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Looking for other Earths? Here’s a list

An astronomer comes up with a top-10 list in the search for Earthlike planets that includes some of the favorite haunts for science-fiction aliens.
Omicron 2 Eridani, also known as Keid or 40 Eridani, is on the list of prospects in the search for Earthlike planets. It's also considered the home star system for Mr. Spock in the original "Star Trek" series.
Omicron 2 Eridani, also known as Keid or 40 Eridani, is on the list of prospects in the search for Earthlike planets. It's also considered the home star system for Mr. Spock in the original "Star Trek" series.ESO

An astronomer involved in a NASA mission to look for Earthlike planets beyond our solar system has winnowed through thousands of stars to come up with a top-10 list that includes some of the favorite haunts for science-fiction aliens.

Actually, the lineup from Margaret Turnbull at the Carnegie Institute of Washington is broken down into two top-five lists: one for the radio-based search for extraterrestrial intelligence, or SETI, and the other for the NASA mission, known as the Terrestrial Planet Finder.

The SETI stars will be on the list of targets for the privately funded Allen Telescope Array in California, which is due to begin limited operation with 42 linked radio dishes this spring. But the top prospects for the Terrestrial Planet Finder are currently in limbo, because NASA has put the mission on indefinite hold.

"It's all but shelved at this point ... pretty much all the research we've talked about is in peril," Turnbull said Saturday during a news briefing on astrobiology, conducted in St. Louis at the annual meeting of the American Association for the Advancement of Science.

Jill Tarter of the California-based SETI Institute said NASA's budget proposal, released this month, would cut funding for astrobiology research by 50 percent. She and the other astronomers at Saturday's session called on Congress to restore funding.

Tarter and her colleagues were particularly concerned about the fate of the Terrestrial Planet Finder, or TPF, and a precursor planet-hunting mission called SIM PlanetQuest. This month's budget proposal would delay SIM's launch until 2015 at the earliest. TPF, which had been set for launch around 2016, has been deferred indefinitely.

"We are facing an increasingly difficult financial threat," Tarter said. Although NASA's official view is that research is being deferred rather than canceled, she said "we are all finite in our lives and our careers. ... Significant delay is in fact cancellation."

Crowd-pleasing corner
Although the search for other Earths and other civilizations is a small and highly speculative corner of astronomical research, it's also arguably the most crowd-pleasing corner. Tarter herself served as the model for the main character in the late astronomer Carl Sagan's best-selling novel "Contact," which was made into a movie starring Jodie Foster. If astrobiologists had a nickel for every time aliens cropped up in popular culture ... well, they wouldn't need to depend on NASA funding.

Turnbull's list serves as a device for targeting the search as well as focusing the imagination. She started out with a database of 19,000 stars surrounded by "habitable zones" where life could conceivably survive. Then she zeroed in on stable stars that were at least 3 billion years old, with masses no more than 1.5 times that of our own sun.

The stars also had to have at least 50 percent of the sun's iron content, because astronomers believe that stellar systems need a minimum of heavy elements in order for planets to form.

"These are places I'd want to live if God were to put our planet around another star," she explained. The list for the SETI search includes:

  • Beta Canum Venaticorum, Turnbull's top prospect. It's a sunlike star about 26 light-years away in the northern constellation Canes Venatici. Astronomers have been looking for planets around the star but have found none to date.
  • HD 10307, another sunlike star about 42 light-years away. It has nearly the same mass, temperature and metal content as our sun — plus a companion star.
  • HD 211415, which has about half the metal content of the sun and is a bit cooler.
  • 18 Scorpii, a popular target for proposed planet searches. The star is almost an identical twin of the sun, Turnbull says.
  • 51 Pegasus, which was the first normal star beyond our solar system known to have a planet. The Jupiterlike planet was detected in 1995, and Turnbull believes 51 Pegasus could harbor Earthlike planets as well.

Tarter said her institute's Project Phoenix had already made an initial check of all five stars, and found nothing. However, when the Allen Telescope Array is on the case, it will be able to look for signals over "three times the frequency range that we looked at before," she said.

Not too dim, not too bright
Turnbull said the top five prospects for the Terrestrial Planet Finder mission were chosen a bit differently, because the TPF's instruments would look for the signature of planets circling around the target stars. The star couldn't be too bright — otherwise the planets themselves would be lost in the star's glare. It couldn't be too dim — otherwise there wouldn't be enough energy to sustain life as we know it. Here are the TPF prospects she came up with:

  • Epsilon Indi A, about 11.8 light-years from Earth, leads Turnbull's list. It's a star somewhat cooler and smaller than our sun, and was recently found to have a brown-dwarf companion. "Star Trek" fans consider it the home of the Andorian race. In the original "Star Trek" series, it was the base of operations for an evil entity called "Gorkon."
  • Epsilon Eridani, 10.5 light-years away, is a star somewhat smaller and cooler than our sun, and is already known to have at least one planet. By some science-fiction accounts, Epsilon Eridani is the parent star for Vulcan, Mr. Spock's home planet on "Star Trek." However, Trekkers have come to favor another star in the same constellation....
  • Omicron 2 Eridani, also known as 40 Eridani, is now cited in most "Star Trek" literature as Mr. Spock's home turf. It's a yellow-orange star about 16 light-years away, and is roughly the same age as our sun.
  • Alpha Centauri B is part of the triple-star system closest to our own sun, just 4.35 light-years away. It's long been considered one of the places in the Milky Way that might offer terrestrial conditions — and it's often cited in science-fiction tales, including Isaac Asimov's Foundation series.
  • Tau Ceti is in the same brightness category as our sun. It's metal-poor, compared to the sun, but long-lived enough for life forms to evolve. It has also served as a locale for science-fiction works ranging from Ursula Le Guin's "The Dispossessed" to the TV show "Earth: Final Conflict."

"If TPF is not canceled, these are the places where we will search," Turnbull said. Assuming that the TPF is eventually funded, the instrument should be able to check as few as 10 or as many as 150 stars in the course of its mission, depending on how extensively NASA wants to study each star, she said.

And if TPF is canceled, it still may be possible to do the search with a different kind of space interferometer called Darwin, which is due to be launched by the European Space Agency in 2015 or later. If it came to that, Turnbull said her criteria could be adjusted to fit Darwin's specifications instead of TPF's.