Ricerca di vita extraterrestre

American Association for the Advancement of Science Washington, D.C.

Earl Lane, 202-326-6431

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Astronomer announces shortlist of stellar candidates for habitable worlds

In the search for life on other worlds, scientists can listen for radio
transmissions from stellar neighborhoods where intelligent civilizations
might lurk or they can try to actually spot planets like our own in
habitable zones around nearby stars.

Either approach is tricky and relies on choosing the right targets for
scrutiny out of the many thousands of nearby stars in our galactic

Margaret Turnbull, an astronomer at the Carnegie Institution of
Washington, has devoted herself to the painstaking search for candidate
stars that may harbor zones of habitability where life – primitive or
otherwise – might thrive. Turnbull announced her shortlist of so-called
“habstars” at the 2006 Annual Meeting of the American Association for the
Advancement of Science in St. Louis.

Out of an initial catalogue of 17,129 “habitable stellar systems” that
Turnbull and her colleagues published in 2003, she selected a handful of
stars that she considers her best bets, based on a variety of screening

Turnbull offered five top candidate stars for those seeking only to listen
for radio signals from intelligent civilizations – the Search for
Extraterrestrial Intelligence or SETI – and five candidates for those who
undertake the demanding job of trying to detect Earth-like planets in
orbit around nearby stars.

Astronomers have found evidence during the past decade for dozens of
planets around nearby stars by studying how an object’s gravity affects
the orbit of the parent star. Virtually all of the discovered planets are
gas giants like Jupiter and are presumed to be inhospitable to life. There
have been hints of smaller, rocky planets like Earth, but definitive
detection of such terrestrial planets likely awaits the deployment of more
capable space-based observatories in about a decade. “It’s impossible to
know the true nature of those planets until we can directly image them,”
Turnbull said.

NASA had a mission on the drawing board called the Terrestrial Planet
Finder, which would consist of two complementary space observatories. The
first, a visible-light coronagraph, had been scheduled for launch around
2016, but the project has been deferred indefinitely, according to NASA’s
2007 budget plan. A precursor planet-finder, called SIM PlanetQuest, has
been delayed until at least 2015.

Turnbull made her habstar choices “purely on the characteristics of the
stars themselves,” she said in an interview. “Stars are not all the same,
and not all of them are like the Sun.”

Her criteria included several related to age. The star has to be at least
3 billion years old, long enough for companion planets to form and complex
life to develop. Variable stars that are prone to lots of flares and
pyrotechnics tend to be too young to meet her criteria. Also, stars more
than 1.5 times the mass of our Sun don’t tend to live long enough to
produce habitable zones.

Turnbull also considered the star’s “metallicity.” Stars and planets form
out of the same parental cloud of dust and gas. If the star doesn’t have
enough iron in its atmosphere, it is likely the parent material did not
contain enough heavy metals for planets to form. Turnbull’s candidate
stars had to have at least 50 per cent of the iron content of the Sun.
Stars with higher metal content also tend to reside in more peaceful
orbits in the plane of the galaxy, Turnbull said. She also stars that,
like our Sun, that reside on the “main sequence” of stellar evolution. No
red giants or white dwarfs allowed.

While her criteria are clearly Sun-centric, Turnbull said they make sense.
“We are intentionally biased toward stars that are like the Sun,” she
said. Like the Sun, such stars tend to be toward the brighter range in
luminosity and are more likely to live long enough for life-supporting
planets to form.

“These are places I’d want to live if God were to put our planet around
another star,” Turnbull said.

The search for signals from extraterrestrial civilizations will benefit
from a new network of radio antennas, called the Allen Telescope Array,
now under development. Forty two of the planned 350 telescopes in the
array should be operational this year.

Turnbull’s top candidate star for such radio scans is beta CVn, a sun-like
star about 26 light-years away in the constellation Canes Venatici (the
Hound Dogs). (One light-year is about 5.9 trillion miles). Astronomers
have been using currently available technology to search the star for
accompanying planets but none has been found so far, Turnbull said. Her
other top candidates for a SETI search:

  • HD 10307, another solar analogue about 42 light-years away. It has
    almost the same mass, temperature and metallicity of the Sun. It also has
    a benign companion star.
  • HD 211415, about half the metal content of Sun and a bit cooler, this
    star is in just a little farther away than HD 10307.
  • 18 Sco, a popular target for proposed planet searches. The star, in the
    constellation Scorpio, is almost an identical twin to the Sun.
  • 51 Pegasus. Already famous. In 1995, Swiss astronomers reported they had
    detected the first planet beyond our solar system in orbit around 51
    Pegasus. An American team soon verified the finding of the Jupiter-like
    object and the rush to find more extra-solar planets was on. Turnbull
    thinks 51 Pegasus could harbor terrestrial planets as well.

Efforts to take direct images of Earth-like planets – the goal of the
planned Terrestrial Planet Finder (TPF) mission – are extremely
challenging. Astronomers want to find Earth-like planets orbiting close
enough to the star – but not too close – for there to be an environment
capable of having liquid water, a key ingredient of life. But such planets
in orbits relatively close to the star simply get lost in the glare of the
host star.

In choosing candidate stars for a TPF mission, Turnbull went for stars
with enough intrinsic luminosity to suggest good prospects for a habitable
zone but not so bright as to overwhelm efforts to images their planets. In
her Goldilocks solution, the best candidates were K-class stars, objects
that are intrinsically dimmer than the Sun.

Turnbull’s top choice is epsilon Indi A, a star only about one-tenth as
bright as the Sun. It is nearby, about 11.8 light-years away in the
constellation Indus. The star is among the top 100 targets for a TPF

Her other TPF candidates:

  • epsilon Eridani. A star somewhat smaller and cooler than our Sun,
    located about 10.5 light-years away in the constellation Eridanus (the
  • omicron2 Eridani. A yellow-orange star about 16 light-years away,
    roughly the same age as the Sun.
  • alpha Centauri B. Part of the closest stellar system to the Sun, just
    4.35 light-years away. Long considered one of the places in the Milky Way
    that might offer terrestrial conditions. This star is part of a triple
    star system.
  • tau Ceti. Unlike the candidates in this group, Tau Ceti is a G-class
    star, the same brightness category as our Sun. Metal-poor compared to the
    Sun but long-lived enough for complex life forms to evolve.

Turnbull acknowledges that it is a toss up when it comes to naming just a
few candidate habstars. “There are inevitable uncertainties in how we
understand these stars,” Turnbull said. “If I took the top 100, it would
be very difficult for me to tell which one is the best.” But the exercise
is worthwhile, she said, and her selection criteria really did drive her
toward a couple top choices and a handful of other candidates.

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Office of News Services University of Colorado-Boulder Boulder, Colorado

Carol Cleland, (303) 492-7619
Jim Scott, (303) 492-3114

Feb. 18, 2006

Search For Alien Life Challenges Concepts, Says CU-Boulder Professor

For scientists eying distant planets and solar systems for signs of alien
activity, University of Colorado at Boulder Professor Carol Cleland
suggests the first order of business is to keep an open mind.

It may be a mistake to try to define life, given such definitions are
based on a single example – life on Earth, said Cleland, a philosophy
professor and fellow at the NASA-funded CU-Boulder Center for
Astrobiology. The best strategy is probably to develop a “general theory
of living systems,” she said.

Many biologists agree the best definition of living systems today is the
“chemical Darwinian definition” involving self-sustaining chemical systems
that undergo evolution at the molecular level, she said. But the theory is
limited in that life on Earth probably resulted from physical and chemical
“contingencies” present at the time of its origin on the planet.

“What we really need to do is to search for physical systems that
challenge our current concept of life, systems that both resemble familiar
life and differ from it in provocative ways,” she said. Cleland
participated in an astrobiology symposium at the annual American
Association for the Advancement of Science meeting held in St. Louis Feb.
16 to Feb. 20.

In 1976, for example, NASA’s Viking 1 spacecraft conducted automated
biology experiments on Mars by mixing soil samples with radioactively
labeled nutrients to determine if metabolic “burps” from possible
extraterrestrial microbes could be detected, she said. Although positive
readings convinced at least some team scientists that life was present, a
subsequent investigation by a second Viking instrument failed to find
evidence of organic molecules on the planet’s surface.

“Initially, the scientists were ready to break out the champagne,” said
Cleland. “But because subsequent investigations yielded baffling results
that didn’t fit the original metabolic definition of life they were
working with, NASA eventually concluded the original signal was not
evidence of life. This is an experiment that is still debated today, and
it’s a classic example of an anomaly.”

Out of more than 100 amino acids, terrestrial life constructs all of its
proteins from only about 20 of them, suggesting a single origin for life
on Earth, said Cleland. “It’s very difficult to generalize about life
based on just one example,” she said.

An article by Cleland and CU-Boulder molecular, cellular and developmental
biology Professor Shelley Copley, published online in the Jan. 16
International Journal of Astrobiology, explores the idea that an
“alternative microbial life” may exist on Earth. Such a “shadow biosphere”
could have a different molecular architecture and biochemistry than known
life and would be undetectable with current techniques like microscopy,
cell cultivation and Polymerase Chain Reaction amplification, the authors

Despite new suites of sophisticated instruments developed in recent years,
the ability of scientists to detect life on Mars or in another solar
system is probably very limited, Cleland said. “If the DNA in an alien
organism was even slightly different than the DNA in life on Earth, with a
different suite of nucleotide bases to encode genetic information, we
probably wouldn’t be able to recognize it.”

So what might be out there? “It’s not too far-fetched to imagine an alien
microbe whose genetic material directly and adaptively changes in response
to different environmental conditions,” said Cleland. “Instead of looking
for life as we know it, scientists may be better served to look for
anomalies, which amounts to looking for life as we don’t know it.”

In the past decade, scientists have discovered more than 170 new planets
around other stars, a number that seems to grow by the month due to clever
new planet-hunting techniques, Cleland said. In the future,
astrobiologists surveying other planets will no doubt encounter non-living
systems that are “really weird,” she said.

“In such cases, it probably is best to suspend judgment,” she said. “The
great strength of science is its tentativeness, and through history, it
has been the careful analysis of anomalies that have eventually changed
scientific paradigms.”