Centauri Dreams
Imagining and Planning Interstellar Exploration
Mass Effect: What Exoplanet Atmospheres Can Tell Us
Let me offer best wishes for the holidays to all Centauri Dreams readers, with thanks for the numerous comments and suggestions over the course of the past year. The schedule this week is abbreviated but I’ll have a new post up on Friday. I’m about to set out to gather the materials I need for a family dinner tonight, but I have time this morning to talk about interesting new work on figuring out the mass of an exoplanet. As you might guess, this is a key measurement, and a tough one to make. The work out of MIT offers an elegant solution.
I yield to no one in my admiration for the tough-minded Sara Seager (MIT), whose career in astrophysics is so movingly described in Lee Billings Five Billion Years of Solitude: The Search for Life Among the Stars
Now this is interesting stuff because learning something of a planet’s mass can help us make the call on whether it is a rocky world capable of supporting life. Radial velocity methods work well when larger planets are at play, or smaller worlds that orbit extremely close to their parent star. But radial velocity is trickier when we’re dealing with small planets orbiting further out. A planet like the Earth, for example, would be hard to analyze using radial velocity alone. We do, however, have the ability to study planetary atmospheres as planets transit their stars, and ingenious analysis may make it possible to extract from this a reading of a planet’s mass.
Thus de Wit’s description of what the duo have been up to:
“With this method, we realized the planetary mass — a key parameter that, if missing, could have prevented us from assessing the habitability of the first potentially habitable Earth-sized planet in the next decade — will actually be accessible, together with its atmospheric properties.”
Image: Artistic rendering of a planet’s transmission spectrum. Credit: MIT.
How? Our old friend HD 189733b comes into play, a transiting ‘hot Jupiter’ some 63 light years away that has been a testbed for the technique called transmission spectroscopy, where scientists analyze the light that passes through the atmosphere to determine properties like temperature and the density of atmospheric molecules. Because we can study an atmosphere, we can study the effect of mass on that atmosphere. The method described in the de Wit/Seager paper works with a standard equation that describes the effect of three factors — temperature, gravitational force and atmospheric density — on the planet’s atmospheric pressure profile, which is a measure of how pressure changes throughout the atmosphere.
Believing that each of these factors could be derived independently from a transmission spectrum, de Wit studied the effects of each using the 18th Century mathematical constant called the Euler-Mascheroni constant. This MIT news release describes the constant as an ‘encryption key’ that helped decode the ways the properties of a planet’s atmosphere are embedded in its transmission spectrum. Turned on HD 189733b, the analysis yielded the same mass measurement that other scientists had obtained by radial velocity methods. Future space-based platforms should be able to turn these methods to much smaller worlds.
My particular interest in red dwarfs is piqued by this study, because radial velocity methods are not well suited for small planets orbiting faint stars. Extending the range of mass measurement through this new transit technique would make the mass of planets transiting red dwarfs that much more discoverable, offering another way of characterizing possibly habitable worlds. The technique is not, of course, limited to red dwarfs, and we can assume that Earth-sized planets orbiting stars not so different from the Sun will be studied using the same methods, once we have space-based instruments like the James Webb Space Telescope operational.
The paper is de Wit and Seager, “Constraining Exoplanet Mass from Transmission Spectroscopy,” Science 20 December 2013 (abstract).
From Brown Dwarfs to NEOWISE
I will admit to an obsession with small, dim stars, one that goes far enough to take in those not-quite stars called brown dwarfs, objects too small to ignite hydrogen fusion. The WISE mission showed us that, at least in our Sun’s neighborhood, brown dwarfs aren’t as common as we once thought, with perhaps one of them for every six main sequence stars. For their part, red dwarfs are the prime currency of the galaxy, accounting for 75 to 80 percent of all stars, so between the two we have a host of venues for planets and, possibly, life. But so much needs to be done before we’ll know if either brown or red dwarfs could really be candidates for astrobiology around them.
These thoughts are triggered by more news from WISE, now in its reactivated incarnation as NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer). The spacecraft came back to life in September after 31 months of hibernation and is now working to help us identify potentially hazardous near-Earth objects, a reminder that brown dwarfs were only a part of a much larger mission to characterize the infrared sky. But before I leave brown dwarfs today, let me run an image Greg Benford passed along, a Jon Lomberg conception of a brown dwarf.
Image: Could a brown dwarf like this form planets that support life? Credit: Jon Lomberg.
If memory serves, I ran this image a long time ago in the course of another brown dwarf discussion, and Greg and I talked about it at the first 100 Year Starship Symposium in Orlando. The Lomberg artwork was on Greg’s mind when he finished his recent story “The Man Who Sold the Stars” (in Starship Century) with a scene set around a dim brown dwarf that was only discovered after a renewed search of old data, including that of the WISE mission. Greg describes a surreal landscape on a planet orbiting the brown dwarf called Redstar:
Stars shone in pale gray here against the inky black. The huge hull of Redstar hung as a burgundy disk cut off by the sea. Here and there across the long panorama of perpetual twilight, slanting rays of a deep Indian red showed floating plants, lapping on the waves in a somber sprawl. Everything glowed with infernal incandescence…. Down from the desolate slope to his left came an echoing cry, long and slow. In the thick air a thing like a huge orange gossamer butterfly fluttered on a thin wind. It swooped across a sky peppered with amber clouds and vanished with deliberate, long flaps of its enormous wings, vanishing behind a low eroded hill.
As we learn more about dim stars and much dimmer brown dwarfs, we have to answer key questions that impinge upon their chances for such life. About red dwarfs, we have to learn whether tidal lock offers stable meteorology anywhere on the surface (various papers have argued that it can), and whether the flare activity particularly in young stars would sterilize the planet or serve as a spur to evolutionary change. About brown dwarfs we have to learn whether these continually cooling objects produce rocky planets in close orbits that could stay warm long enough for life to thrive.
NEOWISE Surveys the Nearby Sky
We won’t have the answers to these questions any time soon. For this morning, then, back to NEOWISE. The image below shows a view from the spacecraft after its recent reactivation. If you’ll study the center of the image, you’ll see the track of an asteroid as captured in a series of exposures. This image shows an area in the constellation Pisces; the asteroid is (872) Holda. A red streak at the top of the image is the track of an Earth-orbiting satellite.
Image: NEOWISE originated as WISE (Wide-field Infrared Survey Explorer), which was put into hibernation in 2011 upon completing its goal of surveying the entire sky in infrared light. WISE cataloged three quarters of a billion objects, including asteroids, stars and galaxies. In August 2013, NASA decided to reinstate the spacecraft on a mission to find and characterize more asteroids. Credit: NASA/JPL.
From 2010 to early 2011, NEOWISE discovered more than 34,000 asteroids and characterized 158,000 throughout the Solar System. Amy Mainzer, principal investigator for NEOWISE at JPL, looks at the future of the mission:
“NEOWISE not only gives us a better understanding of the asteroids and comets we study directly, but it will help us refine our concepts and mission operation plans for future, space-based near-Earth object cataloging missions. The spacecraft is in excellent health, and the new images look just as good as they were before hibernation. Over the next weeks and months we will be gearing up our ground-based data processing and expect to get back into the asteroid hunting business, and acquire our first previously undiscovered space rock, in the next few months.”
NEOWISE uses a 40-centimeter telescope and infrared cameras in going about its work studying asteroid size, albedo, reflectivity and other properties. The original WISE mission captured more than 2.7 million images in various infrared wavelengths and catalogued well over 700 million objects. The primary goal of NEOWISE is to study asteroids and comets that approach within 45 million kilometers. It’s exciting and necessary work, but I do wish we had a second WISE working on brown dwarfs.
Cometary Clues to the Fomalhaut System?
It was only in October of this year that we discovered that a red dwarf star called LP 876-10 was in fact part of the Fomalhaut system. Now known as Fomalhaut C, the diminutive object is making news of its own with the announcement that, like its much larger and brighter counterpart, Fomalhaut A, it hosts a belt of comets. That two stars in the same system could each have a ring of comets raises interesting issues. Grant Kennedy (University of Cambridge), whose team made the discovery using data from the Herschel Space Observatory, explains:
“It’s very rare to find two comet belts in one system, and with the two stars 2.5 light years apart this is one of the most widely separated star systems we know of. It made us wonder why both Fomalhaut A and C have comet belts, and whether the belts are related in some way.”
Image: View of the Fomalhaut triple star system from Earth. The small inset shows a zoom of the newly discovered comet belt around Fomalhaut C as seen at infrared wavelengths by Herschel. The large inset shows a zoom of the much larger comet ring around Fomalhaut A as seen at optical wavelengths by Hubble. Telescope resolving power is lower at the infrared wavelengths observed by Herschel, so the size of the belt around Fomalhaut C is not well known. Image Credit: Grant Kennedy (Cambridge) & Paul Kalas (UC Berkeley).
There are those who speculate that the Oort Cloud might extend halfway to Alpha Centauri, perhaps intersecting with a similar cloud around the triple star system there, but the idea that stars 2.5 light years apart could be members of the same system takes a little getting used to. Proxima Centauri, at some 15000 (plus or minus 700 AU) has undergone extensive scrutiny to discover whether it is gravitationally bound to Centauri A and B — Greg Laughlin and Jeremy Wertheimer (UC-Santa Cruz) make a convincing case that it is. 2.5 light years is a much further stretch, but Fomalhaut C has been demonstrated to be gravitationally bound to Fomalhaut A.
And in case you’re wondering, Fomalhaut B (TW PsA), the second member of this trinary system, is fully 0.91 light years from Fomalhaut itself, a K4-class object whose velocity and age are consistent with its being bound to this system. You’ll recall that Fomalhaut A is also where the first planet to be directly imaged at visible wavelengths was found. The image of Fomalhaut b showed it to be orbiting just inside the outermost of the star’s two debris disks, perhaps as much an accumulation of rubble as a fully-formed planet, but one capable of shaping the debris disk itself. The disk emits considerable infrared radiation and has been well studied.
Image: Artist’s impression of the Fomalhaut system. The newly discovered comet belt around Fomalhaut C is shown to the left. The comet belt around Fomalhaut A is in the distance to the right. The belt around Fomalhaut A is offset slightly, a signature of the elliptical orbits in the belt, which may have been caused by past interactions with the star Fomalhaut C. Credit: Amanda Smith.
The discovery of a cometary belt around Fomalhaut C may be telling us something about encounters within this system in the past. For both the outermost disk around Fomalhaut A and the planet that orbits it move in elliptical orbits, probably the result of encounters with undetected planets or with one of the other two stars, B and C. Encounters like these can brighten a disk by causing the comets within them to collide more frequently and release more dust and ice. The comet belts around both A and C in the Fomalhaut system are bright enough to suggest such encounters. To follow up on the matter will require further study of Fomalhaut’s C’s orbit.
The paper summarizes the situation:
Whether there is any link between the discs around Fomalhaut and LP 876-10 is unclear. If the stars have always remained well separated, the evolution of the two bright debris discs should be no different to random single stars, and their detection in the Fomalhaut system would be attributed to the relative youth of the system (in particular LP 876-10). Alternatively, the wide separation of the companions may lead to complex dynamics; the bright debris discs and eccentric planet may have a common cause due to a past interaction between Fomalhaut and LP 876-10, which stirred up their debris discs, perhap igniting a collisional cascade in a previously quiescent disc… or provoking an instability in the planetary system… that later stirs the disc. Such scenarios are of course speculation, but motivate detailed descriptions of all system components.”
All of which gets us back to Fomalhaut B, a star that lacks a bright debris disk of the sort found around its two companions. Is the very lack of such a disk pointing to past encounters between A and C, with B unaffected by their interactions? Computer simulations and further observations of the Fomalhaut C belt may provide some answers. The paper adds: “At only a few dozen light years from Earth, this remarkable system provides a unique laboratory in which to observe one outcome of star and planet formation in detail.”
The paper is Kennedy et al., “Discovery of the Fomalhaut C debris disc,” published online in Monthly Notices of the Royal Astronomical Society Letters, 17 December 2013 (abstract / preprint).
Possible Planet in Nearby Brown Dwarf System
Has astrometry finally bagged an exoplanet? A new study from Henri Boffin (European Southern Observatory) and colleagues has found compelling evidence that the nearest pair of brown dwarfs to the Sun — WISE J104915.57-531906, otherwise known as Luhman 16AB — is home to a hitherto undetected companion. It’s interesting news not only for the astrometry angle but because Luhman 16AB is no more than 6.6 light years away, making it the third closest system to the Sun after Alpha Centauri and Barnard’s Star.
Image: Luhman 16AB, two brown dwarfs in the Sun’s neighborhood, now considered home to a possible planet. Credit: NASA / JPL / Gemini Observatory / AURA / NSF.
I give prominence to astrometry here because the European Space Agency’s Gaia mission was launched this morning, chartered with creating a three-dimensional map of the Milky Way, but also with finding exoplanets using astrometry as its primary method. While radial velocity measures tiny motion in stars induced by unseen planets, it does so by analysis of the Doppler shift in light from the star. Astrometry actually measures the positions and movement of the stars. It’s all about measuring angles and deriving astrophysical data from the result.
The Hubble Space Telescope used astrometric methods in 2002 to study a planet around Gliese 876 that had already been discovered, but despite many claims, planet discoveries via astrometry have yet to be confirmed. The technique has had a checkered history. William Herschel studied the effects of what he believed to be an unseen companion on the star 70 Ophiuchi in the 18th Century, and numerous claims involving other nearby systems have been made over the years, most recently one involving a Jupiter-class object around the nearby red dwarf VB 10 and a 2010 study of HD 176051. But the changes in stellar position involved are so tiny that we need a new generation of instruments to really put the method to work.
The late, lamented Space Interferometry Mission would have been such an instrument, but of course it never flew. Now we have Gaia to look forward to, and as the mission progresses, we’ll follow its observations with interest. The sunshield has deployed and Gaia is moving toward L2, some 1.5 million kilometres beyond Earth as seen from the Sun. Gaia will measure stars with micro-arcsecond precision. Astrometry is most sensitive to planets with large orbits, which makes it a useful complement to radial velocity methods that are most sensitive to nearer objects.
For our purposes today, though, the Boffin team used the FORS2 instrument on ESO’s Very Large Telescope at Paranal to image Luhman 16AB in the spring and early summer of this year, looking for displacements of the two brown dwarfs in their orbit. Says Yuri Beletsky (Carnegie Institution for Science):
“The two brown dwarfs are separated by about three times the distance between the Earth and the Sun. Binary brown dwarf systems are gravitationally bound and orbit about each other. Because these two dwarfs have so little mass, they take about 20 years to complete one orbit.”
Measuring down to a precision of a few milli-arcseconds, the researchers found small deviations from the expected motion of the two brown dwarfs around each other. The preprint of the upcoming paper in Astronomy & Astrophysics has this to say:
The FORS2 data, once combined with the data of Luhman (2013) and Mamajek (2013), yield a significant improvement of the precision of the parallax derived by Luhman (2013). Yet, the FORS2 positions alone indicate that a two-body system is very unlikely whereas an additional companion would explain the observed wobbles. Such a companion must have a mass lower than the brown dwarfs in the system, because i) it would otherwise have been seen in direct imaging or spectroscopy, and ii) a system with two equal mass brown dwarfs would not be detected as there would be no motion of the photocentre.
We can say little about the putative companion, with Boffin’s team noting only that its likely mass is between a few and up to 30 Jupiter masses, but the paper points out that an object of the latter size should be detectable by adaptive optics given the size of the expected separation. As to the brown dwarfs themselves, their masses are predicted to be 0.04-0.05 solar masses for the primary and 0.03-0.05 for the secondary. The system seems to present itself to us nearly edge-on, making for the possibility of a radial velocity follow-up with a robust signal.
Assuming we really do have a planet here, it would be the second closest exoplanet to the Earth (assuming that Alpha Centauri Bb is eventually confirmed). Previous brown dwarf planets have been found by microlensing and direct imaging, with microlensing able to find relatively close planets and direct imaging helpful at planets with large separation from their host star. Luhman 16AB gives us a chance to study a planet at an intermediate distance as we try to learn more about brown dwarfs and the planet formation processes that may be at work around them.
The paper is Boffin et al., “Possible astrometric discovery of a substellar companion to the closest binary brown dwarf system WISE J104915.57-531906.1,” accepted at Astronomy & Astrophysics (preprint).
New Views of Titan’s Lake Country
Titan has about 9000 cubic kilometers of liquid hydrocarbon, some forty times more than in all the proven oil reservoirs on Earth. That’s just one of the findings of scientists working over the data from recent Cassini flybys of the Saturnian moon. Each flyby snares our attention because this is the only other place in the Solar System that has stable liquid on the surface, even if it’s not water. That’s part of Titan’s fascination, of course, because it’s similar to the Earth in terms of basic interactions between liquids, solids and gases but completely alien in terms of temperatures.
Just how extensive are those seas and lakes we’ve found in Titan’s northern hemisphere? Cassini’s radar instrument has given us our best views to date with the mosaic shown below, one that’s based on multiple images from flybys tracking areas at various angles. Kraken Mare, Titan’s largest sea, and Ligeia Mare, the second largest, appear along with nearby lakes. We learn not only that Kraken Mare is more extensive than first thought, but that almost all the lakes on Titan are in an area some 900 kilometers by 1800 kilometers. A mere three percent of the liquid on Titan is found outside this region. Cassini radar team member Randolph Kirk explains:
“Scientists have been wondering why Titan’s lakes are where they are. These images show us that the bedrock and geology must be creating a particularly inviting environment for lakes in this box. We think it may be something like the formation of the prehistoric lake called Lake Lahontan near Lake Tahoe in Nevada and California, where deformation of the crust created fissures that could be filled up with liquid.”
This JPL news release adds that processes like these on Earth lead to the formation of faults that create basins broken by mountain ranges. Much of present day Nevada was, some 13,000 years ago, flooded by Lake Lahontan in a configuration that resembles, on a smaller scale, Titan’s closely packed seas.
Image: This colorized mosaic from NASA’s Cassini mission shows the most complete view yet of Titan’s northern land of lakes and seas. In this projection, the north pole is at the center. The view extends down to 50 degrees north latitude. In this color scheme, liquids appear blue and black depending on the way the radar bounced off the surface. Land areas appear yellow to white. A haze was added to simulate the Titan atmosphere. Kraken Mare, Titan’s largest sea, is the body in black and blue that sprawls from just below and to the right of the north pole down to the bottom right. Ligeia Mare, Titan’s second largest sea, is a nearly heart-shaped body to the left and above the north pole. Punga Mare is just below the north pole. Credit: JPL.
Note the smaller lakes above and to the left of the north pole, which are about 50 kilometers across or less. Moreover, the new data are finally telling us how deep at least one of the seas is. Because the liquid methane of Ligeia Mare is very pure, Cassini’s radar signal passes through it easily and can detect a signal from the sea floor. The lake turns out to be about 170 meters deep, and in at least one place is deeper than the average depth of Lake Michigan. With northern summer approaching, Titan’s lake country should be entering an interesting meteorological phase for Cassini’s future studies as the atmosphere heats up.
Water Vapor Detected Above Europa
Last week’s look at Europa examined the possibility of primordial impacts there that might have brought organic materials to the moon, focusing especially on clay-like minerals that a JPL team found in data from the Galileo mission. I had barely finished that article before the news from Hubble arrived with observations of water vapor above the southern pole of Europa, a possible indication of water plumes erupting from the moon’s surface. That work ran in Science Express and was reported at the meeting of the American Geophysical Union in San Francisco. Lead author Lorenz Roth (Southwest Research Institute) described it this way:
“By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa. If those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice. And that is tremendously exciting.”
Exactly so, for now we can start thinking about doing something similar to what has been envisioned at Enceladus, flying a spacecraft through a plume to study what’s below the ice. As the comments on last Thursday’s post (Europa: Minerals from an Ancient Impact) have shown, the question of ice thickness on Europa is wide open, and scientists studying the matter are divided on it — once again I point you to Richard Greenberg’s Unmasking Europa (Copernicus, 2008) for a lively defense of thin ice.
Image: This graphic shows the location of water vapor detected over Europa’s south pole that provides the first strong evidence of water plumes erupting off Europa’s surface, in observations taken by NASA’s Hubble Space Telescope in December 2012. Hubble didn’t photograph plumes, but spectroscopically detected auroral emissions from oxygen and hydrogen. The aurora is powered by Jupiter’s magnetic field. This is only the second moon in the solar system found ejecting water vapor from the frigid surface. The image of Europa is derived from a global surface map generated from combined NASA Voyager and Galileo space probe observations. Credit: NASA, ESA, and L. Roth (Southwest Research Institute and University of Cologne, Germany).
Geysers on Europa could indeed be a signature of a thin ice crust, but as reader Andrew Tribick has noted here, they could also flag a lake enclosed within a thick crust. The key question, of course, is just where the water is coming from. “Do the vents extend down to a subsurface ocean or are the ejecta simply from warmed ice caused by friction stresses near the surface?” Roth asks. So far we don’t know. And as to the source of the faint emissions detected by Hubble, the long cracks on Europa’s surface known as lineae may well be the answer. That would correspond to the fissures that the Cassini spacecraft has seen near the south pole of Enceladus.
But whatever the thickness of Europa’s ice, flying through a plume rather than drilling would make our task immeasurably easier. We can also take up Freeman Dyson’s suggestion, noted in the comments by Larry Klaes, that space around Europa should be investigated for the possible remains of aquatic life forms that could have been blown out by impact events.
Another reminder of Enceladus is the fact that the intensity of the plumes detected on Europa varies with the moon’s orbital position. The jets are detected only when Europa is at the farthest point in its orbit from Jupiter, while signs of venting disappear when the moon is closer. Are we looking at tidal flexing caused by Jupiter’s gravitational pull? That’s what one would expect with a subsurface ocean placed in this environment. It’s interesting that Europa’s own gravity, twelve times that of Enceladus, would cause any water vapor to fall back to the surface, leaving surface features near the south pole that may be observable by future spacecraft.
The paper is Roth et al., “Transient Water Vapor at Europa’s South Pole,” published online in Science 12 December 2013 (abstract).
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