Another Earth Just 12 Light-Years Away?

Another Earth Just 12 Light-Years Away?




Sunlike star.
Tau Ceti is so close to us that it's visible to the naked eye.

Credit: Mikko Tuomi/University of Hertfordshire










Astronomers have discovered what may be five planets orbiting Tau
Ceti, the closest single star beyond our solar system whose temperature
and luminosity
nearly match the sun's. If the planets are there, one of them is
about the right distance from the star to sport mild temperatures,
oceans of liquid water,
and even life. Don't pack your bags just yet, though: The discovery
still needs to be confirmed.


Tau Ceti is only 12 light-years from Earth, just three times as far
as our sun's nearest stellar neighbor, Alpha Centauri. Tau Ceti
resembles the sun so
much that astronomer Frank Drake, who has long sought radio signals
from possible extraterrestrial civilizations, made it his first target
back in 1960.
Unlike most stars, which are faint, cool, and small, Tau Ceti is a
bright G-type yellow main-sequence star like the sun, a trait that only
one in 25 stars
boasts. Moreover, unlike Alpha Centauri, which also harbors a G-type
star and even a planet, Tau Ceti is single, so there's no second star
in the system whose gravity could yank planets away.


Astronomer Mikko Tuomi of the University of Hertfordshire in the
United Kingdom and his colleagues analyzed more than 6000 observations
of Tau Ceti from
telescopes in Chile, Australia, and Hawaii. As the researchers will
report in Astronomy & Astrophysics, slight changes in Tau Ceti's motion
through space suggest that the star may be responding to gravitational tugs from five planets that are only about two to seven times as massive as Earth.


If that's right, all five planets lie closer to their star than Mars
does to ours; however, Tau Ceti emits only 45% as much light as the
sun, so each
planet receives less warmth than a planet would at the same distance
from our sun. Tau Ceti's three innermost planets—designated b, c, and
d—are
probably too hot to support life, being so close to the star that
they require only 14, 35, and 94 days to complete an orbit. The farthest
of the three, d, is about as close to Tau Ceti as Mercury is to the sun.


It's the fourth planet—planet e—that the scientists suggest might be
another life-bearing world, even though it's about four times as
massive as
Earth. If you lived there, you'd see a yellow sun in the sky, but
your year would last just 168 days. That's because Tau Ceti e lies
somewhat closer to its
star than Venus does to the sun and thus revolves faster than Earth.
The fifth and outermost planet, designated Tau Ceti f, completes an
orbit every 640
days and is slightly closer to its star than Mars is to the sun.


However, Tuomi's team warns that disturbances on the star itself,
rather than orbiting planets, may be producing the small velocity
changes in Tau Ceti.
"They're really digging deep into the noise here," says Sara Seager,
an astronomer at the Massachusetts Institute of Technology in Cambridge
who was not
part of the team. "The [astronomical] community is going to find it
hard to accept planet discoveries from signals so deeply embedded in
noise."


"They're pushing the envelope," says Gregory Laughlin, an astronomer
at the University of California, Santa Cruz. "Some or even many of
these planets could
go away. But I think that they've done absolutely the best job that
you can do, given the data." Laughlin says it's frustrating that the
most interesting
planets—small ones like Earth—are so challenging to detect: "You
have to get tons and tons and tons of velocity measurements over many
years, and
then you really, really have to take extreme care—as this Tuomi et al. paper does—to get rid of all the systematic noise."


Team member Chris Tinney, an astronomer at the University of New
South Wales in Sydney, Australia, acknowledges the problem. "It's
certainly very
tantalizing evidence for potentially a very exciting planetary
system," Tinney says, but he adds that verifying the discovery may take
10 years, and the
scientists didn't want to wait that long. "We felt that the best
thing to do was to put the result out there and see if somebody can
either independently
confirm it or shoot it down."


If the planets exist, they orbit a star that's about twice as old as our own, so a
suitable planet has had plenty of time to develop life much more advanced than Homo sapiens. That may just explain why no one from Tau Ceti has
ever contacted beings as primitive as us.



Source:-
http://news.sciencemag.org/sciencenow/2012/12/another-earth-just-12-light-year.html