Exoplanet hunting takes time, a fact that is well demonstrated in the case of a newly confirmed gas giant. Eight times as massive as Jupiter, it orbits a star much like the Sun but at a distance vast enough (300 AU) to place it well within the Kuiper Belt if it were in our own system. 1RXS 1609 b was first reported back in September of 2008 when David Lafrenière (now at the University of Montreal) and team used adaptive optics to take direct images and spectra of the object, which can be seen in the image below.
Image: First released in September of 2008: Gemini adaptive optics image of 1RXS J160929.1-210524 and its ~8 Jupiter-mass companion (within red circle). This image is a composite of J-, H- and K-band near-infrared images. All images obtained with the Gemini Altair adaptive optics system and the Near-Infrared Imager (NIRI) on the Gemini North telescope. Credit: Gemini Observatory.
This was thought to have been the first planet directly imaged around a Sun-like star, but the question was whether the object was gravitationally associated with the star, something that only time would tell. There was always the possibility that what the astronomers had found was a chance alignment. Says Lafrenière:
“Back in 2008 what we knew for sure was that there was this young planetary mass object sitting right next to a young Sun-like star on the sky. Our new observations rule out this chance alignment possibility, and thus confirm that the planet and the star are related to each other.”
So what do we know about the planet circling 1RXS 1609, other than its mass? For one thing, the team’s subsequent work has shown that there are no additional planets of Jupiter mass or greater in this system, which makes the planet intriguing in terms of planetary formation models. Time again enters into the equation — as we continue observations for several more years, we should be able to learn more about the planet’s orbit, which will take more than 1000 years to complete. A circular orbit would imply formation at a great distance from the host star.
But a non-circular or unbound orbit would tell a different tale, indicating that the planet may have formed close to its star only to become involved in a gravitational encounter with another object that resulted in its being flung into the outer system. We do know that this is a warm place, with an estimated temperature of 1800 Kelvin (1500 degrees Celsius), much hotter than Jupiter. That’s consistent with the young age of the system (roughly five million years). The contraction of the planet under its own gravity, once ended, will let the object cool down by radiating infrared light, but it may not reach Jupiter levels (160 Kelvin in the cloud tops) for several billion years.
Lafrenière notes that his team was able to obtain a spectrum of the object back in 2008, finding absorption features due to water vapor, carbon monoxide and molecular hydrogen in the atmosphere. We’ve found other planets through direct imaging, including the trio orbiting HR 8799 (more massive and five times as luminous as the Sun), but the odd outlier circling 1RXS 1609 provides plenty of fodder for planet formation theorists. From the preprint on this work:
The planetary mass companion 1RXS J1609-2105 b is the least massive known to date with an orbital separation of a few hundreds of AU… [T]he existence of such a low mass companion orbiting so far away from its star poses a great challenge for star and planet formation models. Indeed, all main modes of formation — core accretion, gravitational instability or binary-like formation — and migration — disk interaction or planet-planet interaction — face some obstacles when trying to account for such a companion.
The paper is Lafrenière et al., “The Directly Imaged Planet around the Young Solar Analog 1RXS J160929.1-210524: Confirmation of Common Proper Motion, Temperature and Mass,” accepted by the Astrophysical Journal (preprint).
The solar sail news continues to be positive, a welcome relief after so many years of delay and frustration. Now that we finally have an operational sail in space, it’s worth noting how the Japanese IKAROS sail differs from earlier sail concepts. For IKAROS is designed to use two kinds of power. The first comes from the momentum applied to the sail by photons from the Sun. The second (and this is just one of the areas where the Japan Aerospace Exploration Agency went in a new direction) is produced by the thin film solar cells built into the membrane of the 20-meter (diagonal) sail.
Remember that we’ve been getting helpful imagery from two cameras that separated from the spacecraft and looked back on its operations. The image shown later in this post was taken by the DCAM2 camera, a cylindrical device about six centimeters in diameter and height that was detached from the spacecraft by a spring, as was DCAM1. JAXA is continuing to measure the power generating capabilities of the solar cells, and intends to verify the acceleration by photon pressure as the mission progresses, all part of mastering the art of space sail navigation.
Addendum: Daniel Fischer, in the comments below this post, states that the camera used in the image below was DCAM1, and that the JAXA materials are mistaken (in the English version) in citing DCAM2.
IKAROS Attitude Adjustment
The camera work helps us to check on the status of another mission concept. Interestingly, IKAROS is equipped with a liquid crystal technology that allows controllers to change the reflection characteristics on the sail surface by turning the power on and off. While the spacecraft carries a small engine to change the sail’s attitude, JAXA is also testing the liquid crystal devices to see whether the sail’s attitude can be changed without using propellant. Project leader Osamu Mori spoke about the technique in an interview published last year:
The solar-powered attitude-control system uses a technology that controls the reflectivity of the sail. It works just like frosted glass: normally, the entire area of the sail will reflect sunlight, but by “frosting” part of the film, we can reduce the reflectivity of that area. When the reflectivity is reduced, that part of the sail generates less solar power. So by changing the reflectivity of the left and right sides of the sail, we can control its attitude.
You can see the technique at work in the image below.
The image shows most of the IKAROS sail membrane. Notice the areas marked on the photo. According to JAXA, the sail attitude control experiment involves turning the power on the liquid crystal devices on and off in synchronization with the spin of the sail. The photo shows a preliminary experiment in which the power is switched on and off to show the difference — with the power off, the light is more diffusely reflected than when it is on, and the images become whiter. The areas enclosed in dashed red lines are powered off; the blue are powered on.
What we’re going to learn is how effective this technique is at attitude adjustment, a critical part of sail operation and navigation. The results will feed into the second IKAROS mission attempt, scheduled at the moment for late in the decade and involving a larger solar sail with a diameter of some 50 meters. IKAROS 2 will carry ion engines as well as the sail and will target Jupiter and the Trojan asteroids. But for now, we have plenty to learn as IKAROS continues on its trajectory into the inner system, where the solar cell technology will be thoroughly tested. These solar cells, after all, will be needed to supply power to the ion engines on the later IKAROS 2.
LightSail-1 Passes Major Test
In further solar sail news, we have word that the Planetary Society’s LightSail-1, a solar sail built around Cubesat spacecraft, has passed its Critical Design Review in a two-day session in Pasadena. That means building the spacecraft’s hardware and software can proceed. Louis Friedman likens the 4.5 kg spacecraft’s technology to the miniaturization of computers within the last twenty years. In this Planetary Society post, he notes that the sail will be of 4.5 micron Mylar film, aluminized and seamed for rip-stop protection, with a total area of 32 square meters. All of this, Friedman adds, would fit inside a carry-on suitcase. He continues:
Each of these components represents advanced technology, but it is more than the manufacturing [of] the spacecraft that makes LightSail-1 a big undertaking. We also have required system engineering, integration, and testing, and the software programming of the circuit boards that serve as the spacecraft’s central computer. We will complete building the spacecraft by the end of this year, but — to increase our reliability and quality assurance — we have stretched out our integration and test schedule so that it runs into the first quarter of 2011.
Will we see a second solar sail in space by mid-2011? Much depends on launch options — LightSail1 requires a minimum altitude of 820 kilometers in order to escape any atmospheric drag effects — but we should see a firm launch decision within the next few months . By that time, too, JAXA should be well along in its acceleration and navigation tests and we will have accumulated a great deal of data about how sails perform in space. One lesson already being driven home is that solar sails, if they can successfully harness the Sun’s abundant photon output, will be an economical way to move payloads within the inner Solar System.
Knowing of my fascination with small red stars, a friend recently asked why they seemed such problematic places for life. M-dwarfs are all over the galaxy, apparently accounting for 75 percent or more of all stars (I’m purposely leaving the brown dwarfs out of this, because we’re still learning about how prolific they may be). Anyway, asked my friend, is it just that a habitable planet would have to be so close to the star that it would always present the same side to it? That’s tidal lock, and it looks as if it would play havoc with any chances for a stable environment.
But maybe not. In the absence of observational evidence, we have to apply models to M-dwarf planets to see what might or might not work, and some very solid modeling out of NASA Ames back in the 1990s showed that there were ways an atmosphere could circulate so as to keep the dark side of the planet from freezing out its atmosphere. This work, by Robert Haberle and Manoj Joshi, was followed by Martin Heath (Greenwich Community College, London), who showed a viable mechanism for getting liquid water circulating between night and day sides. Tidal lock may not be a showstopper after all.
Image: The M-dwarf AD Leonis, a flare star that may offer clues to habitability. Credit: ESO Online Digitized Sky Survey.
No, in recent times the rap against M-dwarf planets has been that their stars are prone to violent convulsions that launch potentially lethal flares into their planetary systems. Many M-dwarfs produce high energy charged particles and short-wavelength radiation from X-rays to ultraviolet. All of this activity can also affect a planet’s atmosphere, so that a key question becomes whether a planet in an M-dwarf’s habitable zone can retain its atmosphere, or whether terrestrial worlds would lose hydrogen and helium and gas giants would erode into Neptune-mass cores.
My friend and I kicked this around before parting company, he returning to studies unrelated to astronomy, while I returned to my office to find a message from Adam Crowl on red dwarfs and flare activity. A new study demonstrates that red dwarf planets may be shielded from these flares after all. As is standard practice in these matters, Antigona Sugura (Universidad Nacional Autónoma de México) and team went to work with computer models, simulating how a 1985 flare from the star AD Leonis would have affected an Earth-like planet orbiting it at 0.16 AU. AD Leonis is an M-dwarf about 16 light years from Earth, and 0.16 AU, about half Mercury’s distance from the Sun, is in the zone where liquid water could exist at the surface.
The results are promising. It turns out that in the simulation, bursts of UV radiation hitting an Earth-like atmosphere produced a thicker ozone layer, protecting the surface. From the preprint:
For an oxygen-rich, Earth-like planet in the habitable zone of an active M dwarf, stellar flares do not necessarily present a problem for habitability. Much of the potentially life-damaging UV radiation goes into photolyzing ozone in the stratosphere, preventing it from reaching the planetary surface. Ozone variations cause temperature fluctuations in the upper atmosphere, but these fluctuations are small, and the climate at the surface is unaffected.
In fact, in a feature on this work in Science, Lucianne Walkowicz (UC-Berkeley), a co-author of the paper, is quoted as saying “Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth.” That’s good news indeed, for AD Leonis wasn’t chosen at random. At less than 300 million years old, it’s young and energetic, one of the most active M-dwarfs known, and the same article notes that the 1985 flare studied was 1000 times as energetic as a similar flare on our Sun.
Are we out of the woods? Not exactly. The paper goes on to note further problems:
Ionizing particles emitted during a flare may be more dangerous depending on how much of the particle flux strikes the planet. The additive effects of repeated flares over the duration of the planet’s lifetime are not well understood– as M dwarfs can be active on timescales of days to weeks, the atmosphere may not return to equilibrium before another flare occurs.
So there’s still plenty to think about regarding M-dwarf planets as abodes for life. But the coronal mass ejections and dangerous flares that characterize younger M-dwarfs don’t necessarily rule out life in the system, based on this work, and it’s also true that as these stars age, they offer lengthy lifetimes of up to 100 billion years (compared to a G-class star like the Sun, whose life will be around 10 billion years) during which life processes can emerge. That long, slow maturation is often accompanied by a decrease in problematic stellar activity.
The paper is Segura et al., “The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M Dwarf,” accepted by Astrobiology and available online as a preprint. And in the ‘work remaining to be done’ category, note this sentence: “…there has not yet been a detailed, dynamic model exploring the
evolution of an Earth-like atmosphere over the course of a flare.”
The space-based Spitzer telescope has performed a new study of brown dwarfs, concentrating on a region in the constellation Boötes. Fourteen of the objects, with temperatures ranging between 450 and 600 Kelvin, have been found. These are cold objects in stellar terms, and in fact are as cold as some of the planets we’ve found around other stars. 450 Kelvin works out to 177 degrees Celsius, or 350 degrees Fahrenheit, the temperature of a moderately hot oven.
In fact, it gives me pause to reflect that the focaccia I baked the night before last needed higher temperatures (500 degrees Fahrenheit) than the coolest of these brown dwarfs can supply. Most of the new objects in the Spitzer study are T dwarfs, the coolest class of brown dwarfs known, defined as being less than 1500 Kelvin (1226 degrees Celsius). One of the dwarfs in this study is cold enough that it may represent the hypothetical class called Y dwarfs, part of a classification created by a co-author of the paper, Davy Kirkpatrick (Caltech).
Concerning the brown dwarf classification system, Kirkpatrick points to the WISE (Wide-Field Infrared Survey Explorer) mission for further validation:
“Models indicate there may be an entirely new class of stars out there, the Y dwarfs, that we haven’t found yet. If these elusive objects do exist, WISE will find them.”
Image: This artist’s conception shows simulated data predicting the hundreds of failed stars, or brown dwarfs, that NASA’s Wide-field Infrared Survey Explorer (WISE) is expected to add to the population of known stars in our solar neighborhood. Our sun and other known stars appear white, yellow or red. Predicted brown dwarfs are deep red. The green pyramid represents the volume surveyed by NASA’s Spitzer Space Telescope — an infrared telescope designed to focus on targeted areas in depth, rather than to scan the whole sky as WISE is doing. The region within 25 light-years from the sun is marked by the blue sphere. Credit: JPL.
Kirkpatrick’s thoughts on WISE are fascinating in their own right (he’s also a member of the WISE science team). It has long been speculated, for example, that a cool object, perhaps a brown dwarf, could be found in nearby space, close enough to cause perturbations to the Oort Cloud. Kirkpatrick notes that his team is now calling this putative object Tyche, the benevolent counterpart to Nemesis, and says that WISE is powerful enough that it will either find Tyche or rule it out. And let me quote WISE project scientist Peter Eisenhardt (JPL) on the mission:
“WISE is looking everywhere, so the coolest brown dwarfs are going to pop up all around us. We might even find a cool brown dwarf that is closer to us than Proxima Centauri, the closest known star… We’ll be studying these new neighbors in minute detail — they may contain the nearest planetary system to our own.”
It’s worth keeping in mind that WISE is studying a volume of space 40 times larger than is covered in the recent Spitzer work. While all fourteen of the objects discovered in this survey are hundreds of light years away (and invisible to visible-light telescopes), their presence implies that there are a hundred or more brown dwarfs within 25 light years of the Sun, and the latter should be close enough to confirm with spectroscopy. The Spitzer team goes so far as to speculate that WISE may find more brown dwarfs within this 25 light year sphere around the Sun than the number of stars known to exist there.
The paper is Eisenhardt et al., “Ultracool Field Brown Dwarf Candidates Selected at 4.5 ?m,” Astronomical Journal Vol. 139, No. 6 (May, 2010). Abstract available.
Combining the assets of multiple telescopes in the technique known as interferometry has a long pedigree. Using a cluster of small telescopes rather than a single gigantic one is a way to achieve high resolution at sharply lower costs. Take a look at this list of astronomical interferometers working from the visible to the infrared and you’ll see how widely spread the technique has become as we’ve moved from earlier long wavelength observations (including the Very Large Array and MERLIN) toward optical installations and submillimeter interferometers and, now under construction, the Atacama Large Millimeter Array.
Observing Earth-like planets from space has often been studied in terms of a space-based array, with separated spacecraft operating in tandem, as in the infrared interferometer concept shown in this image (Credit: JPL). Both the now stalled Terrestrial Planet Finder and the canceled Darwin mission from ESA were looking at interferometry concepts that would have used a technique called nulling to reduce the light of a central star so that the planets orbiting it could be studied. New work may give these ideas renewed life and improve the concept.
For Stefan Martin (JPL) and A.J. Booth (Sigma Space Corp., Lanham, MD) have created a nulling interferometer that combines the light from four different telescopes to achieve effects no previous nulling techniques have been able to equal. Says Martin:
“Our null depth is 10 to 100 times better than previously achieved by other systems. This is the first time someone has cross-combined four telescopes, set up in pairs, and achieved such deep nulls. It’s extreme starlight suppression…And because this system makes the light from the star appear 100 million times fainter, we would be able to see the planet we’re looking for quite clearly.”
100 million times fainter is impressive indeed, and should be weighed against the fact that at infrared wavelengths, host stars can emit 10 million times more infrared than the terrestrial world being sought. The device, under study at JPL, is shown below.
Image: From left to right: JPLers Felipe Santos Fregoso, Piotr Szwaykowski, Kurt Liewer and Stefan Martin with the nulling interferometer testbed at JPL, where the device is built and refined. Image credit: NASA/JPL-Caltech.
The ideal would be to find a terrestrial world circling a nearby star, one whose atmosphere we could hope to study with spectroscopic techniques, looking for the signatures of life. We have to find such planets first, of course, for of the 463 planets now known, none could support life as we know it. But this could change literally overnight as the three ongoing studies of Alpha Centauri A and B continue, with every prospect that we’ll have answers about planets there within the next few years. How Martin and Booth’s technique would cope with the tricky situation around the central binary stars there I don’t know, but rocky worlds around either of them would give even more impetus to the search for other ‘Earths’ in the interstellar neighborhood.
A recent paper on the Planet Detection Testbed is Martin et al., “Demonstration of the Exoplanet Detection Process Using Four-Beam Nulling Interferometry,” Aerospace Conference 2009 IEEE (abstract).