by Paul Gilster | Jun 6, 2014 | Exoplanetary Science |
Although I’m sure I’ll refer to various papers presented at the American Astronomical Society this week in future entries, I’ll close our current look at the Boston meeting with word of two planets that will be falling into their star in short order (at least as astronomers measure time). Kepler-56b and Kepler-56c have a predicted era of death, some 130 million and 155 million years from now respectively. I can’t think of any other exoplanets about which we’ve been able to make such statements, making this a system worth watching as we ponder our own Sun’s future.
Just as the Sun will one day enlarge to red giant status, threatening the inner planets, so Kepler-56 is growing, already reaching four times the size of the Sun. The star has a long way to go as it continues its outward expansion, and the two planets in question are in a perilous position, with Kepler-56b orbiting the host star every 10.5 days, and Kepler-56c every 21.4. Gongjie Li (Harvard-Smithsonian Center for Astrophysics) and team have worked the number’s on the star’s evolution, leading to this prediction about the inner worlds’ demise.
You may recall this system as the first multi-planet system to be found with a tilt, as the orbits of the two inner planets are tipped from the star’s equator. Li told her session at the AAS conference that while the inner planets will be gradually destroyed by the star, heated enough to boil off any atmospheres and stretched by stellar tides along the way, a third known planet in this system, Kepler-56d, will be far enough out to survive. The latter is a gas giant in a 3.3 year orbit, and the researchers now believe that its orbit is tilted relative to the star as well. Such systems demand explanation since we would expect that planets formed from the same gas and dust disk as their host star would orbit in the plane of the star’s equator, as we see in our own system.
Image: Graphical sketch of the Kepler-56 system. The line of sight from Earth is illustrated by the dashed line, and dotted lines show the orbits of three detected companions in the system. The solid arrow marks the rotation axis of the host star, and the thin solid line marks the host star equator. Credit: NASA GSFC/Ames/D Huber.
Li pointed out in Boston that finding the inclination of the outer companion was important in helping us learn about the processes that produced the planetary misalignments. Last October I wrote about Daniel Huber’s work at NASA Ames involving the misalignment of the Kepler-56 inner planets with the host star. We do know of systems where ‘hot Jupiters’ are found that are not aligned with the equatorial plane of their hosts. Is this the result of planetary migration and chaotic close encounters with other planets in the system? If that is the case, then multi-planet systems without hot Jupiters should not show misalignment with the host star.
But we have no hot Jupiters around Kepler-56. Huber’s team used radial velocity measurements from the HIRES instrument at the Keck 10-meter telescope to study the system, seeing the signature of a third planet in a wide orbit (Kepler-56d) that was likewise misaligned from the equatorial plane of the star. It’s that outer planet that could in Huber’s view, have torqued the orbits of the inner planets out of the star’s equatorial plane. From the paper:
The inner planetary orbits would stay aligned with one another because of strong coupling between their orbits, resulting in a misalignment of the two co-planar transiting planets with the host star. Dynamical simulations that include a third companion in an eccentric orbit inclined to the equatorial plane of the host star confirm that such a mechanism can reproduce the architecture of the Kepler-56 system.
At 3000 light years from the Earth, Kepler-56 turns out to be a fascinating system. Huber found the rotation axis of the star tilted about 45 degrees to our line of sight at the same time that we were seeing the two transiting inner planets (Kepler-56d was detected solely by radial velocity methods). Both Li’s and Huber’s earlier work point to misalignment like this not being caused by hot Jupiters, and leave open for study how Kepler-56’s tilting mechanism could be at play around other stars. The important work ahead is to further constrain the inclination of the outer planet, Kepler-56d.
The Huber paper is “Stellar Spin-Orbit Misalignment in a Multiplanet System,” Science Vol. 342, No. 6156 (18 October 2013), pp. 331-334 (abstract).
by Paul Gilster | Jun 5, 2014 | Exoplanetary Science |
On Tuesday I mentioned the work of Lars A. Buchhave, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), in connection with the Kepler-10c discovery. The latter is the so-called ‘mega-Earth’ now found to be seventeen times as massive as our own planet, with a diameter of about 29,000 kilometers. A larger population of solid planets with masses above 10 times that of Earth was suggested in the Kepler-10c paper (see Introducing the ‘Mega-Earth’ for more on this), with reference to Buchhave’s ongoing work.
Let’s take a closer look at what Buchhave is doing, because the intriguing fact is that planets four times the size of Earth and smaller comprise about three-quarters of the planets found by the Kepler mission. How large a role does the metallicity of the host star play in planet formation?
At the ongoing meeting of the American Astronomical Society in Boston, Buchhave explained his research methods, which involve measuring ‘metals’ — in astronomical parlance the elements heavier than hydrogen and helium — in stars hosting exoplanets. His team analyzed more than 2000 high-resolution spectra of Kepler Objects of Interest, yielding the metallicities and other parameters of 405 stars orbited by 600 exoplanet candidates.
A statistical examination of the results followed, with this outcome: Planet-hosting stars fall into three groups that can be defined by their compositions. Buchhave’s team found two dividing lines, one at 1.7 times Earth’s radius, the other at 3.9 times the radius of Earth. The inference is that planets smaller than 1.7 Earth radius are completely rocky, while those above 3.9 Earth radius are most likely gas giants. [Addendum: See kzb’s comment below re a mistake I made in the initial version of this post.]
That interesting region between 1.7 and 3.9 times the size of Earth is where we find the so-called ‘gas dwarfs,’ planets whose cores accreted gas from the protoplanetary disk but failed to grow into gas giants of Jupiter-class or larger. Says Buchhave:
“It seems that there is a ‘sweet spot’ of metallicity to get Earth-size planets, and it’s about the same as the Sun. That makes sense because at lower metallicities you have fewer of the building blocks for planets, and at higher metallicities you tend to make gas giants instead.”
From the paper:
…the observed peak in the metallicity–radius plane at 1.7R? suggests that the final
mass and composition of a small exoplanet is controlled by the amount of solid material available in the protoplanetary disk. A higher-metallicity environment promotes a more rapid and effective accretion process, thereby allowing the cores to amass a gaseous envelope before dissipation of the gas. In contrast, lower-metallicity environments may result in the assembly of rocky cores of several Earth masses on timescales greater than that inferred for gas dispersal in protoplanetary disks (<10 Myr), yielding cores without gaseous hydrogen–helium atmospheres.
To produce small, terrestrial worlds, then, stars with metallicities similar to the Sun are favored, while stars with gas dwarfs are likely to be those that are slightly more metal-rich. The stars most likely to produce gas giants contain the most metals, generally fifty percent more than the Sun. The finding is intriguing but comes with the caveat that Kepler is best at finding planets relatively close to their star, and the metallicity thesis needs to be tested over a wider range of orbits.
Image: Three kinds of planetary outcome are suggested by the metallicity of the host star, according to new work by Lars A. Buchhave and team. Credit: David Aguilar, CfA.
Whether or not Buchhave’s work can explain a Kepler-10c will depend upon gathering a larger sample of such worlds to work with. But so far his analysis implies that there is no evident cutoff in size for rocky worlds, the data showing that the mass and radius indicating the transition from rocky to gaseous planets should increase with orbital period:
Although additional data are required to confirm this relationship, the fit is apparently consistent with a critical core mass that increases with orbital period and an atmospheric fraction of 5%… If correct, this predicts the existence of more massive rocky exoplanets at longer orbital periods.
Thus the farther a planet is from its star, the larger it can grow before the accretion of a thick atmosphere turns it into a gas dwarf. When we have the observational tools to examine a wide range of planets in outer-system orbits, we may be able to confirm or refute Buchhave’s suggestion that truly massive ‘super-Earths’ like Kepler-10c may not be uncommon.
The paper is Buchhave et al., “Three regimes of extrasolar planets inferred from host star metallicities,” Nature 509 (29 May 2014), pp. 593-595 (abstract / preprint).
by Paul Gilster | Jun 4, 2014 | Exoplanetary Science |
The red dwarf known as Kapteyn’s Star — the name comes from the 19th Century Dutch astronomer Jacobus Kapteyn — is about thirteen light years from Earth in the southern constellation of Pictor, close enough that a small telescope can pick it out. Kapteyn’s efforts at cataloguing the star in 1898 revealed that it had the highest proper motion of any star then known, a position it lost with the discovery of Barnard’s Star in 1916. Because I find stellar encounters interesting, I’ll note that about 11,000 years ago, Kapteyn’s Star would have come within seven light years of the Sun. It has been moving away from us ever since.
Kapteyn’s Star belongs to the galactic halo, a dispersed population of stars on elliptical orbits that wraps around the galactic disk and bulge. It is the nearest known halo star to the Sun. We now know, through the efforts of an international research team, that at least two planets are found here. Kapteyn c is a ‘super-Earth’ with an orbit of 121 days, while Kapteyn b comes in at about five Earth masses and orbits the star every 48 days, potentially making it warm enough for liquid water to exist on the surface. Bear in mind that this is the 25th nearest star to the Sun, a star with a history that evidently includes birth in an original dwarf galaxy now known as Omega Centauri, which was long ago absorbed and disrupted by its encounter with the Milky Way.
Image: Kapteyn’s Star and its planets likely come from a dwarf galaxy now merged with the Milky way. The bottom right panel shows characteristic streams of stars resulting from such a galactic merging event. Credit: Victor Robles, James Bullock, and Miguel Rocha at University of California Irvine and Joel Primack at University of California Santa Cruz.
The paper explains this intriguing background and estimates the star’s age:
The age of Kapteyn’s star can be inferred from its membership to the Galactic halo and peculiar element abundances (Kotoneva et al. 2005). The current hierarchical Milky Way formation scenario suggests that streams of halo stars were originated as tidal debris from satellite dwarf galaxies being engulfed by the early Milky Way (Klement 2010, and references therein). This scenario is supported by the age estimations of the stars in the inner halo (~10-12 Gyr, Jofré & Weiss 2011), and globular clusters. The work by Eggen and collaborators… established the existence of such high-velocity, metal-poor moving groups in the solar neighborhood. Kapteyn’s star is the prototype member of one of these groups…
So we’re dealing with an ancient star with an age not likely to be less than 10 billion years, and perhaps much older (the obvious upper cut-off being 13.7 billion years). The paper speculates that detecting two super-Earths around Kapteyn’s Star is consistent with the idea that low-metallicity stars tend to produce low-mass planets rather than gas giants, an idea supported by the small number of low mass planets found in radial velocity searches of metal-rich stars.
It’s also interesting to consider, as the paper does, that at the likely age of the Kapteyn’s Star system, most G and K dwarfs would be evolving away from the main sequence into giants, which would make detecting planets around them through Doppler methods all but impossible because of the high level of solar activity. The result: If we want to learn about the oldest planetary systems in the galaxy, we’ll need to turn to low-mass stars like this one, which remain on the main sequence and will stay there for a long time to come.
If you’re a science fiction fan, you’ll want to check out this news release from the University of London, which interweaves within the text a short story by Alastair Reynolds called ‘Sad Kapteyn.’ The tale describes in just a few paragraphs the arrival of a robotic interstellar probe at the Kapteyn’s Star planetary system. Reynolds is always worth reading (the Revelation Space novels in particular), and I quote him here as he recounts the AI probe’s discovery of a long-dead civilization on Kapteyn b:
“Continent-sized craters mar Kapteyn b, and I wonder if they speak of some truly awesome catastrophe – a cosmic accident, or something worse? Whatever the case, the builders of these cities are long gone. Perhaps they were dead even before Kapteyn’s star was snatched from the clutches of its mother galaxy.
“At the risk of inferring too much from too little data, I can’t help indulging in a little speculation. I too was the product of a technological civilisation, with the capability to transform a planet, to colonise other moons and worlds, to build daunting structures. The people of Kapteyn b were clearly more advanced than you, my own builders – but given time, you too could have transformed a world in this manner.
“Something to think about, isn’t it?”
The paper is Anglada-Escudé et al., “Two planets around Kapteyn’s star: a cold and a temperate super-Earth orbiting the nearest halo red-dwarf,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Jun 3, 2014 | Exoplanetary Science |
Building public interest in deep space is a long-term goal for most of us in the interstellar community, and the release of the film Interstellar this fall may set off a new round of discussion among reviewers and movie fans alike. Also helpful is the DVD release of the Neil deGrasse Tyson Cosmos series, given Tyson’s performance and the stunning visuals that communicate the majesty and power of the universe around us.
But I think it’s encouraging that while these blockbuster media releases work their magic, what used to be staid scientific conferences frequented only by specialists are turning into media events of their own. The American Astronomical Society is currently meeting in Boston, with exoplanet papers that I’m already seeing discussed well outside the usual venues. The more we see the excitement and sheer scope of the exoplanet hunt communicated to the public, the more likely we’re building the kind of interest among young people that may one day turn into scientific careers and — dare I say it — funding for missions that will fly beyond our system’s edge.
Maybe Dimitar Sasselov (Harvard-Smithsonian Center for Astrophysics) had a bit of media buzz in mind when he described a rocky world known as Kepler-10c as “the Godzilla of Earths.” The finding is exciting enough that he can, in any case, be forgiven the hyperbole. Kepler-10c is located about 560 light years from us in the constellation Draco, orbiting its host star every 45 days. A second world in this system, the star-hugging Kepler-10b, was the first rocky planet detected by Kepler and confirmed by radial velocity follow-ups from Earth. With a diameter of about 29,000 kilometers, Kepler-10c looked to be another ‘mini-Neptune,’ a world with a thick atmosphere covering a much smaller rocky core.
The surprise came when a research team led by Sasselov’s colleague Xavier Dumusque at the CfA went to work with the HARPS-North instrument on the Telescopio Nazionale Galileo (Canary Islands) to measure the mass of the planet. The result: Kepler-10c weighs fully 17 times as much as the Earth, demonstrating it is made of rocks and other solids. A rocky planet on this scale is beyond the designation ‘super-Earth.’ This CfA news release calls it a ‘mega-Earth,’ a world that we would have expected to have become the core of a Neptune- or Jupiter-class gas giant.
Image: The newly discovered “mega-Earth” Kepler-10c dominates the foreground in this artist’s conception. Its sibling, the lava world Kepler-10b, is in the background. Both orbit a sunlike star. Kepler-10c has a diameter of about 18,000 miles, 2.3 times as large as Earth, and weighs 17 times as much. Therefore it is all solids, although it may possess a thin atmosphere shown here as wispy clouds. Credit: David A. Aguilar (CfA).
The paper on this work notes that “Kepler-10c might be the ?rst ?rm example of a population of solid planets with masses above 10 M?.” And it goes on to discuss the work of Lars A. Buchhave, a CfA astronomer who likewise presented at the AAS meeting in Boston, whose work supports the idea that orbital period correlates with the size at which a planet makes the transition from rocky to gaseous:
If this model is correct, it implies that the core mass limit for gas accretion should increase with orbital period. A recent study from Buchhave et al. (2014, in press.), analyzing hundreds of Kepler candidates, seems to agree with this theoretical prediction. With a period of 45.29 days, a radius of 2.35 R?, and a density higher than Earth, Kepler-10c would be at the limit of the transition from terrestrial to gaseous planets observed by Buchhave et al. (2014, in press.). Kepler-10c might be the ?rst object con?rming that longer period terrestrial planets can be more massive than ones with shorter periods.
The implication is that Kepler-10c is not likely to be the last ‘mega-Earth’ we find. And this is interesting:
We note that Kepler-131b (Porb = 16 days, 16.1 ± 3.5 M?, 2.4 ± 0.2 R?, Marcy et al. 2014) lies in the same location of the mass-radius diagram as Kepler-10c. However the mass determination of Kepler-131b is not as robust as for Kepler-10c and more data are needed to con?rm the high density of this planet. Measuring precisely the mass of several other long-period Kepler candidates orbiting bright stars could test this speculation. This experiment has just been started as a new observational program on HARPS-N.
Observational errors dominated by photon noise are a problem with a star as faint as Kepler-10, and the paper adds that future transit searches focusing on bright stars will allow the high-quality radial velocity findings needed to measure planetary masses to greater precision. The TESS (Transiting Exoplanet Survey Satellite) mission should provide numerous candidates, as should the ESA’s PLATO mission (PLAnetary Transits and Oscillations of stars).
Meanwhile, we may be learning something new about the early universe, for the Kepler-10 system is some 11 billion years old. Huge, rocky planets could evidently form despite the fact that heavy elements were scarce in this era. Says Sasselov: “Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life.” Even stars as old as this one, then, shouldn’t be ruled out when we look for astrobiology candidates, a finding that expands the parameters of our search for life elsewhere in the universe.
The paper is Dumusque et al., “The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet,” accepted for publication in The Astrophysical Journal (preprint).
by Paul Gilster | Jun 2, 2014 | Culture and Society |
In our email conversations leading up to my publishing Human Universals and Cultural Evolution on Interstellar Voyages, Cameron Smith confirmed that there were few anthropologists engaged in studying long-term spaceflight. The same can be said for historians and sociologists, although we do have some prominent names devoting themselves to changing that. Kathleen Toerpe is doing splendid work with the Astrosociology Research Institute (I’m hoping for a new report from her soon in these pages), while Interstellar Migration and the Human Experience (1985) made a determinedly multidisciplinary effort to study our place in the cosmos.
The latter book was actually the proceedings of the Conference on Interstellar Migration held in Los Alamos in 1983, and it remains a storehouse of insights into spaceflight’s effect on humanity. Presenters at the 100 Year Starship symposia have thus far been multidisciplinary as well, with representation from biologists, philosophers, writers and psychologists as well as scientists working the ‘hard science’ fields of propulsion and aerospace engineering. Of course, the place where starflight is most vividly presented from all angles of human experience is in science fiction, which over the years has explored many of the issues raised by, for example, putting a human crew into a multi-generational starship and shooting for Alpha Centauri.
Reading Cameron Smith’s essay, though, had me thinking all weekend about some of the fictional situations great writers have gotten their interstellar crews into. A trip that takes a thousand years inspires thoughts of generations that eventually forget their mission and may not realize they are on a starship. This is familiar turf, worked by Brian Aldiss in his novel Non-Stop (1958), and of course famously portrayed by Robert Heinlein in Orphans of the Sky, which grew out of two novellas in Astounding Science Fiction. Ever the bibliographer, I must mention both: “Universe,” in ASF May, 1941, and the sequel “Common Sense,” which ran in October of the same year.
But if crews that forget where they are is a classic trope of science fiction, somewhat less so is the different kind of ignorance portrayed by J. G. Ballard’s crew in “Thirteen to Centaurus,” a story mentioned by Centauri Dreams commenter Alex Tolley over the weekend. Here the issue is how human crews react to long-haul spaceflight, and we learn early on in the story that the narrative being told by the authoritative ship’s doctor is not exactly what it seems. Here he’s speaking with a young man who has come to believe that his ‘world’ may be a ship:
“You’d more or less guessed before I told you. Unconsciously, you’ve known all about it for several years. A few minutes from now I’m going to remove some of the conditioning blocks, and when you wake up in a couple of hours you’ll understand everything. You’ll know then that in fact the Station is a spaceship, flying from our home planet, Earth, where our grandfathers were born, to another planet millions of miles away, in a distant orbiting system. Our grandfathers always lived on Earth, and we are the first people ever to undertake such a journey. You can be proud that you’re here. Your grandfather, who volunteered to come, was a great man, and we’ve got to do everything to make sure that the Station keeps running.”
Abel nodded quickly. “When do we get there — the planet we’re flying to?”
Dr. Francis looked down at his hands, his face growing somber. “We’ll never get there, Abel. The journey takes too long. This is a multi-generation space vehicle, only our children will land and they’ll be old by the time they do. But don’t worry, you’ll go on thinking of the Station as your only home, and that’s deliberate, so that you and your children will be happy here.”
This all seems familiar for a time, but of course this is J. G. Ballard, in whose hands the tools of science fiction take evocative new form. Normally I would worry about spoiler alerts, but this is a short piece and the reader learns early on that things are not what they seem, so I feel comfortable saying that Dr. Francis is himself lying, that the ship is a long-term Earthbound experiment, and that its crew is being used to study the problems humans will have when we really do send ships to another star. The boy Abel is smart enough to start seeing through even this subterfuge, and our friend Dr. Francis learns he has an agonizing decision to make.
Published in the April, 1962 issue of Amazing Stories — then under the leadership of Cele Goldsmith, whom I consider one of science fiction’s great editors — “Thirteen to Centaurus” was also dramatized by the BBC in 1965 as part of its ‘Out of the Unknown’ series. If you’re not a collector of old magazines, you can track it down in 1963’s collection Passport to Eternity, or more easily still in 2010’s The Complete Stories of J. G. Ballard.
It would be interesting to know how much of an influence the Ballard tale played on Bruce Sterling, whose story “Taklamakan” won a 1999 Hugo Award. Here simulation again comes into play, this time in a remote cave under the Taklamakan desert. There, within three enormous structures each the size of a high-rise building, what one of the characters calls ‘a dry-run starship experiment’ is ongoing, an entire, isolated community having been chosen for the task, which has aspects of both ethnic cleansing and scientific study. The characters ‘Spider Pete’ and Katrinko talk over the ramifications:
“You suppose these guys really believe they’re inside a real starship?”
“I guess that depends on how much they learned from the guys who broke out of here with the picks and the ropes.”
Katrinko thought about it. “You know what’s truly pathetic? The shabby illusion of all this. Some spook mandarin’s crazy notion that ethnic separatists could be squeezed down tight, and spat out like watermelon seeds into interstellar space. . . . Man, what a come-on, what an enticement, what an empty promise!”
“I could sell that idea,” Pete said thoughtfully. “You know how far away the stars really are, kid? About four hundred years away, that’s how far. You seriously want to get human beings to travel to another star, you gotta put human beings inside of a sealed can for four hundred solid years. But what are people supposed to do in there, all that time? The only thing they can do is quietly run a farm. Because that’s what a starship is. It’s a desert oasis.”
And maybe you’ll remember one last story that traded off isolation and experimentation on human crews. The very first episode of Rod Serling’s series The Twilight Zone was “Where Is Everybody?” First broadcast in October of 1959, the story, written by Serling, stars Earl Holliman as a man who finds himself in a town where no one seems to exist, even though evidence of recent occupation is everywhere. We learn at the end that the character Holliman plays, Mike Ferris, is an astronaut in training. He’s been inside an isolation room for almost 500 hours during a simulated trip to the moon. Can the human mind handle deep space in tiny spacecraft?
The message of the show is ‘yes,’ as Ferris looks out and sees the Moon when he is being taken on a stretcher to the hospital. His hallucinations haven’t altered his determination. He yells into the sky:
“Hey! Don’t go away up there! Next time it won’t be a dream or a nightmare. Next time it’ll be for real. So don’t go away. We’ll be up there in a little while.”
