Innumerable as the Starrs of Night,
Or Starrs of Morning,
Dew-drops, which the Sun
on every leaf and every flouer
Beauty is truth, truth beauty,
— that is all
Ye know on earth, and all
ye need to know.
E = M
Energy is eternal delight.
Impearls: The Earths of Alpha Centauri
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As a Board member of the Friends of the U.C. Santa Cruz Library, I was invited this year to assist in judging UCSC's annual Graduate Research Symposium, in which the university's graduate students present personal or poster presentations concerning their thesis research for prestige, prizes, and trophies.
The winner of this year's entire event was a female graduate student, Javiera Guedes of the Astronomy and Astrophysics department at UCSC, who presented a talk on “The Earths of Alpha Centauri,” concerning the likelihood that both principal stars of the binary Alpha Centauri system possess planets, which we should be able to start discovering (as detection technology has steadily improved, and given a determined search) within the next several years.
I wrote up a brief report on Guedes' talk for a mailing list, only to be subsequently invited by editor Kevin Langdon (also endorsed by Javiera) to expand that piece for the Mega Society's online journal Noesis's upcoming special issue on Astronomy and Space.
University of California at Santa Cruz astronomy and astrophysics graduate student Javiera Guedes (first author), together with her coauthors, have published a fascinating piece in The Astrophysical Journal titled the “Formation and Detection of Terrestrial Planets around α [Alpha] Centauri B” 1 — which in my view deserves far wider audience and consideration than it can receive in that journal, however prestigious and renowned a scientific journal it assuredly is.
The subject of that paper, the binary Alpha Centauri star system (also known as Rigil Kentaurus or Toliman), at some 4.4 light years (or about 1.3 parsecs) distant from the Sun, is the closest extrasolar stellar system to our own Solar System and Earth. The brightest star in that system Alpha Centauri A is quite similar to our Sun in mass (at ∼1.105 solar masses), and extremely similar in color and thus temperature (classed like the Sun as a spectral type G2 V, a so-called “yellow dwarf”), whilst its companion Alpha Centauri B is only slightly smaller (∼0.934 times the Sun's mass) and a bit redder and therefore cooler (spectral type K1 V) than the Sun. One might note that the Alpha Centauri system (at about 5.6–5.9 Gyr) is between 1 and 1.3 billion years older than our Sun and Solar System, while it's about half again as rich in “metals” (as astronomers regard them: i.e., elements heavier than hydrogen and helium) as our own system.
Though it has a third, much smaller (∼0.1 solar mass) spectral type M “red dwarf” companion star known as Proxima Centauri — swinging at an enormous distance (perhaps a fifth of a light year) away from its principals — however ignoring Proxima, Alpha Centauri is essentially a close binary star system; and thus one might imagine that Alpha Centauri's two principal stars A and B's gravitational interference on each other would forestall prospects for any stable planets circling either star. As it happens, however, those primary components of Alpha Centauri are not actually all that close, orbiting each other some 23 astronomical units (23 times the distance between the Earth and the Sun, abbreviated AU) apart from each other — equivalent to B (or A) circling between the orbits of Uranus and Neptune (in our Solar System) with regard to the other — and as a result planets orbiting beyond what would be the orbit of Mars here, up to some 3 AU away from its primary (or well into our asteroid belt) are not ruled out around either star; moreover any planets (if they exist) are computed with high probability to be stable for the requisite billions of years time. Moreover, planets have already been discovered orbiting other roughly similar binary stars (e.g., γ [Gamma] Cephei, HD 41004, and Gliese 86) having basically equivalent separations from each other.
Indeed, Alpha Centauri A and B would probably even have performed a positive perturbative role with regard each other's incipient planetary systems, similar to that which the gas giants Jupiter, Saturn, and beyond are thought to have played in planetary evolution here in our Solar System, to wit providing “perturbations allow[ing] for the accretion of a large number of planetary embryos into a final configuration containing 3–4 bodies.” (Note that we omit end-note references in all quotes from The Astrophysical Journal article.)
Alpha Centauri B, as a cooler, “quieter,” less variable and flare-prone star than Alpha Centauri A (or the Sun for that matter), as a result is somewhat easier than A to detect any planets circling round. Thus it is on B that the authors concentrate their attention, estimating that after only about three years of “high cadence” observations (watching B on basically every night that there's good seeing, which could be close to 300 days a year), one could detect (using the so-called Doppler or radial-shift detection method) a planet of only some 1.8 Earth-masses circling within B's so-called “habitable zone,” while somewhat smaller worlds ought to become apparent in only a couple of years more.
Whilst it's also sometimes possible to detect extrasolar planets by observing their transit (or eclipse) across the disk of their primary star as seen from Earth, that method requires that the plane of any planets' orbits be closely aligned with the direction of our Sun with respect to that system — which is obviously extremely unlikely when attempting to locate worlds circling any particular star — and thus such an approach is suitable only for statistical surveys of a great many stars, not for finding the planets of any specific suns.
In addition to evaluating how Alpha Centaurian planets could be observed from the perspective of Earth, the authors conducted a number of computed simulations (eight in all) of possible routes to planetary system formation, starting from initial circumstances “mimic[ing] conditions at the onset of the chaotic growth phase of terrestrial planet formation in which collisions of isolated embryos, protoplanets of approximately lunar mass, dominate the evolution of the disk. During this phase, gravitational interactions among planetary embryos serve to form the final planetary system around the star and clear out the remaining material in the disk. At the start of this phase, several hundred protoplanets were presumed to orbit the star on nearly circular orbits.” Each run of the simulation “populate[d] the disk with N = 400 to N = 900 embryos of lunar mass […].”
Simulation number 7 (see Figure 3), specially exemplified herein and in The Astrophysical Journal article (known as r600_1 there), started with 600 embryos.
All bodies in the simulations interact only through gravity and the evolution of their positions and velocities with time were calculated using the MERCURY code, designed for the presence of a binary companion and allowing planetary embryos to collide and stick together to form larger planets.
The investigators “focus[ed] on terrestrial planet formation around α Cen B […].” As they note, “[P]lanet formation around α Cen A is expected to be qualitatively similar.”
Figure 2 illustrates how simulations of the evolution of a planetary system surrounding Alpha Centauri B typically progressed (using simulation 7):
The authors describe the foregoing figure thusly:
Figure 3 illustrates the results of all eight Alpha Centauri B system evolution simulations that the authors performed. The especially illustrated simulation used herein appears as number seven near the bottom, whilst for comparison our Solar System is shown to scale at top.
We see that realistic astrophysical simulations predict that planets surrounding Alpha Centauri B (as well as a similar system circling A) are quite likely. What will it take to actually find such worlds, if they do exist?
As noted earlier, due to the extreme unlikelihood of any specific stellar planetary system's equivalent of our “plane of the ecliptic” (the plane in which its planets' orbits generally circle) exactly lining up on edge as seen from Earth, the transit method for detecting extrasolar planets cannot be applied (other than by the remotest chance) for locating worlds orbiting specific suns — leaving only the “Doppler wobble” approach available for finding planets in more particular circumstances. Even for that method to work, the plane of a given star's planetary orbits must not directly face the Sun (i.e., the axis of that plane mustn't be oriented directly toward or away from the Sun), as there has to be some planetary radial velocity toward or away from the Earth for us to detect. Inasmuch as theoretical considerations imply that the orbital plane of planets circling either star of a close binary system should in general be aligned with the orbits of the stars themselves as they revolve about each other — and since in the case of the Alpha Centauri system, its two stars' orbital plane can be observed to be inclined to the line of view from here in the Sol System by a mere 11 degrees (the axis of that plane being almost perpendicular to the line of sight from the Earth) — thus planets circling either A or B are nearly ideal for detection from Earth using the radial-velocity technique.
Indeed, as the authors of this study conclude: “α Cen B is overwhelmingly the best star in the sky for which one can contemplate mounting a high-cadence [nightly] search” for extant terrestrial worlds, among other things because “α Cen B is exceptionally quiet, both in terms of acoustic p-wave mode oscillations and chromospheric activity.”
They note that “[t]he radial velocity [Doppler] detection of Earth-mass planets near the habitable zones of solar-type stars requires cm s−1 [centimeter per second velocity] precision,” whereas Alpha Centauri A exhibits (rather Sun-like) oscillatory noise on the order of 1 to 3 m s−1 (meters per second), which would effectively swamp attempts to detect planets circling A using near-term technology. Alpha Centauri B, on the other hand, as a fundamentally quieter star, displays peak amplitude noise on the order of 0.08 m s−1 (8 cm/second), which is also far higher in frequency than the periods of any potential terrestrial planets to be detected. As a result, a “focused high cadence approach involving year-round, all-night observations would effectively average out the star's p-mode oscillations.”
Observations also reveal that Alpha Centauri B exhibits much less chromospheric variability associated with stellar flares than does A (the former modifying its x-ray brightness only within a factor of two over a couple of years time whilst A could be observably seen to vary by an order-of-magnitude factor of ten).
The paper further points out that:
Moreover, Alpha Centauri A and B being so close to each other in space as well as physically similar to one another allows parallel observations of the two stars to reveal concurrent variations which, seen in both, allow identification of systematic artifacts in the observational process, that can thus be filtered out of any meaningful results. Furthermore, as the study notes, the position of Alpha Centauri at about −60° declination in our southern sky is nearly perfect for virtually continuous night-by-night (“high cadence”) observations from two existing vantage points, the Las Campanas Observatory together with the Cerro Tololo Inter-American Observatory, both in Chile, either of which ought to provide up to almost 300 viewing days a year (60 days a year being basically unavailable while Alpha Centauri annually passes behind the Sun, plus a few more days lost as a result of bad weather).
Inasmuch as the proportionate density of binary, roughly solar-mass component star systems in this part of the galaxy is only about 0.02 per cubic parsec (1 cubic parsec = ∼35 cubic light years), since at this time the Alpha Centauri system hovers a mere 1.33 parsecs away from us, we're very lucky here in the Solar System having α Cen proceeding so close nearby during this era for us to perform this highly desirable search upon.
As The Astrophysical Journal paper concludes: “All these criteria make α Cen B the ideal host and candidate for the detection of a planetary system that contains one or more terrestrial planets.” Indeed, “our current understanding of the process of terrestrial planet formation strongly suggests that both principal components of the α Cen system should have terrestrial planets.”
Given that extremely tantalizing possibility, what will it take to find at least those worlds orbiting Alpha Centauri B, if they exist? As the authors make note:
At this point well over three hundred planets have been discovered circling other stars beyond the Sun — all thus far found, due to hitherto operative technical limitations, necessarily being much larger than Earth and thus far from being really terrestrial in type. The Alpha Centauri system offers the opportunity to refine those limits downward towards worlds much closer in size, and thus potentially in habitability, to the Earth.
Figures 4 and 5 below illustrate how such a high-cadence search over a period of several years could zero in closer and closer towards identifying any planets of Alpha Centauri B that are truly terrestrial in scale.
As the capability for detecting truly terrestrial-type planets circling round nearby stars approaches, we're on the cusp of an adventure grander by far than Columbus's voyages to the New World or other great discoveries of the age of exploration, not only for its tremendous scientific value (finding what variety of worlds so-called “terrestrial” planets can form, not to speak of the enormous significance of possibly discovering other independently evolved organisms inhabiting them), but also for the sake of the future history of mankind, along with the ultimate fate of all life dwelling on — but presently restricted to this single egg-basket of — our planet Earth.
In a discussion such as this of potential planets circling the component stars of the Alpha Centauri system, special recognition is due the late biochemist and most prolific science fiction and fact writer Isaac Asimov, for it was he who, just a half-century ago (in June, 1959), penned his far-sighted essay “The Planet of the Double Sun” 2 concerning the possibility of just such worlds existing. A quarter-century later, in 1985 he wrote another essay on the subject of life near Alpha Centauri called “The Double Star&rdquo 3; whilst in 1976 Asimov published an entire book on Alpha Centauri, The Nearest Star. 4
In regard to the intrinsic value of such planets, it's worth noting the ending of Asimov's affecting science fiction novel The End of Eternity 5, which serves as the introduction to his famous Galactic Empire and Foundation series of stories. In the context of this tale, when it is realized that ready access to the universe is at hand for humanity (provided they take a critical step, namely make a certain change to the past), the principal protagonist wonders aloud what good it would really do if they should indeed accomplish it:
Whereupon his erstwhile enemy, more recent ally, and soon to be spouse replies:
Now, on the cusp of the fortieth anniversary of mankind's (as representative of all life on Earth) first visit in all the billions of years history of Earthly life to another planet, it's time to get on with it.
Let's find those worlds!
Acknowledgments and References
Many thanks to talented astronomy and astrophysics graduate student Javiera Guedes for her support and suggestions as well as permission to use the figures (indeed her own modifications to one of the figures) from her and her coauthors' article in The Astrophysical Journal. One might note that Ms. Guedes herself will personally be conducting observations of Alpha Centauri later on this year. Kudos to her and the other investigators in this study, and best wishes in the great search!
1 J. M. Guedes, E. J. Rivera, E. Davis, and G. Laughlin (all at the University of California at Santa Cruz, Astronomy and Astrophysics department), E. V. Quintana (SETI Institute, Mountain View, CA), and D. A. Fischer (San Francisco State University, Physics and Astronomy department), “Formation and Detection of Terrestrial Planets around α Centauri B,” The Astrophysical Journal, Vol. 679, Issue No. 2 (2008), pp. 1581-1587; doi: 10.1086/587799.
2 Isaac Asimov, “The Planet of the Double Sun,” The Magazine of Fantasy and Science Fiction, 1959-06, Mercury Press, New York. Collected in Fact and Fancy, Doubleday & Co., Garden City, NY, 1962; also in Asimov on Astronomy, Anchor Press, Garden City, NY, 1975.
3 Isaac Asimov, “The Double Star,” American Way, American Airlines, 1985-09-03. Collected in The Dangers of Intelligence And Other Science Essays, Houghton Mifflin, Boston, 1986.
4 Isaac Asimov, Alpha Centauri, The Nearest Star, Lothrop, Lee & Shepard Co., New York, 1976.
Isaac Asimov, The End of Eternity, Doubleday & Co., Garden City, NY, 1955, p. 187.
UPDATE: 2009-07-19 19:40 UT: Version 2: Updated via suggestions from Javiera Guedes.
UPDATE: 2009-07-28 06:00 UT: Version 3: At the time of submission for publication in Noesis.
UPDATE: 2009-08-03 19:00 UT: Version 4: Further updates to the Noesis publication version, including adding the Glossary.
UPDATE: 2009-08-04 15:30 UT: Version 5: A few corrections.
UPDATE: 2009-08-05 03:40 UT: Version 6: Minor fix.
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