The search
for alien life outside our solar system has been made a little less daunting
thanks to a new list drawn up by astronomer Margaret Turnbull that includes the
known stars most likely to support habitable stellar systems.
Turnbull,
an astronomer at the Carnegie Institute of Washington, listed 10 of what she believes
are likely to be habitable stellar systems, or "habstars," capable of
supporting Earth-like planets and life. The list was presented at the recent
annual meeting of the American Association for the Advancement of Science in St. Louis.
Five of the
stars on the list are thought of as good candidates for SETI astronomers
seeking only to listen for radio signals from intelligent alien civilizations.
They will be included in a list of targets for the Allen
Telescope Array, a network of 42 linked radio dishes that is expected to go
online this spring in California.
The other
five are for NASA's Terrestrial
Planet Finder (TPF), a planned space telescope that would attempt to
directly image Earth-like planets around nearby stars. The TPF mission was
scheduled for launch around 2016, but is currently on hold indefinitely,
according to NASA's
2007 budget plan.
Selection
criteria
Turnbull's
list of top-10 habstars were winnowed down from a pool of about 19,000
potentially habitable star systems that she and SETI scientist Jill Tarter
created in 2003. A star is considered habitable if it is thought able to
support a rocky planet with liquid water on its surface.
To narrow
the selection, Turnbull chose only those stars that were at least 3 billion
years old, which had masses that were no more than 1.5 times that of our Sun
and metal contents that were at least 40 percent that of our Sun.
Stars that
were less than 3 billion years old are thought to be too young for planets and
life to evolve. Stars that are more than 1.5 solar masses tend to burn out
before life can emerge and stars with low-metal contents probably formed from
clouds that didn't have enough heavy metals to make rocky planets in the first
place.
"Our first
and favorite targets are stars that are very similar to the Sun, stars that
look like they're taking care of a planet that has Earth-like life on it,"
Turnbull said. "These are places I'd want to live if God were to put our planet
around another star."
For now,
there are no known Earth-sized planets in favorable orbits around Sun-like
stars other than our own. But astronomer are confident such worlds will be
found when technology allows.
SETI
stars
Turnbull's
top habstar candidates for the Allen Telescope Survey:
- Beta
Canum Venaticorum: a Sun-like star about 26 light years away in the
constellation Canes Venatici. This was Turnbull's top choice for SETI.
- HD 10307:
a near replica of the Sun but with a companion star. Located about 42
light-years away, this star has almost the same mass, temperature and
metal-content as the Sun.
- HD
211415: has about half the metal content of the Sun and is a little
cooler; just slightly farther away than HD 10307.
- 18 Sco: a
near-identical twin of the Sun, located in the constellation Scorpio.
- 51
Pegasus: The first planet beyond our solar system was detected around this
star in 1995. Although that planet was a gas giant, Turnbull thinks 51
Pegasus could harbor rocky planets as well.
All of
these stars have already been surveyed by SETI's Project
Phoenix in the past without success, but the Allen Telescope Array will
have up to five times Project Phoenix's frequency range, making it more likely
that a habitable planet around these stars will be detected if one exists.
TPF
stars
For the TPF
mission, Turnbull chose the following five stars:
- Epsilon
Indi A: Turnbull's top TPF mission choice; this star is only about
one-tenth as bright as the Sun and about 11.8 light-years away in the
constellation Indus.
- Epsilon
Eridani: This star is a bit smaller and cooler than our Sun; it is located
about 10.5 light-years away in the constellation Eridanus.
- Omicron2
Eridani: A yellow-orange star about 16 light-years away that is roughly
the same age as our Sun.
- Alpha
Centauri B: This triple star system is located just 4.35 light-years away
and one of the Sun's closest stellar neighbors.
- Tau Ceti:
This star is a G-class star and is in the same brightness category as the
Sun. Despite being relatively metal-poor, it is long-lived enough for
complex life forms to evolve.
If NASA's
TPF mission is cancelled, Turnbull said there is a chance her list could be
adjusted for the European Space Agency's Darwin
mission, which is expected to launch in 2015 with similar goals.
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