by Paul Gilster | Dec 27, 2012 | Exoplanetary Science |
In his new article on Alpha Centauri in Astronomy & Geophysics, Martin Beech (Campion College, University of Regina) noted that the Alpha Centauri stars seem to go through waves of scientific interest. Beech used Google’s Ngram Viewer to look for references to the system in both the scientific literature as well as general magazines and newspapers, finding that there is a 30-year interval between peaks of interest. The figure is suspiciously generational, and Beech wonders whether it reflects an awakening of interest in this nearby system as each generation of scientists and publishers arises.
I mentioned on Christmas Eve that the Beech paper was a real gift for the holidays, and for those of us who try to track developments about Alpha Centauri, it certainly is, drawing together recent work and commenting with care on the findings. The big issue for now is the existence of planets around these stars, a question Centauri B b will begin to answer if it can be confirmed. Everyone from astrophysicists to science fiction authors has noted at one time or another that we may have planets around all three of these stars, no doubt fueling that thirty-year spike Beech identified.
Image: Despite the overexposure of Centauri A and B that fuses the two stars into one at top left, I like it because it reveals Proxima Centauri (shown by the arrow at bottom right), an indication of how distant this nearest star to the Sun is from the two larger stars. Credit: 1-Meter Schmidt Telescope, ESO.
I remember a long-ago sixth grade afternoon when I asked the teacher, after a presentation about astronomy, whether the nearest star had planets like ours around it. After class, she slipped me a book to look at whose title is long lost to memory, but I recall it being stated with confidence that planets were all but impossible around binary stars. Now we know better, for we have planets around binaries elsewhere. As for Alpha Centauri, it was in 1997 that Paul Wiegert and Matt Holman showed that planetary orbits were viable here out to about 4 AU around Centauri A and B.
That would seem to rule out gas giants, which presumably would have formed beyond the snow line at roughly the same 4 AU and beyond, but terrestrial planets in closer orbits are still allowed. Beech runs through the recent scholarship for Centauri A and B, most of which shows the likelihood of planet formation, though in at least one case with a serious restriction:
Guedes et al.(2008) and Thébault et al. (2009) additionally find that planets should have formed around a Cen B. Assuming an initial co-planar distribution of planetesimals, Guedes et al. argue that multiple 1-2 MEarth planets might reside within the region 0.5-1.5 au from a Cen B. Taking into account the details of planetesimal encounters and impact velocities, however, Thébault and co-workers argue that planet formation is only favoured within a restricted zone of 0.5 au about a Cen B. At this stage it will be the discovery of additional planets, with orbits well beyond that of a Cen Bb, that will guide the development of next-generation planetesimal accretion models.
For that matter, can we assume that the current orbital characteristics of Centauri A and B are those of the initial system? In the paper by Philippen Thébault and team referenced above, the authors point out that the odds of Alpha Centauri having gone through a close encounter with a nearby star system that could have affected stellar orbits may be as high as 1 in 2. It’s regrettable, then, that we have no knowledge of the cluster that spawned Alpha Centauri or the effects such encounters might have had. We do know that this system formed from a different cluster than the Sun — our stars are close now but the condition is temporary, and Alpha Centauri was likely 1 to 2 billion years old when our Solar System was beginning to form.
You’ll find most of the papers Beech refers to, the recent ones anyway, discussed in earlier Centauri Dreams posts, and you can use the search function to track them down by author. The same is true of work on Proxima, the M-dwarf that accompanies Centauri A and B, or at least is located at roughly the same distance and shares the same proper motion. It’s hard to believe that the three don’t form a single system, especially given their common chemical composition, but it has always been a tough call going back to the work of Joan Gijsbertus Voûte, who announced the first parallax findings about Proxima in 1917. Voûte pondered whether the three stars were physically connected or ‘members of the same drift,’ and Beech notes that despite the following century of observations, we are still working on the same question.
It’s also been amply demonstrated that both gas giants and rocky worlds can form around M-dwarfs, so while we’re gradually ruling out larger planets around Proxima, we’re far from being able to declare there are no terrestrial planets orbiting it. Here again we’ve run through the numbers in Centauri Dreams in recent memory, but Beech sifts through the findings, which increasingly exclude planets of Jupiter-mass, and he notes in particular the work of Michael Endl and team, which argues that any planet greater than two Earth masses within Proxima’s habitable zone should have been detected by now. That leaves room for smaller worlds in interesting places.
We can also exclude various kinds of planets around Centauri A and B:
With respect to a Cen AB, the five-year radial velocity survey starting in 1992 conducted by Endl et al. (2001) revealed the following constraints: for a Cen A there are no planets more massive than 1 MJupiter within 2 au, and no 2 MJupiter or larger mass planets within 4 au; for a Cen B there are no planets more massive than 1.5 Jupiter within 2 au, and no planets more massive than 2.5 MJupiter within 4 au. On the larger scale, a deep CCD imaging survey of the region immediately surrounding a Cen AB revealed no co-moving companions with masses greater than 15 MJupiter at distances 100-300 au (Kervella and Thévenin 2007).
Thus the search goes on, and Beech believes the current surveys will be able to dampen the noise in the radial velocity signal of an Earth mass planet in a 1 AU orbit around Centauri A or B with about five more years of data gathering. Although the HARPS team at La Silla was first with an Alpha Centauri planet candidate, the hunt for an Earth-class world in a habitable orbit has only intensified. Dedicated Alpha Centauri search programs are in progress not just via HARPS but also through a Yale University program at Cerro Tololo (this one backed by the Planetary Society as well) and through the HERCULES spectrograph at Mt. John Observatory in New Zealand.
I’ve only touched on the highlights of this rich paper, whose bibliography alone makes it worth seeking out. The reference is Beech, “A journey through time and space: Alpha Centauri,” Astronomy & Geophysics, Volume 53, Issue 6, pp. 6.10-6.16 (abstract).
by Paul Gilster | Nov 13, 2012 | Exoplanetary Science |
The discovery of Centauri B b, a small planet with a mass similar to Earth, continues to percolate in the news even if the initial buzz of discovery has worn off. Science News gives the new world a look in a recent article, noting the fact that with an orbital period of 3.236 days, this is not a place even remotely likely for life. Surface temperatures in the range of 1200 degrees Celsius are formidable obstacles, but of course the good news is the potential for other planets around Centauri B and, indeed, around its larger companion.
Centauri A may well host interesting worlds, but it’s a tough study because it’s given to the kind of stellar activity that can more readily mask a planetary signature than the quieter Centauri B. Even so, we can imagine the possibility of two planetary systems in close proximity, a scenario that would surely propel any technological civilization around one to investigate the other. We don’t have the driver for spaceflight in our system that an Earth-like world around Centauri B might have, a second habitable planet breathtakingly close around another star.
If we’ve ruled out planets larger than Neptune around any of the three Alpha Centauri stars, that leaves the door open for the small worlds that could be the most interesting if one or more turned up in the habitable zone, and it’s worth noting that on this score, Proxima Centauri is still in the game. But right now the incredibly tricky detection of Centauri B b needs confirmation, which could be delivered by Debra Fischer (Yale University). You’ll recall that Fischer has been working at Cerro Tololo (CTIO) in Chile to develop the high-resolution spectrometer known as CHIRON, commissioned in March of 2011, as part of her team’s search for rocky Alpha Centauri planets.
Leaving Copernicus Behind
Centauri B b might also be confirmed through detection of a transit, the chances being estimated in the region of 10% and perhaps, according to Greg Laughlin (UC-Santa Cruz) as high as 25%. While the necessary work continues, let’s move beyond the Alpha Centauri stars for a moment to talk about Laughlin’s latest work with Eugene Chiang. Exoplanet hunters have learned through experience to question assumptions, the most obvious of which is that our Solar System is in some sense ‘normal.’ Or as Laughlin writes on systemic, “There is an intriguing, seemingly anti-Copernican disconnect between the solar system and the extrasolar planets.”
Image: Exoplanet hunter Greg Laughlin, whose latest work re-examines how super-Earths form close to their stars. Credit: UC-Santa Cruz.
Maybe the reason exoplanets so often surprise us is that we base our thinking on our own Solar System, and the minimum-mass Solar nebula from which it grew, considering this a template. The rest of the galaxy may have other ideas. Consider that close-in super-Earths are common. Planets like these, showing up in abundance in Kepler data and Doppler velocity surveys, are a challenge to explain. Laughlin and Chiang say that more than half, if not nearly all Sun-like stars have planets with radii between 2 and 5 times that of Earth and orbital periods of less than 100 days. The researchers write:
Super-Earths are not anomalous; they are the rule that our Solar System breaks. In a sense, the burden of explaining planetary system architectures rests more heavily on the Solar System than on the rest of the Galaxy’s planet population at large.
The problem, then, is that our Solar System has no planets inside Mercury’s 88-day orbit. Is it possible our Solar System did not undergo the same kind of formation history that may be the dominant mode in the galaxy? To explore this, the researchers look at migration issues, for it is commonly thought that short-period planets formed several AU out from their stars and then migrated to their present location. But disk migration is poorly understood, and while it may be necessary to explain hot Jupiters, Laughlin and Chiang say it may not be the mechanism to explain the majority of planetary systems with super-Earths in inner orbits.
The alternative: Forget orbital migration and consider the possibility that super-Earths form right where they are, in circumstellar disks that extend inward from 0.5 AU. The researchers go to work on constructing a new template, the Minimum Mass Extrasolar Nebula (MMEN), which allows them to explore how such planets could form near their star:
Our order-of-magnitude sketches in this regard are promising. In-situ formation at small stellocentric distances has all the advantages that in-situ formation at large stellocentric distances does not: large surface densities, short dynamical times, and the deep gravity well of a parent star that keeps its planetary progeny in place.
The basic properties of planets forming at their current orbital distance are made clear:
In-situ formation with no large-scale migration generates short-period planets with a lot of rock and metal and very little water. The accretion of nebular gas onto protoplanetary cores of metal produces H/He-rich atmospheres of possibly subsolar metallicity that expand planets to their observed radii. Retainment of primordial gas envelopes against photoevaporation leads to planets that can be similar in bulk density to Uranus and Neptune while being markedly different in composition. Close-in planets are not water worlds.
Laughlin and Chiang believe there should be observational consequences to such predictions, making the theory falsifiable. Hot young stars should lack close-in super-Earths because they would be too hot for planetesimal-building dust to survive. Brown dwarfs and M dwarfs should have close-in super-Earths and Earths orbiting them. Close-in planets grown from close circumstellar disks should also have orbital planes aligned with the equatorial planes of their host stars. I’ll send you to the paper to go through the entire list of predictions, all of which should allow these ideas to be probed, but I do want to mention one last prediction having a bearing on Centauri B b. For if Laughlin and Chiang are right, then binary systems offer a good test.
After all, close binary systems should make planetary migration extremely difficult. The close companion would disrupt planet formation at large distances from the star. Planets orbiting Centauri B inside the 0.5 AU boundary would be incompatible with migration, and of course, we now have such a planet, along with the likelihood of finding more. Here I drop back to the Science News article, which quotes Laughlin on Centauri B: “I think that the odds that there’s an interesting planet, a truly interesting planet in the system, are very high, given that this one is there.” And if he’s right, that interesting, potentially habitable world may serve as further evidence for the theory that such planets formed right where they are found.
The paper is Chiang and Laughlin, “The Minimum-Mass Extrasolar Nebula: In-Situ Formation of Close-In Super-Earths,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Oct 16, 2012 | Exoplanetary Science |
For much longer than the nine years Centauri Dreams has been in existence, I’ve been waiting for the announcement of a planetary discovery around Centauri B. And I’m delighted to turn the first announcement on this site over to Lee Billings, one of the most gifted science writers of our time (and author of a highly regarded piece on the Centauri stars called The Long Shot). Lee puts the find into the broader context of exoplanet research as we turn our gaze to the nearest stars, those that would be the first targets of any future interstellar probes. On Thursday I’ll follow up with specifics, digging into the discovery paper with more on the planet itself and the reasons why Centauri B was a better target than nearby Centauri A. I’ll also be offering my own take on the significance of the find, which I think is considerable.
by Lee Billings
For much of the past century, astronomy has been consumed by a quest to gaze ever deeper out in space and time, in pursuit of the universe’s fundamental origins and ultimate fate. This Old Astronomy has given us a cosmological creation story, one which tells us we live in but one of innumerable galaxies, each populated with hundreds of billions of stars, all in an expanding, accelerating universe that began 13.7 billion years ago and that may endure eternally. It’s an epic, compelling tale, yet something has been missing: us. Lost somewhere in between the universe’s dawn and destiny, passed over and compressed beyond recognition, is the remarkable fact that 4.5 billion years ago our Sun and its worlds were birthed from stardust, and starlight began incubating the planetary ball of rock and iron we call Earth.
Somehow, life emerged and evolved here, eventually producing human beings, creatures with the intellectual capacity to wonder where they came from and the technological capability to determine where they will go. Uniquely among the worlds in our solar system, the Earth has given birth to life that may before the Sun goes dim reach out to touch the stars. Perhaps, on other worlds circling other suns, other curious minds gaze at their night skies and wonder as we do whether they are alone. In this coming century, a New Astronomy is rising, one that focuses not on the edge of space and the beginning of time but on the nearest stars and the uncharted worlds they likely hold. It will be this New Astronomy, rather than the Old, that will at last complete the quest to place our existence on Earth within a cosmic context.
In a major leap forward in this enterprise, today a European planet-hunting team announced their discovery of an alien world about the same mass as Earth. This alone would be noteworthy, for of all the “exoplanets” now known beyond our solar system, only a very few, and very recently, have been shown to at all resemble our own. But there is more to the story. This particular exoplanet resides in a three-day orbit around the dusky orange star Alpha Centauri B, a member of the Sun’s closest neighboring stellar system. There are two other stars in the system as well, the yellow Sun-like star Alpha Centauri A and the red dwarf star Proxima Centauri.
Astronomers began discovering exoplanets about two decades ago, finding at first a few per year. Since then, the pace of discovery has dramatically accelerated. Today there are more than 750 confirmed exoplanets, and a single NASA mission, the Kepler spacecraft, has detected more than 2,300 additional candidates that await confirmation. Most of these exoplanets are far too large, too hot, or too cold to support life as we know it, but a handful appear to be “Goldilocks” worlds the right size and the right distance from their stars where liquid water could flow in streams and pool in seas, worlds where carbon-based organisms could potentially thrive. The discovery of more Goldilocks worlds appears inevitable — statistics from the Kepler mission and other sources suggest that somewhere between ten to thirty percent of stars harbor potentially habitable planets. Among the planet-hunters, the question is no longer whether life exists elsewhere in the universe, but rather how far removed the next-nearest living world might be.
At a distance of just over 4.3 light years, the stars of Alpha Centauri are only a cosmic stone’s throw away. To reach Alpha Centauri B b, as this new world is called, would require a journey of some 25 trillion miles. For comparison, the next-nearest known exoplanet is a gas giant orbiting the orange star Epsilon Eridani, more than twice as far away. But don’t pack your bags quite yet. With a probable surface temperature well above a thousand degrees Fahrenheit, Alpha Centauri B b is no Goldilocks world. Still, its presence is promising: Planets tend to come in packs, and some theorists had believed no planets at all could form in multi-star systems like Alpha Centauri, which are more common than singleton suns throughout our galaxy. It seems increasingly likely that small planets exist around most if not all stars, near and far alike, and that Alpha Centauri B may possess additional worlds further out in clement, habitable orbits, tantalizingly within reach.
Anyone in the Southern Hemisphere can look up on a clear night and easily see Alpha Centauri — to the naked eye, the three suns merge into one of the brightest stars in Earth’s sky, a single golden point piercing the foot of the constellation Centaurus, a few degrees away from the Southern Cross. In galactic terms, the new planet we’ve found there is so very near our own that its night sky shares most of Earth’s constellations. From the planet’s broiling surface, one could see familiar sights such as the Big Dipper and Orion the Hunter, looking just as they do to our eyes here. One of the few major differences would be in the constellation Cassiopeia, which from Earth appears as a 5-starred “W” in the northern sky. Looking out from Alpha Centauri B b and any other planets in that system, Cassiopeia would gain a sixth star, six times brighter than the other five, becoming not a W but a sinuous snake or a winding river. Cassiopeia’s sixth bright point of light would be our Sun and its entire planetary system.
Image: Alpha Centauri as seen by the Cassini orbiter above the limb of Saturn. Credit: NASA/JPL/Space Science Institute.
Despite its close proximity, as with nearly all other known exoplanets, Alpha Centauri B b is as yet unseen. It was detected indirectly, via a periodic 50-centimeter-per-second wobble its orbit raises in the motions of its star, in a painstaking process that took four years of nightly monitoring and careful analysis. The wildly successful Kepler mission finds the bulk of its candidates by a different technique, looking for the minuscule diminution of a star’s light when, by chance, a planet in its orbit transits across the star’s face and casts a shadow toward Earth. Both of these discovery methods can constrain the most basic properties of a planet: its orbit, its mass, and perhaps its size and bulk composition. But neither can readily reveal whether or not any potentially habitable planet is actually a place much like Earth. To do that really requires taking a planet’s picture. Even if that picture amounted to only a meager clump of pixels, astronomers might discern within it a planet’s rotational period — the length of its days — as well as clouds, oceans, and continents. The reflected planetary light would also contain spectroscopic signatures of atmospheric gases. Carbon dioxide would suggest a rocky planet, and water vapor would hint at oceans or seas. Detecting oxygen and methane — gases produced on Earth by living things — would further suggest that the distant planet was not only habitable, but inhabited.
Viewed over interstellar distances in visible light, the Earth is some ten billion times fainter than the Sun, meaning that for every photon bouncing off Earth’s atmosphere or surface, ten billion more are flying out from our star. About the same ratio would apply for any habitable planet around Alpha Centauri’s stars. Distinguishing such faint planetary light from that powerful stellar glare is rather like spotting a firefly hovering a centimeter away from the world’s most powerful spotlight, when the spotlight is in Los Angeles and you are in New York. To see the firefly, that overwhelming ten-billion-to-one background light must be suppressed.
Amazingly, on paper and in laboratory studies, astronomers have already devised multiple ways to do exactly this for potentially Earth-like planets that may exist around nearby stars. Most of these methods require an entirely new multi-billion-dollar space telescope, though a few proposals exist to augment NASA’s upcoming James Webb Space Telescope with starlight-suppressing technology at an estimated cost of $700 million. Considering that three years ago a film about life on Alpha Centauri’s planets — James Cameron’s Avatar — grossed some $2 billion in box-office receipts, it stands to reason there is public appetite to spend at least that much on space telescopes to search for the real thing.
Matt Mountain, the director of the Space Telescope Science Institute in Baltimore, Maryland, likes to quip that the discovery of life beyond our solar system could be to this coming century what Neil Armstrong’s lunar footprints were to the last. Yet today NASA is not seriously funding any life-finding telescopes, and has no real plans to do so in the future. The agency instead is spread thin and lacking any unified direction, strapped for cash and struggling to avoid obsolescence while it maintains the International Space Station, builds a new fleet of rockets to replace the retired Space Shuttles, and completes the James Webb Space Telescope. Yet obsolescence is precisely what NASA will embrace if it fails to invest now in the next giant leap required for this New Astronomy. Of all the scientific institutions and agencies upon this planet, at present NASA alone has the resources to build a telescope capable of directly imaging and characterizing any Earth-like planets around nearby stars. Unless it does so, as the list of potentially habitable planets grows long in years to come, all that shall grow along with it will be our ignorance of what those distant worlds are actually like and what may live upon them.
Meanwhile powerful, influential Old Astronomy, which has revolutionized our understanding of the universe at its largest scales, is wary of the New, and at times has acted quite deliberately against it. Alas, government-funded Big Science is too often a zero-sum game, one in which money that could go toward looking for life on exoplanets around nearby stars would be taken from cosmological efforts to study far-distant galactic clusters and the expansion of the universe. In a perfect world we would fully fund both quests simultaneously. But our world — with its economic instabilities, rising temperatures, growing populations, and plummeting biodiversity — seems to grow more imperfect by the day, in ways that no knowledge of dark energy or dark matter is likely to ever assuage. The New Astronomy is different. We do not yet know whether planets like ours and creatures like us are in fact common or rare in the cosmos, but by trying to find out, we will unavoidably learn just how precious our planet truly is. Perhaps, with luck, discoveries following from today’s announcement will help us finally kick off from our small blue footstool, and find our way among the stars.
Lee Billings is working on a book about the search for other Earth-like planets, forthcoming from Penguin/Current next fall.
by Paul Gilster | Sep 24, 2012 | Culture and Society |
I’m always looking for ways to relate interstellar distances to common objects on Earth, knowing that misunderstandings about the vast scale of the universe are common. Sir John Herschel (1792-1871) talked about dropping a pea off the side of a ship after every mile on an ocean voyage the distance of the nearest star, telling his readers that it would require ‘a fleet of 10,000 ships of 600 tons burthen, each starting with a full cargo of peas.’ Herschel was the first to try to measure the distance to Alpha Centauri, and while his numbers weren’t as accurate as ours, he captured for his era the disconnect between Earthly distances and the heavens. Larry Klaes weighs in this morning with yet another way to study the issue, using an exhibition on the interplanetary scale that has now been extended Herschel-style to the ultimate voyage.
By Larry Klaes
Since its dedication in 1997, the Sagan Planet Walk has become a landmark feature of downtown Ithaca, New York. The various monuments, or “stations”, representing the major places in the Solar System from the Sun to Pluto serve first as a memorial to the late Cornell astronomer and science popularizer Carl Sagan. Perhaps best known for his Cosmos television series which premiered on PBS in 1980, Sagan passed away in December of 1996 at the age of 62 after two decades of teaching at the university.
The Sagan Planet Walk also serves as a continuing educational tool about our celestial neighborhood. Among its astronomy informational features is the Walk’s physical demonstration of just how far apart objects in our Solar System really are on a scale that the general public can relate to. When reduced from its actual radius of five billion miles, the Solar System’s scale becomes clear in everyday terms. The gray marble Sun station resides prominently in the Ithaca Commons with the planets from Mercury to Mars nearby, while the Pluto station sits just outside the Ithaca Sciencenter nearly one mile away. The Sciencenter is the primary developer of the Sagan Planet Walk.
One question that has often been asked by folks who hiked the Sagan Planet Walk is just how far away the stars nearest to our Sun would be on this scale. The answer is that a station for this feature of the Walk would have to be located in tropical Hawai’i over five thousands miles west of temperate Ithaca. That is how far one would have to travel to reach the representation of our nearest suns, the Alpha Centauri system, which exists 4.3 light years away, or over 25 trillion miles across interstellar space.
Image: The beginning of the Sagan Planet Walk, a gray marble monument symbolizing the Sun. Bring your walking shoes if you’re headed for the Pluto station. Alpha Centauri will require plane tickets. Credit: Andrew Alden.
For the last two years, the Sciencenter has been partnering with Cornell, the University of Hawai’i, and NASA to place an Alpha Centauri station on the Big Island of Hawai?i at the ‘Imiloa Astronomy Center on the Hilo campus of the University of Hawai?i. Lead funding was provided by the NASA Space Grant Program of New York (based at Cornell) and Hawai?i (based at the University of Hawai?i), with additional major funding provided by ?Imiloa and the Sciencenter.
A dedication ceremony to mark this newest and most distant addition to the Sagan Planet Walk will be held on September 28 at the ‘Imiloa Astronomy Center to coincide with the annual Polynesian Voyaging Festival at the hands-on science center, which focuses on astronomy and the culture of Polynesian voyaging. The features of the festival are to include presentations of double-hulled, ocean-going canoes, live demonstrations, and other activities that celebrate the proud heritage associated with centuries of Pacific Ocean crossings without instruments. Among those present at the dedication will be Charlie Trautmann, executive director of the Sciencenter and adjunct professor of engineering at Cornell.
While retaining the basic features common to all the stations of the Sagan Planet Walk – placards with interesting facts and a proportional model of each world that invites visitors “to contemplate the enormous, awe-inspiring scale of the Universe and our place within it” – the Alpha Centauri station also reflects the culture of its surroundings (Kamailehope is the Hawaiian name for Alpha Centauri).
The centerpiece of the exhibition is a large stone Hawaiian figure, sculpted in native Hawaiian volcanic stone by world-renowned Hawaiian artist Rocky Jensen. On either side of this sculpture are four graphic panels which detail the scale of the Solar System in both English and Hawaiian, the connection with the Sagan Planet Walk in Ithaca, and how the Alpha Centauri stars were utilized by Polynesian sailors to cross the Pacific in their open, double-hulled canoes without the need for astronomical tools. Native Hawaiian school children will learn about their heritage and the history of navigation thanks to an eight-foot wide circular star compass which completes the station.
Image: The Alpha Centauri station of the Sagan Planet Walk, fittingly found under Polynesian skies. Credit: Ithaca Sciencenter.
“The inclusion of the Alpha Centauri station has been fifteen years in the making, so we’re excited to see it coming to fruition and completing the Sagan Planet Walk,” said Teresa Smith, Public and Media Relations Manager at the Sciencenter.
The day after the Hawai’i dedication ceremony, the Sciencenter will hold its own celebration of the Sagan Planet Walk expansion to our nearest stellar neighbors by offering a free tour of the Walk, guided by Greg Sloan of Cornell’s Astronomy Department. Tour members will meet up at the Sun station on the Commons and make their way through the Solar System to the Pluto station at the Sciencenter, where prepared Hawaiian refreshments will await.
The newly expanded Sagan Planet Walk is now the largest such exhibition on Earth, measuring five thousand miles from end to end. This eclipses the previous record of 66 miles held by the artwork in the Stockholm, Sweden subway tunnels.
by Paul Gilster | Apr 19, 2012 | Deep Sky Astronomy & Telescopes |
Alpha Centauri is irresistible, a bright beacon in the southern skies that captures the imagination because it is our closest interstellar target. If we learn there are no planets in the habitable zones around Centauri A and B, we then have to look further afield, where the next candidate is Barnard’s Star, at 5.9 light years. Centauri A and B are far enough at 4.3 light years — that next stretch adds a full 1.6 light years, and takes us to a red dwarf that may or may not have planets. Still further out are Tau Ceti (11.88 light years) with its problematic cometary cloud, and Epsilon Eridani (10.48 light years), a young system though one thought to have at least one planet.
A warm and cozy planet around the K-class Centauri B would be just the ticket, and the planet hunt continues. One thing we’ve learned in the past decade is that neither Centauri A or B is orbited by a gas giant — planets of this size should have shown up in the data by now. We’ve also learned that stable orbits reach out maybe 2 AU from either star. Remember that while Centauri A and B are separated by almost 40 AU at their widest point, they close to within 11 AU, thus disrupting outer orbits, as demonstrated by computer simulations. We should expect planets, if they exist, to be no further out than the main asteroid belt in our own system.
Helping the Centauri Planet Hunt
Debra Fischer (Yale University) has been working on the Alpha Centauri problem at Cerro Tololo (Chile) in addition to her efforts at improving instrument sensitivity for planet hunting at the Keck and Lick observatories. The goal is to reach the precision needed to turn up planets the size of the Earth with radial velocity methods. If we’re going to get a Centauri detection, odds are it favors Centauri B because A does not seem to be as stable as B, and the latter is more likely to be the first to yield what Fischer calls the ‘tiny whisper’ that would flag an Earth-like world. Usefully, the 79 degree orbital plane of these stars means that planets in this system, assuming they share this tilt, should be generating a reflex velocity close to the line of sight from the Earth.
Image: The view from the Cerro Tololo Inter-American Observatory, where Debra Fischer’s work continues. Credit: San Francisco State University.
Radial velocity methods, in other words, should work here if we can attain sufficient sensitivity. The detection effort calls for telescope time at the Cerro Tololo Inter-American Observatory this spring and summer, and The Planetary Society is campaigning to raise money to support the effort. What Fischer needs is 20 nights of observing time, but the team’s NASA and NSF grants cannot be used to pay for telescope time, which at Cerro Tololo runs to $1650 per night. A total of $33,000 will do it, then, money the community should be able to raise. Have a look at the Planetary Society’s donation page and let’s see if we can’t make this happen.
Anyone involved with The Planetary Society is probably already aware of Fischer’s work with astronomer and Tau Zero practitioner Geoff Marcy (UC-Berkeley) on FINDS Exo-Earths (Fiber-optic Improved Next generation Doppler Search for Exo-Earths). The collaboration has resulted in a high-end optical system installed on the 3-meter Lick Observatory telescope and is now feeding the FINDS 2 effort to provide advanced optics for the Keck Observatory in Hawaii. Marcy and Fischer are working with a fiber optics array that adjusts light entering the telescope’s spectrometer and an adaptive optics system that offers the best signal to noise ratio.
FINDS worked out well at the Lick Observatory, improving the ability to detect Doppler velocities from the pre-existing 5 meters per second down to the 1 meter per second range, allowing us to detect smaller planets. Fischer and Marcy are hopeful of attaining precisions down to 0.5 meters per second with their work at Keck, which should get us into the range of Earth-sized planets. FINDS 2 will then be used with Keck to provide follow-up data about planets found by the Kepler mission, ruling out false positives in the ongoing hunt for planets like our own. The work on FINDS has led directly into the commissioning of a new spectrometer at Cerro-Tololo.
The Cerro-Tololo instrument is now tuned up and perfectly positioned for the study of Centauri A and B. The system is a valuable target whether or not we find a habitable world (or even two) there. We don’t know whether a rocky world around Centauri A or B would have oceans, because the mechanisms for delivering water to inner planets in a binary system are unknown. What Alpha Centauri provides on our doorstep is a look at planet formation in a close binary system — recall that as many as half of all stars are in binary pairs, so these are questions with a profound relevance to our understanding of the galactic distribution of planets.
What we find around Alpha Centauri may also drive future space research. We’re gaining good information about the statistical spread of planets in the galaxy through the Kepler mission, but the logical follow-on is a mission to investigate the closest stars to learn whether habitable planets are to be found within 100 light years or less, and ultimately to gather spectroscopic data about their atmospheres. The discovery of an interesting world 4.3 light years away would encourage these plans and, in the long term, energize the idea of interstellar probes. But the near term is now, and we may have results on Centauri A and B in short order. Please join in the effort to fund Debra Fischer and team as the Alpha Centauri hunt continues.
