Science:
Tau Ceti, at 12 light-years away, is the closest single star
system with a temperature and luminosity similar to those of
the Sun. From an analysis of more than 6000 observations, Mikko
Tuomi of the University of Hertfordshire in the UK and his
colleagues have determined that there may be as many as five
planets orbiting Tau Ceti. Slight changes in the star's motion
through space indicate that all five have masses between 2 and
7 times that of Earth, and all are located closer to their star
than Mars is to our Sun. The fourth planet from Tau Ceti orbits
at a distance similar to that of Venus. Because Tau Ceti emits
only 45% as much light as the Sun, this puts the fourth planet
in the habitable zoneâmdash;the region of
space where water can exist in liquid form. Because of the
small size of the variations in Tau Ceti's motion, confirmation
that the variations are caused by orbiting
planetsâmdash;rather than disturbances on the
star itselfâmdash;will require an improvement
in our ability to identify low-mass planets.
