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Four planets in 'habitable zones' spotted within spitting distance of Earth

Astronomers say they used a new statistical technique to find four possible super-Earths orbiting in the habitable zone of two stars within 22 light-years of Earth, Gliese 667C and tau Ceti.

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There, the planets range in mass from twice to five time's Earth's mass. Orbit times for the planets range from seven days for the innermost to 91 days for the outermost. Here again, the fourth planet out – an object with twice Earth's mass and a travel time of one orbit every 39 days – is the habitable-zone winner.

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The results from Tuomi's team were published online Wednesday by the journal Astronomy and Astrophysics. Dr. Gregory has submitted a formal paper reporting his results to the Monthly Notices of the Royal Astronomical Society in Britain and has posted a draft of the paper on the research server arXiv.

The two efforts were based on so-called radial-velocity measurements of the host stars. In essence, these are measurements of a star's back and forth motion as a planet orbits and its gravity tugs on the star. The motion appears as a back-and-forth wobble in the star's spectrum.

A big planet close in has a more pronounced effect than either a small planet close in or more-distant planets. For Earth-scale planets in habitable zones, the effect can be quite weak. This opens the door to confusion, because the star's own activity – star spots, flares, prominences, and other features – can also produce weak wobbles in the star's spectrum.

A star's "noise" lacks an important feature: the consistent, regular repetition of a planet's orbit. But that feature can be hard to detect amid a star's noise, dubbed "star jitter."

Rather than using a traditional approach of starting with the hunt for a planet-imparted wobble, Vogt explains, Tuomi devised a mathematical approach that focused on a star's noise.

To test the approach, the team needed a very bright star that had been a target of lots of radial-velocity measurements from different observatories and that had yielded no evidence of a planet. No planet must mean all jitter. Tau Ceti fit the bill.

The team combined 5,943 radial-velocity measurements gathered at observatories in Chile, Australia, and Hawaii over time spans of up to 13-1/2 years. The largest number of measurements came from a high-precision HARPS spectrometer at the European Southern Observatory's 3.6 meter telescope at La Silla. The team then looked for spectral signatures of the star's jitter across all three sets of observations and how that jitter changed over time.

Tuomi then developed a set of competing mathematical simulations of the star, each of which yielded the jitter patterns the team saw in the star. Then he introduced signatures of faux planets in each model to see which model had the highest probability of mimicking the star's jitter – and of revealing the faux planets.

The team applied the winning model of tau Ceti's star jitter to the full range of data coming from the star, and out popped the five candidate planets.

"These were signals underneath the noise that none of us had really known how to deal with," Vogt says. "These are very weak signals, and out they came."

"We have only now started to realize how important it is to model the stellar jitter as realistically as possible," Tuomi adds in an e-mail exchange.

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