An Overview of Tau Ceti a Star with Inhabitable Planets

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Tau Ceti is a yellow star in the constellation Cetus, which is visible from both the Northern and Southern hemispheres. The star's name is a combination of a Greek letter and the name of the constellation. In 1603, Johann Bayer compiled the Uranometria star catalog, and labeled the stars in each constellation with Greek letters. The stars in his catalog were usually given letters in order of descending brightness, but sometimes they were given letters by position. In Greek mythology, the constellation represents the sea monster slain by Perseus while rescuing Andromeda. In biology, the order Cetacea includes whales, dolphins and porpoises. The star is also known by several other names and numbers, including HD 10700 used in the recent report, which is from the Henry Draper catalog.

Tau Ceti is just 11.9 light-yeas away,  in the direction given by stellar coordinates right ascension 01h 44m 04s and declination -15° 56' 15”.  It appears in the sky near the star Mira, which is much larger and farther away. It is spectral type G8.5 V, smaller and less active than the sun. The star's mass is 0.78 that of the sun.  At 5.8 billion years old, it is also older than the sun, and has less metal in it. It rotates once every 34 days, which is important in the recent report of planets.

Tau Ceti is the second closest  type G star to the sun, after Alpha Centauri A. The sun is also a type G star, so type G stars have been thought to be among the most likely to have planets that support life. So Tau Ceti has been a popular home to alien planets in science fiction, and a target of astronomical searches for life, because of it's proximity to Earth and spectral type.

In The Caves of Steel (1954) by Isaac Asimov, a planet orbiting Tau Ceti was mankind's first extrasolar planetary settlement, Aurora. In Time for the Stars (1956) by Robert Heinlein, the Starship Lewis and Clark finds a number of planets including Constance, which is later colonized, in orbit around Tau Ceti. Wikipedia has a more complete list of fiction that mentions Tau Ceti. UFOlogists have also claimed it is the home of the “Tau Cetians” a humanoid race that is fighting the “Greys.” During the 1960s, Project Ozma, led by Frank Drake, used the Green Bank radio telescope to conduct a SETI investigation into Tau Ceti and Epsilon Eridani for signs of life, but no signals were found.

In December 2012, the discovery of five planets orbiting the star Tau Ceti was announced by an international team of astronomers from the United Kingdom, Chile, United States and Australia. Scientists had discovered a dusty debris disk at least 10 times more massive than our solar system’s Kuiper Belt in 2004. But they had not previously discovered any planets, and it has been observed enough that no large planets were expected to be found. Astronomers led by Mikko Tuomi combined more than 6,000 observations from the UCLES spectrograph on the Anglo-Australian Telescope, the HIRES spectrograph on the Keck Telescope, and spectra from the HARPS spectrograph based in Chile. It should be noted there could be three (based on HARPS data alone) or five (* based on the combined data) planets. HD 10700 e and maybe HD 10700 f are the ones that are at a distance which might be habitable, and they are less certain than the others. Their signal is below the threshold until all the data is combined. There are several strong signals with periods between 3.7 and 5.5 days which are not considered to be planets. Also, the strong signal at 35 days could be an artifact of the star's spin.

And the planets are:
Planet Year (in Earth Days)  Distance (from Tau Ceti) Mass (compared to Earth)

HD 10700 b 13.9 days 0.105 AU 2.0 Me

HD 10700 c 35.4 days 0.195 AU 3.1 Me

HD 10700 d 94 days 0.37 AU 3.6 Me

HD 10700 e* 168 days 0.55 AU 4.3 Me

HD 10700 f* 642 days 1.35 AU 6.6 Me

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