAPPIST-1b, c, d, e, f, g, and h. TRAPPIST-1b represents the planet closest to the red dwarf, with h being the most remote. The TRAPPIST-1 solar system is far more cramped than our own, with each of the seven worlds orbiting closer to their star than Mercury does to the Sun.
According to density measurements taken by the team, it is likely that the innermost six exoplanets have a rocky composition. Climate models suggest that TRAPPIST-1b, c and d are likely too hot to maintain liquid water across all but a fraction of their surfaces.
The seventh planet in the system, TRAPPIST-1h, is likewise considered unlikely to host liquid water. However, the team has not ruled out the possibility of liquid water on the outermost planet, as it is possible that a phenomenon known as tidal heating, which is thought to be the driving force behind the eruptions taking place on the Jovian moon Io, could be warming the exoplanet.
However, without question the most exciting of the exoplanets are TRAPPIST-1e, f, and g, which reside within the red dwarf's habitable zone (HZ). It is possible that the rocky planets within this region could support oceans of liquid water, making them an attractive breeding ground for life outside of our solar system.
The TRAPPIST-1 solar system is currently being targeted for follow up observations by the venerable Hubble Space Telescope, which will attempt to discover whether any of the newly confirmed worlds play host to an atmosphere. The star will also be a high-priority target for the next generation of planet hunting observatories, including the ESO's European Extremely Large Telescope, and the NASA/ESA/CSA James Webb Space Telescope.
The following NASA video offers a look at what life might be like on a TRAPPIST-1 planet.