Simulating Moons Around Terrestrial Planets

Just how the Moon originally formed is under renewed scrutiny given the finding that it contains larger amounts of water than previously thought. We’ll look at that issue in depth another time, because it’s far from resolved. The generally accepted account of the Moon’s formation involves a giant impact with a planetary embryo that has been called Theia. The name is a nod to the Greek story of the titan that gave birth to Selene, the Moon goddess. After its formation, the Moon would have been closer to a much more quickly rotating Earth, inducing huge tidal forces that may have had repercussions on the evolution of the earliest life on the planet.

All of this has a further bearing on life’s emergence because a large moon can affect the tilt of a planet’s rotation relative to its orbit around the star. The term for this degree of tilt is ‘obliquity,’ and its effects on global climate can be profound. If there is little or no tilt, the poles become colder and heat flows in their direction. Increasing the obliquity means that the poles get more sunlight during half of the year while the equatorial regions cool twice a year. The influence on climate is inescapable, as is the fact that obliquity will be unique for each planetary situation.

A new paper by Sebastian Elser (University of Zurich) looks at this issue in terms of the Earth’s history and the probability of giant impacts among planets in general. What we know now is that the Earth’s tilt varies about 1.3 degrees around the figure of 23.3 degrees, with a period of roughly 41,000 years. Elser and team note that without the Moon, the Earth’s obliquity would experience large variations. Venus, which has no moon, shows a retrograde spin, which the Elser paper finds may have been induced by spin-orbit resonances and tidal effects.

Obliquity can vary enormously with time. The tilt of Mars’ rotation ranges from 0 to 60 degrees in less than 50 million years, and earlier work has indicated that the obliquity of an Earth without its Moon would range from 0 to as much as 85 degrees (complete references on these numbers can be found in the paper, cited below). Large moons, then, may be a major player in keeping climatic conditions stable. The Elser paper explores the impact history of planets to see how many would be likely to have a companion like the Moon, using simulations of planets forming in the habitable zone. The history and evolution of such Moons is then modeled.

The results show that large moons are not unlikely:

Under these restrictive conditions we identify 88 moon forming events in 64 simulations… On average, every simulation gives three terrestrial planets with different masses and orbital characteristics and we have roughly 180 planets in total. Hence, almost one in two planets has an obliquity stabilizing satellite in its orbit. If we focus on Earth-Moon like systems, where we have a massive planet with a final mass larger than half of an Earth mass and a satellite larger than half a Lunar mass, we identify 15 moon forming collisions. Therefore, 1 in 12 terrestrial planets is hosting a massive moon.

Assuming, then, that an Earth-class planet forms in the habitable zone around another star, the chances of its being orbited by a moon large enough to stabilize its orbital tilt is roughly 10 percent. The simulations used here, based on 2010 work by Ryuji Morishima (Swiss Federal Institute of Technology) and colleagues, produce numerous habitable ‘Earths,’ so the question of the importance of the Moon’s stabilizing influence becomes significant. We also have to untangle the issue of the water content of lunar magma, called into play by new work by Erik Hauri (Carnegie Institution of Washington). We’re looking at water levels 100 times higher than first supposed, challenging the giant impact theory of the Moon’s formation, which predicted very low lunar water content. Clearly, untangling all this will involve, among other things, sample returns from planets and other bodies that will teach us more about our system’s history.

Hauri speaks to this question himself:

“Water plays a critical role in determining the tectonic behavior of planetary surfaces, the melting point of planetary interiors and the location and eruptive style of planetary volcanoes. I can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which have been mapped not only on the Moon but throughout the inner solar system.”

The paper on planet/moon simulations is Elser et al., “How common are Earth-Moon planetary systems?” accepted for publication in Icarus (preprint). On the issue of water on the Moon, see Hauri et al., “High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions,” published online by Science on 26 May 2011 (abstract). On the Moon’s stabilizing effects in general, see Laskar et al., “Stabilization of the Earth’s Obliquity by the Moon,” Nature 361, 615-617 (1993). Abstract available.


‘Blue Stragglers’ in the Galactic Bulge

I’m fascinated by how much the exoplanet hunt is telling us about celestial objects other than planets. The other day we looked at some of the stellar spinoffs from the Kepler mission, including the unusual pulsations of the star HD 187091, now known to be not one star but two. But the examples run well beyond Kepler. Back in 2006, a survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) used Hubble data to study 180,000 stars in the galaxy’s central bulge, the object being to find ‘hot Jupiters’ orbiting close to their stars.

But the seven-day survey also turned up 42 so-called ‘blue straggler’ stars in the galactic bulge, their brightness and temperature far more typical of stars younger than those around them. It’s generally accepted that star formation in the central bulge has all but stopped, the giant blue stars of the region having exploded into supernovae billions of years ago. Blue stragglers are unusual because they are more luminous and bluer that would be expected. They’ve been identified in star clusters but never before seen inside the core of the galaxy.

Image: Peering deep into the star-filled, ancient hub of our Milky Way (left), the Hubble Space Telescope has found a rare class of oddball stars called blue stragglers, the first time such objects have been detected within our galaxy’s bulge. Blue stragglers — so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed — were first found inside ancient globular star clusters half a century ago. Credit: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI).

The galactic bulge is a tricky place to study because foreground stars in the disk compromise our view. But the SWEEPS data led to a re-examination of the target region, again with Hubble, two years after the original observations were made. The blue stragglers could clearly be identified as moving at the speed of the bulge stars rather than the foreground stars. Of the original 42 blue straggler candidates, anywhere from 18 to 37 are now thought to be genuine, the others being foreground objects or younger bulge stars that are not blue stragglers.

Allan Sandage discovered blue stragglers in 1953 while studying the globular cluster M3, leading scientists to ask why a star would appear so much younger than the stars around it. Stars in a cluster form at approximately the same time and should therefore show common characteristics determined by their age and initial mass. A Hertzsprung-Russell diagram of a cluster, for example, should show a readily defined curve on which the stars can be plotted.

Blue stragglers are the exception, giving the appearance of stars that have defied the aging process. One possibility is that they form in binaries, with the less massive of the two stars gathering in material from the larger companion, causing the accreting star to undergo fusion at a faster rate. More dramatic still would be the collision and merger of two stars — more likely in a region where stars are dense — which would cause the newly formed, more massive object to burn at a faster rate.

Scientists will use the blue straggler data to tune up their theories of star formation. Lead author Will Clarkson comments on the work, which will be published in the Astrophysical Journal:

“Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood. While the consensus is that the bulge largely stopped forming stars long ago, many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge.”

I’ll note in passing that Martin Beech (University of Regina) has suggested looking at blue stragglers in a SETI context, noting that some could be examples of astroengineering, the civilization in question using its technology to mix shell hydrogen into the inner stellar core to prolong its star’s lifetime on the main sequence. It’s an interesting suggestion though an unlikely one given that we can explain blue stragglers through conventional astrophysics. In fact, blue stragglers point to an important fact about the field some are calling ‘interstellar archaeology’ — gigantic astroengineering may be extremely difficult to tell apart from entirely natural phenomena, in which case Occam’s razor surely comes into play.

For the recent blue straggler discoveries, see Clarkson et al., “The First Detection of Blue Straggler Stars in the Milky Way Bulge,” in press at the Astrophysical Journal (preprint). On the possible application of blue stragglers to SETI, see Beech, “Blue Stragglers as Indicators of Extraterrestrial Civilizations?” Earth, Moon, and Planets 49 (1990), pp.177-186. And Greg Laughlin (UC-Santa Cruz) looks at blue stragglers as targets for photometric transit searches in this post on his systemic site.


On the Calendar: Exoplanets and Worldships

Be aware of two meetings of relevance for interstellar studies, the first of which takes place today at the Massachusetts Institute of Technology. There, a symposium called The Next 40 Years of Exoplanets runs all day, with presentations from major figures in the field — you can see the agenda here. I bring this up because MIT Libraries is planning to stream the presentations, starting with Dave Charbonneau (Harvard University) at 0900 EST. Those of you who’ve been asking about Alpha Centauri planet hunts will be glad to hear that Debra Fischer (Yale University), who is running one of the three ongoing Centauri searches, will be speaking between 1130 and 1300 EST.

The poster for this meeting reminds me of the incessant argument about what constitutes a habitable planet. It shows two kids in a twilight setting pointing up at the sky, their silhouettes framed by fading light reflected off a lake. One of them is saying ‘That star has a planet like Earth.” An asterisk reveals the definition of Earth-like for the purposes of this meeting: “…a rocky planet in an Earth-like orbit about a sun-like star that has strong evidence for surface liquid water.” So this time around we really do mean ‘like the Earth,’ orbiting a G-class star and harboring temperatures not so different from what we’re accustomed to here.

And no, that doesn’t rule out more exotic definitions of habitability, including potential habitats around M-dwarfs or deep below the ice on objects far from their star. But finding an ‘Earth’ fitting the symposium’s definition would seize the public imagination and doubtless inspire many a career in science. The forty years referred to in the title of the meeting is a prediction that within that time-frame, we’ll be able to point to a star visible with the naked eye and know that such a planet orbits it. While following the events online, you might also want to track writer Lee Billings (@LeeBillings on Twitter), who’s in Cambridge for the day. Lee’s insights are invariably valuable.

Addendum: Geoff Marcy, amidst many a pointed comment about NASA’s priorities, also discussed in his morning session a probe to Alpha Centauri, as reported by Nature‘s newsblog:

On the back then of these serious policy criticisms came Marcy’s provocative idea for a mission to Alpha Centauri. He appealed to US President Barack Obama to announce the launch of a probe that would send back pictures of any planets, asteroids and comets in the system in the next few hundred years, with the US partnering with Japan, China, India and Europe to make it happen. “It would jolt NASA back to life,” he declared. Maverick it might sound, but many in the room seemed to take the idea in the spirit of focusing minds on the ultimate goal of planet-hunting; to take humanity’s first steps towards reaching out to life elsewhere in the universe.

Into the Generational Deep

The other conference, this one deep in the summer, takes us into the domain of far future technology. Some of the great science fiction about starships talks about voyages that last for centuries — I’m thinking not only of Heinlein’s Orphans of the Sky (1963, but drawing on two novellas in Astounding Science Fiction from 1941), but also Brian Aldiss’ Non-Stop (1958) and the recently published Hull Zero Three, by Greg Bear. In each case, we’re dealing with people aboard a vessel that is meant to survive for many human generations, a frequent issue being to identify just what is going on aboard the craft and what the actual destination is.

Call them ‘generation ships’ or ‘worldships,’ vast constructs that are built around the premise that flight to the stars will be long and slow, but that human technology will find a way to attain speeds of several percent of the speed of light to make manned journeys possible. A surprising amount of work has been done addressing the problems of building and maintaining a worldship, much of it appearing in the Journal of the British Interplanetary Society. Now the BIS is preparing a session dealing explicitly with worldships. Here’s their description of the event:

In 1984 JBIS published papers considering the design of a World ship. This is a very large vehicle many tens of kilometres in length and having a mass of millions of tons, moving at a fraction of a per cent of the speed of light and taking hundreds of years to millennia to complete its journey. It is a self-contained, self-sufficient ship carrying a crew that may number hundreds to thousands and may even contain an ocean, all directed towards an interstellar colonisation strategy. A symposium is being organised to discuss both old and new ideas in relation to the concept of a World Ship. This one day event is an attempt to reinvigorate thinking on this topic and to promote new ideas and will focus on the concepts, cause, cost, construction and engineering feasibility as well as sociological issues associated with the human crew. All presentations are to be written up for submission to a special issue of JBIS. Submissions relating to this topic or closely related themes are invited. Interested persons should submit a title and abstract to the Executive Secretary.

The session will run on August 17, 2011 from 0930 to 1630 UTC at the BIS headquarters on South Lambeth Road in London. The call for papers is available online. My own thinking is that a worldship is almost inevitable if we find no faster means of propulsion somewhere down the line. Imagine a future in which O’Neill-style space habitats begin to create a non-planetary choice for living and working. If we develop the infrastructure to make that happen, it’s not an unthinkable stretch to see generations that have adapted to this environment moving out between the stars.

Would their aim be colonisation of a remote stellar system? Perhaps, but my guess is that humans who have lived for a thousand years in a highly customized artificial environment may choose not to plant roots on the first habitable planet they find. They may explore it and study its system while deciding to stay aboard their familiar vessel, eventually casting off once again for the deep. In any case, worldships offer an interesting take on how we might make interstellar journeys relying not so much on startling breakthroughs in physics as steady progress in engineering and the production of energy. The London session should be provocative indeed.


OSIRIS-REx: Sampling an Asteroid

Asteroid 1999 RQ36 may or may not pose a future problem for our planet — the chances of an impact with the Earth in 2182 are now estimated at roughly one in 1800. But learning more about it will help us understand the population of near-Earth objects that much better, one of several reasons why the OSIRIS-REx mission is significant. The acronym stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer, a genuine mouthful, but a name we’ll be hearing more of as the launch of this sample-return mission approaches in 2016.

The target asteroid, 575 meters in diameter, has been the subject of extensive study not only by ground-based telescopes including the Arecibo planetary radar but also by the Spitzer Space Telescope. We know that 1999 RQ36 orbits the Sun every 1.2 years and crosses the Earth’s orbit every September, with a shape and rotation rate that are well understood. OSIRIS-REx will carry pristine samples of carbonaceous materials of a quality never before analyzed in our laboratories back to Earth, using a sample collecting device that will inject nitrogen to stir up surface materials for capture and storage on the journey back.

This University of Arizona video gives an overview of the mission:

Dante Lauretta (University of Arizona) is deputy principal investigator for OSIRIS-REx:

“OSIRIS-REx will usher in a new era of planetary exploration. For the first time in space-exploration history, a mission will travel to, and return pristine samples of a carbonaceous asteroid with known geologic context. Such samples are critical to understanding the origin of the solar system, Earth, and life.”

The 60 gram sample will be collected in a surface contact that lasts for no more than five seconds. In fact, it’s hard to describe this as a landing. A little over a year ago I quoted Joseph Nuth (NASA GSFC) on the problems of sampling a quickly rotating object of this size. Think in terms of two spacecraft trying to link rather than one trying to land on a surface:

“Gravity on this asteroid is so weak, if you were on the surface, held your arm out straight and dropped a rock, it would take about half an hour for it to hit the ground,” says Nuth. “Pressure from the sun’s radiation and the solar wind on the spacecraft and the solar panels is about 20 percent of the gravitational attraction from RQ36. It will be more like docking than landing.”

The spacecraft will spend more than a year orbiting the asteroid before collecting the samples for return to Earth in 2023. You’ll recall the Japanese Hayabusa spacecraft’s mission to Itokawa, with the first return of asteroid materials in June of 2010. 1999 RQ36 may be a more interesting target given its carbonaceous composition. The idea is to go after an asteroid rich in organics, the kind of object that might have once seeded the Earth with life’s precursors.

OSIRIS-REx should also provide useful data on the Yarkovsky effect, which induces uneven forces on a small orbiting object because of surface heating from sunlight. You can imagine how tricky the Yarkovsky effect is to model given the variables of surface composition, but learning more about it will be helpful as we learn to tighten the precision of projected asteroid orbits. That, in turn, can help us decide whether or not a particular object really does pose a threat to the Earth at some future date.

Meanwhile, we’re keeping a close eye on the Dawn mission, now closing on Vesta and eventually destined to orbit Ceres. Both missions will add significantly to our knowledge of asteroids and their role in Solar System development, but Dawn will not return samples of either of its destinations to Earth. OSIRIS-REx also recalls the Stardust spacecraft, which returned particles from comet Wild 2 in 2006. And like Stardust, OSIRIS-REx will be capable of an extended mission if called upon — Stardust (renamed NExT, for New Exploration of Tempel 1) performed a flyby of previously visited comet Tempel 1 earlier this year.


Beyond the Kepler Planets

Kepler is a telescope that does nothing more than stare at a single patch of sky, described by its principal investigator, with a touch of whimsy, as the most boring space mission in history. William Borucki is referring to the fact that about the only thing that changes on Kepler is the occasional alignment of its solar panels. But of course Borucki’s jest belies the fact that the mission in question is finding planets by the bushel, with more than 1200 candidates already reported, and who knows how many other interesting objects ripe for discovery. Not all of these are planets, to be sure, and as we’ll see in a moment, many are intriguing in their own right.

But the planets have center stage, and the talk at the American Astronomical Society’s 218th meeting has been of multiple planet systems found by Kepler, after a presentation by David Latham (Harvard-Smithsonian Center for Astrophysics). Of Kepler’s 1200 candidates, fully 408 are found in multiple planet systems. Latham told the conference that finding so many multiple systems was a surprise to a team that had expected to find no more than two or three.

To discover this many multiple systems requires planetary orbits to be relatively flat in relation to each other. In our Solar System, for example, some planetary orbits are tilted up to seven degrees, meaning that no one observing the system from outside would be able to detect all eight planets by the transit method. What Kepler has uncovered are numerous multiple systems whose planetary orbits are much flatter than our own, tilted less than a single degree.

Image (click to enlarge and animate): Multiple-planet systems discovered by Kepler as of 2/2/2011; orbits go through the entire mission (3.5 years). Hot colors to cool colors (red to yellow to green to cyan to blue to gray) are big planets to smaller planets, relative to the other planets in the system. Credit: Daniel Fabrycky.

Interestingly, multiple planet systems may give us the help we need to detect small, rocky worlds. While the radial velocity method helps us find larger objects orbiting a star, terrestrial-class worlds are small enough that their radial velocity signal is hard to detect. With a multiple planet systems, astronomers will be able to use transit timing variations, measuring how gravitational interactions between the planets cause tiny changes to the time between transits. Latham’s colleague Matthew Holman notes the power of such a signal:

“These planets are pulling and pushing on each other, and we can measure that. Dozens of the systems Kepler found show signs of transit timing variations.”

Using the transit method, Kepler should be able to identify small planets in wider orbits around their stars, including those that may be in the habitable zone, but transit timing variations may flag the presence of such planets and play a role in the intense follow-up that will produce a confirmation. We have exciting times ahead of us as Kepler continues its mission. Meanwhile, what accounts for the flatness of the planetary orbits in these multiple planet systems? Latham gives a nod to the fact that most of them are dominated by planets smaller than Neptune. Jupiter-class worlds cause system disruptions that can result in tilted orbits for smaller planets.

“Jupiters are the 800-pound gorillas stirring things up during the early history of these systems,” said Latham. “Other studies have found plenty of systems with big planets, but they’re not flat.”

A Catalog of Eccentric Objects

Kepler’s treasure trove includes far more than planets, as an interesting article in Science News points out (thanks to Antonio Tavani for the pointer to this one). After all, the observatory is looking at tens of thousands of stars to produce its planetary finds, and in most cases, planets aren’t lined up in such a way that they can be seen from Earth, if they exist. But Science News quotes Geoff Marcy (UC-Berkeley) on the variety of stars being seen: “There are so many stars that show bizarre, utterly unexplainable brightness variations that I don’t know where to begin.”

Consider the English amateur Kevin Apps, who became curious about a red dwarf in the Kepler field that was not among the 156,000 chosen for full investigation. Apps discovered that he could get light curves for the system from data produced by Kepler’s initial commissioning phase. The light curve showed dips spaced 12.71 days apart, an intriguing find that led him to contact professional astronomers who went to work on the system themselves. The result: The red dwarf turned out to be not a single star but a widely spaced binary of two M-dwarfs, with a massive object in orbit around the larger of the stars, evidently a brown dwarf.

Kepler keeps turning up oddities. A star called KIC 10195926, for example, twice the mass of the Sun, shows ‘torsional modes’ in its rotation — the star’s northern and southern halves spin at different rates, trading off which spins fastest. This is the first time such torsional modes have been seen. The star has now been classed as an Ap star — A-peculiar — with a strong magnetic field. It’s the subject of a paper in Monthly Notices of the Royal Astronomical Society.

The star HD 187091, about a thousand light years from the Earth, is twice as massive as the Sun. Kepler’s light curve showed a 42-day cycle, with the star’s brightness rising to a peak and then quickly subsiding, with numerous secondary brightness variations between the peaks. It turns out this is not a single A-class star, as previously believed, but two stars of nearly the same size in a highly elliptical orbit. Let me quote from Science News on this, drawing on the magazine’s interview with William Welsh (San Diego State University):

The brightening occurs as the stars, tidally warped by their gravity at closest approach into slight egg shapes, roast one another on their facing sides and heat up. And that explains the spike in brightness, the team reported online in February at The more surprising revelation of Kepler’s data is that one, and perhaps both, pulsate furiously at rates that are precise multiples of their rate of close encounters, in some cases pulsing exactly 90 and 91 times for each orbit. “Nobody had ever seen, or even thought, something like this could happen,” Welsh says. Discovering that a star’s rapid pulsations are not always driven by internal processes, but can be paced by a tidal metronome from a partner star, offers a new window into stellar dynamics and structure.

How many more such surprises will Kepler give us? An extended Kepler mission (and we might be able to get an additional four or five years beyond the 2013 original mission end date) should yield interesting objects galore. The follow-up to Kepler might be the European Space Agency’s Plato (Planetary Transits and Oscillations of Stars), which would, like Kepler, examine star fields for lengthy periods of time, but would also be able to swivel and look at different stellar fields. Perhaps the success of Kepler will give Plato the boost it needs for a liftoff in this decade.