by Paul Gilster | Nov 6, 2013 | Exoplanetary Science |
Thinking about the Kepler results now under discussion in these pages, one thing that stands out is that for most of the Kepler planets, we have no idea of their density. A transit can tell you about the size of the planet crossing in front of its star, but following up the detection with ground-based telescopes is crucial, because radial velocity studies can put some boundaries on its mass. With both size and mass in hand, you can determine the density, and that tells us whether a detected world is rocky like the Earth or a water world or an ice giant like Neptune.
In a recent interview with Popular Mechanics, David Latham (Harvard-Smithsonian Center for Astrophysics) described TESS, the Transiting Exoplanet Survey Satellite, and its role in the planet hunt beyond Kepler. With so many space observatories either cancelled (Space Interferometry Mission) or on indefinite hold (Terrestrial Planet Finder), it’s heartening to have this mission in the pipeline. And because TESS is going to be looking for planets transiting nearer, brighter stars than those available in the Kepler field — Kepler’s stars range from 600 to 3,000 light years from Earth — its targets will be easier to follow up with ground-based telescopes, so those all important mass measurements that lead to a planet’s density can be performed.
Image: An artist’s conception of the TESS mission, which will conduct an all-sky survey looking for transiting exoplanets. Credit: MIT Kavli Institute for Astrophysics & Space Research.
In a time of strapped budgets, Latham says the 100 man-years of work that went into the Kepler processing pipeline will be converted over to TESS, with much of the code re-used. The mission also gets an obvious boost from the experience Kepler has given us in processing vast amounts of data. TESS will be looking at the entire sky, making its field of view 400 times larger than Kepler, but it will spend about a month at a time on each location, with later follow-ups possible. Having identified the best candidate stars, TESS can turn the work over to space observatories like the James Webb Space Telescope or next generation instruments on the ground for the kind of deeper investigations that may one day characterize a small exoplanet’s atmosphere.
Think of TESS, then, as a spotter mission. In a backgrounder from the Kavli Institute for Astrophysics and Space Research (MIT), George Ricker discussed TESS’s role in finding targets that are suited for follow-up through optical and infrared telescopes. Ricker is interested in multi-planet systems that can be studied when one or more of the planets transit the host star. Their gravitational interactions cause slight changes by accelerating or decelerating these worlds in the course of their orbits, with even small planets that we can’t detect affecting the transit time of larger planets in the system. TESS should help here:
Measuring transit time variations is particularly important because if you find a large Neptune- or Saturn-sized planet that appears to be tugged a little bit, you can make an estimate of how massive the planet is that is actually doing the tugging. Measuring transit time variations, which is a technique enabled by Kepler data, is a bit like successively unnesting a Russian matryoshka doll – you go a few layers down and you can find smaller and smaller planets in the system. That’s one of the things that doing transit time variations, during the TESS mission, will enable us to do.
The TESS team calculates that their observatory could detect as many as 2700 planets, including perhaps hundreds of Earth-sized worlds. Ricker notes that while the passage of the Earth in front of the Sun as viewed from outside our Solar System would dim the light of our star by about 85 parts in a million, TESS will be sensitive to drops of roughly 40 parts in a million. The observatory will be studying approximately two million stars brighter than 12th magnitude, and that includes red dwarfs relatively near the Sun, targets to which the TESS researchers will be paying particular attention. Ricker again:
What we envision is that after two years we will complete a census of all the transiting extrasolar planets in the sky that have a period of less than about a month. This will be particularly valuable for studying the small M stars that are so abundant in our galaxy. We will have a good sample of planet periods extending out to about a year, and this will cover a wide variety of solar types – from stars that are hotter than the sun to stars that are a lot colder than the sun, and for which the habitable zones range from small distances from the stars to large distances from the stars.
All of that should be enough to provide the James Webb Space Telescope with plenty of targets, and offer a solid follow-up list to Earth-based observatories. Scheduled for launch in 2017, TESS was conceived from the ground up with the philosophy of using components from the commercial semiconductor industry that can be modified to work in space, with the CCDs onboard manufactured at MIT. Using components from the commercial sector rather than developing all instruments in-house keeps the costs down, achieving a launch slot for TESS when other missions have fallen victim to the budget ax. TESS is scheduled to fly in 2017 aboard an Orbital Sciences Pegasus XL rocket released from a Lockheed L-1011 aircraft.
by Paul Gilster | Nov 5, 2013 | Exoplanetary Science |
The widely circulated Kepler results, announced yesterday, tell us that over twenty percent of Sun-like stars in the Milky Way have Earth-sized planets in the habitable zone, where liquid water could exist on the surface. Work out the math and it turns out that the nearest Sun-like star with a planet like ours in the habitable zone is probably on the order of twelve light years away, an energizing thought for those of us who ponder future technology and interstellar probes. Imagine: One in five Sun-like stars with a planet the size of Earth in the zone where liquid water can exist.
Image: Analysis of four years of precision measurements from Kepler shows that 22±8% of Sun-like stars have Earth-sized planets in the habitable zone. If these planets are as prevalent locally as they are in the Kepler field, then the distance to the nearest one is around 12 light-years.zone. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.
But how did we get here? Kepler, which was launched in 2009 to look for planets transiting their stars, examined over 150,000 stars for four years and turned up more than 3000 planet candidates. It’s been a fascinating ride, but finding ‘hot Jupiters’ and Neptune-class worlds and even intriguing super-Earths always reminded us that the primary goal was to learn what fraction of stars have Earth-sized planets at just the right temperatures for life. Ideally, retrieving data for G-class stars like the Sun would have helped us look for close twins of our planet.
Kepler’s malfunctions challenged but did not end that effort. The team involved in the present work includes Geoff Marcy and Erik Petigura (UC-Berkeley) and Andrew Howard (University of Hawaii, Manoa), who have been working with the 10-meter instruments at the Keck Observatory (Mauna Kea) to obtain data from the HIRES spectrograph, focusing on 42,000 stars in the Kepler field that are only slightly smaller and cooler than the Sun. 603 planets turned up, with 10 being between one and two Earths in diameter and orbiting in the habitable zone. Remember that only a small number of systems are oriented so that transits occur as viewed from Earth. The team’s algorithms yielded the estimate of 22 percent of all Sun-like stars with Earth-sized planets in their habitable zones, plus or minus eight percent depending on the habitable zone definition.
On the latter point, Erik Petigura defined the habitable zone for this study as that region where a planet receives between four times the light the Earth receives from the Sun and one-quarter of that amount. Kepler’s stuck reaction wheels have meant that extending the mission to analyze G-class stars like the Sun was not possible. Instead, the potentially habitable planets the team found in its survey all occur around K-class stars (Alpha Centauri B is the nearest example of a K-class star, though not in the Kepler field). The team’s analysis demonstrates that the results for K stars can be extrapolated to G-class stars, and thus we arrive at the 22 percent figure.
Telling us how common potentially habitable planets are around Sun-like stars is prime-time for Kepler, and now we have a reading that’s highly encouraging. As we look forward to missions to characterize exoplanet atmospheres and look for the signatures of life in their spectra, we can now assume that only a few dozen nearby stars will need to be observed before we detect an Earth-sized planet in the habitable zone, a fact that will play into the design of telescopes for such missions. Geoff Marcy, though, is quick to point out that just because a planet is Earth-like in size and in the habitable zone defined here, it isn’t necessarily life-bearing:
“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms. We don’t know what range of planet types and their environments are suitable for life.”
The caution is understandable, and we are a long way from being able to make the kind of observations that help us pin down the presence of life on an exoplanet. We also need to remember that without information about the mass of these planets, we can’t say anything about their density and thus can’t be sure that they are in fact rocky worlds like our own. The discovery that Kepler-78b has the same density as the Earth, announced just last week, does tell us that at least some of these planets are likely to be rocky.
In any case, the idea that there are tens of billions of potentially habitable worlds in a galaxy of 200 billion stars is exhilarating, as Andrew Howard notes:
“It’s been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then, we have learned that most stars have planets of some size orbiting them, and that Earth-size planets are relatively common in close-in orbits that are too hot for life. With this result, we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way Galaxy.”
We should also put these findings in a broader context. Red dwarf stars are not included in the study, but they represent 75 percent of the stars in the Milky Way. The question of whether life could exist around such a star, given the problems of tidal lock and stellar flare activity, is an open one, but we do know from previous work that 15 percent of these stars are expected to have Earth-sized planets in their own habitable zones (this is based on work by David Charbonneau and Courtney Dressing at the CfA; Ravi Kopparapu at Penn State obtained an even higher estimate). We don’t know yet whether life exists on any of the worlds around any of the stellar classes, but it does appear that the cosmos is stuffed with planets where the great natural experiments that lead to life can be run again and again.
The paper is Petigura et al., “Prevalence of Earth-size planets orbiting Sun-like stars,” Proceedings of the National Academy of Sciences, published online 4 November 2013 (abstract). Dennis Overbye’s report in the New York Times is well worth reading. I love this quote in Overbye’s article from Geoff Marcy: “This is the most important work I’ve ever been involved with. This is it. Are there inhabitable Earths out there? I’m feeling a little tingly.” Me too.
by Paul Gilster | Nov 4, 2013 | Deep Sky Astronomy & Telescopes |
I haven’t yet read Stephen Baxter’s new novel Proxima, but because of my admiration for his previous books, it’s at the top of my reading list. Judging from the Amazon description, Proxima gets into issues that for me make red dwarfs utterly compelling. What would a habitable planet look like around such a star, tidally locked so that its sun never moved in the sky? What would it be like to move around this world, going from a warm substellar point toward twilight and then a frigid night on the dark side?
Given that this M-class red dwarf is 18,000 times fainter than the Sun, you wouldn’t expect it to make much of an impression in photographs. The one above (credit: European Southern Observatory) is instructive because it puts the entire Alpha Centauri system in context. At top left we have Centauri A and B, which are bright enough to merge together and appear as a single bright object. At the lower right is the arrow indicating Proxima Centauri, so faint as to be barely visible. Proxima is 4.218 light years from Earth and roughly 15,000 AU from Centauri A and B.
The image below gives us a brighter look. Taken by the Hubble Space Telescope, it isolates Proxima from the other two stars, but even here the red dwarf is a point-like object whose image is distorted by diffraction spikes produced within the telescope itself.
Image: The closest known star, Proxima Centauri is here imaged by the Hubble instrument. Credit: ESA/Hubble & NASA.
Proxima should remain a main sequence star for another four trillion years, about 300 times the current age of the universe. The planet search here is inconclusive, but here’s what we’ve got so far: Work by Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), using seven years of high precision radial velocity data from the UVES spectrograph at the European Southern Observatory, can identify no planet of Neptune mass or above out to about 1 AU from the star. No super-Earths larger than 8.5 Earth masses have been detected in orbits of less than 100 days.
As for the habitable zone where liquid water could exist on the surface of a planet, it is thought to be between 0.022 and 0.054 AU, which would produce orbits between 3.6 and 13.8 days. The Proxima investigations have yet to find anything here, but the most we can say is that super-Earths of 2-3 times the mass of the Earth in circular orbits have been ruled out. That still leaves a lot of possibilities to be investigated as we refine our techniques and apply them to this intriguing star. Radial velocity capability down to one Earth mass is getting closer here.
Image: Comparative sizes of the Alpha Centauri stars, with the Sun thrown in for comparison. Credit: Wikimedia Commons.
Because Proxima is a flare star that can experience sudden changes in brightness, any life evolving on a planet around it would have to have found a way to protect itself from such radiation. Even so, the star’s nearness to the Sun has made it a staple in science fiction, from Murrray Leinster’s 1935 story “Proxima Centauri”, published in Astounding Stories through Heinlein’s Orphans of the Sky (1963), where it was the logical destination for a starship, to the new Stephen Baxter novel. I’ll have more to say about the latter in upcoming posts. A search through the archives here will pull numerous articles on Proxima, including The Proxima Centauri Planet Hunt and Proxima Centauri: Looking at the Nearest Star.
by Paul Gilster | Nov 1, 2013 | Culture and Society |
Not long ago I pulled a wonderful 1950 film out of my collection for a long-overdue viewing. I remember 711 Ocean Drive from late night television airings, and when it popped up a few years back on a local cable channel, I made a recording. Edmund O’Brien and Joanne Dru are the key players in this gritty tale about an electronics expert who gets drawn into big-time crime, and the ending, which takes place at the Hoover Dam straddling the Nevada/Arizona border, is simply terrific, with O’Brien taking the fall after his shady dealings have been exposed.
Image: On the run at the Hoover Dam in Joseph Newman’s 711 Ocean Drive.
Titanic forces, vast engineering, gunplay in the desert, all artfully directed by Joseph M. Newman — what more could you want? And now, thanks to Les Johnson, I connect Hoover Dam not only with a film noir classic but with long-term thinking and starflight. Johnson, speaking in his role as a science writer with deep connections to science fiction, told the recent Eve Online conference in Las Vegas that we as a species need to think big and think long if we are to realize our interstellar dreams. Hoover Dam was the example he used to remind us what it will take to reach the stars.
Deep Time and Future Engineering
I’m always looking for examples of long-term thinking in our history, because as a species, we actually do this pretty well. The Egyptian pyramids are an obvious example, and so are many European cathedrals, some of which were generational in their construction. Some cathedrals went up relatively quickly, to be sure — the main structure at Chartres took a mere 25 years. Others, like Lincoln or Notre-Dame in Paris, were a century or more in the making. The foundation stone at Cologne was laid in 1248 and by the time of the Reformation, the structure remained unfinished, to be completed only in 1880, having become a national project.
Johnson’s invocation of the Hoover Dam is a reminder that not all the great projects were undertaken by civilizations long gone. As a culture, we often seem to think in short time-frames, but we do have the engineering skills to do much better. This Discovery News story quotes Johnson as saying “this dam was built in the 1930s, but the design life of the dam itself (not the power systems) is 2,000 years … that’s foresight! That’s engineering planning. That’s something we shouldn’t be afraid of today when planning for our (species’) future.”
Johnson is well known in these pages not only because of his work at NASA’s Marshall Space Flight Center, but also for his non-NASA activities, which include writing and editing as well as frequent public appearances. Going Interstellar (Baen, 2012), which he edited with science fiction author Jack McDevitt, is a collection of fiction and non-fiction that belongs on your shelf if you share my passion for interstellar flight. And Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2007), which he wrote with Greg Matloff and Giovanni Vulpetti, is a wise introduction to the possibilities and the problems of building sails in space.
Methods like nuclear pulse (think Orion) and antimatter have their advantages, but all require us to carry fuel onboard, and in the case of antimatter, creating enough of that fuel — not to mention storing it — is a major problem. But Johnson can point to sail successes that tell us we’re moving into the era of space applications. The IKAROS sail put Japan into space first with a functioning sail, and NASA’s NanoSail-D deployed a smaller sail in 2011, a year after IKAROS. We now look forward to NASA’s 1200 square meter Sunjammer mission in 2015, even as the Planetary Society continues to develop its Lightsail 1. They’re a long way from interstellar applications, but these missions show us that sails are swiftly climbing the ladder of technological readiness.
Thinking long and thinking big work together. Building the kind of sail that could be laser-boosted into interstellar speeds would mean creating a sail as big as Nevada itself, and Johnson pointed out that the energy output needed for it would equal the energy output of the entire human race today. The point is that the much smaller sail experiments we run today can build toward a future where such structures become possible because of the technologies we’ll create in coming centuries. And if we continue our work with that long-term perspective in mind, we can speak of starflight in ways that do not violate known physics even if they demand huge engineering.
An Icelandic Perspective
The world of gaming seems a good place to stretch our concepts, and in the case of Eve Online, the setting is itself enormous. We’re talking about an MMORPG, which stands for Massively Multiplayer Online Role-Playing Game, and this one has a community of half a million, with a canvas that stretches across the galaxy and is stuffed with star systems ready to be explored.
Eve Online comes out of one of my favorite places, Iceland, and reminds me of travel experiences there over the past thirty years. Back in the 1970s, working on medieval linguistics in grad school, I went to Iceland under the urging of a wonderful professor named George Lane, who was himself fluent in Icelandic and had inspired my own interest in the language. Strolling through the numerous bookshops of Reykjavik, I found that modern editions of the great medieval works, from the sagas to the poetic Eddas, were readily available. The language itself retained much of the medieval structure in ways that most modern tongues do not.
And maybe it was walking around at the site called Thingvellir (Þingvellir) on a foggy late afternoon in August that started teaching me about perspective. It was here that Iceland’s general assembly, the Alþing, first met in about 930 AD, on land that is situated at the boundaries of tectonic plates, a fissure zone that runs through Iceland itself. I walked about a mile from the small hotel and looked back across the valley as a bit of Sun emerged and a rainbow arced across the landscape. It was as if a Viking past reached all the way into the present day, and that day was itself the outgrowth of physical processes that came up out of the Earth’s deep core.
We carry the past with us wherever we walk but the sense of ‘deep time’ that overcame me that day is sometimes lost in the rush to complete day-to-day tasks. I think Les is right that we need to recover it as we look toward a human destiny among the stars. “Thingvellir is a place of echoes,” said a friend when I got back to Reykjavik. She was an English writer who had travelled often there and she knew what she was talking about. We must look back, far back, and then forward into a deep future, building our bridges as if our ancestors were crossing them.
