Candidate Stars for Terrestrial Worlds

by Paul Gilster on February 20, 2006

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:

  • Epsilon Indi A: almost 12 light years away, it’s about a tenth as bright as the Sun
  • Epsilon Eridani: about 10.7 light years from Earth and somewhat smaller than Sol
  • Omicron2 Eridani: the same age as the Sun, some 16 light years out
  • Alpha Centauri B: the closest target, with the added virtue that Alpha Centauri A is close enough that the planet hunt around B may also tell us much about Centauri A’s potential planets
  • Tau Ceti: metal-poor but old enough for complex life to have evolved
  • 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.

    { 7 comments }

    anthonares February 20, 2006 at 22:40

    Dr. Turnbull gave a great talk last year as part of the NASA Astrobiology Institute’s Director’s Seminar Series. Here’s the link. Unfortunately, she hasn’t posted her slides up there, but suffice to say the talk was a great primer on what scientists are really looking for when they try and say which systems are most likely to harbor life.

    One of the key concepts that she had to offer was the idea of the evolving habitable zone of a star. Many stars are not only short lived, but their luminosity varies so much over their lifetimes that it infeasible that life could truly evolve beyond single-celled organisms. There are many, many other concerns as well which I’m sure went into her selection of these five most likely candidates.

    Administrator February 21, 2006 at 10:20

    Evolving habitable zones offer interesting speculation. Especially when you contrast stars with more short-lived habitable zones with the potential zones around M-class red dwarfs, which could be stable (depending on life’s adaptation to solar flare activity) for far longer than any G or K-type star. As we continue the exoplanet hunt (and learn more about astrobiology), it will be intriguing to see what we find out about how long it takes life to develop in a given environment. On that score, we still have much to learn here on Earth!

    Timothy J Mayes September 30, 2006 at 3:27

    I think there is good chance of earth like planets at alpha centauri
    A & B
    tim

    Dave September 24, 2007 at 11:00

    Evolving habitable zones affect every star including our own, I’m convinced of two things: We don’t understand enough about the origin of life on Earth to extrapolate to how quicly or slowly it can develop elsewhere, and that current climate changes on Earth could have natural as well as man made causes-and one of those natural causes could be changes in our own star’s energy output. We have to face the reality that we don’t understand all the mechanisms that make a star tick, and sometimes we can overlook something because of an all too hastily made presumption about how something works. For instance, even if I’m 99% sure inteligient life couldn’t develop around a blue supergiant, I’d check it out anyways in case my theories about how ling it takes life to develop and how long such a star can live are in need of a bit of tweaking.

    ljk November 9, 2007 at 13:26

    ASTRONOMERS DISCOVER SUN’S TWIN AT McDONALD OBSERVATORY

    FOR IMMEDIATE RELEASE

    November 9, 2007

    FORT DAVIS, Texas – Peruvian astronomers Jorge Melendez of
    The Australian National University and Ivan Ramirez of The
    University of Texas at Austin have discovered the best
    “solar twin” to date, using the 2.7-meter Harlan J. Smith
    Telescope at McDonald Observatory. Their findings suggest
    that the Sun¹s chemical composition is not unique, as some
    previously thought.

    The star, HIP 56948, is more like the Sun than any yet seen,
    and is 200 light-years away in the constellation Draco, the
    dragon. The star may be a billion years older than the Sun.

    Only three solar twins were previously known: 18 Scorpii,
    HD 98618, and HIP 100963. But while they were all like the
    Sun in many ways, there was one major difference: the amount
    of lithium they contained. They all had several times more
    than the Sun. Astronomers wondered if the Sun was unique in
    its low amount of lithium.

    The discovery of this new solar twin puts that question to
    rest: it has the same low lithium content as the Sun. The
    study turned up another solar twin, HIP 73815, that contains
    a similarly low amount of lithium.

    The question of chemical peculiarities in the Sun is related
    to the “anthropic principle” — is there something special
    about the Sun that has allowed life to spring up in our
    solar system? Their findings don¹t answer that completely,
    but they do show that when it comes to the Sun¹s chemical
    composition, the answer is an emphatic “no.”

    Melendez’ and Ramirez’ findings suggest the opposite,
    so-called “Copernican” view: It is possible that life is
    common elsewhere in the universe. They suggest that stars
    like HIP 56948 would be good targets for SETI (Search for
    Extra-Terrestrial Intelligence) researchers.

    The star already has been studied by the McDonald
    Observatory Planet Search led by University of Texas at
    Austin astronomer Bill Cochran. His team found that, like
    our Sun, HIP 56948 does not host any “hot Jupiter” planets,
    those massive, short-period planets orbiting close to their
    parent stars, so common among the more than 200 stars found
    to date that host one or more planets.

    Searches for “solar twins” are important because astronomers
    use the Sun as a baseline for many other types of studies.
    But they cannot study the Sun the same way they do the
    distant stars. It’s too close, and too bright.

    The solar twins discovered at McDonald will be useful for
    many areas of astrophysics. In particular, they will help
    astronomers who study the chemical compositions of stars, as
    well as validate theoretical models of stars’ interiors, and
    theoretical models of stellar evolution.

    END

    NOTE TO EDITORS: High-resolution artwork to accompany this
    release is available online:

    http://mcdonaldobservatory.org/news/releases/2007/1109.html

    Joel September 18, 2010 at 8:25

    A general question in response to ljk’s posted article:

    In other sun-like stars with higher Lithium content, from where does the Lithium originate?

    [From Wikipedia: "Certain orange stars can also contain a high concentration of lithium. Those orange stars found to have a higher than usual concentration of lithium (such as Centaurus X-4) orbit massive objects—neutron stars or black holes—whose gravity evidently pulls heavier lithium to the surface of a hydrogen-helium star, causing more lithium to be observed."]

    So is it just that all the sun-like stars have the same amount of Lithium, and in certain ones it is being gravitationally dragged to the surface?

    Did these other stars form in regions of high cosmic ray flux, creating more Lithium?

    Paul Gilster September 18, 2010 at 8:59

    Quite an interesting question! And we also have the recent work on lithium depletion in Sun-like stars with planets versus those without them:

    http://www.centauri-dreams.org/?p=10206

    I don’t have the answer to your question re cosmic ray flux, but maybe some of our resident astronomers will have some thoughts on that.

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