by Paul Gilster | Jul 10, 2007 | Exoplanetary Science |
The Voyager Interstellar Mission sounds like something out of Star Trek, but it is in fact the extended mission of the doughty spacecraft that taught us so much about the outer Solar System. An extended mission can be just as valuable, and sometimes more so, than the original — think about the continuing adventures of our Mars rovers, working well beyond their projected timelines. In Voyager’s case, we’re learning much about how the Solar System behaves as it moves through the interstellar medium, and about the heliopause, where the Sun’s solar winds effectively lose their dominance over the winds from other stars.
Now the Deep Impact spacecraft, which provided such spectacular scenes of Comet Tempel 1, will acquire an extended mission of its own, and in two parts. The one that catches my eye is called Extrasolar Planet Observation and Characterization (EPOCh), which will turn the spacecraft loose on the study of several nearby bright stars already known to have gas giant planets around them. The collected data should tell us more about the planets and their atmospheres as they transit between Deep Impact and the stars they orbit. We may also learn whether some of these planets have moons or rings, with the possibility of finding smaller, hitherto undetected planets in orbit around the same stars.
At the end of 2006, Emily Lakdawalla discussed EPOCh on the Planetary Society Weblog with principal investigator Drake Deming (NASA GSFC), an e-mail conversation that yielded this description of the mission:
“EPOCh will do photometry of giant planets transiting several nearby bright stars (it’s like Kepler in that respect, but Kepler is locked-in to a particular field in Cygnus, and won’t look at nearby bright transiting systems). We exploit the fact that the Deep Impact telescope is out of focus (allows us to get better photometry). We are sensitive to terrestrial planets via their perturbations on the transit times of the giant planets (which we measure very precisely).”
Out of focus? It turns out that Deep Impact’s high-resolution camera does indeed have a flaw, but it’s one that may prove advantageous to the mission, as Deming goes on to explain:
“We are doing very precise photometry, measuring the brightnesses of the stars. As the giant planets pass in front of them (“transit” the stars) we will see the dip in the star’s light. This dip lasts for several hours, but we want to time its occurrence very precisely (to about 1 second accuracy). It’s those changes in the time of the giant planet transits that will indicate the presence of terrestrial planets. That means we have to measure the stellar brightness very accurately, so that the curve of brightness versus time is very “smooth”, i.e. has high signal-to-noise ratio, and we can find the center time accurately. But to get high signal to noise, we have to collect lots of photons from the star. That’s where the defocus helps. Each pixel of the CCD has a limited capacity to collect photons before it saturates. With a defocused image, we have about 75 pixels collecting light for us, so we can collect lots of photons in each exposure without saturating, and that gives us the high signal-to-noise ratio that we need.”
This interesting search will occur as Deep Impact proceeds toward the second goal of its extended mission, a flyby of the comet Boethin planned for December 5, 2008. Both investigations are intriguing ways to stretch existing resources, and they’re complemented by an extended mission for the Stardust spacecraft, which will revisit Comet Tempel 1. Deep Impact’s encounter with Tempel 1 was on July 4, 2005; Stardust, now flying as the New Exploration of Tempel 1 (NExT) mission, is to arrive on February 14, 2011. Encounters with Tempel 1 seem irresistibly drawn to holidays.
When you’ve got a budget as tight as NASA’s, it’s important to find ways to stretch your dollars, and according to the agency’s Alan Stern (who knows something about dollar-stretching through his work as principal investigator for the New Horizons mission), these extended missions will accomplish their work for about 15 percent of the cost of a new mission built from scratch. Thus do two seasoned spacecraft acquire new targets that had previously been unplanned, including an extrasolar study that could prove productive indeed, as EPOCh’s sensitivity should exceed that of existing ground and space-based observatories.
Addendum: Dr. Deming just responded to my note asking about the target list for EPOCh, saying that his team was still evaluating the best candidates. He also said the observations would involve three stars, all relatively nearby, and all, of course, orbited by transiting giant planets.
by Paul Gilster | Jul 9, 2007 | Exoplanetary Science |
The challenge of working with a small sample of exoplanetary systems — and one tilted toward those detectible through radial-velocity methods — is that building up solid models of planet formation is tricky. I’m thinking about this in terms of the recent planetary conference at Santorini, and also recalling work performed at the University of Texas, where Michael Endl and team have looked into the relationship between planets around red dwarfs and the metallicity of their stars.
It’s an intriguing question and one that only continuing observations can nail down. Metallicity refers to the presence of elements higher than hydrogen and helium in a star’s composition, something we can determine through spectroscopic analysis. Endl and co-author Fritz Benedict, as originally noted in this post, worked with graduate student Jacob bean on a study of three dwarfs known to have planets: Gliese 876, Gliese 436 and Gliese 581, noting their lower values of metallicity compared to stars of spectral types F, G and K (our Sun is a G-type star).
Most red dwarfs studied in our surveys thus far show low metallicity, and the number of high-mass planets found around them is small. Are higher levels of metallicity necessary for gas giants to form? It sounds perfectly logical: More dust in the protoplanetary disk should encourage planetary formation, so we should expect few gas giants around red dwarfs, as seems to fit current observation. This backs the core accretion model of planet formation, in which planets build up rocky cores as they make their way through crowded protoplanetary disks, eventually becoming massive enough to begin the process of accumulating dense atmospheres.
But every exception to the rule helps us understand the rule better, and draws the comparison between it and alternatives like the disk instability model. We already have two gas giants around Gliese 876. And now the California & Carnegie team have come up with two more gas giants, both around the star GJ 317. Steinn Sigurðsson (Pennsylvania State) noted this find at the Santorini conference. GJ 317 is an M3 red dwarf some 10 parsecs from here with about a fourth the Sun’s mass. The planetary masses are 1.2 and 0.8 Jupiter masses respectively, with orbital periods of 673 and 2700 days. “Bit of a theory buster,” muses Sigurðsson, “high mass planets in wide orbits around a low mass metal poor star…”
Maybe not so much a theory buster as a theory tweaker. But that’s how we learn things, finding the anomalies and figuring out how to account for them. The result is a sounder theory that can encompass oddball worlds where we do run into them. We need a larger sample of M dwarfs to develop broader patterns for mass and metallicity, and we need time — perhaps we’re going to begin finding more gas giants in wide orbits around M dwarfs as our data accumulate. Ultimately, we’ll use such observational data to help tune up our target list for planet-finder missions looking for terrestrial worlds around such stars.
The Extrasolar Planets Encyclopedia offers what is known about GJ 317b and GJ 317c. Note that the latter is still a work in progress as radial-velocity data accumulate. The Texas work is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68 (available online).
by Paul Gilster | Jul 7, 2007 | Exoplanetary Science |
In another decade or so, we should have space-based telescopes actively looking for life around other stars by studying the atmospheres of exoplanets. In the beginning, it will make sense to look for bio-chemistries similar to our own. This isn’t some kind of species chauvinism but simple realism. We know more about how life works on Earth than it might in far more extreme environments, so we’ll turn first to Earth analogues, seeking the bio-signatures of carbon-based metabolisms on worlds with liquid water.
But as we explore our own Solar System, the situation will continue to evolve. If life exists on Enceladus, or Ceres, or in some bizarre Kuiper Belt ecosystem, it’s not going to be operating on the same principles as life here on Earth. These aren’t Earth analogues, and moreover, they are places for which we have the possibility of lander and rover exploration within the forseeable future. We’ll want to widen our range so we don’t overlook a form of life that isn’t immediately recognizable.
A new report from the National Research Council comes to the same conclusion, underlining how important it is that we be open to life forms other than those we can extrapolate from our own environment. Thus this clip:
“…no discovery that we can make in our exploration of the solar system would have greater impact on our view of our position in the cosmos, or be more inspiring, than the discovery of an alien life form, even a primitive one. At the same time, it is clear that nothing would be more tragic in the American exploration of space than to encounter alien life without recognizing it.”
How to recognize it? One way to prepare is to continue to explore extreme environments on our own planet, where we’ve already found life in deep oceanic vents and deserts as hostile as the Atacama. How life adapts to places where resources are incredibly scarce may tell us much about how living things might have adapted to hostile conditions below, for example, the surface of Mars.
But the report goes further, stressing that we need to move beyond the idea of water as crucial for life. Could there be places on Mars more suited to life than those where water once flowed? For that matter, what about different biochemistries altogether? According to the study, liquids like ammonia might serve as bio-solvents for living things far different than any we’ve so far imagined. A world like Titan, which the report pegs as the Solar System’s most likely home for what it calls ‘weird life,’ may well have capacious mixtures of liquid water and ammonia in its interior.
What we find in the Solar System should give us an idea how flexible we can be in our conception of life’s range around other stars. It may be that our early exoplanet work will routinely find bio-signatures in the spectra of terrestrial-type worlds, keeping us so busy analyzing the results that other environments are pushed to the back burner. But my guess is we’ll eventually find puzzling data that indicates life has taken unusual directions in far more exotic places, a finding that should be at once inspiring and humbling.
The report is The Limits of Organic Life in Planetary Systems, published by National Academies Press and available here.
by Paul Gilster | Jul 6, 2007 | Outer Solar System |
We’re getting a closer look at Saturn’s moon Hyperion, the result of data analysis following Cassini’s flyby in September of 2005. Using near-infrared and ultraviolet spectroscopy, researchers have been able to analyze the moon’s surface composition, with results suggestive of water and carbon dioxide ices as well as an analysis of dark material indicating hydrocarbons. That’s a mix of materials not unlike what we’ve found in comets and probably similar to what we’ll detect in Kuiper Belt objects.
Here’s Dale Cruikshank (NASA Ames), lead author on the paper:
“Of special interest is the presence on Hyperion of hydrocarbons — combinations of carbon and hydrogen atoms that are found in comets, meteorites, and the dust in our galaxy. These molecules, when embedded in ice and exposed to ultraviolet light, form new molecules of biological significance. This doesn’t mean that we have found life, but it is a further indication that the basic chemistry needed for life is widespread in the universe.”
We already knew about frozen water on Hyperion, based on earlier ground-based observations, but Cassini also found solid carbon dioxide mixed with ordinary ice that is chemically attached to other molecules. “We think that ordinary carbon dioxide will evaporate from Saturn’s moons over long periods of time,” adds Cruikshank, “but it appears to be much more stable when it is attached to other molecules.”
Image: This is a color map of the composition of a portion of Saturn’s moon Hyperion’s surface about 75 kilometers (45 miles) on a side. In this map, blue shows the maximum exposure of frozen water, red denotes carbon dioxide ice (“dry ice”), magenta indicates regions of water plus carbon dioxide, yellow is a mix of carbon dioxide and an unidentified material. This map was made with data from the Visual and Infrared Mapping Spectrometer aboard the Cassini spacecraft during its flyby of Hyperion in September 2005. Credit: NASA/JPL/University of Arizona/Ames.
More on Hyperion’s hydrocarbons in this news release. Meanwhile, the same issue of Nature contains results from Cassini imaging and radio data collecting during the same flyby. The moon’s sponge-like appearance is attributed to extremely low density. Hyperion turns out to be about half as dense as water. Using the tiny deflection in Cassini’s orbit caused by the flyby, Italian scientists were able to estimate the moon’s mass. This combined with volume data collected by imaging provided density figures.
The porous moon tends to compress under the impact of incoming debris, but what material is ejected from such craters probably doesn’t fall back to the surface due to Hyperion’s low gravity. That gives craters there a crisp, well-defined look that sets them apart from those on denser worlds.
Image: The odd, sponge-like surface of Hyperion. Note the dark centers in some craters. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA.
The papers are Cruikshank et al., “Surface composition of Hyperion,” Nature 448 (5 July 2007), pp. 54-56 (abstract) and Thomas et al., “Hyperion’s sponge-like appearance,” Nature 448 (5 July 2007), pp. 50-56 (abstract).
by Paul Gilster | Jul 6, 2007 | Culture and Society
The tenth edition of the weekly Carnival of Space is now available, with stories ranging from space-based solar power to Mars Society volunteers in the Canadian high arctic and a look at the latest in spacesuits.
