Reflections on Visible Exoplanets

The images of planets around Fomalhaut and HR8799 carried more clout than I expected, with traffic to the site quadrupling when the story ran, and substantial coverage from major media outlets as well. I ran the exciting images of both stars and their companions, but because I enjoy astronomical artwork, I now want to include the visualization below, showing Fomalhaut b surrounded by a large ring of autumnal russet and gold. Note, too, the extensive debris disk surrounding the distant star. Orbiting every 872 years, Fomalhaut b lies some 2.9 billion kilometers inside that disk’s inner edge.

Credit: ESA, NASA, and L. Calcada (ESO for STScI).

Greg Laughlin (University of California, Santa Cruz) was surprised at the even-handed media treatment of HR8799, considering the brightness of Fomalhaut (‘A star with a name like a rocket’), not to mention the acknowledged skills of the Hubble Space Telescope’s media office. But while HR8799 isn’t exactly a household word, the faint object trumps Fomalhaut in sheer numbers. Imagine being able to announce that you’ve imaged not one but three planets at the same time! We’re getting better at this, and as more eyes focus on young systems bright in the infrared, we’ll doubtless be snaring more examples of multiple worlds forming around their stars.

Laughlin is an ace exoplanet hunter whose systemic site should be on your priority list, and I was glad to see that he turned his attention to the question of planetary formation around these two stars, for the new planets are gas giants that make us take another look at planet formation. A news release from the Hubble site noted this point in relation to Fomalhaut b:

The planet may have formed at its location in a primordial circumstellar disk by gravitationally sweeping up remaining gas. Or, it may have migrated outward through a game of gravitational billiards where it exchanged momentum with smaller planetary bodies. It is commonly believed that the planets Uranus and Neptune migrated out to their present orbits after forming closer to the sun and then gravitationally interacted with smaller bodies.

The conventional model (insofar as anything can be considered ‘conventional’ in this fast-moving field) of gas giant formation is that it occurs through core accretion, meaning that a massive solid core eventually builds up out of much smaller pieces. Early dust and debris gradually become rocky planetesimals which, in turn, collide and coalesce. But does this have time to happen in the distant outer reaches of a nascent solar system, a place where the disk is thinner and intense stellar radiation can cause it to dissipate quickly? Gravitational instability offers the counter-model: Instabilities in the debris disk cause quick collapse of matter into planets that can form in a matter of a few millennia.

Laughlin’s take on this relates the new planets to an interesting exoplanet detection by Bunei Sato:

Sato’s detection in early 2007 of a 7.6 Jupiter-mass planet orbiting Epsilon Tauri (2.7 solar masses) in the Hyades is probably a good example of the type of planet that’s showing up in these new images, and Eps Tau b provides good support for the case that this category of objects arose from gravitational instability. The Hyades were a tough environment for planet formation via core accretion, due to the intense UV radiation that caused the disks to lose gas quickly…

And there are ways we might firm up this model:

Remnant debris disks would be expected around young stars that had massive enough disks to trigger gravitational instability. Also, in general, the more massive the star, the more massive the disk. And finally, if the planets formed via gravitational instability, one wouldn’t expect a bias toward high metallicity. If this idea is correct, as more of these planets are imaged, there shouldn’t be a metallicity correlation with the parent star.

Both models of planet formation seem robust in particular environments. Laughlin notes that, if the planets that circle HR8799 really were formed by gravitational instability, then our own Solar System’s gas giants, which formed through core accretion, would have less in common with them than with ‘hot Jupiter’ systems like 51 Peg, where the relevant gas giant was also the result of core accretion. Odd to think we have something in common with such bizarre systems… For more, check this helpful page of links to HR8799 materials, including images and movies, that the discovery team has made available. But look fast — I have no idea how long this temporary page will be up.

Science Fiction: Future Past

Be sure to have a look at New Scientist‘s special coverage of science fiction, from which this (in an article by Marcus Chown):

“As well as a mere storytelling device, science fiction often articulates our present-day concerns and anxieties – paradoxically, it is often about the here and now rather than the future. As Stephen Baxter points out…, H. G. Wells’s ground-breaking 1895 novella The Time Machine – famous for popularising the idea of time travel – was more concerned with where Darwinian natural selection was taking the human race than with the actual nuts and bolts of time travel. In the 1968 novel Stand on Zanzibar, John Brunner imagined the dire consequences of overpopulation. Arthur C. Clarke’s The Lion of Comarre explored the terrible allure of computer-generated artificial realities, which – god forbid – people might actually choose over the far-from-seductive messiness of the real world.

All of these books are about imagining where present-day, often worrying, scientific and technological trends might be leading us. They can act as a warning or, at the bare minimum, cushion us from what American writer Alvin Toffler so memorably described as ‘future shock.'”

Chown’s point is well taken. I’ve long believed that science fiction is less predictive than diagnostic, telling us more about the era in which it is written than about the future. Better to say that science fiction is the way we, in our own particular times and places, work out possible futures given the scenario we see before us. Can a truly ‘futuristic’ science fiction — one that makes no reference to its own provenance, but tries to depict the future while remaining free of the political and sociological baggage of the time from which it emerged — even be written? If so, how?

Addendum: In my view, the writer who came closest to the ‘futuristic’ goal outlined above was Paul Linebarger, who wrote under the name Cordwainer Smith. More on this remarkable man here.

Life’s Traces in Mineral Evolution

Now here’s a comprehensive task for you. Take about a dozen primordial minerals found in interstellar dust grains and figure out what processes — physical, chemical, biological — led to the appearance of the thousands of minerals we find on our planet today. The job was undertaken by Robert Hazen and Dominic Papineau (Carnegie Institution Geophysical Laboratory) and colleagues, and it produced startling results: Of the roughly 4300 known types of minerals on Earth (fifty new types identified each year), up to two-thirds can be linked to biological activity.

Mineral evolution? In a sense, although Hazen is quick to qualify the statement:

“It’s a different way of looking at minerals from more traditional approaches. Mineral evolution is obviously different from Darwinian evolution — minerals don’t mutate, reproduce or compete like living organisms. But we found both the variety and relative abundances of minerals have changed dramatically over more than 4.5 billion years of Earth’s history.”

Those early dust grains gave us a set of basic chemical elements, but it took the temperature and pressure changes as the Solar System coalesced to produce a wider range of minerals. Hazen and Papineau’s work suggests that sixty different minerals appeared in this era, with many more forming when volcanic activity and water eventually came into play on planetary surfaces. Mars and Venus, with about five hundred mineral species in their surface rocks, offer a glimpse of this phase of mineral formation. On Earth, plate tectonics, creating movement among continents and churning the ocean basins, produced new chemical environments that brought the mineral count up past a thousand.

The key to true mineral diversification, though, is life. Because so many important minerals are oxidized weathering products, they owe their existence to an oxygen-rich atmosphere, itself the result of photosynthesis. The lovely blue and green minerals azurite and malachite, for example, could only form in an oxygen-rich environment. The development of microorganisms and plants also plays into the formation of clay minerals, while ocean dwelling creatures with shells and skeletons develop deposits of minerals like calcite. In every respect, the byplay between life and minerals is complex. In a useful video describing his work, Hazen explains:

“The interplay between minerals and life works both ways. It turns out that the origin of life may have been absolutely dependent on certain minerals. Mineral surfaces are the perfect place to concentrate, to select, to organize, to make larger structures like polymers, chains of molecules that have biological function. So minerals may have played a key role in life’s origins. but by the same token — and this is what I find so amazing — life played a key role in the minerals’ origin. If you have a world where you have no oxygen, perhaps Mars, certainly Mercury, these minerals will not form. there’s no way to produce them. Life has a role in the origin of minerals just the way minerals have a role in the origin of life.”

Image: This lustrous azurite under intense light shows transparency and overgrowth on malachite clusters just below the surface. Only the presence of oxygen can produce these minerals. Origin: New Cobar Mine, New South Wales, Australia. Credit: Vic Cloete.

All of this leaves a record that can be examined through the science of mineralogy and it’s one that could be exploited as we examine other worlds. We could, in other words, see evidence of life in the mineral diversity we find on a distant planet. From the paper on this work:

Arguably the single most important cause for mineralogical diversification is Earth’s surface oxygenation associated with biological activity, which may be responsible directly or indirectly for more than two thirds of all known mineral species. Thus, for at least the last 2.5 billion years, and possible since the emergence of life, Earth’s mineralogy has evolved in parallel with biology. Accordingly, remote observations of the mineralogy of other moons and planets may provide crucial evidence for biolgical influences beyond Earth.

In Hazen’s view, the connection between the sciences could not be more profound, and the story of the development of individual minerals on a world like ours is inseparable from the changes wrought by biology. This is a provocative way of looking at mineralogy that stresses the development of a mineral identity over time, with obvious ramifications for planetary history. The paper is Hazen, Papineau et al., “Mineral evolution,” American Mineralogist Vol. 93, pp. 1693-1720 (in press).

Exoplanet Images: Two Observational Coups

Are we really moving beyond indirect detection methods to being able to produce actual images of extrasolar planets? Apparently so, as witness the first direct images of multiple planets around a normal main sequence star. And on the same day, we have the announcement of a visible light image of a Jupiter-class planet orbiting the star Fomalhaut, one suspected for several years because of the sharply defined inner edge of the dust belt around the star. A planet in an elliptical orbit affecting the debris disk had been thought to be offsetting the inner edge of the belt.

Let’s go to the planets found around the dusty young star HR8799 first. They range from seven to ten times the mass of Jupiter. Bruce Macintosh (Lawrence Livermore National Laboratory), one of the authors of a new paper on the achievement in Science Express, explains its significance:

“Every extrasolar planet detected so far has been a wobble on a graph. These are the first pictures of an entire system. We’ve been trying to image planets for eight years with no luck and now we have pictures of three planets at once.”

Radial velocity methods can tell us much, including not only the presence of a planet but also something about its mass and orbit. But these methods work best when the separation between the star and the planet is relatively small, usually within the range of 5 AU. What we have here are three worlds quite a distance from their primary, at 24, 37 and 67 AU respectively. By comparison, Neptune is about 30 AU from the Sun in our Solar System. The star, a blue A-class object, retains a sizable disk, and its planets are young enough to be throwing a bright infrared signature. The results of observing same appear immediately below:

: Gemini Observatory discovery image using the Altair adaptive optics system on the Gemini North telescope with the Near-Infrared Imager (NIRI). Image shows two of the three confirmed planets indicated as “b” and “c” on the image. “b” is the ~7 Jupiter-mass planet orbiting at about 70 AU, “c” is the ~10 Jupiter-mass planet orbiting the star at about 40 AU. Due to the brightness of the central star, it has been blocked and appears blank in this image to increase visibility of the planets. Credit: Gemini Observatory.

HR8799 is relatively nearby at 130 light years, visible using binoculars or a small telescope (or even via the naked eye, depending on your seeing conditions). Close enough to find, and perhaps image, other planets in this interesting system? It’s a distinct possibility, and with ever more powerful adaptive optics systems going into place via the Gemini Planet Imager, we’ll boost our capabilities a hundred times. Says Macintosh:

“I think there’s a very high probability that there are more planets in the system that we can’t detect yet. One of the things that distinguishes this system from most of the extrasolar planets that are already known is that HR8799 has its giant planets in the outer parts – like our solar system does – and so has ‘room’ for smaller terrestrial planets – far beyond our current ability to see – in the inner parts.”

Interesting indeed, and note that we’re already doing spectroscopy to study these three planetary atmospheres. Bear in mind, too, that the work on this star is part of a survey of eighty young, dusty stars located not far from the Sun. Because the HR8799 planets showed up after observations on only a few, we may discover that Jupiter-class worlds at these separations are not uncommon among more massive stars.

Imaging planets around young stars like this one is by no means an easy task, but as we move to older systems with planets well beyond their early formation stage, the game will only get tougher. HR8799, a star with a dust disk more massive than any star within 300 light years from Earth, is a harbinger of necessary instrumentation tune-ups to come as we extend the search for exoplanetary images from Earth into advanced space missions. But for now, what a job by astronomers using adaptive optics at Gemini North and the Keck Observatory!

As to Fomalhaut b, clearly defined in the Hubble Space Telescope image below, the evidence seems strong indeed that we are looking at a actual planet. James Graham (UC Berkeley), a co-author of the paper on this work, has little doubt: “It will be hard to argue that a Jupiter-mass object orbiting an A star like Fomalhaut is anything other than a planet.”

Image: This 2006 Hubble Space Telescope optical image shows the belt of dust and debris (bright oval) surrounding the star Fomalhaut and the planet (inset) that orbits the star every 872 years and sculpts the inner edge of the belt. A coronagraph on the Advanced Camera for Surveys blocks out the light of the star (center), which is 100 million times brighter than the planet. Credit: Paul Kalas/UC Berkeley; STScI.

Its brightness also argues that Fomalhaut b has a particularly intriguing property. Paul Kalas (also at UC Berkeley), has this to say:

“To make this discovery at optical wavelengths is a complete surprise. If we’re seeing light in reflection, then it must be because Fomalhaut b is surrounded by a planetary ring system so vast it would make Saturn’s rings look pocket-sized by comparison. Fomalhaut b may actually show us what Jupiter and Saturn resembled when the solar system was about a hundred million years old.”

The researchers believe they can constrain the planet’s mass to between 0.3 and 2 Jupiter masses, noting that a more massive object would destroy the dust belt around Fomalhaut. Like HR8799, Fomalhaut is a young star — about 200 million years old — and will have a short lifetime of perhaps a billion years. Sixteen times brighter than the Sun, the star would appear about as bright from the new planet as our Sun does from Neptune even though the distance to HR8799 is four times greater. Fomalhaut b is also a mysterious place in at least one sense: It has dimmed by half a stellar magnitude from 2004 to 2006, perhaps an indication of a hot outer atmosphere affected by convection cells.

So there we are. Not just an exoplanet directly imaged in visible light, but one with a ring system that may well resemble what Jupiter’s must have been like before the large Galilean satellites coalesced. Here again, note the observational constraints. The radial velocity method would not have allowed Kalas and team to have made this detection — the planet is simply too far from its star and too low in mass. And while infrared would seem to be the tool of choice to detect hot young worlds like this one, the planet’s great distance from its star made a visible light image possible.

The Fomalhaut b paper is Kalas et al., “Optical Images of an Exosolar Planet 25 Light-Years from Earth,” Science Express November 13, 2008 (abstract). The paper on the triple find around HR8799 is Marois et al., “Direct Imaging of Multiple Planets Orbiting the Star HR 8799,” Science Express (same date). The abstract is here. This Hubble news release is also helpful.

An Inflatable Sail to the Oort Cloud

Want to get to the outer Solar System quickly? Try this on for size: Two and a half years to reach the heliopause, six and a half years to get to the Sun’s inner gravitational focus (550 AU), with arrival at the inner Oort Cloud in no more than thirty years. A spacecraft meeting those targets is moving at 403 kilometers per second, roughly twenty times as fast as anything we’ve put into space before. Such a mission could perform useful astrophysical observations enroute, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin.

The combined Oort Cloud explorer/gravity focus probe grows out of work by Gregory Matloff and Roman Kezerashvili (CUNY), Italian physicist Claudio Maccone and Les Johnson (NASA MSFC). Matloff, of course, has been studying solar sail technologies for decades, looking at missions that could reach velocities in the range of 0.003c-0.004c, with metallic sails that, parachute-like, pull a payload attached to diamond-strength cables. The cables (and the sail itself) can be wound around the payload enroute to provide cosmic-ray shielding and, in the case of true interstellar missions, redeployed upon arrival at a destination star.

That’s a familiar sail concept, but it’s one with a problem: Such designs do not scale well. In fact, as you increase the size of the spacecraft, the proportion of its mass that is devoted to cable rises rapidly with payload. Because of his long-standing interest in ‘generation’ ships, capable of carrying human colonies on millennia-long journeys to the stars, Matloff has a natural interest in refining the sail so it can be used in ever more ambitious missions. It’s natural to turn to the idea of inflatable beryllium sails, hollow-body sails that feature sail surfaces just tens of nanometers in thickness, with the body inflated by a low-pressure gas like hydrogen. Unlike the ‘parachute’ concept, the payload would be mounted on the space-facing surface — the inflatable sail is a ‘pusher’ model.

The concept goes back to Joerg Strobl, who first published it in a 1989 paper for the Journal of the British Interplanetary Society. And it’s a design that seems to scale well if properly deployed. The team studied two configurations, one a generation ship with inflated sail radius of 541.5 kilometers, a payload of 107 kg, and a separation between the sail faces of one kilometer. A second is a near-term extrasolar probe with sail radius of 937 meters, a 30 kg payload and a 1.8 meter separation. The numbers show how well the concept adjusts to different missions:

From the point of view of kinematics, mechanical stress, and thermal effects, the hollow-body solar photon sail scales well. Both con?gurations had a spacecraft areal mass density of 6.52 × 10?5 kg/m2, a peak internal gas pressure of 1.98 × 10?4 Pa, and a peak perihelion temperature of 1412 K. If fully in?ated at the 0.05 AU perihelion of an initially parabolic solar orbit, both had a peak radiation-pressure acceleration of 36.4 m/s2 and exited the solar system at 0.00264c after an acceleration duration less than one day.

The new paper looks hard at the issues these designs face, including problems with the proposed 0.05 AU close pass by the Sun and the effects of solar radiation on sail materials and the hydrogen fill gas. The result is a modification of the near-term concept discussed above, with perihelion adjusted to 0.1 AU. The greater distance lowers the sail temperature considerably and reduces the need to replace hydrogen fill gas that will have diffused through the sail walls at higher temperatures. Even so, the team calculates that the gas must be replenished some fifty times during this solar acceleration process. The challenge is manageable:

To err on the side of caution, it is assumed here that a hydrogen reserve of 100 times the required ?ll gas mass is carried aboard the spacecraft. This amounts to only 170 grams of hydrogen. If hydrogen ?ll gas is dissociated from water as required, no more than about one kilogram of water is required. Even water-storage and dissociation equipment will not add more that a few kilograms to the payload and have a very small effect on spacecraft performance.

Also manageable is the constant ionization of beryllium sail atoms during the acceleration period, the result of solar ultraviolet radiation. The surface of the sail will lose electrons and become positively charged. And because the tensile strength of beryllium degrades with temperature, the sail could burst from electrostatic pressure at the earlier 0.05 AU perihelion. Increasing the perihelion distance lowers the electrostatic pressure dramatically and makes the mission feasible.

Can a beryllium sail of this description be launched from the surface, or does it demand space manufacture? We don’t know the answer to that yet, or to the question of whether beryllium is indeed the best material for the sail walls. It seems clear that an inflatable sail will be more massive than other designs despite its advantages in scalability, and it’s also more likely to experience significant damage from micro-meteorites. Plenty of questions remain as we work out the various sail designs and rigging arrangements that may make a fast mission to the Oort Cloud a reality in, the paper suggests, the post-2040 time frame.

The paper is Matloff, Kezerashvili, Maccone and Johnson, “The beryllium hollow-body solar sail: exploration of the Sun’s gravitational focus and the inner Oort Cloud,” now available online.