Centauri Dreams
Imagining and Planning Interstellar Exploration
AAS: 8 New Planets in Habitable Zone
One way to confirm the existence of a transiting planet is to run a radial velocity check to see if it shows up there as a gravitationally induced ‘wobble’ in the host star. But in many cases, the parent stars are too far away to allow accurate measurements of the planet’s mass. What Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) did in the case of eight new candidates possibly in their stars’ habitable zones was to use BLENDER, a software program he and Francois Fressin developed that runs at NASA Ames on the Pleiades supercomputer.
A BLENDER analysis can determine whether candidates are statistically likely to be planets. Torres and Fressin have applied it before in the case of small worlds like Kepler 20e and Kepler 20f, important finds because both were exoplanets near the size of the Earth. Using the software allowed the researchers to create a range of false-positive scenarios to see which could reproduce the observed signal. A nearby binary star system, for example, could cause a dimming of the star’s light that might be mistaken for a planet. The Pleiades supercomputer allowed the team to work through almost a billion different scenarios, which in the case of Kepler 20e showed that it was 3,400 times more likely to be a planet than a false positive.
Applying the same techniques to the eight new planet candidates, Torres and team went on to spend a year doing follow-up work in adaptive optics imaging, high-resolution spectroscopy and speckle interferometry to characterize the new systems. We learn from all this that all eight of these worlds meet the team’s standards for verifiability. All orbit at a distance where liquid water could occur on their surfaces, while two are, as researchers told a meeting of the American Astronomical Society today, more similar to Earth than any exoplanets we’ve yet found.
The similarity in question refers to the size and composition of the two planets rather than other broad characteristics like the star they orbit. Unlike our G-class star, the primary star for Kepler-438b is a red dwarf, while Kepler-442b orbits a K-class star. Kepler-438b receives about 40 percent more light than Earth (Venus receives twice the solar flux of Earth), while Kepler 442b gets about two-thirds the light of Earth. The team gives the latter a 97 percent chance of being in the habitable zone, while the former’s chances are calculated at 70 percent.
Image: This artist’s conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning “planetary nebula.” Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. Credit: David A. Aguilar (CfA).
Kepler-438b, 470 light years from Earth, is in a 35-day orbit, while Kepler 442b (1100 light years away) completes an orbit around its star every 112 days. Four of the eight newly found planets are in multiple-star systems, although in each case, according to this CfA news release, the companion stars are far enough away not to exert a significant influence on the observed planets.
A key question is whether these really are rocky worlds — without a measurement of planetary mass, their composition is unknown. Torres and colleagues think that Kepler-438b, with a diameter about 12 percent larger than Earth, has a 70 percent chance of being rocky. Kepler 442b is about a third larger than Earth, but by the team’s reckoning has a 60 percent chance of being rocky. So these are intriguing possibilities, but it has to be said that habitability remains no more than an inference. “We don’t know for sure whether any of the planets in our sample are truly habitable,” says second author David Kipping (CfA). “All we can say is that they’re promising candidates.”
Oceans on a Larger ‘Earth’
We often think about how thin Earth’s atmosphere is, imagining our planet as an apple, with the atmosphere no thicker than the skin of the fruit. That vast blue sky can seem all but infinite, but the great bulk of it is within sixteen kilometers of the surface, always thinning as we climb toward space. Now a presentation by graduate student Laura Schaefer (Harvard-Smithsonian Center for Astrophysics) at the 225th meeting of the American Astronomical Society in Seattle points out that, like the atmosphere, water is also a tiny fraction of what makes up our planet.
A small enough fraction, in fact, that although water does cover seventy percent of the Earth’s surface, it makes up only about a tenth of one percent of the overall bulk of a world that is predominantly rock and iron. Dimitar Sasselov (CfA), co-author of the paper on this work, thinks of Earth’s oceans as a film as thin as fog on a bathroom mirror. But we’ve seen recently that water isn’t strictly a surface phenomenon. The Earth’s mantle, in fact, holds several oceans of water pulled underground by plate tectonics and subduction of the ocean seafloor.
What Schaefer presented at the AAS is a report on her computer simulations of the planet-wide recycling that keeps Earth’s oceans from disappearing. Volcanic outgassing from the mantle, primarily at the mid-ocean ridges, keeps water returning to the surface even as subduction returns water to the mantle. The cycle maintains the oceans over aeons. The question for the researchers was whether similar cycles occur on super-Earths, and how long it would take an ocean to form after the cooling of a planet’s crust during its formation period.
The results are encouraging for those hoping to find stable oceans on super-Earths. Planets two to four times Earth’s mass turn out to be better at maintaining their oceans than Earth itself. Super-Earth oceans can persist for ten billion years unless destroyed by a red giant primary star as it nears the end of its life. The largest planet in these simulations — five times Earth’s mass — took a billion years to develop its ocean in the first place, however, the result of a thicker crust and lithosphere and the resultant delay in volcanic outgassing.
Image: This artist’s depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years. Credit: David A. Aguilar (CfA).
We have nothing to compare the timeframe of life’s development on Earth with, having no data on life elsewhere. But if we took our model as the norm, says this CfA news release, we would be wise to look for life on older super-Earths, those perhaps a billion years older than the Earth, given the lag time in getting those oceans into play. Sasselov notes:
“It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet’s atmosphere. So, it takes time for life to become detectable.”
My own guess is that once we do develop the ability to study exoplanet atmospheres on the level of Earth-sized worlds, we’ll run into surprises on this front as well, depending on how typical the experience of getting life started on Earth really was. In any case, screening for older planets as the best targets for complex life seems like a rational procedure, but especially with super-Earths for whom surface water may be a slow-developing resource.
The paper is Schaefer and Sasselov, “Persistence of oceans on Earth-like planets,” American Astronomical Society, AAS Meeting #225, #406.04 (abstract).
Stars Passing Close to the Sun
Every time I mention stellar distances I’m forced to remind myself that the cosmos is anything but static. Barnard’s Star, for instance, is roughly six light years away, a red dwarf that was the target of the original Daedalus starship designers back in the 1970s. But that distance is changing. If we were a species with a longer lifetime, we could wait about eight thousand years, at which time Barnard’s Star would close to less than four light years. No star shows a larger proper motion relative to the Solar System than this one, which is approaching at about 140 kilometers per second.
The Alpha Centauri stars are the touchstone for close mission targets, but here again we could make our journey shorter with a little patience. In 28,000 years, having moved into the constellation Hydra, these stars will have closed to less than 3 light years from the Sun. Some time back, Erik Anderson discussed star motion in his highly readable Vistas of Many Worlds (Ashland Astronomy Studio, 2012), where I learned that the star Gliese 710, currently 64 light years out in the constellation Serpens, is headed squarely in our direction. Wait around for 1.3 million years or so and Gl 710 will push right through the Oort Cloud, with who knows what results in the inner system. A new paper considers these matters and tunes up the numbers on stellar encounters.
Image: Could a passing star dislodge comets from otherwise stable orbits so that they enter the inner system? Credit: NASA/JPL-Caltech).
A close pass from a star is bound to cause effects elsewhere in the Solar System, as Coryn Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg) notes in his latest paper. Such an encounter can disrupt cometary orbits in the Oort, sending them into the inner system. Earth’s catalog of impact craters, which contains almost 200 known craters and doubtless should include many awaiting discovery, some of them beneath the oceans, is a reminder of what can happen. Nor should we forget that if we really drew the wild card, a close star turning supernova could have disastrous effects on surface life. So how many stars are problematic?
Bailer-Jones identifies the key candidates in this paper, assuming an Oort Cloud that extends to about 0.5 parsecs (1.6 light years), but he notes that a star passing even as close as several parsecs could produce significant cometary disruptions if the star were massive and slow enough. The author worked with 50,000 stars from the Hipparcos astrometric catalog in hopes of fine-tuning earlier studies of passing stars, but he notes that the search can’t be considered complete because radial velocities are not available for all stars and many are fainter than the Hipparcos work could detect. Further analysis will be needed using upcoming Gaia data.
But studying stars within a few tens of light years from the Solar System, Bailer-Jones finds forty that at some point were or will be within 6.4 light years of the Sun — the timeframe here extends from 20 million years in the past to 20 million years in the future. Fourteen stars, in fact, come within 3 light years of the Sun, with the closest encounter being with HIP 85605, which is currently about 16 light years away in the constellation of Hercules. The paper cites “…a 90% probability of [the star] coming between 0.04 and 0.20 pc” somewhere between 240,000 and 470,000 years from now, but Bailer Jones notes that this encounter has to be treated with caution because the astrometry may be incorrect. Future Gaia data should resolve this.
If HIP 85605 were to close to 0.04 parsecs of the Sun, it would be .13 light years out, or roughly 8200 AU, a close pass indeed. But one thing to keep in mind: Oort Cloud perturbation is not an unusual phenomenon, and the situation we are dealing with today is partially the result of encounters with stars that have occurred in the past. We have no data on the time between stellar encounters like these and the subsequent entry of comets into the inner system, making it all but impossible to link a specific passing star with a rise in the rate of Earth impacts. Bailer-Jones discusses all this on his website at the MPIA, where he notes the following:
A close encountering star is likely to perturb the Oort cloud sufficiently to increase the flux of comets entering the inner solar system. Let’s not forget, however, that this kind of perturbation is happening all the time due to the gravitational effect of the Galaxy as whole, and due to stars which [were] encountered even earlier. That is, there is a “background” of comets entering the inner solar system which we cannot necessarily associate with a particular stellar encounter. This is also because the time between an encounter and the time that comets enter the inner solar system could be many or even many tens of millions of years, much longer that than the typical time between close encounters.
Gl 710 is generally cited as the star making the closest encounter in previous studies, and Bailer-Jones sees a 90 percent probability that it passes within 0.10 to 0.44 parsecs, meaning an Oort Cloud passage in 1.3 million years. Looking into the past, the star gamma Microscopii, a G6 giant, encountered the Sun 3.8 million years ago, probably the most massive encounter within one parsec or less. Some encounters are recent: Tiny Van Maanen’s star, a white dwarf, passed near our Sun as recently as 15,000 years ago. While data from the Gaia mission will help us improve the parameters of this catalog of passing stars, Bailer-Jones believes the Gaia results will also make it possible to investigate the link between stellar encounters and impacts in a broad, statistical sense, helping us better understand the history of Earth impacts.
The paper is Bailer-Jones, “Close Encounters of the Stellar Kind,” accepted at Astronomy & Astrophysics (preprint).
Happy New Year from Centauri Dreams
And for those of you who’ve been asking about the videos of presentations at the Tennessee Valley Interstellar Workshop, they’re now online. 2015, with New Horizons at Pluto/Charon and Dawn at Ceres, is shaping up to be an extraordinary year. Here’s to the continuing effort to advance the human and robotic effort in deep space.
Dawn: Beginning Approach to Ceres
Speaking of spacecraft that do remarkable things, as we did yesterday in looking at the ingenious methods being used to lengthen the Messenger mission, I might also mention what is happening with Dawn. When the probe enters orbit around Ceres — now considered a ‘dwarf planet’ rather than an asteroid — in 2015, it will mark the first time the same spacecraft has ever orbited two targets in the Solar System. Dawn’s Vesta visit lasted for 14 months in 2011-2012.
We have the supple ion propulsion system of Dawn to thank for the dual nature of the mission. In the Dawn version of the technology, xenon gas is bombarded by an electron beam. The resulting xenon ions are accelerated through charged metal grids out of the thruster. JPL’s Marc Rayman, chief engineer and mission director for the mission, explained thruster design in one of the earliest of his Dawn Journal entries:
Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second… Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion’s extraordinary efficacy.
Slow but steady wins the race. For the same amount of propellant, a craft equipped with an ion propulsion system can achieve ten times the speed of a probe boosted by today’s conventional rocketry, says Rayman, but on the other hand, an ion-powered spacecraft can manage to carry far less propellant to accomplish the same job, which is how missions like Dawn can be executed. It’s also true that one of Dawn’s thrusters pushes on the spacecraft with about the force of a piece of paper pushing on a human hand on Earth. Dawn isn’t exactly the spacecraft equivalent of a Ferrari — at full power, the vehicle would go from 0 to 60 miles per hour in a stately four days.
Fortunately, space is a zero-g environment without friction, so the minuscule thrust has a chance to build up. ‘Acceleration with patience’ is Rayman’s term. In addition to enhancing maneuverability, ion thrusters are also durable. Dawn’s three thrusters have completed five years of accumulated thrust time, more than any other spacecraft. If all goes well at Ceres, we can’t rule out an extended mission that might include other asteroid targets, just as we hope for a Kuiper Belt object encounter for New Horizons after its 2015 flyby of Pluto/Charon.
Image: An artist’s concept shows NASA’s Dawn spacecraft heading toward the dwarf planet Ceres. Dawn spent nearly 14 months orbiting Vesta, the second most massive object in the main asteroid belt between Mars and Jupiter, from 2011 to 2012. It is heading towards Ceres, the largest member of the asteroid belt. When Dawn arrives, it will be the first spacecraft to go into orbit around two destinations in our Solar System beyond Earth. Credit: NASA/JPL-Caltech.
Keep an eye on Rayman’s Dawn Journal as the Ceres encounter approaches. His latest entry goes through the historical background on the dwarf planet’s discovery, and includes the fact that the Dawn team has been working with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon the name given to it by its discoverer. Astronomer Giuseppe Piazzi found Ceres in 1801 and named it after the Roman goddess of agriculture. The plan going forward is for surface detail like craters to be named after gods and goddesses of agriculture and vegetation, drawing on worldwide sources of mythology.
Deep space has been yielding unexpected results since the earliest days of our exploration, and with Dawn approaching Ceres it’s instructive to recall some of the discoveries the Voyagers made as they moved into Jupiter space, starting with the surprisingly frequent volcanic activity on Io. Ceres will doubtless yield data just as intriguing, says Christopher Russell (UCLA), principal investigator for the Dawn mission:
“Ceres is almost a complete mystery to us. Ceres, unlike Vesta, has no meteorites linked to it to help reveal its secrets. All we can predict with confidence is that we will be surprised.”
Not quite twice as large as Vesta, Ceres (diameter 950 kilometers) is the largest object in the asteroid belt, and unlike Vesta, it apparently has a cooler interior, one that may even include an ocean beneath a crust of surface ice. We’ll know more soon, for Dawn has emerged from solar conjunction and is communicating with Earth controllers, who have programmed the maneuvers for the next stage of operations, which includes the Ceres approach phase. At present, the spacecraft is 640,000 kilometers from the dwarf world, approaching it at 725 kilometers per hour.
Long-Distance Spacecraft Engineering
I find few things more fascinating than remote fixes to distant spacecraft. We’ve used them surprisingly often, an outstanding case in point being the Galileo mission to Jupiter, launched in 1989. The failure of the craft’s high-gain antenna demanded that controllers maximize what they had left, using the low-gain antenna along with data compression and receiver upgrades on Earth to perform outstanding science. Galileo’s four-track tape recorder, critical for storing data for later playback, also caused problems that required study and intervention from the ground.
But as we saw yesterday, Galileo was hardly the first spacecraft to run into difficulties. The K2 mission, reviving Kepler by using sophisticated computer algorithms and photon pressure from the Sun, is a story in progress, with the discovery of super-Earth HIP 116454 b its first success. Or think all the way back to Mariner 10, launched in 1973 and afflicted with problems including flaking paint that caused its star-tracker to lose its lock on the guide star Canopus. The result: A long roll that burned hydrazine as thrusters tried to compensate for the motion. Controllers were able to use the pressure of solar photons on the spacecraft’s solar panels to create the torque necessary to counter the roll and re-acquire the necessary control.
The Messenger spacecraft also used pressure from solar photons as part of needed course adjustment on the way to Mercury, and now comes news of yet another inspired fix involving the same craft. Messenger was on course to impact Mercury’s surface by the end of March, 2015, having in the course of its four years in Mercury orbit (and six previous years enroute) used up most of its propellant. But controllers will now use pressurization gas in the spacecraft’s propulsion system to raise Messenger’s orbit enough to allow another month of operation.
The helium in question was used to pressurize the propellant tanks aboard the spacecraft. Let me quote Stewart Bushman (JHU/APL), lead propulsion engineer for the mission, on just what is going on here:
“The team continues to find inventive ways to keep MESSENGER going, all while providing an unprecedented vantage point for studying Mercury. To my knowledge this is the first time that helium pressurant has been intentionally used as a cold-gas propellant through hydrazine thrusters. These engines are not optimized to use pressurized gas as a propellant source. They have flow restrictors and orifices for hydrazine that reduce the feed pressure, hampering performance compared with actual cold-gas engines, which are little more than valves with a nozzle.”
Bushman adds that stretching propellant use is not the norm:
“Propellant, though a consumable, is usually not the limiting life factor on a spacecraft, as generally something else goes wrong first. As such, we had to become creative with what we had available. Helium, with its low atomic weight, is preferred as a pressurant because it’s light, but rarely as a cold gas propellant, because its low mass doesn’t get you much bang for your buck.”
Image: A compilation of Messenger images from Mercury in 2014. Next April, Messenger’s operational mission will come to an end, as the spacecraft depletes its fuel and impacts the surface. However, the last few months of operations should be rich, including science data obtained closer to the planet’s surface than ever previously accomplished. Credit: JHU/APL.
So we gain an extra month to add to Messenger’s already impressive data on the closest planet to the Sun. The spacecraft’s most recent studies, begun this past summer, have involved a low altitude observation campaign looking for volcanic flow fronts, small scale tectonic effects, layering in crater walls and other features explained in this JHU/APL news release. Growing out of this effort will be the highest resolution images ever obtained of Mercury’s surface.
The additional month of operations will allow a closer look at Mercury’s magnetic field. “During the additional period of operations, up to four weeks, MESSENGER will measure variations in Mercury’s internal magnetic field at shorter horizontal scales than ever before, scales comparable to the anticipated periapsis altitude between 7 km and 15 km above the planetary surface,” says APL’s Haje Korth, the instrument scientist for the Magnetometer. Korth also says that at these lower altitudes, Messenger’s Neutron Spectrometer will be able to resolve water ice deposits inside individual impact craters at the high northern latitudes of the planet.
That’s a useful outcome and it grows out of sheer ingenuity in using existing resources. What’s fascinating in all these stories is that when we send a spacecraft out, we have frozen its technology level while our own continues to expand and accelerate. Think of the Voyagers, still operational after their 1977 launches, and imagine the kind of components we would use to build them today. The trick in resolving spacecraft problems and extending their missions is to keep the interface between our latest technology and their older tools as robust as possible. That involves, it’s clear, not just hardware and software, but the power of the human imagination.
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