Lending even more credibility to the argument that the galaxy is full of stars with planetary systems, a paper published in January's Astronomy and Astrophysics outlines a group of worlds in orbit around Tau Ceti. The discovery also is of some moment due to the fact that Tau Ceti is the closest Sol-like star to our own. And to make the discovery even more exciting to astronomers -- one of the five planets in the star's train is believed to be in the "Goldilocks," or habitable, zone.
Prof. Hugh Jones of the University of Hertfordshire and his colleagues studied data gathered from several sources, the Harps, AAPS, and HiRes planet hunting missions. The discovery came via the radial velocity method (Doppler spectroscopy, or more colloquially, the "wobble method"), where the data is checked to find discernible shifts in light emitted by the star, which is an indication of said star being acted upon by outside entities (i.e. other stars and/or planets). Jones told BBC News that there was more data on Tau Ceti than any other star and the team was able to "refine" their methods of detection in their search for extrasolar planets.
Jones et. al. introduced "fake planets" into the data and reduced the "noise" around them to make them more discernible. In so doing, the noise reduction produced other entities that stood out from the data -- other worlds.
"Putting all that together, we optimised a noise-modelling strategy which allows us to recover our fake signals - but in the process of doing that, we actually saw that we were finding signals as well," Jones explained.
Of those signals, five were found to be planets. An additional planet, according to the the abstract of the paper (posted online), was initially thought to have been discovered but further study indicated that the signal correlated to "the chromospheric emission from the star."
One of the five planets, designated HD 10700e, orbits about half the distance from Tau Ceti as the Earth does from the Sun. Tau Ceti being smaller and dimmer than the Sun places the planet in what is referred to as the Goldilocks (or habitable) zone, where liquid water might possibly form.
BBC News also noted that the radial velocity method had produced positive results in another (as yet unpublished) study as well. Philip Gregory at the University of British Columbia in Canada announced he had discovered a trio of planets in the habitable zone of Gliese 667C, a member star of a triple triple-star system, 22 light-years away.
Tau Ceti is 12 light years from Earth, the closest star in the same magnitude class and much like our own Sun (in that it is a single star).
Still, there is at least one planet closer to Earth. According to a report published in October on Nature.com, a planet was discovered circling Alpha Centauri B, one of the three stars in a star system closest to Earth. Although the planet was comparably Earth-massed and similar in type, it rotated around its parent star closer than does Mercury in our own Solar System, making the world most likely having the consistency of lava, at least on the side believed to permanently face its host star.
Alpha Centauri B and its planet, Alpha Centauri Bb, are located just short of 4.4 light years away.
The planet was also discovered using the radial velocity method.
More than 850 extrasolar planets have been detected since 1995, according to The Extrasolar Planets Encyclopedia. Of those, almost 500 have been found using the radial velocity method to date.
***** Author's note: Title reflects inaccurate description of Tau Ceti as "nearest star to Earth," when it should more properly be designated as the "nearest Sun-like star to Earth" (not considering the Sun itself).