by Paul Gilster | May 22, 2009 | Deep Sky Astronomy & Telescopes |
When we talk about the diameter of the Milky Way, it’s usual to cite a figure of about 100,000 light years. But the much more diffuse halo of stars surrounding the galaxy actually extends almost twice as far. You would expect to find more or less the same situation in other galaxies, but new observations of the giant galaxy M87 have turned up a surprising fact: Its halo of stars is much smaller than expected. It’s true that the halo is three times the size of that around the Milky Way, but its diameter of a million light years is still much smaller than anticipated given the size of the parent galaxy.
Mysteries like this seem just the thing for the weekend, so consider the possibility, raised in the paper on this work, that the truncated halo is the result of a collapse of dark matter in the Virgo Cluster, where M87 resides. The Virgo Cluster is approximately 50 million light years from us and contains hundreds of galaxies of all descriptions, including spirals like the Milky Way. Another possibility: A close pass by the galaxy M84, which could have perturbed M87 about a billion years ago.
Image: The Virgo cluster of galaxies taken with the Palomar Observatory 48-inch Schmidt telescope as part of the Digitized Sky Survey 2. The giant elliptical galaxy Messier 87 is seen in the centre, while Messier 84 and 86 are the two bright galaxies forming part of the small group on the centre right of the image. New observations obtained with ESO’s Very Large Telescope have shown that the halo of stars around Messier 87 has been truncated, possibly because of some interaction with Messier 84. The observations also reveal that Messier 87 and 86 are moving towards each other. Credit: European Southern Observatory.
How to measure the motions of stars in the haloes of distant galaxies? Researchers in this study used a spectrograph on the European Southern Observatory’s Very Large Telescope in Chile to measure planetary nebulae on the outskirts of M87 and also in intergalactic space within the surrounding Virgo Cluster. Such nebulae show strong emission lines, making them easier to detect at great distances and allowing for precise radial velocity measurements. They offer clues to the number and type of stars in outer galaxy regions and help us account for their motions.
We don’t know that dark matter is the culprit here, but it’s interesting to note that similar effects believed to be caused by dark matter have been found in other galaxies, detected through gravitational lensing. The authors point out, however, that the data gathered to this point are insufficient to draw a conclusion between dark matter and other scenarios. Their thinking is that further study of planetary nebulae with a much larger sample will be needed to get an accurate picture of what is going on.
The paper is Doherty et al., “The Edge of the M87 Halo and the Kinematics of the Diffuse Light in the Virgo Cluster Core,” accepted for publication in Astronomy & Astrophysics and available online. More in this European Southern Observatory news release.
by Paul Gilster | May 21, 2009 | Asteroid and Comet Deflection |
None of us would have wanted to be around during the Late Heavy Bombardment, that frenetic bashing of our planet as the young Solar System worked out its debris problems between 4.1 and 3.8 billion years ago. The Hadean period was a time when enormous asteroids pummeled our world over a span lasting as long as 200 million years, an ongoing series of events one would have assumed lethal for whatever organisms may have evolved by then.
But was the Late Heavy Bombardment really the deadly rain we’ve always assumed? A new paper in Nature questions the idea, basing its results on computer modeling of the Earth’s heating during the bombardment. Oleg Abramov and Stephen J. Mojzsis (University of Colorado) argue that our planet’s surface would likely have been sterilized during this period, but microbial life below the surface or in underwater conditions would almost certainly have survived.
“Our new results point to the possibility life could have emerged about the same time that evidence for our planet’s oceans first appears,” said Mojzsis, principal investigator of the project.
If so, it’s conceivable that life has been through a continuous process of development here, rather than one marked by sudden extinctions and re-starts. The theory is given weight by our growing understanding of the role of hydrothermal events deep beneath the oceans, where early life might well have lingered. Abramov thinks the finding has implications for life elsewhere:
“Even under the most extreme conditions we imposed on our model, the bombardment could not have sterilized Earth completely. Our results are in line with the scientific consensus that hyperthermophilic, or ‘heat-loving,’ microbes could have been the earliest life forms on Earth, or survivors from an even more ancient biosphere. The results also support the potential for the persistence of microbial biospheres on other planetary bodies whose surfaces were reworked by the bombardment, including Mars.”
It takes but a glance at the Moon through a small telescope to see the extent of the bombardment on a surface that has not, because of internal activity, resurfaced itself in billions of years. And it’s worth speculating, as we look at nearby stars like Tau Ceti, that the abundance of impactors that may be present in a system like this may not preclude the continuing development of life. That’s a conclusion that’s cheering to this writer, for life once started seems determined to last.
The paper is Abramov and Mojzsis, “Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment,” Nature 459 (21 May 2009), pp. 419-422 (abstract).
by Paul Gilster | May 20, 2009 | Exoplanetary Science |
Finding Earth-like planets around any star would be a stunning feat, and either Kepler or CoRoT may deliver such news before too long. But how much more exciting still if we find a planet like this around a star as close as Centauri B? After all, the Centauri stars are our closest stellar neighbors, close enough (a mere 40 trillion kilometers!) to conjure up the possibility of a robotic mission there and, if we play our propulsion cards right in the future, perhaps a manned trip as well.
A Radial Velocity Long Shot
But can we pick up the faint signature of a terrestrial world in this system, given that it would be akin to ‘detecting a bacterium orbiting a meter from a sand grain — from a distance of 10 kilometers’? The phrase is Lee Billings’, from his fine essay in SEED called The Long Shot, on an ongoing project to do just that. Most radial velocity surveys are spread out over numerous stars, picking off close-in worlds whose traces should be obvious in short periods of time.
Gregory Laughlin (UC-Santa Cruz), on the other hand, armed with a planet hunter’s insights, a passion for the Centauri system, and a realization that patience could tease out faint signals like these, traded ideas with Debra Fischer (San Francisco State) on the possibility of devoting years to an Alpha Centauri search. Fischer is now hard at work, using a telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. She works with a decommissioned spectrometer and other vintage equipment. Call it ‘Alpha Centauri on the cheap.’
It may take promising early data to get even this modest setup funded after National Science Foundation funds run out in November, but we’ll take the funding problem one step at a time. For now, the precision work continues, with software Fischer herself coded being used to filter out distortions of weather, instrumentation and stellar activity on the target stars to hunt for the minute shifts in wavelength that could signal the breakthrough discovery. If she pulls this off, Fischer’s patience may become legendary.
Image: Centauri planet-hunter Debra Fischer. Credit: NASA.
A Parallel Hunt, and Controversy
We may be talking three to five years here, and in the meantime, Fischer’s work is being paralleled by Michel Mayor and Stéphane Udry using the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory facilities at nearby La Silla. But ‘matched’ isn’t the best word — Mayor’s team isn’t as fixated on Alpha Centauri as Fischer’s because HARPS can’t be committed to a single, intensive project. If we’re talking an Earth-mass planet in Centauri B’s habitable zone, Fischer should find it first. A larger world may be claimed by Mayor.
You must read this essay. Billings is a wonderful writer whose scientific clarity is matched by a novelist’s eye for detail. And he’s working with a fascinating bunch of scientists, including Philippe Thébault, whose recent papers, discussed in these pages, have made the case that planets could not form around Centauri A and B because the relative velocities of planetesimals there would prevent their further growth. You have to love a scientist who speaks so openly about his own results, which differ sharply from Greg Laughlin’s conclusions on the Centauri system. Here’s Thébault on the matter, as quoted by Billings:
“If you ask a doctor about a fatal diagnosis he makes for a cancer patient, of course he wishes he is wrong. It is the same for me. It appears to be very difficult to form planets around close binary stars. I don’t wish for such a universe—I wish for another universe where planet formation is always very easy. I hope that Greg is right and that I’m wrong.”
We won’t know for a while, but it’s interesting that the binary system HD196885, a close match to Alpha Centauri, has a known gas giant at 3.5 AU. With refreshing candor, Thébault says he can’t explain that result: “…It shouldn’t be there.” On the flip side, the astronomer Peter van de Kamp spent decades looking for planets around Barnard’s Star, only to learn that the effect he was observing was an aberration in his instruments. The hunt continues in search of hard data.
Image: Alpha Centauri in context. Note that the three Centauri stars appear here as a single light source. Credit: Akira Fujii/David Malin.
What Centauri Means
Billings has his own thoughts on the significance of a find, and he also asks planet hunter Geoff Marcy for his take on the project:
The discovery of habitable worlds around any star would be front-page news, but finding them around our next-door neighbors would catalyze a scientific and cultural revolution, an immense rising wave of effort to learn whether our sister stars’ habitable planets were in fact inhabited. The ripples would spread beyond science to touch and change our literature and art, our politics and religion, perhaps even aiding our struggles to unite, survive, and expand as a species. The chain of chance that brought us into existence would swing to point outward to the stars, strengthening our resolve to someday reach them.
“If planets are found around Alpha Centauri, it’s very clear to me what will happen,” Marcy said. “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital camera, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”
Well, maybe. Or maybe we’ll start with something closer to home, such as putting new emphasis on a planet hunter mission that can get spectroscopic observations of planetary atmospheres around these stars, a hellishly difficult challenge, but considerably less expensive than a one-shot flyby. That’s a question that can only be resolved by technical advances in coming years. While we work on the relevant technologies, what an encouraging thought that early in the next decade, we should have hard evidence for the presence or absence of Centauri planets. Billings again:
Alpha Centauri is today what the Moon and Mars were to prior generations—something almost insurmountably far away, but still close enough to beckon the aspirational few who seek to dramatically extend the frontiers of human knowledge and achievement. For centuries, it has been a canonical target of the scientific quest to learn whether life and intelligence exist elsewhere. The history of that search is littered with cautionary tales of dreamers whose optimism blinded them to the humbling, frightful notion of a universe inscrutable, abandoned, and silent.
Let’s hope today’s dreamers will be vindicated in their hopes for planets in this fascinating system. The confirmation of a terrestrial world there would add robustly to our theories of planetary formation. The study of its atmosphere could tell us whether our nearest neighbor also sheltered life. And what was true decades ago still holds: As an inspirational target driving propulsion research for the ultimate next generation mission, there’s nothing like Centauri.
by Paul Gilster | May 19, 2009 | Astrobiology and SETI |
I’m glad to see the phrasing of the key question used in the SETI Institute’s ‘Earth Speaks’ project. Assuming we one day detect a signal from an extraterrestrial civilization, the Institute asks, ‘Should we reply, and if so, what should we say?’ Given the apparent ease with which broadcasts to the stars have been made in the last few years, advertising everything from snack foods to movies, this question might easily have been ‘What should we say when we respond to an extraterrestrial signal?’
When or if? I come down on the side of the ‘if’ formulation, because the question deserves a global response, one reflecting a broad range of disciplines and perspectives. Such a response takes time to build. Another thing I like about ‘Earth Speaks’ is that it will give us an interesting take on our own species. The plan here is to encourage people to submit messages, pictures and sounds online, using the Internet to solicit ideas.
Fine-Tuning an Interstellar Greeting
The site is here, where the project further defines itself:
People from around the world are invited to submit pictures, sounds, and text messages that they would want to send to other worlds. The project aims to foster a dialogue about what we should say to extraterrestrial intelligence, as well as whether or not we should be sending intentional messages.
According to the SETI Institute, ‘Earth Speaks’ will use some kind of message tagging, evidently inserted by its contributors, to categorize the incoming material, which can then be broken down by demographic variables. Commonalities and differences in content should offer perspective on how interstellar messaging is viewed by people around the world (those who are connected to the Internet, that is). Says the Institute’s Doug Vakoch:
“By studying the tags used by many different people, we can capture the major themes that run through thousands of individual messages. That sets the stage for creating interstellar messages that begin to portray the breadth and depth of the human experience.”
I’d prefer to say that it creates a cultural resource through which we can get a better understanding of our global differences as we confront the possibility of future contact. In any case, as the Allen Telescope Array moves into its survey of the galactic plane, SETI advocates believe the chances of a detection are increasing. I’ll wager we won’t have one in this century, but the prospect is enticing and can be a useful tool in energizing public interest about our place in the cosmos.
Broadening the Debate
So is this the beginning of a larger public debate about whether or not we should send messages to other worlds? The SETI Institute has no current plans to transmit such messages, but powerful signals have been sent in the past from the Evpatoria Planetary Radar in the Ukraine, beginning with the Cosmic Call transmissions of 1999 and 2003, and including the so-called Teen Age Message to the Stars in 2001. A recent Evpatoria transmission was beamed at the star Gliese 581. We should also note the 1974 signal sent from Arecibo to the star cluster M13, and the recent flurry of commercially inspired message traffic.
Image: The RT-70 planetary radar site in Evpatoria.
In fact, given the global nature of both SETI and METI (Messaging to Extraterrestrial Intelligence), I would favor a wider approach that links this SETI Institute initiative to similar efforts among other SETI/METI proponents and skeptics. The Internet conversation about the merits of messaging the stars is provocative but the public is largely unaware of its complexities. Can ‘Earth Speaks’ grow into not only a database of public responses but a larger conversation that brings in experts from many disciplines to debate these matters online? Note, for example, Tibor Pacher’s Faces from Earth project, a reflection of the ways the Net is growing communities whose interplay can serve this purpose.
by Paul Gilster | May 18, 2009 | Tau Zero Foundation |
Tau Zero’s Kelvin Long seems to be everywhere these days, his most recent publication being a summary of the interstellar sessions at the UK Space Conference, held in early April. You can read that one here, where you’ll discover that Long also provides a thorough backgrounder on the Tau Zero Foundation, its goals and vision for the future. Some of these goals are much discussed in these pages — to make incremental progress toward the robotic and human exploration of the stars by using philanthropic funding to support credible research by Tau Zero ‘practitioner’ scientists.
Other goals include practical ways to expand the public perception of interstellar issues, including supporting students through scholarships, offering educational products, and organizing sessions at established conferences. Echoes naturally arise from the Breakthrough Propulsion Physics project that Marc Millis once managed for NASA, but in fact Tau Zero hopes to take a significantly different course, and one with a broader charter. From Long’s summation:
The Tau Zero Foundation is interested in all aspects of interstellar research. This includes the seeming impossible faster-than-light travel schemes such as Warp Drive, through to the seemingly simple solar sails which were recently described in a book by practitioner Greg Matloff titled Solar Sails: A Novel Approach to Interplanetary Travel, by Praxis Publishing Limited. Matloff has also initiated an interstellar precursor mission study with the help of Claudio Bruno and others. This is for an IAA Commission 3 study for a probe to the outer heliosphere to be launched around the 2023 timeframe. This shows that the work of the Foundation is not just a continuation of the NASA BPP work but is broad ranging in scope.
We should note that while hunting up philanthropic funding provides its own challenges, it also gets the organization out from under the political and bureaucratic bonds imposed by large government organizations, and thus offers significant advantages. We’ve recently seen another side of what we hope will become a regular Tau Zero activity, which is the publication of books and articles that gather recent research and communicate it to the public. Here Long is again center stage, as he is working on the upcoming book Interstellar Travel: Going from Mars to the Stars, to be published by Springer in 2010. And as if that weren’t enough, he heads up the Project Icarus starship effort, a joint activity between the British Interplanetary Society and Tau Zero.
The six presentations from the UK Space Conference are summarized in Long’s current article, and having read it, you’ll want to know that you can access all of the conference presentations at Tibor Pacher’s peregrinus interstellar site. And speaking of Tau Zero activities, the interstellar bet between Tibor and myself continues to show a satisfying tilt in my direction, with 73 percent of those voting agreeing with me that 2025 is too early to expect the launch of a true interstellar mission with a mission duration of less than 2000 years. Tibor and I will be talking about the bet and other matters at the upcoming Sixth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, to be held this July in Aosta, Italy. The current Aosta program is available and, thanks to Tibor, now easy to access online.
