by Paul Gilster | Feb 19, 2008 | Deep Sky Astronomy & Telescopes |
We usually think of gravitational lenses in terms of massive objects. When light from a distant galaxy is magnified by a galactic cluster between us and that galaxy, we get all kinds of interesting magnifications and distortions useful for astronomical purposes. But gravitational lensing isn’t just about galaxies. It happens around stars as well, as we saw recently with the discovery of a solar system with planets analogous to Jupiter and Saturn in our own system. That find was made with the help of a single star crossing in front of another, the resulting magnification allowing the signature of two planets around the closer star to be seen.
Interestingly enough, some of the earliest work on solar sails in interstellar environments came out of the attraction of taking advantage of the Sun’s own gravitational lens. Push some 550 AU out and you reach the point where solar gravity focuses the light of objects on the other side of the Sun as seen from a spacecraft. Note two things: At 550 AU, electromagnetic radiation from the occulted object is boosted by a factor of roughly 108. Secondly, gravity-focused radiation does not behave like light in a conventional optical lens in one important sense. The light does not diverge after the focus as the spacecraft continues to move away from the Sun. Indeed, the focal line extends to infinity.
The Italian aerospace company Alenia Spazio (based in Turin) began investigations into inflatable sail technologies as long ago as the 1980s. Since then, physicist Claudio Maccone has continued to investigate a mission he calls FOCAL, a probe to the gravity focus. Maccone sees such a mission as inevitable, for it takes advantage of an asset every technological civilization will ultimately want to exploit. Here’s how he puts it in his 2002 book The Sun as a Gravitational Lens: Proposed Space Missions:
As each civilization becomes more knowledgeable they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass… This lens is the civilization’s star; in our case, our Sun. The gravity of each star acts to bend space, and thus the paths of any wave or particle, in the end creating an image just as familiar lenses do….Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise.
Maccone is the FOCAL mission’s most eloquent spokesman; his continuing travels and presentations on its behalf are part of the discovery process for our own civilization as we begin to see the possibilities opening up in nearby interstellar space. Another part of the discovery process is the distribution of news like the recent Hubble Space Telescope findings, which have demonstrated 67 new gravitationally lensed galaxies. Clearly, we are only beginning to understand the power of lensing to help us make sense out of even the most distant parts of the cosmos.
Hubble’s latest finds are part of a survey of a single 1.6 square degree field of sky with various space-based and Earth-based observatories. A team of European astronomers using Hubble’s Advanced Camera for Surveys (ACS) identified the new lenses, which were found around massive elliptical or lenticular galaxies without spiral arms or discs.
Here again we run into the issue of scale. Many of the gravitational lensing observations made thus far have involved not just single galaxies but whole clusters. Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) notes the difference:
“We typically see the gravitational lens create a series of bright arcs or spots around a galaxy cluster. What we are observing here is a similar effect but on much smaller scale – happening only around a single but very massive galaxy.”
Four of the discovered lenses have produced ‘Einstein rings,’ which occur when a complete circular image of a background galaxy is formed around a foreground galaxy. Finding such lenses isn’t easy, and involves going through a catalog of more than two million galaxies by eye to identify possible candidates. Given the odd shapes that lensed galaxies can assume, filtering out a genuine lensing event from the observation of an unusually shaped galaxy takes time. But the European team now plans to train robot software on the lenses thus far found, hoping to identify still more.
Image: An Einstein ring can be seen in this image from the COSMOS project. An Einstein ring is a complete circle image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the Hubble Space Telescope are all aligned perfectly. Credit: NASA, ESA, C. Faure (Zentrum für Astronomie, University of Heidelberg) and J.P. Kneib (Laboratoire d’Astrophysique de Marseille).
The universe, it would seem, continually tries to tell us about itself through lensing inherent in the effect of mass upon spacetime. We’re a long way from such an infrastructure, but imagine the consequences for astronomy of having a wide variety of observational tools moving on spacecraft within the Sun’s gravity focus to examine targets ranging from the earliest galaxies to nearby stars and their planets. Such an outcome would depend, of course, upon our mastering propulsion technologies that can reach such distances within a single human lifetime. But it’s clear that the benefits of going interstellar won’t be found just around other stars. There’s plenty of work to do within 1000 AU of home.
by Paul Gilster | Feb 18, 2008 | Exoplanetary Science |
About two weeks ago we looked at the work of Michael Meyer (University of Arizona), whose team examined over 300 Sun-like stars (spectral types F5-K3) at mid-range infrared wavelengths. A wavelength of 24 microns detects warm dust, material at temperatures likely to be found between 1 and 5 AU from the parent star. The headline that day was Meyer’s contention that many if not most such stars produce terrestrial planets. Now Meyer is presenting these findings at the annual meeting of the American Association for the Advancement of Science, doubtless putting the exoplanet hunt back in the daily papers, at least for a day.
Bear in mind that in using the term ‘terrestrial’ we’re talking about small, rocky worlds like the inner planets of our Solar System. That could include worlds like our own, of course, but could also include hellish places like Mercury and Venus and their analogues around other stars. Nonetheless, it’s exciting to think that the chances of rocky planet formation are high enough to make Earth-like worlds a likely outcome around a large number of stars.
Image: This artist’s concept illustrates the idea that rocky, terrestrial worlds like the inner planets in our solar system may be plentiful, and diverse, in the universe. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech).
The stars surveyed by Meyer’s team were grouped by age. But does the warm dust found around ten to twenty percent of the stars in the four youngest age groups really indicate Earth-like planets? What we know is that stars older than 300 million years don’t show the kind of warm dust Meyer’s younger stars do, a possible indication that planet formation is complete. Now Thayne Currie and Scott Kenyon (Smithsonian Astrophysical Observatory) have performed a separate study on dust around ten to thirty-million year old stars, with results suggesting similar processes.
Here is the scenario: Warm dust should be detectable during planet formation because the collision of small rocky bodies in the circumstellar debris disk is building larger and larger objects, creating warm dust as the result of the activity. Says Kenyon: “Our work suggests that the warm dust Meyer and colleagues detect is a natural outcome of rocky planet formation. We predict a higher frequency of dust emission for the younger stars, just as Spitzer observes.”
So rocky planets may indeed be forming, but around what percentage of stars? The Spitzer data are susceptible to various interpretations, but if you want to shoot for the most favorable outcome for terrestrial worlds, you can look at them this way (the words are Meyer’s):
“An optimistic scenario would suggest that the biggest, most massive disks would undergo the runaway collision process first and assemble their planets quickly. That’s what we could be seeing in the youngest stars. Their disks live hard and die young, shining brightly early on, then fading. However, smaller, less massive disks will light up later. Planet formation in this case is delayed because there are fewer particles to collide with each other.”
Now we’re in the sixty-percent plus zone Meyer flagged in his original paper. But we won’t know whether that or the conservative twenty percent figure is correct without the kinds of observation the Kepler mission will shortly commence. And then there’s the Giant Magellan Telescope, scheduled for completion in 2016 at its site at Las Campanas, Chile. Remarkably, astronomers are already talking about using its seven 8.4-meter primary mirrors to image Earth-like planets from the ground. The GMT will produce images ten times sharper than the best the Hubble Space Telescope can offer, another exceptional tool that should help us snare a terrestrial world by 2020 (although I bet an M-dwarf transit bags one much sooner).
The complete Meyer reference is in our previous coverage. The Currie/Kenyon study is Currie et al., “The Rise and Fall of Debris Disks: MIPS Observations of h and chi Persei and the Evolution of Mid-IR Emission from Planet Formation,” accepted by The Astrophysical Journal and available online. This Spitzer Space Telescope news release summarizes both studies.
by Paul Gilster | Feb 16, 2008 | Exotic Physics |
When Hollywood met MIT last month in Cambridge, MA I suspect most of the students who jammed the on-campus lecture hall to discuss the new movie Jumper were thinking about Star Trek‘s famed transporters. After all, Jumper is a movie about a man who learns at a completely unexpected moment that he can teleport himself anywhere he wants to go. The Enterprise’s transporters could get you to your destination in a hurry, too, and presumably invoked some of the same mechanisms, the gist of which were explained in the discussion by MIT physicists Max Tegmark and Edward Farhi, with Hollywood contribution by director Doug Liman and Hayden Christensen, respectively director and star of the film.
What came to mind first for me, though, wasn’t Star Trek but the Harry Harrison collection One Step from Earth (Macmillan, 1970). Harrison’s stories wove together a future around the premise that beaming matter to destinations near and far would soon be invented. His book begins with the first interplanetary transmission (an adventurer sent to to Mars in 1993) and proceeds to encounters with alien life and interstellar war. Want to get somewhere quickly? It was easy in Harrison’s world, as he says in his introduction:
A matter transmitter is very easy to use. Justs dial your number, there, as simple as a telephone, and wait until the ready light comes on. Then step forward, you won’t feel a thing. Just walk through the MT screen as though it were a door…
The entertaining outtakes from Jumper that I’ve seen operate a bit differently than Harrison’s matter transmitters, though. For one thing, there is no screen to step through. We see Hayden Christensen and cohorts simply pop out of existence, then re-emerge, perhaps a few feet away (to dodge a blow) or at the top of the Sphinx, to take in the view. In the trailer, Christensen’s character talks about ‘my day so far’: coffee in Paris, a doze on Kilimanjaro, surfing the Maldives, a stop in Rio, and back for the NBA finals. A marvelous capability if you can find it, but actions have consequences and more than a few things soon begin to go wrong.
We know that quantum teleportation not only exists but has been demonstrated in laboratory conditions, with successful teleportation of a photon over a distance of several miles. Yet quantum teleportation carries its own price. To carry it out, you have to begin with two entangled particles, making a measurement that destroys the quantum state of one, but reconstructs that state (after transmitting information about the measurement) in the other. It would be as if, mused MIT’s Farhi, you destroyed Hayden Christensen and transmitted his egregiously complex quantum state information to a bag of elementary particles located somewhere else. Out would emerge the distinguished actor in a new place, but of course the original would have been lost along the way.
The movie, then, isn’t making any assumptions about quantum teleportation, but in the intriguing MIT discussion, it became clear that if you wanted to go way out on a theoretical limb, it might be possible to invoke wormholes to explain the jumps it depicts. Until recently, the belief was that wormholes might exist according to the laws of physics but could not remain stable without the help of negative energy, and were thus impossible to harness for such functions. On the other hand, we’re now talking about dark energy, a mysterious contributor to the universe’s continuing expansion, and conceivably the stuff by which a wormhole’s mouth could be fixed in place.
Just how you would manipulate that wormhole to go wherever you chose is quite a question, but I like the chutzpah shown by director Doug Liman, who didn’t let this get in the way of telling a good yarn. He wanted, he told the MIT crowd, to know what it would look like if this kind of teleportation happened in front of their eyes, and the effects he uses to suggest the result are ingenious, including a teleportation from underwater into a library complete with onrush of water and associated debris. And since we have no ability to harness dark energy or, indeed, any knowledge of how it operates, having Hayden Christensen possessed by a miraculous power seems as good a plot device as any; no one would argue that the movie is depicting a known phenomenon.
If it were possible, and if it could be done at interstellar distances, imagine the consequences of teleportation. Harry Harrison must have been working with wormholes himself, because he puts no speed of light limitation on his matter transmitters, of the sort required by quantum teleportation. Here’s a snip from the eponymous title story “One Step From Earth,” as our hero on Mars watches his colleague come through the other side:
Otto’s hand appeared even before the voice ended. It took the radio waves nearly four minutes to reach Mars, but the matter transmission was almost instantaneous, since it went through Bhattacharya space where time, as it is normally constituted, does not exist. Otto’s arm dropped limply and Ben took him by the shoulders, a dead weight that he eased to the ground. Rolling him over Ben saw that his eyes were closed. But he seemed to be breathing regularly. He was probably unconscious. Transmission shock, they called it…
Transmission shock indeed, and who wouldn’t experience it when stepping from one planet directly onto another? Let me recommend One Step from Earth, still a fun read after all these years. As for Jumper, I haven’t seen it, but I’ve seen enough scenes from it thanks to Twentieth Century Fox’s press materials that I plan to take it in. Whatever kind of plot you superimpose onto this mind boggling idea (and the plot looks more than a little contrived), the notion of teleportation itself carries its own magic, and while we are a long way from making this kind of jump possible, I am not the one to say that down the road we won’t discover things just as exotic. Working out all the implications is not a task for the faint of heart, but Harry Harrison might still be able to pull it off.
by Paul Gilster | Feb 15, 2008 | Exoplanetary Science |
Finding solar system analogs is tricky business, as we saw yesterday when examining the discovery of Jupiter and Saturn-class worlds around a distant star. That find, I notice, is getting some attention in the popular media as an indication that our Solar System may not be unique. But take a look at the gas giant recently found around HD 154345 if you want to see an even closer analog to our own system.
HD 154345b is a single world, to be sure, but it orbits a G8 dwarf much more like the Sun than the diminutive star examined yesterday, and it’s a close match for Jupiter not only in size but orbital position. The planet’s minimum mass is 0.95 Jupiter’s, and its 9.2 year circular orbit carries it around its star at a distance of some 4.2 AU. Sound familiar? What’s happening around HD 154345 is more or less what a distant astronomer using our current technologies would see if observing our Solar System.
Rather than using microlensing, the discovery team here put radial velocity techniques to work, with data gathered at the Keck Observatory (Hawaii). Recall that finding ‘hot Jupiters’ is a much simpler proposition, because their tight orbits make the radial velocity signature much easier to spot. In this case, it has taken ten years to tease out the signal of a true Jupiter analog. And as our collected data for other radial velocity projects continues to accumulate, we’re gradually widening our perspective on extrasolar planetary systems to include these farther and perhaps more representative objects.
Thus far in the game, though, we’re looking at only twelve other nearby planets with orbits over six years in duration. And the current find is truly a gem. As the discovery paper notes:
This system joins 55 Cnc in demonstrating that the architecture of the Solar System – a dominant, Jupiter-mass planet at 4-5 AU in a circular orbit with only lower-massed objects interior – while rare, is not unique.
Notice another problematic feature of using radial velocity methods on planets with long orbits. Given the time frames involved, we’re dealing with a single orbit that has consumed a solid nine years, making it difficult to rule out other planets in still more distant orbits that may be detectable over longer time ranges. What it takes to break such a logjam is long-term observation. Transit methods? The large orbital distance makes that possibility unlikely, but we can’t rule out planets further inside HD 154345b’s orbit that could be caught in a transit.
And, of course, by flagging a Jupiter-class planet in a Jupiter-like location, we’ve added another item on our list for future space-borne observation. This G8 star is roughly 59 light years from Earth, bright enough and near enough to make it attractive for such work. Any planetary system with a Jupiter analog in this kind of orbit cries out for investigation of its interior regions, in the hope of discovering the presence of terrestrial worlds in the habitable zone there.
The paper is Wright, Marcy et al., “The Jupiter Twin HD 154345b,” accepted at Astrophysical Journal Letters and available online.
by Paul Gilster | Feb 14, 2008 | Exoplanetary Science |
We always have to watch our preconceptions, an early one in the exoplanet game being that solar systems around other stars would look pretty much like our own. Then we started the whole exoplanet discovery binge by finding planets around a pulsar, of all things, and went on to the terrifically odd world of ‘hot Jupiters,’ whose existence had not been predicted by most theorists. Now we’ve gotten used to the idea that solar systems come in huge variety, but finding one that looks more or less like ours would still be comforting, and would make it seem more likely that there are other ‘Earths’ out there, perhaps teeming with life.
Today and tomorrow we look at two such finds, noting the resemblance to what we have around Sol and pondering the implications for the broader planet search. First up is not a single but a double planetary find, two worlds that inhabit a place much like that of Jupiter and Saturn in our Solar System. Not only is this an intriguing discovery in itself, but note the distance involved: We’re dealing with a system fully 5000 light years from Earth. The only method that will snare that kind of catch is gravitational microlensing, a method triumphantly vindicated by these recent findings.
Microlensing happens when a star crosses in front of a more distant star, the gravity of the nearer object magnifying the light being received from the more distant. Few examples of microlensing thus far can equal this one, which resulted in a magnification of 500 times, revealing the presence of the two planets. The planets actually orbit the nearer star, but cause their own changes to the light intensity of the more distant object. We’ve found Jupiter-class worlds with microlensing before, but the additional planet is the result of the unusual opportunity afforded by this event. Says Scott Gaudi (Ohio State University):
“This is the first time we had a high-enough magnification event where we had significant sensitivity to a second planet — and we found one. You could call it luck, but I think it might just mean that these systems are common throughout our galaxy.”
If common, such systems give credence to the idea that systems like ours may eventually start filling up our exoplanet catalogs, but do note the differences: The two worlds, assumed to be gas giants, are roughly 80 percent as big as Jupiter and Saturn, and the star they orbit is roughly half the size of Sol, releasing a mere five percent as much light as the Sun. But the similarities are also noteworthy. The smaller of the planets is twice the distance from its star as the larger (this is the case of Jupiter and Saturn in our Solar System), and the closer planetary orbits mean that the temperatures of these worlds are similar to those of Jupiter and Saturn as well.
Image: Microlensing works by using the gravity of the near star (shown at the bottom left), and its orbiting planets as a lens, magnifying light from a background star. By studying how the complex patterns of magnified light change as the near star and the two planets of the planetary system move in front of the distant star (top right), we can determine the properties of the planets and their star. Credit: KASI/CBNU/ARCSEC.
Intriguing news for planet formation theorists? Believe it. Gaudi again:
“The temperatures are important because these dictate the amount of material that is available for planet formation. Most theorists think that the biggest planet in our solar system formed at Jupiter’s location because that is the closest to the sun that ice can form. Saturn is the next biggest because it is in the next location further away, where there is less primordial material available to form planets.
“Theorists have wondered whether gas giants in other solar systems would form in the same way as ours did. This system seems to answer in the affirmative.”
But get this: Gaudi believes that next generation microlensing experiments will be able to find Solar System analogs down to the Mercury level. That means all known planets but one in our system, and would include any terrestrial worlds in the exosystem under investigation. The work grows out of observations made by the Optical Gravitational Lensing Experiment, with follow-up by the Microlensing Follow Up Network (MicroFUN), which helped to coordinate worldwide observations.
You can call this a solar system analog or a ‘scaled solar system’ (Gaudi uses both terms), though I would prefer to wait until we know more about this system before using that term. Nor do we have any way of knowing whether the optimistic thought that such systems are common will really hold. But since this is the first time microlensing has been able to operate at this level of magnification, that does mean that the first time we were able to see a potential Solar System analog with microlensing, we did. Exciting stuff, which coupled with the discovery of a close Jupiter analog (and here the word ‘analog’ really fits) that we’ll describe tomorrow points to how fast we’re moving toward detecting planetary environments not dissimilar from our own.
The paper is Gaudi et al., “Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing,” Science Vol. 319, No. 5865 (February 15, 2008), pp. 927-930 (abstract).
