Nearest Sun-like star orbited by four Earth-sized planets

Sun-like stars seen as best hopes for harboring Earth-like worlds.
By Laurel Kornfeld | Aug 11, 2017
Four roughly Earth-sized planets have been discovered orbiting the star Tau Ceti, the closest Sun-like star to our solar system, located approximately 12 light years away.

Visible to the naked eye, Tau Ceti is estimated to be about 78 percent as massive as the Sun and approximately 5.8 billion years old.

Using the radial velocity method, which involves searching for planets by measuring gravitational tugs on a star, a research team led by Fabo Feng of the University of Hertfordshire in the UK detected four planets orbiting Tau Ceti, two of which are located in the star's habitable zone.

Two of the planets have about 1.7 Earth masses, and the other two have about 3.9 Earth masses. To date, they are the smallest planets discovered orbiting a Sun-like star.

Proxima Centauri b, the nearest exoplanet to our solar system, orbits a dimmer, less massive red dwarf star.

Designated Tau Ceti g, h, e, and f, the planets have estimated orbital periods of 20 days, 49.3 days, 160 days, and 600 days, respectively.

The researchers also detected a signal that could indicate a planet with an orbital period of 90 days, but that has not been confirmed.

Tau Ceti e and f orbit in the inner and outer edges of the star's habitable zone, where temperatures could allow liquid water to exist on their surfaces.

"We are now finally crossing a threshold, where, through very sophisticated modeling of large combined data sets from multiple independent observers, we can disentangle the noise due to stellar surface activity from the very tiny signals generated by the gravitational tugs from Earth-sized orbiting planets," said Stephen Vogt of the University of California at Santa Cruz.

Scientists measuring stellar wobbles caused by gravitational tugs can now detect movements as small as 30 centimeters per second. Detection of Earth analogs will require the ability to detect movements of 10 centimeters per second, said Feng, lead author of a study on the findings published in the Astrophysical Journal.

Located further from Tau Ceti, these planets are less likely to be tidally locked to the star--having one side always face the star and the other side always face away from it--than planets orbiting smaller red dwarfs.

However, the Tau Ceti planets may still not be habitable because a large disk of debris has been detected orbiting the star, from which asteroids and comets could regularly bombard them.

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