14 Top Candidates for Humanity’s First Interstellar Missions, in order of distance:
- ALPHA CENTAURI A and B
R_{A} = 4.34 light years, proper motion = ? arc-sec/yr
Triple star system visible from the Southern Hemisphere. A and B form a spectroscopic (close) binary, separated by 1-3.3 billion km, eccentricity = 0.52, period ~80 years. Third member of system, ALPHA CENTAURI C aka PROXIMA CENTAURI (the closest star to our Sun at 4.24 l-y) forms a visual (distant) binary with first pair, having a separation of ~1 trillion miles and a period of ~1 million years. It is probably too small (spectral class M_{5} star) to host a habitable planet, unless that planet is lucky enough to have a stabilizing moon of the necessary mass and period. System's proximity to Earth (almost 3X closer than the next good candidate) with two stable Sun-like stars, each with noninterfering high-quality ecoshells, make the Alpha Centauri system by far the most promising target for the first interstellar mission, having both the best chance and the shortest trip. Significantly lucky for us, no?
A: right ascension, theta = 14 h 39 m 37 s; declination, Ø = -60° 50' 02". Spectral class = G_{2} V. Mass: 1.08 Sol. Probability of habitable planet at A: 0.054
B: right ascension, theta = 14 h 39 m 35 s; declination, Ø = -60° 50' 13". Spectral class = K_{1} V. Mass: 0.88 Sol. Probability of habitable planet at B: 0.057
Joint probability of at least one habitable planet at Alpha Centauri: 0.107
? OPHIUCHI or BARNARD’S STAR
R_{A} = 5.88 light years, largest proper motion in sky = 10.3 arc-seconds per year, or 88 km/sec; right ascension, theta = 17? h ? m ?.? s; declination, Ø = -0?° ?' ?". Two known jovian planets: m_{1} >= 0.7 M_{Jupiter} @ 3 AU, period = 12 years, and m_{2} >= 1.15 M_{Jupiter} @ 5 AU, period = 26 years. BORDERLINE:
Spectral class K?_{?}. Probability of habitable planet: ‹ 0.001?
LALANDE 21185 A
R_{A} = 8 light years, proper motion ? arc-sec/yr; right ascension, theta = ? h ? m ?.? s; declination, Ø = ?° ?' ?". One known superjovian planet: >= 8 M_{Jupiter} @ ? AU, period = ? years. BORDERLINE:
Spectral class K?_{?}. Probability of habitable planet: ‹ 0.001?
- EPSILON ERIDANI
R_{A} = 10.8 light years, proper motion = ? arc-sec/yr; right ascension, theta = ~03 h ? m ?.? s; declination, Ø = -20?° ?' ?";
Spectral class = K_{0}. Mass: 0.80 Sol. Probability of habitable planet: 0.033
- TAU CETI
R_{A} = 11.7 light years, proper motion = ? arc-sec/yr; right ascension, theta = ~01 h ? m ?.? s; declination, Ø = -15?° ?' ?". Independent system, but close neighbor (~2 l-y?) to Epsilon Eridani above.
Probability of habitable planet: 0.036
Joint probability of at least one habitable planet in Epsilon Eridani/Tau Ceti neighborhood: 0.07
- 70 OPHIUCHI A and B
R_{A} = 17.3 light years, proper motion = ? arc-sec/yr
Spectroscopic (close) binary, separated by 1-3 billion km, eccentricity = 0.50, period = 87.85 years. Possible dark companions.
A: right ascension, theta = 17? h ? m ?.? s; declination, Ø = -0?° ?' ?". Spectral class = G_{5?} V. Mass: 0.90 Sol. Probability of habitable planet at A: 0.057
B: right ascension, theta = 17? h ? m ?.? s; declination, Ø = -0?° ?' ?". Spectral class = K_{5} V. Mass: 0.65 Sol. Probability of habitable planet at B: 0.001 (only if satellite-stabilized within life zone).
Joint probability of at least one habitable planet at 70 Ophiuchi: 0.057
- ETA CASSIOPEIÆ A and B
R_{A} = 18.0 light years, proper motion = ? arc-sec/yr
Spectroscopic (close) binary, separated by 3-10 billion km, eccentricity = 0.53, period ~500 years. Possible third companion.
A: right ascension, theta = 1? h ? m ?.? s; declination, Ø = +70?° ?' ?". Spectral class = F_{9} V. Mass: 0.94 Sol. Probability of habitable planet at A: 0.057
B: right ascension, theta = 1? h ? m ?.? s; declination, Ø = +70?° ?' ?". Spectral class = K_{6} V. Mass: 0.58 Sol. Probability of habitable planet at B: 0.001 (only if satellite-stabilized within life zone).
Joint probability of at least one habitable planet at Eta Cassiopeiæ: 0.057
- SIGMA DRACONIS
R_{A} = 18.2 light years, proper motion = ? arc-sec/yr; right ascension, theta = 14? h ? m ?.? s; declination, Ø = +80?° ?' ?"; Spectral class = G_{9} V. Mass: 0.82 Sol. Probability of habitable planet: 0.036
- 36 OPHIUCHI A and B and C
R_{A} = 18.2 light years, proper motion = ? arc-sec/yr.
Triple star system towards center of Milky Way. Orbital elements unknown.
A: right ascension, theta = 18? h ? m ?.? s; declination, Ø = -0?° ?' ?". Spectral class = K_{2} V. Mass: 0.77 Sol. Probability of habitable planet at A: 0.023
B: right ascension, theta = 18? h ? m ?.? s; declination, Ø = -0?° ?' ?". Spectral class = K_{1} V. Mass: 0.76 Sol. Probability of habitable planet at B: 0.020.
C: right ascension, theta = 18? h ? m ?.? s; declination, Ø = -0?° ?' ?". Spectral class = K_{6} V. Mass: 0.63 Sol. Probability of habitable planet at C: 0.001 (only if satellite-stabilized within life zone).
Joint probability of at least one habitable planet at 36 Ophiuchi: 0.042
- HR 7703 A and B
R_{A} = 18.6 light years, proper motion = ? arc-sec/yr.
Binary star system in Sagittarius. Orbital elements unknown.
A: right ascension, theta = 19? h ? m ?.? s; declination, Ø = -30?° ?' ?". Spectral class = K_{2} V. Mass: ~0.76 Sol. Probability of habitable planet at A: 0.020
B: right ascension, theta = 19? h ? m ?.? s; declination, Ø = -30?° ?' ?". Spectral class = M_{5} VI?. Mass: ~0.5 Sol. Probability of habitable planet at B: 0.001 (only if satellite-stabilized within life zone).
Joint probability of at least one habitable planet at HR 7703: 0.020
- DELTA PAVONIS
R_{A} = 19.2 light years, proper motion = ? arc-sec/yr; right ascension, theta = 20? h ? m ?.? s; declination, Ø = -60?° ?' ?". Spectral class = G_{7} V?. Mass: ~0.98 Sol. Probability of habitable planet: 0.057
- 82 ERIDANI
R_{A} = 20.9 light years, proper motion = ? arc-sec/yr; right ascension, theta = 4? h ? m ?.? s; declination, Ø = -30?° ?' ?". Spectral class = G_{5} V?. Mass: ~0.91 Sol. Probability of habitable planet: 0.057
- BETA HYDRI
R_{A} = 21.3 light years, proper motion = ? arc-sec/yr; right ascension, theta = 09? h ? m ?.? s; declination, Ø = -15?° ?' ?". Spectral class = G_{1} V?. Mass: 1.23 Sol. Probability of habitable planet: 0.037
- HR 8832
In Cassiopeia. R_{A} = 21.4 light years, proper motion = ? arc-sec/yr; right ascension, theta = 1? h ? m ?.? s; declination, Ø = +70?° ?' ?". Spectral class = K_{3} V?. Mass: ~0.74 Sol. Probability of habitable planet: 0.011
Notes on Finding Extrasolar Planets, and What's Coming to this Page:
Techniques:
Radial velocimetry: is an indirect detection method measuring doppler shift in primary star's spectrum. This method only sets a lower limit on planet's mass due to random inclination of the target's orbital plane to our line of sight. Probable lower mass limit of radial velocity method for finding extrasolar planets: 0.05 Jupiter ( ~ Neptune). Effectiveness also drops strongly with orbital diameter. Minimum mass discovered so far: 0.25 Jupiter ( ~ 0.80 Saturn).
Astrometry observes induced wobble directly, not indirectly like doppler techniques.
Transit observations indicate size and possible composition of extrasolar planets by measuring magnitude and duration of dimming as well as absorption curve during transit in front of star.
Direct visual observation is very difficult and will require major advances in optical interferometry and adaptive optics. At present, the smallest thing which has been imaged is protoplanetary accretion disks, 20-200 AU in diameter.
The quoted probabilities above are calculated theoretical chances for finding at least one habitable planet at the target system, defined as a preexisting life-bearing world suitable for man without major terraforming. Given what we have learned about the early emergence of life on Earth, its tenacity and exploitation of extreme environments, the chances of finding life of some kind must approach certainty. The probablities were calculated by Dole assuming a minimum age of 3 billion years, however, due to those discoveries since the early 1960s about life's early emergence, the set of stars which live long enough to host habitable planets might be expanded to perhaps the F_{0} class, possibly even the low mass end of A-class. Therefore, more candidates from our stellar neighborhood may be added soon, for instance Altair (Alpha Aquilae).
The presence of “hot superjovian” extrasolar planets as discovered by Marcy and Butler may remove some candidates from this list due to the disruptive effects of gas giants in inner systems. The results of this cross-indexing will be posted soon, as well as another list of borderline candidates.
Spectral class is only approximately correlated with stellar mass, due to the fact that luminosity increases with age on the main sequence. The red giant phase (indicated by the Roman numeral suffixes Ia-III) as a star leaves the Main Sequence would destroy any habitable planets that did exist.
At some future time, I'll figure out a possible path of a "Grand Tour" similar to the one Voyager took, but on an interstellar scale.
Credit goes out to Stephen Dole's amazing seminal work Habitable Planets for Man, RAND Research Study 1964. Wow.
Other Sources:
2. ed. Finney, Ben & Jones, Eric, Interstellar Migration and the Human Experience, (Univ. Calif. Press, Berkeley: 1985).
2. MacVey, John, Journey to Alpha Centauri, (Macmillan, New York: 1965).
2. ed. Sagan, et al, Communications with Extraterrestrial Intelligence, (MIT Press, Cambridge: 1973).
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