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HabStars: Speeding Up In the Zone by Astrobiology Magazine staffwriter

Profile: Jill Tarter
The inspiration for the main character of Carl Sagan's novel Contact," Jill Tarter holds the Bernard M. Oliver Chair for SETI at the SETI Institute (SETI is the Search for Extraterrestrial Intelligence).

Tarter attended Cornell University, earning a bachelor of engineering physics degree with distinction. She earned a master's degree and a Ph.D. in astronomy from the University of California at Berkeley. Her major field of study was theoretical high-energy astrophysics.

As a graduate student at Berkeley, she became involved in SERENDIP, a small commensal search for radio signals from extraterrestrial civilizations using the Hat Creek Observatory 85-foot telescope. After completing a Nuclear Regulatory Commission resident associateship at NASA's Ames Research Center, Tarter joined the newly formed SETI Program Office at Ames.

In 1984 she helped found the non-profit SETI Institute in Mountain View, CA. Tarter served as the project scientist for NASA's SETI High Resolution Microwave Survey (HRMS) until its termination by Congress in 1993. Today she heads the SETI Management Group at the SETI Institute.

Jill Tarter and Margaret Turnbull have a wish list. To maximize the search for extraterrestrial technological signals, the SETI researchers know where the next generation radio telescopes should start pointing.

Their HabCat, or Catalog of Nearby Habitable Systems, was created from what is known about habitable stars, or 'habstars,' near our sun. Seventy-five percent are within 140 parsecs, or around 450 light years. These Sun-like, habitable stars have just the right distance, constancy, and light to qualify in a forthcoming enlarged radio search.

A former list of 2000 targets guided the search for Project Phoenix, a privately funded All-Sky-Survey that continued NASA's High Resolution Microwave Survey (HRMS). In 1993, that mission commenced to search for continuous and pulsed radio signals from extrasolar civilizations. Each year, the Phoenix Project gets telescope time to observe 200 stars using the world's largest capable dish, the Arecibo Observatory in Puerto Rico. All that is about to change in the next two years.

Tarter and Turnbull are preparing for much bigger and faster searches. In 2005, a joint effort by the SETI Institute and the University of California-Berkeley expects to increase the speed of this search by 100 times or more (~20,000 stars per year). But even without their help, this faster search would begin to exhaust nearby habstars.

The new shortlist of habstars has grown nearly 9-fold.

Their article, "Target Selection for SETI: I. A Catalog of Nearby Habitable Stellar Systems," published in the Astrophysical Journal, identifies 17,129 potentially habitable hosts for complex life. The co-authors plan a follow-up article that will prioritize which habstars to target first.

The creation of the Catalog of Habitable Stellar Systems was motivated by the ongoing and rapid development of a network of 350 radio antenna dishes. Called the Allen Telescope Array (or ATA), the network ties together 6.1 meter (~20 foot) diameter dishes for a total surface area as large as eight football fields. In 2005, the telescope will be completed using commercial satellite dishes and be located 290 miles northeast of San Francisco.

Tarter and Turnbull winnowed down about 120,000 Hipparcos stars to those that could be habitable to life as we know it for observation during the first few years of ATA operation.

Arecibo Telescope
Arecibo. World's largest dish, radio telescope. Puerto Rico.
In evaluating Sun-like targets as suitable hosts for communicating life forms, one recurring theme is defining habitability. At the very least, candidate stars need terrestrial planets. For life to develop on Earth, liquid water and certain heavy atomic elements like phosphorus were also needed. Stars with very low metal content probably formed from a cloud that did not have enough heavy metals to make planets or life forms. These needs refined the HabCat list to only those stars exceeding a lower limit on stellar metallicity, about 40 percent that of the sun.

Planets and water might be enough for simple life, but stars also must remain nearly constant in brightness over billions of years for complex life to have time to develop. On Earth, single cells may have developed after only 800 million years or so, but the fossil record indicates that it took another 3 to 4 billion years before multi-cellular life flourished.

The authors write that accurate luminosities are "perhaps the most important information we use in determining the habitability of nearby stars" for complex life, because luminosity indicates which phase of life the star is in, and that in turn dictates how long the star will remain stable.

Our own sun follows an 11-year, "bright and dark" cycle (the sunspot cycle), yet its luminosity fluctuates over that time by only 2 parts per 10,000 (0.02 percent). A drop in solar radiance of half a percent or less locked terrestrial life into the coldest years of the Little Ice Age (1550-1700 A.D.).

Turnbull and Tarter took on the daunting task of evaluating 118,218 nearby stars, using membership criteria of constant luminosity and potential habitable zones. A database search gave them their first cut, which they call the "Celestia sample." If a star fluctuated by 3 percent in its luminosity, the level of variability detectable to Hipparcos, complex life would be imperiled. Stars that Don't Twinkle A star's twinkle in the night sky is due to the distorting effects of our atmosphere. If stars like our sun actually varied or twinkled that much, life as we know it would cease to exist.

For that reason, Tarter and Turnbull are not interested in sifting through what they term, "the cataclysmic, eruptive, pulsating, rotating, or X-ray" stars. If those stars had planets, their climate would prove a quick killer. Such stars are relatively common, so after eliminating such stars from the search, the Celestia sample totaled about half of the original list, or 64,120 candidate stars.

The remaining stars could be narrowed down even further based on mass, color, and age. The final list of candidates needed to have just the right size and composition. If, for example, a habstar expanded or shrank over the required 3 billion years for complex life to develop, then orbiting planets would experience ice ages and runaway greenhouses.

Allen Telescope Array
Allen Telescope Array (ATA)
Another two-thirds (64%) of the HabCat candidates fell off this wish list because, while it may be possible for advanced life to survive, the stars posed a threat to well-known biological sensitivities. For example, the excessive ultraviolet (UV) radiation of a star could cause DNA to fragment or mutate. However, Tarter and Turnbull note that these assumptions may change in the future with additional data.

The scientists also excluded young star systems. Young stars typically have high rotations, they emit soft X-rays above and beyond their estimated temperatures, or they haven't burned through enough light elements to produce metals (heavy elements like iron).

The survivors in the catalog at this point numbered only one in six stars, or 20,814 potential candidates.

Favorable Stars As Loners

After eliminating the variable, low metallicity or very young stars from HabCat, Turnbull and Tarter took on the binary and multiple star systems. As disquieting as a dual sunrise and sunset might prove to our own biorhythms, there are a great number of multiple star systems where such broken days occur. The fraction of solar-type stars in binary or multiple systems has been estimated to be two out of every three stars. These stars have fewer stable orbits for hosting planets with liquid water, and planets would have a higher danger of either spinning out of the system (ejection) or spiraling into one of the stars (accretion). Very elliptical orbits also cause climatic chaos.

Turnbull and Tarter examined the "habitable zone" (where liquid water can exist on an Earth-like planet) around each potential habstar that was part of a binary or multiple sytem. By eliminating systems where the habitable zones were not stable, about 2200 of 3500 stars in binary or multiple systems were kept in the HabCat.

The HabCat inventory was nearly finished, their final cut done.

Profile: Margaret Turnbull
Margaret Turnbull's career has included construction, setup, testing and repair of detectors and readout system for an Antarctic Muon and Neutrino detector array.

At the University of Wisconsin in space physics, payload construction, she was involved in detector preparation for the X-ray Quantum Calorimeter sub-orbital launch experiment.

At Lowell Observatory in Arizona, she participated in the examination of main-belt and Earth-crossing asteroid orbital parameters and trends of ephemeris uncertainty with orbital parameters.

She has done Monte Carlo modeling of high-resolution Hubble Space Telescope images of protostars in the Taurus-Auriga dark cloud at the Harvard-Smithsonian Astrophysical Observatory. And in the radio domain, she has analyzed large-scale HI 21 cm emission from Seyfert galaxies imaged by the Very Large Array, and conducted a search for obvious distortions caused by unseen companion galaxies.
It Takes a Planet to Feed a Village

Even if a star is classified as "habitable," the orbiting planets might not be. Our neighboring planet, Venus, has a surface hot enough to melt lead. Presumably, any carbon-based intelligence capable of communication needs some solid ground from which to broadcast.

In 1993, Project Phoenix began looking at 2,000 candidate stars for signs of life. Since then, a revolution in astronomy has taken place. Scientists have discovered and catalogued an ever-increasing number of extrasolar planets.

Of the 17,000 or so habstars in our neighborhood of the galaxy, 55 harbor already-discovered planets. Owing to the challenges of detecting a relatively tiny pale blue dot, all of these planets are hugely massive. The least massive one identified so far (HD 49674), is about a tenth the mass of our own Jupiter -- a so-called gas giant.

As Turnbull and Tarter note: "Despite the broad array of data used to assemble this catalog, this exercise has forced us to admit that we are defining habitability from a position of considerable ignorance...For SETI this humbling situation is amplified when we consider that we have no indisputable definition for life itself, to say nothing of the precise conditions that are necessary and sufficient for life to evolve into a technological civilization detectable by a SETI search program."

Star field
In a universe brimming with stars, the search is on if life exists elsewhere
Turnbull and Tarter took time from their busy preparations to outline for Astrobiology Magazine the highlights of how they built HabCat and what they plan next.

Turnbull (Steward Observatory, Univ. Arizona, Tucson) and Tarter (SETI Institute, Mountain View, California) broadly described their insights into galactic habitability.

Interview with Margaret Turnbull and Jill Tarter

Astrobiology Magazine (AB): How did work on the new HabCat begin?

Jill Tarter (JT): This paper [Target Selection for SETI] is part of [Margaret's] thesis in Astrobiology at the University of Arizona. The need for a larger target list is driven by the desire to do simultaneous radio astronomy and SETI observing at the same time.

AB: What was the primary motivation -- to expand the 2000 Phoenix targets to 17,000 HabCat targets for the Allen Telescope Array (ATA)? Does this derive in part from the enhanced search capabilities expected from the new array?

Margaret Turnbull (MT): Absolutely. Whereas Project Phoenix has been observing about 200 stars per year, the ATA will be capable of observing 10,000 or more, at three times the frequency coverage and four times the spectral resolution of Project Phoenix.

Observing this many targets annually means that HabCat will have to be supplemented by even larger target lists, and currently we are assembling a list of about 100,000 stars from the Tycho-2 catalog. Such a list will not be nearly as "refined" as HabCat due to a severe lack of parallax and spectral data, but we will do our best to keep our targets on the main sequence.

JT: With 6-meter telescopes, the primary field of view of the array is about 3 degrees across at 1 gigahertz, but it shrinks by factor of ten at 10 gigahertz, so to have enough targets to always find one or more in the preferred field of view requires very large lists. The lifecycle of the ATA will be long, and we'll start at lower frequencies where the beamsize is larger.

AB: Where is the Allen Telescope Array in its current development timeline?

JT: ATA completion is probably 2005.

MT: The Prototype Test Array at Hat Creek now has three antennas. Ultimately it will consist of 350 individual 20-foot antennas, providing a larger total collecting area than the Green Bank Telescope in West Virginia and higher resolution than the Arecibo dish.

AB: How did the target list prioritize its selections for HabCat?

MT: We are currently working on a prioritization algorithm for all of our SETI targets, including several subsets of objects not included in HabCat, to be published in a second paper.

AB: Your first paper weights candidate stellar systems for X-ray luminosity, rotation, spectral types, kinematics, metallicity, and Strömgren photometry, correct? That's the long equation for acceptance.

MT: For the HabCat stars, inclusion in the catalog was based on the X-ray and other data you mentioned, but priority will be assigned based on location in the color-magnitude diagram [CMD] and on distance. A plot of the CMD score would look like a bull's-eye centered on the sun's absolute visual magnitude and B-V color, with the highest score at the center. The distance scoring will go as the inverse square of distance, or the detectability of a given signal.

We are still considering how to weight these two factors, but most likely the distance score will dominate within a certain distance. One idea is that, given the first terrestrial transmissions of a century ago, HabCat stars within the possible ETI [extraterrestrial intelligence] response distance of 50 light years should have scores dominated by distance.

AB: What is the significance of excluding, or annotating the catalog with, binary and ternary stellar systems? Is this because an orbital instability is incompatible with habitability criteria?

MT: In fact, we included in HabCat approximately 2,200 stars that are members of known binary or triple systems, and each of these systems has passed our test for dynamical stability of the habitable zone. For 3,500 candidate habstars in multiple systems, we mapped out the location of the habitable zone according to work done by Kasting, Whitmire and Reynolds [1993, Icarus, 101,108].

To determine if that location is dynamically stable in the presence of the companion stars, we used the stability criteria developed by Holman and Wiegert [1999, Astrophysical Journal, 117, 621]. It turned out that many binary and triple systems were habitable, including 21 tight binaries where the habitable zone encompasses both stars.

AB: So what's planned in forthcoming projects?

MT: This work falls within the broader themes of, first, the overall habitability of the Milky Way, and, second, the remote sensing of biosignatures. My dissertation work includes topics such as characterizing the Earthshine signal, including spectral signatures of life; constraining the fraction of habitable stars in the solar neighborhood; identifying suitable targets for the Terrestrial Planet Finder mission; and examining the concept of a habitable belt in the Galaxy.
The number of HabCat stars, as a function of distance for M-type stars (solid red histogram), K stars (dark-hatched green histogram), G stars (light-hatched violet histogram), F stars (horizontal-lined yellow histogram), and all stars (open blue histogram). Click image for larger view. Inset, Allen Telescope Array. Credit: Turnbull, Tarter

JT: We were sorry to see FAME [Full-sky Astrometric Mapping Explorer] cancelled, since it would have provided 40 million parallaxes [or star positions].

[To further refine HabCat], in FAME's absence we'll have to use reduced proper motions and color-color relationships from the Tycho-2 Catalog while waiting for SIM [NASA's Space Interferometry Mission] and GAIA [European Space Agency's Galactic Census Project].

Editor's Note: For HabCat the current best set of stellar distances comes from what astronomers call Hipparcos parallax measurements which give values accurate to ~ 1 milli-arcsecond. Several new astrometric satellites are now being planned to measure star distances.

The most ambitious planned star cataloguing projects are NASA's Space Interferometry Mission and ESA's Galactic Census Project, or GAIA mission, which may yield large numbers of parallaxes with precisions better than ~ 10 micro-arcseconds. SIM is scheduled to operate from 2006 to 2011 while GAIA, if accepted by ESA, could launch in 2009 with a 5 year lifetime.

SIM would provide astrometric measurements of 10,000 stars and GAIA would measure around 10 billion positions. The next scheduled astrometry mission, however, was the now cancelled FAME (Full-sky Astrometric Mapping Explorer), which was scheduled to be launched in 2004.

What's Next

Conservatively as catalogued so far, the preferred habstars must be 3 billion years old, stable, and support liquid water on the surface of whatever planets are properly distanced. These planets reside in a habitable zone, satisfying the precondition for complex life to develop. Three out of four reside within a communication neighborhood spanning around 450 light years.

When the Allen Telescope Array turns on in 2005, it will be capable of searching to the farthest of 17,000 habstars, just beyond 300 parsecs. For those search distances, an electromagnetic communication, if detected, would have begun broadcasting a millennium ago, just about 1000 AD on a terrestrial calendar (a transmission originating from a distance of 978 light-years from Earth).
NASA's Astrobiology Magazine