Researchers from the University of California, Santa Cruz have identified four Earth-sized planets in orbit around the closest Sun-like star to the Solar System, tau Ceti. The star is slightly smaller than the Sun, and less active, and is about 12 light years away. The planets were discovered by improving the methods used to observe slight wobbles in tau Ceti and using noise reduction tools to eliminate wobbles caused by the movement of the stellar matter on the surface of the star.
The finding allows researchers to identify Earth-sized exoplanets more easily. Co-author of the study Steven Vogt says, "We are now finally crossing a threshold where, through very sophisticated modeling of large combined data sets from multiple independent observers, we can disentangle the noise due to stellar surface activity from the very tiny signals generated by the gravitational tugs from Earth-sized orbiting planets."
Two of the planets have masses as small as 1.7 times the mass of the Earth. These are among the smallest exoplanets discovered. The outer two planets in the system, tau Ceti-f and tau Ceti-e are super Earths, and in the habitable zone of the star. This means that liquid water could exist on the surface of the planets, making them prime candidates for exoplanets that could harbour life as it is known on Earth. There is a disk of debris surrounding the star, which means the planets would be exposed to more bombardments by asteroids and comets, which probably reduces the habitability on these planets.
The tau Ceti system is an ideal candidate for the search of extraterrestrial life as it has conditions very similar to our own Solar System. The planets in the tau Ceti system are not likely to be tidally locked to their host stars, which means a day and night cycle on the planets, similar to Earth. On dwarf stars with planetary systems, such as Proxima Centauri and TRAPPIST-1, the Earth-sized planets are tidally locked to the stars, with one side experiencing permanent day and the other side perpetually stuck in a never-ending night. Earth-sized exoplanets in habitable zones have been discovered in orbit around both our closest stellar neighbour, Proxima Centauri, as well as the closest Sun like stellar neighbour, tau Ceti.