Our nearest single
Sun-like star hosts five planets - one of which is in the "habitable
zone" where liquid water can exist, astronomers say.
Tau Ceti's planetary quintet - reported in
an online paper that will appear in Astronomy and Astrophysics - was found in existing planet-hunting data.
The study's refined methods of sifting through data should help find even more far-flung worlds.
The star now joins Alpha Centauri B as a nearby star
known to host planets.
Tau Ceti lies 12 light-years distant; Alpha Centauri B, just
four. In both cases, the planets were found not by spying them through a
telescope but rather by measuring the subtle effects they have on their
host stars' light.
"It's a star on which we have a lot of data - an order of magnitude
more data than we have for pretty much any other star," Prof Jones told
BBC News.
"It's a good test case for how low can we go, what size of signals can we pick up."
The team started with data from three planet-hunting missions:
Harps,
AAPS, and
HiRes, all of which had data on Tau Ceti.
The trick to honing the technique was to put in "fake
planets" - to add signals into the messy data that planets should add -
and find ways to reduce the noise until the fake planets became more and
more visible in the data.
"Putting all that together, we optimised a noise-modelling
strategy which allows us to recover our fake signals - but in the
process of doing that, we actually saw that we were finding signals as
well," Prof Jones said - actual planets.
The quintet includes planets between two and six times the Earth's mass, with periods ranging from 14 to 640 days.
One of them, dubbed HD 10700e, lies about half as far from
Tau Ceti as the Earth is from the Sun - and because Tau Ceti is slightly
smaller and dimmer than our Sun, that puts the planet in the so-called
habitable zone.
It is increasingly clear that in existing data from radial velocity measurements there may be evidence of many more planets.
On Monday, Philip Gregory at the University of British Columbia in Canada posted
an as-yet unpublished paper
to the arXiv repository, claiming to have seen three planets in the
habitable zone of Gliese 667C, one of three stars in a triple-star
system, 22 light-years away.
It is also clear that in almost every direction we look and
in every way that we look, there are planets around stars near and far.
