I like the overview of the discovery provided at Scientific American.
Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth — a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.
With a minimum mass just 4.3 times that of Earth, this potential planet would be the smallest yet found in the habitable zone of a sun-like star if it's confirmed, researchers said.
"This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets," study co-author Steve Vogt, of the University of California, Santa Cruz, said in a statement. "They are everywhere, even right next door."
The five planet candidates are all relatively small, with minimum masses ranging from 2 to 6.6 times that of Earth. The possibly habitable world, which completes one lap around Tau Ceti every 168 days, is unlikely to be a rocky planet like Earth, researchers said.
"It is impossible to tell the composition, but I do not consider this particular planet to be very likely to have a rocky surface," lead author Mikko Tuomi, of the University of Hertfordshire in England, told SPACE.com via email. "It might be a 'water world,' but at the moment it's anybody's guess."
Universe Today's Nancy Atkinson goes into more detail about Tau Ceti's background and the methodology of the study.
Tau Ceti has long been a target of both detailed astronomical study and hopeful science fiction, since it is among one of the 20 closest stars to Earth. It is also easily visible to the naked eye and can be seen from both the Northern and Southern Hemisphere. During the 1960′s, Project Ozma, led by SETI’s Frank Drake, probed Tau Ceti for signs of life by studying interstellar radio waves with the Green Bank radio telescope. Science fiction authors like Robert Heinlein, Isaac Asimov and Frank Herbert used Tau Ceti as destinations and focal points in their books.
Scientists know this star has a dusty debris disk at least 10 times more massive than our solar system’s Kuiper Belt, and it has been observed long enough that no planets larger than Jupiter have been found.
An international team of astronomers from the United Kingdom, Chile, United States, and Australia, combined more than six-thousand observations from the UCLES spectrograph on the Anglo-Australian Telescope, the HIRES spectrograph on the Keck Telescope, and reanalysis of spectra taken with the HARPS spectrograph available through the European Southern Observatory public archive.
Using new techniques, the team found a method to detect signals half the size of previous observations, greatly improving the sensitivity of searches for small planets.
“We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches,” said lead author Mikko Tuomi of the University of Hertfordshire. “This significantly improved our noise modeling techniques and increased our ability to find low-mass planets.”
The paper is "Signals embedded in the radial velocity noise Periodic variations in the τ Ceti velocities", also available here.
The question of whether any of these world are habitable is open to question. At one extreme, Enzo, in the comments at Centauri Dreams' post, suggests that Tau Ceti e--the planet most likely to support Earth-like conditions--orbits Tau Ceti too closely to avoid overheating. At the other, a press release from the University of Puerto Rico at Arecibo's Planetary Habitability Laboratory suggests that Tau Ceti f, the outermost planet discovered, might be barely within the habitable zone.