by Paul Gilster | May 18, 2006 | Exoplanetary Science |
Finding three planets around a single star is newsworthy in itself, but when the planets are Neptune-class things get more interesting. And when one of these worlds is found to be in the star’s habitable zone, Centauri Dreams definitely drops everything for a closer look. Not only that, but the system around HD 69830, a Sun-like star some 41 light years away, is also the home of an asteroid belt, making the comparison with our Solar System that much closer.
Here’s what we know, as reported in a paper in the May 18 Nature: The orbital periods of the three planets are 8.67, 31.6 and 197 days, with that outer world located near the inner edge of the zone where liquid water could exist. In terms of mass, this planet is not Earth-like; in fact, the measurements show the new planets to be between 10 and 18 times the mass of Earth. So what we’re probably detecting in the habitable zone is a planet with a rocky/icy core surrounded by a dense atmosphere. We know nothing, of course, about possibly habitable moons.
A rocky composition is suggested for both inner planets as well. In terms of distance, the three worlds are 0.08, 0.19, and 0.63 AU from their star, but the location of the asteroid belt is unknown. The possibilities are between the two outermost planets or farther out than 0.8 AU. Remember that this work was all accomplished by radial velocity investigations, and much remains to be learned as we untangle the results of these measurements.
What grand work this is. The researchers, led by Michel Mayor and Christophe Lovis (Geneva Observatory) used the European Southern Observatory’s HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph mounted on the 3.6-meter La Silla instrument in Chile. HARPS has demonstrated a remarkable long-term precision of 1 m/s. Take a look at the image below to get an idea of how its data are manipulated.
Image: The HARPS radial velocity measurements of HD 69830 are folded with the orbital periods of the three discovered planets: 8.67, 31.6 and 197 days, respectively. In each case, the contribution of the two other planets has been subtracted. The solid line shows the best fit to the measurements, corresponding to minimum masses of 10.2, 11.8 and 18.1 Earth masses. Note that the full span of the vertical axis is only 13 m/s! Error bars indicate the accuracy of the measurements. The integration time was 4 minutes on average for the first 18 measurements (shown as open dots), and was increased to 15 minutes for the remaining points (full dots). The latter measurements are therefore of much higher quality. Credit: European Southern Observatory.
Centauri Dreams‘ take: What astronomers can do with the HARPS instrument is simply mind-boggling, as I am reminded every time I look into its operations. Radial velocity measurements depend on our detecting how the presence of a planet exerts a pull on its star. The velocity variations are tiny, in this case between 2 and 3 meters per second, roughly the speed of a brisk walk. Images like the one above aren’t glamorous — they don’t show us great, ringed planets or green terrestrial Earths — but the story they tell is spectacular. I often write about the breakthroughs we’ll make by imaging exoplanets directly and analyzing their spectra, but let’s not forget the continuing excellence of radial velocity research, or its ever-higher levels of sensitivity.
The paper is Lovis, Mayor, Pepe et al., “An extrasolar planetary system with three Neptune-Mass Planets,” in Nature 441 (18 May 2006), pp. 305-309.
by Paul Gilster | May 17, 2006 | Exoplanetary Science |
Yesterday’s post on UMBRAS and occulter technology focuses attention on the characteristics of light, some of them counter-intuitive but well demonstrated. And since we’ve also been talking recently about the nearby star Epsilon Eridani, I’ve chosen an image of that star to illustrate some of the problems with planetary detections. What you see below is via Massimo Marengo (Harvard-Smithsonian Center for Astrophysics), who has done such outstanding recent work on untangling the riddle of Epsilon Eridani’s debris disk.
This is a false color image with red, yellow, green and blue representing different infrared wavelengths. I ran the same image last summer, when Marengo posted it on his own weblog (he had used it to illustrate his team’s work in a presentation at the American Astronomical Society meeting in San Diego).
Image: A false-color infrared image of Epsilon Eridani. Credit: Massimo Marengo (CfA).
What I want to single out here are the artifacts in the image. The red/orange cross is produced by electronic effects inside the collector, while the green and blue spikes are caused by diffraction in the optical system. In other words, this is one ravishingly beautiful image, but it’s nothing like what the eye would see if you were able to view Epsilon Eridani close up, and not just because it’s working at infrared wavelengths. The idea behind both coronograph and occulter studies is to find ways to reduce or eliminate these distortions. Diffraction, for example, happens because light is actually bent around an intervening object; using a basic telescope with an occulting disk in its focal plane (a coronagraph), you wind up with a series of concentric rings and a bright spot in the center, all of which need to be suppressed.
It’s a tricky challenge, and one being studied in many ways, from using square apertures and varying the shape of the occulting object to creating deformable mirrors that reduce scattered light. But external occulters have significant advantages, including elimination of the scattering problem, theoretically better light suppression, and the ability to use them with any properly placed telescope. Moreover, the target can be placed anywhere in the image plane. A complete list of pros and cons contrasting internal coronagraphs with external occulters can be found at the UMBRAS site.
In any event, the more I think about the recent Terrestrial Planet Finder funding woes, the more I think the situation will result in better science. Terrestrial Planet Finder seemed, not so long ago, to be firming up around an internal coronagraph design that was more costly than competing occulter possibilities and, it seems likely, not capable of the same level of performance, at least when compared to the larger occulter options. We should all be glad that external occulter designs and new missions are now back in the hunt as we try to design the technologies that will give us our first actual images of distant exoplanets.
by Paul Gilster | May 16, 2006 | Exoplanetary Science, Research Tools |
‘Umbras’ is Latin for ‘shadows,’ and it becomes a fitting acronym for projects to block the light of stars so that astronomers can see the planets around them. The unwound acronym is Umbral Missions Blocking Radiating Astronomical Sources, which refers to both an imaging technique and a class of space missions. The basic idea is this: deploy a space telescope flying in formation with a second, distant companion spacecraft that carries an occulting screen. We’re looking for direct pictures of planets by reducing a star’s glare, and there are a number of projects aimed at making them, including one we’ve discussed here many times, the New Worlds Imager mission championed by Webster Cash.
I pulled both images in this post from the UMBRAS Web site, where these ideas are explored as a way of pooling talent in the disparate occulter community. Remember, almost everything we know about exoplanets has come from radial velocity studies, microlensing and planetary transits. At best, we are studying variations in starlight that provide solid evidence, but do not yield images or spectroscopy. The next big step in exoplanet observation will surely be direct imaging, even as we continue to make new discoveries with our time-tested methods.
The images are telling. The first (above) shows a bright star whose light obscures the field of view. Any planets around such a star would be drowned in the starshine. The image at left shows the light reduced orders of magnitude by an occulter. Now we can see the light of a possible planet around this star, and if this were an actual photograph, we would be able to measure the planet’s motion and, possibly, take stellar spectra. Clearly, this kind of view takes us into a new realm of exoplanet discovery, and the effort going into finding the best technologies to make this happen should pay off in superior results.
A coronagraph is another way to reduce starshine, but an occulter is not built into the telescope and is thus able to reduce scattered light due to the various optical surfaces and apertures involved. The potential for a clean image is dramatically enhanced. A key problem is to reduce diffraction, which is what happens when light bends around the edges of an object to converge on the opposite side. From the UMBRAS site:
If we consider a square occulter 45 m on a side at a distance of 16,000 km from the telescope, it has a width of 0.58 arcsec. A pattern of light from a blocked star will be visible within the shadow of the occulter. The diffraction pattern within the shadow area will be surrounded by a series of bright and dark (null) fringes with bright diffraction spikes due to straight edges of the occulter.
The goal, then, is to suppress not just the central light from the star being studied, but also to suppress or alter the shape of the diffraction pattern so that planets near the star can be studied. All of this is here conceptualized within the framework of an UMBRAS mission, based on a xenon-based ion thruster propulsion system that would station both telescope and occulter in an orbit around one of the Lagrangian points. This is far enough from the Earth to allow the spacecraft to maintain its alignment without disruptive effects caused by Earth’s gravitational pull. Some design concepts can be found here.
And how would such a mission take shape, once past initial testing? Larger missions might involve the James Webb Space Telescope, as we saw earlier in a study of Webster Cash’s ideas, or (depending on budgetary constraints) smaller space telescopes could be dedicated to the project. With a scalable architecture, an UMBRAS mission might involve relatively small occulters, too, with a screen as minimal as 5-8 meters across, although such a mission would be more technology demonstrator than full-scale mission. Even so, it should be able to observe at least some of the easier targets.
This post is intended to call your attention to the fact that occulter technologies are being approached from a number of different angles. The UMBRAS site offers numerous background papers that should be of use. The key question before us is whether or not an occulter design will be chosen for a planet-finding mission in the relatively near future. From my own reading of Cash’s work and the UMBRAS papers, it seems likely that occulters, especially if capable of working with JWST and therefore not needing a separate telescope, will offer a way to image exoplanets that is much less expensive than older Terrestrial Planet Finder designs.
But more on this tomorrow. As Ian Jordan (Space Telescope Science Institute) has reminded me, the history of occulters is a long and interesting one. I want to probe a little deeper into this subject, and we should look at some of the other ideas posted on the UMBRAS site.
by Paul Gilster | May 15, 2006 | Exoplanetary Science |
In Centauri Dreams‘ imagination, the name Epsilon Eridani is magic. Like many of us, my earliest speculations about life on other worlds always came back to the nearby, Sun-like stars like Tau Ceti, Epsilon Eridani and Centauri A and B. Frank Drake used the first two as his targets for Project Ozma in 1960, an effort that continues to inspire SETI work today. And Epsilon Eridani is joined by Vega, Fomalhaut and Beta Pictoris as the first stars found by the Infrared Astronomical Satellite (IRAS) to have a cool debris disk somewhat analogous to our own Kuiper Belt.
The fact that this K2 star is likely to be orbited by the closest exoplanet to our Sun is also exciting. Its planet seems to be slightly larger than Jupiter, with estimates ranging from 0.8 to 1.6 Jupiter masses, and an eccentric orbit varying from 5.3 to 1.3 AU (here again we see how important it is to establish the effect of gas giants on terrestrial worlds in the habitable zone). At 10.5 light years from us, Epsilon Eridani offers an unusual close look at the final phases of planetary formation.
Epsilon Eridani is, moreover, a young star (less than a billion years old) with a disk featuring interesting clumpy structures suggestive of planetary interactions with the dust. Some computer models, still inconclusive, make the case for a Neptune-class planet as the cause. Using the Spitzer Space Telescope to study the debris disk for more planetary companions in the system’s outer regions is thus fruitful science, as a team led by Massimo Marengo (Harvard-Smithsonian Center for Astrophysics) has demonstrated.
Spitzer is one of our most productive instruments, the source of the only three detections of thermal radiation from exoplanets (all three are eclipsing ‘hot Jupiters’). Brown dwarfs and Jupiter-sized planets can be investigated since they are characterized by strong molecular absorptions in the mid-infrared range. They become accessible targets thanks to the sensitivity of Spitzer’s InfraRed Array Camera (IRAC). What we still lack is what this team would like to provide, a direct detection of gas giants orbiting their star at Jupiter-like distances and beyond.
No new planets here, but Marengo’s team was able to establish limits on planetary detection using the IRAC instrument. Spitzer, they find, can detect young gas giants with a mass as low as several Jupiters when they are orbiting their star at distances comparable to the Kuiper belt. Moreover, planets in highly elliptical orbits during the era of planetary formation can be detected down to a single Jupiter mass. These are useful boundaries for future work, and although the data at the time of publication were insufficient to verify the existence of any new planets around Epsilon Eridani, some infrared sources need further study to determine their nature by measuring if they have a common proper motion with Epsilon Eridani.
Expect more work on Epsilon Eridani, including an upcoming paper from the same team based on results from Spitzer’s InfraRed Spectrograph and Multiband Imaging Photometer, to be published soon. The paper above is Marengo, Megeath, Fazio et al., “A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star Epsilon Eridani,” available here.
by Paul Gilster | May 13, 2006 | Exotic Physics |
We seem to be awash in exotic physics, an administrative category I created on this site only a couple of days ago to house the trillion-year crunch story and the ‘light in reverse’ work at the University of Rochester. It seems an appropriate time, then, to look at an investigation reported in the Physical Review Letters that takes us, like Alice through the looking glass, into the universe before the Big Bang. Penn State researchers are behind this study, combining quantum physics tools that Einstein didn’t have with general relativity to punch through to a universe on the other side.
So let’s talk again about what might have been there before the Big Bang. This analysis says the previous universe had a spacetime geometry much like our own expanding universe, except that it was a contracting universe. Gravititational forces were pulling the previous universe together until the quantum properties of spacetime caused gravity itself to become repulsive. What follows is aptly described as the ‘Big Bounce.’ Let me quote from a Penn State news release on this work:
The research team used loop quantum gravity, a leading approach to the problem of the unification of general relativity with quantum physics, which also was pioneered at the Penn State Institute of Gravitational Physics and Geometry. In this theory, space-time geometry itself has a discrete ‘atomic’ structure and the familiar continuum is only an approximation. The fabric of space is literally woven by one-dimensional quantum threads. Near the Big-Bang, this fabric is violently torn and the quantum nature of geometry becomes important. It makes gravity strongly repulsive, giving rise to the Big Bounce.
Out of which we draw some exotic conclusions. Says Abhay Ashtekar (director of the Institute for Gravitational Physics and Geometry at Penn State):
“Using quantum modifications of Einstein’s cosmological equations, we have shown that in place of a classical Big Bang there is in fact a quantum Bounce. We were so surprised by the finding that there is another classical, pre-Big Bang universe that we repeated the simulations with different parameter values over several months, but we found that the Big Bounce scenario is robust.”
Of course, the idea of one universe feeding into another has been around for a while, and science fiction readers will recall wonderful tales like Poul Anderson’s 1970 novel Tau Zero, in which the runaway ramscoop ship Leonora Christine ends its journey in the only way possible, threading through the lens of gravitational collapse into an entirely new universe. But this is the first time the properties of spacetime geometry in such a universe have ever been deduced. And the big news is that the pre-Big Bang universe this study deduces wasn’t a sea of quantum foam or a matrix of energies indescribable by our mathematics. It was a classical universe, like ours.
Image: The figure represents our expanding universe as the right branch of the arc. Our time now is located at the 1.8 grid mark on the right side of the drawing. According to Ashtekar’s team’s calculations, when looking backward throughout the history of the universe, ‘time’ does not go to the point of the Big Bang but bounces to the left branch of the drawing, which describes a contracting universe. Singh explains, “The state of the universe depicted by its wavefunction is shown in space (\mu) and time(\phi). The big bang singularity lies where space vanishes (goes to zero). Our expanding phase of the universe is shown by the right branch which, when reversed backward in time, bounces near the Big Bang to a contracting phase (left branch) and never reaches the Big Bang.” Credit: Abhay Ashtekar/Penn State.
The paper is Ashtekar, Pawlowski, and Singh, “Quantum Nature of the Big Bang,” Physical Review Letters 12 April 2006, available here.
