Red Dwarfs: Dust, Details and Habitability

Budding astrobiologists should be thinking about the significance of red dwarf stars as they approach their careers. Let’s say, as pure speculation, that one out of every thousand stars in class M has a planet in the habitable zone. That works out to 75 million potentially habitable planets around these stars in our galaxy alone.

Planet around a red dwarf

Note the assumptions I’m making. First, I peg M dwarfs at 75 percent of the galactic population. That figure is widely in use and I’ve just run across it again in a new paper by Paul Shankland (US Naval Observatory), David Blank (James Cook University, Australia) and colleagues, about which more in a moment. Another assumption: That the Milky Way holds about one hundred billion stars. That’s low-balling the number, I think, because estimates seem to start at that figure and go up to four or five times as high. So my 75 million potentially habitable planets, while just a guess, may not be totally off the wall.

Image: An artist’s impression of a gas giant orbiting a red dwarf. Credit: NASA/ESA/STScI/G Bacon.

Now assume one out of every thousand G-class stars like the Sun has a planet in the habitable zone. Using the same assumption for stellar population in the galaxy and figuring that G-class stars represent approximately three percent of the stars in the Milky Way, we wind up with three million potentially habitable planets around them. Clearly those of us who dwell on odd, rotating planets in which there would be alternating times of light and darkness at the surface (!) and all too little of that helpful solar flare activity are very much in the minority.

We do slightly better with K stars (think Centauri B, for example, or Epsilon Eridani). Somewhat cooler than the Sun, these orange stars are more plentiful than G-class, making up about 15 percent of the galactic population. That leaves us, again using the one in a thousand assumption, with 15 million potentially habitable planets. K stars help, but even when we add them to the G-class, we wind up with substantially fewer habitable planets than around M-class dwarfs.

But note another assumption I’ve made above, one that could stand some scrutiny. I’ve set up one in a thousand as a figure for habitable planet occurrence without any reference to how planets form in the first place. A key question is whether we can assume roughly similar methods of planet formation around M dwarfs as around G stars and the other stellar types. We’re studying models like core accretion and gravitational instability as we develop consistent theories for all this, but our knowledge of what goes on around M dwarfs remains sparse.

What do we need to move forward on these questions? Aside from a lot more planetary detections around these stars, we need to learn more about dust disks around M dwarfs. It’s an interesting fact that a 2007 search around 123 late-type red dwarfs using the Spitzer Space Telescope produced no new detections. That leaves us with only a few M dwarf dust disks, the one around AU Microscopii being particularly well-resolved. We’ve also found them around Gl 842.2 and Gl 182.

Where to look for the next dust disk as we relate M dwarf planet formation to what we see happening around stars in other stellar classifications?

Dust disks around nearby stars

Image: Dust disks around the M dwarf AU Microscopii (left) and the G2 star HD 107146. 32 light years away, AU Microscopii is only twelve million years old. Note the gaps in the disk in both images, where planets may have cleared a path. Credit: G. Illingworth (UCO-Lick)/ACS Science Team.

The devil, it’s often said, is in the details. A friend, exasperated with my enthusiasm for tiny red stars, commented recently that he was tired of all the painstaking numbers — he simply wanted to know whether there was another habitable planet nearby or not. Ah, but it’s just such painstaking numbers that will tell us the tale. We would have no planetary detections at all without gigabytes of slowly accumulating data, from which the signature of distant worlds is finally drawn. A better statement might be, the truth is in the details. Painstaking as they might be, details are the stuff of discovery.

And sometimes even negative results are revealing. The nearby red dwarf Gliese 876, a fascinating, multi-planet system, has shown no evidence of a dust disk, as discussed in the above-mentioned Shankland/Blank paper. This team used the Very Large Array (VLA) and Australia Telescope Compact Array (ATCA) to search for microwave emissions from such a disk. What we learn is not so much that Gl 876 has no disk whatsoever, but that if there is one, it has to fit within a newly tightened series of constraints. From the paper:

…a negative detection (as is the result here) suggests that a thin dust mass could still exist about Gl 876 if the dust density were below the detection threshold of the individual VLA or ATCA resolution-per-pixel, or the overall upper mass limit. The yet-poorly understood dust density, temperature, spectral index, opacity and optical thinness muddies our understanding of the mechanisms at play, and inclination would be a factor in each of these. Still, there are other possibilities for our null result. Another may be that the formation and evolution of systems (disks and planets) is different in M dwarfs. At the least, any unexpanded composition in the system would lead to misunderstood opacities, albedos, radii, or black body behavior.

Flag that latter possibility: The formation and evolution of systems may be different around M dwarfs. No one can say at this point that it is or isn’t, but the fact that we don’t know reminds us that to this date we have only six M dwarfs with known planetary systems, and very little information about the disks that give rise to planets in the first place. Broadening that information may involve using different instrumentation, which should be turned upon the known M dwarf planets. Note this comment (internal references deleted for brevity):

It is also worth mentioning that because of the low luminosity of M dwarfs… that leads to cooler dust about them, sub-millimeter or millimeter telescopes may be more sensitive than mid-infrared ones in detecting such planet-associated-disks. We would encourage observations at these wavelengths be explored further. In particular, we recommend that the six M dwarfs found with planets so far be comprehensively checked for dust, to include Gl 876 (with greater sensitivity than us), Gl 436, Gl 674. Gl 849, Gl 581, and GJ 317. Understanding the dust in such planet-bearing systems… may be a key not just to making a ?rst exo-Earth detection, but will more importantly offer a broader understanding of planets and their formation about the populous M-type stars.

By putting limits on the amount of debris that might be found in the Gl 876 system, these astronomers have also given a nudge to the theory that close-in gas giants may have an effect on clearing out a dust disk. But this is one among a range of possibilities discussed in a paper that advances our knowledge of what is possible around one nearby red dwarf and its implications for planets elsewhere. The paper is Shankland, Blank et al., “Further Constraints on the Presence of a Debris Disk in the Multiplanet System Gliese 876,” accepted by the Astronomical Journal (abstract).

Notes & Queries 3/29/08

Did short supplies of oxygen and molybdenum slow down the evolution of animal life? Ancient oceans low on molybdenum would create problems for bacteria that use the element to convert atmospheric nitrogen into a form useful for living things. Brian Wang muses over these matters in his entry in the latest Carnival of Space, referring to a recent Nature paper and moving on to look at potential oceans in the Solar System, from Titan to Callisto, Ganymede, Enceladus, and of course, Europa.

Can life could develop in such places, and if so, how long would it take? Brian frames the question in relation to the Fermi paradox. Perhaps the universe takes a lot longer to evolve complex life than we have been assuming, with implications for what we might find on planets around other stars. We’re shooting in the dark on these questions, unable to say whether life exists off-planet in our own Solar System, but the day may not be so far off when results around nearby planets give us another evolutionary laboratory in which to study biology’s ability to adapt.

Plenty of interesting posts fill this week’s Carnival, but I was particularly taken by Stuart Atkinson’s reflections on Cumbrian Sky, recalling his early fascination with space and reflecting on how we have viewed Mars, and by extension many other astronomical objects, over the past few decades. It’s a long post, filled with reminiscence and reminding us that we can sometimes become all too blasé about the spectacular imagery now flooding the Internet from our space probes. Broadband has changed the landscape. Using his connection and the IAS Viewer available from the HiRISE site, Stuart can see Mars as never before:

“I can go to the HiRISE site, select a picture from the gallery, open it up with the IAS Viewer and literally look down upon single dust-covered boulders, stones and rocks, as if I was being flown over the cratered plains by Peter Pan or Superman.”

Read this post to be reminded of just how remarkable our tools have become, and see if you don’t recognize yourself, as I did, in the space-crazed youth described here.
The Space Access ’08 conference in Phoenix is in its final day, and I notice that bloggers like Henry Cate, Rand Simberg and Clark Lindsey are keeping close watch on events. While the focus is on radically cheaper space transportation, I’ve seen some familiar names from the interstellar community on the agenda, from Gerald Nordley (discussing Tethers Unlimited) to Leik Myrabo, whose work on beamed energy propulsion can translate in the short term into efficient launch systems for Earth orbit, and in the long term into laser propulsion for deep space missions.

LaserMotive‘s Jordin Kare is also presenting, which reminds me of no end of interesting ideas, not the least of which is Kare’s SailBeam, a proposal to send tiny ‘micro-sails’ pushed by laser to drive an interstellar craft. The idea is a form of pellet propulsion of the sort first proposed by Clifford Singer back in 1979 and later developed into Gerald Nordley’s exquisite ‘snowflake’ pellets, using nanotechnology to steer their own course. Kare’s take was to cross pellets with lightsails, with each micro-sail becoming part of the fuel stream for the outbound spacecraft.

Why not a full sail? Kare realized that if you cut a large sail into tiny pieces and accelerate those fragments one after the other, you could bring the same amount of mass up to speed using a much less demanding optical system. The small sails can be accelerated much faster close to their power source, an idea that does away with deployment and maintenance issues in large sailcraft. The interstellar vehicle could use an onboard laser to vaporize the sails into plasma as they approached, deploying a pusher plate or magnetic field to absorb the energy.

Talk about acceleration — Kare’s diamond film sails would accelerate to close to light speed within seconds under an acceleration of thirty million gravities. The study Kare did for NASA’s Institute for Advanced Concepts, called “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” is still available at the NIAC site even though the Institute is no longer in operation. For more, see this earlier Centauri Dreams post.

What I need to do next, now that Space Access ’08 has taken me so far afield from my earlier posting plans today, is to treat Kare’s interesting ‘fusion runway’ concept, one that would use impact fusion to accelerate a spacecraft to speeds that would make an interstellar journey possible. But I’m low on time, so we’ll get to that one down the road. How can I not discuss a propulsion system its designer refers to as the ‘Bussard Buzz Bomb?’ I’ll explain the origins of that name in the upcoming article.

Intriguing Temperatures on Enceladus

Cassini’s recent pass through the plumes of Enceladus resulted in a number of intriguing finds, perhaps the most interesting of which is the temperature along the ‘tiger stripes.’ These are the fissures from which Enceladus’ famous geysers erupt. Cassini’s Composite Infrared Spectrometer found them to be warm along almost their entire length, reaching no less than minus 93 degrees Celsius (-135 F). The warmest regions correspond to two visible geyser locations. The contrast in temperatures is striking: The differential between these regions and other areas on Enceladus is a whopping 93 degrees Celsius (200 F).

Heat map of Enceladus

This heat map gives a sense of what we’re dealing with. The brightest fracture, known as Damascus Sulcus and visible at lower left in the image, shows the highest temperatures. In this image, the false color infrared data are superimposed on a grayscale mosaic of the south pole that dates back to the summer of 2005. The map was made at a distance of between 14,000 and 32,000 kilometers starting sixteen minutes after closest approach as the spacecraft receded.

Image: Heat radiating from the entire length of 150 kilometer (95 mile)-long fractures is seen in this best-yet heat map of the active south polar region of Saturn’s ice moon Enceladus. The warmest parts of the fractures tend to lie on locations of the plume jets identified in earlier images, shown in the annotated version with yellow stars. The measurements were obtained by the Cassini spacecraft’s Composite Infrared Spectrometer from the spacecraft’s close flyby of the moon on March 12, 2008. Credit: NASA/JPL/Space Science Institute.

John Spencer (Southwest Research Institute), a scientist on the spacecraft’s Composite Infrared Spectrometer team, notes the significance of the new findings:

“These spectacular new data will really help us understand what powers the geysers. The surprisingly high temperatures make it more likely that there’s liquid water not far below the surface.”

Surprising indeed, and encouraging, for we’re looking at an Enceladus with unusual warmth and possible liquid water. Moreover, Cassini’s pass through the plumes led to the detection of organic chemicals, a mix reminiscent of cometary materials. The density of volatile gases, water vapor, carbon dioxide and carbon monoxide, along with organic materials both simple and complex, was twenty times what had been expected.

Materials in plume

Image: Enceladus’ plume was found to have a comet-like chemistry by Cassini’s Ion and Neutral Mass Spectrometer during its fly-through of the plume on Mar. 12, 2008. Water vapor, methane, carbon monoxide, carbon dioxide, simple organics and complex organics were identified in the plume. The graph shows the chemical constituents in percentage of abundance found in comets compared to those found in Enceladus’ plume. Credit: NASA/JPL/SwRI.

In a NASA feature on the Saturnian moon, Chris McKay (NASA Ames) discusses microbial ecosystems that could be models for life on Enceladus today:

There are three such ecosystems found on Earth that would conceivably be a basis for life on Enceladus. Two are based on methanogens, which belong to an ancient group related to bacteria, called the archaea — the rugged survivalists of bacteria that thrive in harsh environments without oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems, which pull their energy from the chemical interaction of different rocks. The third ecosystem is powered by the energy produced in the radioactive decay in rocks, and was found deep below the surface in a mine in South Africa.

But if life may be feasible on Enceladus, how would it begin? Organic chemicals, notes McKay, were part of the raw materials from which the moons of Saturn formed. Other ingredients could have arrived on comets or interplanetary dust. Add a heating mechanism — tidal heating is one candidate, assuming an earlier oblong orbit, or earlier tidal relationships with another moon — and you could eventually produce a subsurface aquifer rich in organic materials, what McKay calls a ‘prebiotic soup.’

Every time we look at Enceladus the excitement seems to build, and the next flyby isn’t until August! Another passage through the plumes would certainly add to our data, while it’s clear that Enceladus is also moving up on the interest scale in terms of future missions. A lander that could take plume samples and explore the area near the fissures would be optimal. We can only guess when budget realities might make such a mission possible, but until then the data harvest from our existing orbiter is splendid, with analysis of the organic compounds in the plume and the sources of Enceladus’ energy likely to occupy us for years to come.

Life’s Precursors: The Interstellar Connection

Was the early Earth seeded with amino acids from deep space? The variety of molecules found between the stars makes the supposition provocative, but finding interstellar amino acids has been a challenge. Various amino acids have indeed been found in meteorites, but it has been argued that these could have been produced right here in the Solar System within asteroids. Yet laboratory experiments have shown that amino acids can form among the molecules found in interstellar clouds, including such important ones as glycine, alanine and serine.

What’s next is to identify amino acids in the interstellar medium, and we’re coming close. Ponder this: Since 1965, more than 140 molecules have been identified in space, both in interstellar clouds and circumstellar disks, many of them organic or carbon-based. Now researchers from the Max Planck Institute for Radio Astronomy in Bonn have detected amino acetonitrile (NH2CH2CN), a potential precursor of the simplest amino acid, glycine. The odds are rising that the processes that spread life are commonplace: Stars and planets forming within interstellar clouds where amino acids can be found would be subject to infalls of these materials, perhaps enhancing the chances of life elsewhere in the universe. (Addendum: I’ve added the ‘perhaps’ in that sentence as a result of the interesting comment thread that has developed on this topic; see below).

The focus of the Bonn team’s investigations has been Sagittarius B2. Located about 100 parsecs (326 light years) from galactic center, and some 8000 parsecs from Earth, this is a highly active region of star formation — massive, complex, and packed with interesting chemistry. Sagittarius B2 is composed of two dense cores of star formation separated by two parsecs, one of which is possessed of a chemistry so rich that it has been christened the Large Molecule Heimat (LMH) because of the numerous detections of complex molecules made there.

The Bonn team analyzed 3700 spectral lines from complex molecules using the IRAM 30-metre telescope in Spain, with results confirmed by instruments in France and Australia. The Institute’s Karl Menten sees the discovery as a sign of progress in our understanding of the regions where stars are born:

“Finding amino acetonitrile has greatly extended our insight into the chemistry of dense, hot star-forming regions. I am sure we will be able to identify in the future many new, even more complex organic molecules in the interstellar gas. We already have several candidates!”

Such molecules emit hundreds of weak spectral lines, producing spectra so crowded that untangling the components is an extraordinary challenge. And while the hunt for glycine has had a long and inconclusive history, the identification of amino acetonitrile gives us a molecule chemically related to glycine, one whose status as an amino acid precursor is argued in the paper on this work. Conclusive proof of amino acids in interstellar clouds seems closer than ever, an indication that life’s building blocks may predate the formation of the systems they seed.

The paper is Belloche et al., “Detection of amino acetonitrile in Sgr B2(N),” accepted by Astronomy & Astrophysics and available online.

TESS: All Sky Survey for Transiting Planets

I’ve never met George Ricker, but in at least one respect I believe he thinks the way I do. Ricker is senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, and he’s someone who can connect the exoplanetary systems we study with places we might eventually go. As witness this comment in a discussion of a planned satellite-based observatory being designed at the Institute:

“Decades, or even centuries after the TESS survey is completed, the new planetary systems it discovers will continue to be studied because they are both nearby and bright. In fact, when starships transporting colonists first depart the solar system, they may well be headed toward a TESS-discovered planet as their new home.”

It’s wonderful to see a ‘when’ rather than an ‘if’ when referring to starships, even though everyone concerned can appreciate the blue-sky nature of the comment. For my part, I’ll take whatever the physics will bear, from close-up imagery of terrestrial exoplanets to robotic probes pushed by fusion or lightsails, and just maybe down the line, vessels carrying human crews. It’s a goal worth dreaming about.

Transiting 'hot Jupiter'

TESS refers to the Transiting Exoplanet Survey Satellite, which has caught a bit more media buzz than might be expected because Google is among its sponsors. Remember when we thought of Google as a search engine? Now it’s all over the place, mapping not only the planet but also the heavens through Google Sky. The synergy between the data collection capabilities of wide-field digital cameras and a company that makes its money sifting through information is obvious. TESS will use six such cameras, covering the sky in its entirety and measuring the brightness of some two million stars in total.

Image: Artist’s conception of a transiting ‘hot Jupiter.’ An all-sky survey like TESS should track down more of these, with the potential for landing smaller terrestrial-class worlds as well. Image copyright Mark A. Garlick /

The targets: G, K and M-class stars. Make the assumption that one star in a thousand makes a transit as seen from Earth. Observe two million stars and you’re looking at a couple of thousand transiting worlds, and perhaps more depending on how accurate our emerging views of planetary formation are. Design work for TESS is scheduled to be completed this year (you can read more about that and the scientific partnerships MIT has entered into on this project in this news release). For more specifics, David Latham (Harvard-Smithsonian Center for Astrophysics) offered a helpful description of TESS as well as the Kepler mission in a presentation he gave at the California Institute of Technology in 2007. The CfA is partnering with MIT on the TESS project.

Will TESS-discovered planets form the target list for our first interstellar probes? Conceivably, although that assumes that funding to build and launch the satellite is forthcoming. The earliest launch date is 2012. My own hunch is that within that time we may well have identified an Earth-sized planet that is unqualifiedly in the habitable zone of an M-dwarf, one whose orbital position is far less ambiguous than either Gliese 581 c or d. For that matter, if Greg Laughlin’s team can work out the arrangements, we may even have good radial velocity data on rocky worlds around Centauri B. So I wouldn’t put too much emphasis on TESS being first, but the more eyes we have for the exoplanet hunt the better.