Five Planets Around Nearby Star Tau Ceti; One in Habitable Zone

The location of Tau Ceti in the night sky. Credit: University of Hertfordshire

Look up in the sky tonight towards the southeast in the constellation Cetus. There’s a naked-eye star named Tau Ceti that lies about 12 light-years away from Earth, and astronomers have discovered a system of at least five planets orbiting Tau Ceti, including one in the star’s habitable zone.

While the recent discovery of a Earth-sized planet around the triple star system Alpha Centauri is the closest planet that has been discovered at just 4.3 light years away, this new discovery is the closest single sun-like star that we know of to host of an entire system of planets. The five planets are estimated to have masses between two and six times the mass of the Earth, making it the lowest-mass planetary system yet detected. The planet in the habitable zone of the star has a mass around five times that of Earth, making it the smallest planet found to be orbiting in the habitable zone of any Sun-like star.

“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,” said astronomer Steve Vogt from UC Santa Cruz, coauthor of the paper describing the discovery. “We are now beginning to understand that nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than 100 days. This is quite unlike our own solar system, where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.”

An artist’s impression of the Tau Ceti system. (Image by J. Pinfield for the RoPACS network at the University of Hertfordshire.)

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.”

Tau Ceti e is the planet in the habitable zone, and its year is about half as long as ours. An independent study of the data from the system done by Abel Méndez at the University of Puerto Rico at Arecibo says that the fifth planet, Tau Ceti f, may also be in the habitable zone.

While over 800 planets have been discovered orbiting other worlds, planets in orbit around the nearest Sun-like stars are particularly valuable to study, the team said.

“Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not-too-distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy,” said James Jenkins of Universidad de Chile, a visiting fellow at the University of Hertfordshire.

The team’s paper that has been accepted for publication in Astronomy & Astrophysics.

Read the team’s paper: Signals embedded in the radial velocity noise (pdf file) or here on arVix

Sources: University of California Santa Cruz, University of Hertfordshire