Margaret Turnbull (Carnegie Institution of Washington) has a job Centauri Dreams can’t help but envy. The astronomer is a specialist in identifying stars that have habitable zones — stars, in other words, where life is possible. Back in 2003, Turnbull and colleagues published a list of 17,129 such stars, based on factors such as age (how long does it take life to develop?), stellar mass (larger stars may not live long enough to produce productive habitable zones) and metallicity (a measure of the heavy metals needed for planetary formation).
Narrowing a galaxy of between one and two hundred billion stars down to 17,129 candidates is no small feat, but Turnbull has now gone one better, choosing the top five candidate stars for those engaged in SETI, the search for extraterrestrial intelligence. That list involves choosing stars where technological civilizations are most likely to have developed, but Turnbull complements it with a second list of six stars likely to have Earth-like planets in orbit about them. For the latter, we wait, of course, for the again delayed Terrestrial Planet Finder and similar programs. As Turnbull says, “It’s impossible to know the true nature of those planets until we can directly image them.”
Terrestrial Planet Finder or no, these lists are fascinating. For the SETI search, Turnbull likes Beta CVn, a Sol-like star in Canes Venatici that is roughly 26 light years away (around which, it must be added, no planets have yet been found). The other SETI candidates are HD 10307 (42 light years away, a Sun-like member of a binary system); HD 211415 (cooler than the Sun, and with half its metal content); 18 Sco (a twin of the Sun in Scorpio) and 51 Pegasus (immortalized in the first detection of an exoplanet around a main sequence star).
But the list of top candidate stars for Terrestrial Planet Finder is, in my judgment, more interesting. Here Turnbull chooses K-class stars a bit dimmer than the Sun, reasoning that their inherent brightness is not high enough to complicate the planet hunt. The choices for TPF are these:
Turnbull’s work on candidate stars was presented last Saturday at the 2006 annual meeting of the American Association for the Advancement of Science in St. Louis.
Centauri Dreams‘ take: In the list above, two stars stand out. Tau Ceti seems somewhat less likely as a home for life given studies indicating heavy cometary bombardment would be likely in this system. It is, nonetheless, a G-class star like the Sun that has elicited intense interest ever since Frank Drake turned the telescope at Green Bank (West Virginia) towards it in the first SETI attempt.
As to Centauri B, this seems a likely and outstanding choice. Paul Wiegert and Matt Holman showed in 1997 that stable orbits can exist within 3 AU of Alpha Centauri A or B, while the habitable zone around Centauri A should extend from 1.2 to 1.3 AU, with a zone around Centauri B of 0.73 to 0.74 AU. These findings, along with the proximity of the Centauri system to our own, make it an obvious candidate for close scrutiny and, one day, an interstellar probe. Not mentioned here is Proxima Centauri, but this M-class red dwarf is intriguing on its own, and as more and more work suggests that red dwarfs could offer stable habitable zones, we may see its fortunes rise. Turnbull didn’t choose it, among other reasons, because she wanted to work with brighter stars.
On Terrestrial Planet Finder itself, I’m now seeing launch dates as late as 2020, though these are only guesses given the current budgetary limbo. I will be talking to Webster Cash (University of Colorado at Boulder) about his New Worlds Imager technology later this week, and will post the interview here. Cash’s designs seem far and away the most effective (and affordable) for the task of ferreting out Earth-like worlds. The TPF budgetary delays may be a blessing in disguise in at least one sense — they may offer Cash an unexpected opportunity to convince NASA of the advantages of his breakthrough imaging system.