Astronomers say they have uncovered evidence for what could be
four super-Earth planets orbiting within the habitable zones of two
stars within 22 light-years of Earth.
Three of those candidate planets are among a tightly packed
clutch of five that orbit Gliese 667C, part of a triple-star system
22 light-years away in the constellation Scorpius. The other
possible planet is one of five orbiting tau Ceti, a sun-like star 12
light-years away in the constellation Cetus.
Taken together, the detections not only add to accumulating
evidence that planets look to be more common than stars and that
planets in habitable zones could be more common than previously
thought, some of researchers reporting the finds say.
The finds also illustrate the power of improved statistical tools
to boldly uncover candidate planets where no planet had been found
The evidence for these candidate planets requires independent
confirmation, the researchers caution. Still, the tools represent "a
real breakthrough," says Steven Vogt, an astronomer at the
University of California at Santa Cruz and a member of the team
reporting the results for tau Ceti. The approach the team took
leaves only about one chance in 3 million that the detections could
herald something other than a planet.
Since the mid-1990s, astronomers have bagged more than 850 extra-
solar planets. The ultimate goal is to find rocky planets with Earth-
like masses orbiting within their stars' habitable zones a region
where under the right conditions, liquid water can form stable pools
on the surface. Liquid water is considered an essential ingredient
for organic life.
Different groups of astronomers had aimed three telescopes for
various lengths of time at tau Ceti and found nothing. Led by Mikko
Tuomi at the University of Hertfordshire in Britain, the team
reporting this latest analysis applied relatively new statistical
tools to the combined data from these telescopes.
The result: "Five planets came out: boom, boom, boom, boom, boom
... as clear as a bell," Dr. Vogt says.
Tau Ceti has about 78 percent of our own sun's mass, but its
composition is quite similar, Dr. Tuomi's team reports. Its
candidate planets range from 2 to 6.6 times Earth's mass.
The innermost object orbits the star once every 14 days, while
the outermost takes 642 days to make its circuit. The fourth planet
from the star, with a 168-day orbit, travels well within a zone
where liquid water could remain stable on the planet's surface, the
team estimates. However, the results don't speak directly to what
the planets are made of.
A similar story has played out for Philip Gregory, an astronomer
at the University of British Columbia in Vancouver. Previous
researchers had found two planets orbiting Gliese 667C, a red dwarf
with 31 percent of the sun's mass. Using a broadly similar
statistical approach, he reports detecting the initial two, plus
three more planets. Three of the five fall within the star's
habitable zone, he estimates.
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. …