by Paul Gilster | Jul 7, 2006 | Exoplanetary Science |
A starshade shaped like a daisy? Centauri Dreams remains entranced with the concept, known as New Worlds Imager and now getting renewed attention thanks to the efforts of astronomer Webster Cash (University of Colorado, Boulder). We’ve seen before that Cash is hoping to land a NASA Discovery-class mission for a starshade that would block the light of a nearby star to reveal the planets around it. The starshade would work in tandem with a telescope mounted on a separate spacecraft 15,000 miles away, with the shade being moved as needed to place it into the line of sight of stars of interest. The result: the ability to image planetary systems including terrestrial worlds, and even to analyze exoplanetary atmospheres.
Cash’s latest thoughts on the subject appear in the July 6 issue of Nature, where he describes a starshade some 50 yards in diameter and its associated space telescope. Both could be launched into an orbit roughly a million miles from Earth, where shade and telescope could be moved as needed to image various star systems. An alternative Cash has also considered is to use the James Webb Space Telescope, scheduled for a 2013 launch, as the ‘eyes’ of the starshade concept, doing away with the need for a separate telescope while enhancing the capabilities of the mission. Cash’s $400 million proposal to NASA is a standalone starshade that would do just that.
Image: A starshade would obscure the light of a star to allow its planets to become visible. Credit: Webster Cash; University of Colorado, Boulder.
We saw several days ago that Greg Laughlin (University of California, Santa Cruz) has worked out the (admittedly burdensome) parameters of using the European Southern Observatory’s HARPS (High Accuracy Radial velocity Planet Searcher) instrument to track potential terrestrial worlds around Alpha Centauri. So it may be that finding our first Earth-sized world can be managed from Earth, but the beauty of Cash’s space-based concept is that it is both inexpensive and capable of extending our reach to numerous nearby stars. A Cash-style starshade would cost a fraction of earlier Terrestrial Planet Finder mission concepts, and would do many of the needed tasks better.
Imagine being able to see all major planets from the habitable zone outward, with a good view of debris disks and possibly comets. “Photometric variations might show the presence of surface features like oceans and continents,” Cash writes. “Follow-up spectroscopy of the detected planets would enable classification by type, and the presence of water would be clearly visible in atmospheric absorption lines. Atmospheric markers (like free oxygen absorption lines) could potentially provide the first evidence of life outside our Solar System.”
The beauty of New Worlds Imager is in the way it suppresses light from the central star. That light can be, as Cash notes in the Nature paper, 1010 times brighter than any Earth-like planets around the star. Handling light suppression within the instrument, as was originally envisaged for the Terrestrial Planet Finder Coronagraph (TPF-C), can help, but diffraction and scattering of light are difficult and expensive to suppress in such a system, and coronagraphs are not as sensitive to outer system planets and debris disks, issues that seem best addressed by a external starshade. Cash’s occulter is small and cheap, but as he says in Nature, “When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs.”
The paper is “Detection of Earth-like planets around nearby stars
using a petal-shaped occulter,” Nature 442 (6 July 2006), pp. 51-53, with abstract available here. Be aware, too, of another key paper: Kasdin, Vanderbei, Spergel et al., “Extrasolar planet finding via optimal apodized and shaped pupil coronagraphs,” which ran in the Astrophysical Journal 582, 1147-1161 (2003). This one showed the possibility of suppressing diffraction close to the needed levels.
Centauri Dreams thanks Anthony Kendall, author of the excellent Anthonares weblog, for his help in gathering material for this story.
by Paul Gilster | Jul 6, 2006 | Tau Zero Foundation |
An interview with Marc Millis, founder of the Tau Zero Foundation, was posted yesterday on Alan Boyle’s Cosmic Log on the MSNBC Web site. After discussing the so-called ‘antigravity’ phenomenon known as the Podkletnov effect, which has been called into serious question by recent studies that found no evidence for it, Millis went on to discuss other, more intriguing research. From the interview:
Millis is more interested in research into the Woodward effect – “a transient inertia effect” that could eventually have implications for propulsion, if verified – as well as a more recent study of “a fairly large gravitomagnetic effect, too large to be explained with general relativity as we understand it so far.”
He cautioned that “we’re not talking about an immediate propulsive effect, and it might be a measuring artifact.” But at least the research illustrates that there are still mysteries out there that could someday turn those science-fiction dreams into practical starflight.
Centauri Dreams will have more to say about Woodward’s explanation of inertia and its connection to Mach’s principle in a future post.
by Paul Gilster | Jul 5, 2006 | Exoplanetary Science |
So much good material has run lately on planet-hunter Gregory Laughlin’s systemic site that Centauri Dreams feels seriously remiss in returning to it so infrequently. So there is catching up to do, but we focus today on Laughlin’s new work, with UCSC graduate student Jeremy Wertheimer, on an intriguing question about Proxima Centauri. Is the tiny star in fact gravitationally bound to Centauri A and B?
Surprisingly, much recent work has suggested otherwise, including a 1993 paper by Robert Matthews and Gerard Gilmore that set the tone for Proxima research in that decade. But Laughlin notes that the European Space Agency’s Hipparcos satellite has firmed up our knowledge of the position, distance and velocity of nearby stars, enough to demand a new look at this question. After all, Proxima is roughly 15,000 AU from the Centauri binaries, and shows only a small velocity relative to them. It would seem unlikely these stars would not be bound into a triple system, and Laughlin and Wertheimer make a strong case that they are.
Image: The red dwarf star Proxima Centauri in infrared light (the brightest star below, left from center). Could Proxima’s orbit dislodge cometary materials, delivering volatiles to planets in habitable orbits around Centauri A and B? Credit: Digitized Sky Survey U.K. Schmidt Image/STScI.
But here’s where this question gets truly fascinating: we have two studies of the Centauri stars with regard to planetary formation that suggest positive things for habitable planets there. First, a 1997 study by Paul Wiegert and Matt Holman describes stable orbits for terrestrial worlds within 4 AU of either Centauri star. And a 2004 paper by Jack Lissauer and Elisa Quintana, discussed here on Centauri Dreams, shows that terrestrial planet formation within the binary system is possible. The remaining problem for habitability is that such worlds are likely to be dry, there being no stable areas for planetesimal formation beyond 4 AU from which volatiles could be delivered to the inner worlds.
But if Proxima is in a bound orbital relationship with Centauri A and B, that picture changes. From Laughlin’s paper on this work:
“One might therefore wonder about the habitability of putative terrestrial planets in the system. A possible concern with respect to habitability might arise because any planets orbiting the α-Centauri binary may be depleted in volatiles. If Proxima had been bound to the system during its formation stages, then it may have gravitationally stirred the circumbinary planetesimal disk of the α Centauri system, thereby increasing the delivery of volatile-rich material to the dry inner regions.”
The paper is Laughlin and Wertheimer, “Are Proxima and Alpha Centauri Gravitationally Bound,” as yet unpublished but submitted to The Astronomical Journal. Be sure to read Laughlin’s report on this work on the systemic site (and while you’re there, be sure to check his sly resolution of the Fermi Paradox). He and Wertheimer calculate that Proxima should orbit the Alpha Centauri stars about once every million years, with the semi-major axis of the orbit being roughly 1/6th of a light year. If Proxima is indeed stirring the Centauri planetesimal soup, dislodging comets and delivering interesting materials to the inner systems, then the odds on habitability go up. And on that score, it’s humbling and energizing to consider that Proxima, and probably the entire Centauri system, was 2 billion years old when the Sun formed.
So you can see that binding Proxima Centauri gravitationally to the Centauri A and B stars is much more than an abstract calculation. It could spell the difference between terrestrial, life-harboring worlds in that system and dry, desert planets. With current calculations showing three to five terrestrial worlds should be feasible around both stars, we may one day find that the Centauri system is an astrobiologist’s paradise. Could a two-Earth mass Centauri B “b” planet be detected with current instruments? Laughlin suggests one possibility for doing so, a reminder that even with today’s technology, we are not so far from the first verification of a terrestrial exoplanet.
by Paul Gilster | Jul 4, 2006 | Deep Sky Astronomy & Telescopes |
When I was growing up, ‘blob’ was a word associated with a classic horror movie starring none other than Steve McQueen. Today, blobs are starting to show up in astronomical discussions. Exactly what they are is unknown, but they seem to be as large as galaxies and marked by low luminosity. The latest, an apparently energetic but not very bright object some 11.6 billion light years away, is fully twice the size of our Milky Way and emits the energy of some two billion suns. It is, nonetheless, invisible in images from telescopes looking all the way from the infrared to the x-ray wavebands.
How do you find invisible blobs? Astronomers working with the European Southern Observatory’s Very Large Telescope used a narrow-band filter with the FORS1 spectrograph that allowed them to observe emissions from hydrogen atoms. Applying energy to hydrogen atoms causes their electrons to make a quantum leap to a higher energy level. Upon return to their initial state, the electrons release excess energy in the form of photons, with the various possible transitions resulting in characteristic spectral lines. All of which makes the blob identifiable while leaving its enigmatic nature intact. Says ESO astronomer Kim Nilsson:
“We have tried to explain this blob using the most common explanations, such as the illumination by a galaxy with an active nucleus or a galaxy that produce stars at a frantic rate, but none of them apply. Instead, we are led to the conclusion that the observed hydrogen emission comes from primordial gas falling onto a clump of dark matter. We could thus be literally seeing the building up of a massive galaxy, like our own, the Milky Way.”
Nilsson and team’s work is available as “A Lyman-alpha blob in the GOODS South field: evidence for cold accretion onto a dark matter halo,” a letter to Astronomy and Astrophysics available here (PDF warning). It’s still more evidence of how little we know about one of astronomy’s central mysteries, the nature of dark matter and its role in shaping what we see in the cosmos.
by Paul Gilster | Jul 3, 2006 | Exoplanetary Science |
Back in 2003, while researching Centauri Dreams, I interviewed physicist Geoffrey Landis at Glenn Research Center in Cleveland. At that time, Landis’ office was packed with Mars images, apropos for a man who had done so much work on rover technology. I asked him whether, after all this study, Mars had taken on the aspect of a real place to him, like Cleveland. Not surprisingly, he said that it had, and he credited 3-D images from Mars Pathfinder for that. Wearing glasses, Landis said, “It was as if you were standing on Mars. You could see ups and downs, ridges and valleys. That changed the view of Mars from another planet to a place you could go out and walk around.”
We’re a long way from 3-D close-ups, but I suspect some astronomers are starting to feel that way about Beta Pictoris, a young star some 63 light years away in the southern constellation Pictor that first drew attention to itself because of excess infrared radiation. A warm circumstellar disk was surely the cause, and indeed, such a disk was imaged by ground-based telescopes in 1984, revealing itself to be nearly edge-on to Earth. Since those observations, the Hubble space telescope found what seemed to be a warp in the disk, one studied extensively by Sara Heap at GSFC.
Is the warp evidence of more than one disk? The image below makes a strong case, showing in visible light a distinct secondary disk tilted about four degrees from the main one. It took the use of a coronagraph with Hubble’s Advanced Camera for Surveys to get this image, blocking out light from the star itself to reveal the disk structure (Beta Pictoris puts out a lot of light; in fact, it’s nine times more luminous than the Sun). The secondary disk is visible out to 24 billion miles from the star.
Image: This Hubble Space Telescope view of Beta Pictoris clearly shows a primary dust disk and a much fainter secondary dust disk. The secondary disk extends at least 24 billion miles from the star and is tilted roughly 4 to 5 degrees from the primary disk. The secondary disk is circumstantial evidence for the existence of a planet in a similarly inclined orbit. Credit: NASA, ESA, D. Golimowski (Johns Hopkins University), D. Ardila (IPAC), J. Krist (JPL), M. Clampin (GSFC), H. Ford (JHU), and G. Illingworth (UCO/Lick) and the ACS Science Team.
What’s causing this unusual disk situation? An unseen planet is the most likely culprit, a gas giant perhaps 20 times the mass of Jupiter. In orbit within the secondary disk, the planet would be tapping the primary disk for its building materials. And that leads to an interesting thought: what if Beta Pictoris is not so unusual? Perhaps planetary systems often form this way, from two or even more disks whose interactions are fertile ground for planetesimals. Listen to what David Golimowski (Johns Hopkins) has to say about this; he led the team that made this find:
“The Hubble observation shows that it is not simply a warp but two concentrations of dust in two separate disks. The finding suggests that planetary systems could be forming in two different planes. We know this can happen because the planets in our solar system are typically inclined to Earth’s orbit by several degrees. Perhaps stars forming more than one dust disk may be the norm in the formative years of a star system.”
And that several degree spread around Beta Pictoris isn’t really so different from the several degree separation of planets in our own Solar System’s plane. There is even some evidence from work at the Keck II Observatory in Hawaii of a possible third disk, one the size of our own Solar System that Golimowski’s team couldn’t see because it was covered by the coronagraph in Hubble’s Advanced Camera for Surveys. Whatever the case, Beta Pictoris, that unique laboratory for the study of planet formation, is becoming more and more of a ‘place’ all the time, one that will be visited by astronomers for years to come as it slowly gives up its secrets.
The paper is Golimowski, Ardila, Krist et al., “Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk Around ? Pictoris,” Astronomical Journal 131:3109-3130 (June, 2006).
