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On Debris Disks and Super-Earths

The red dwarf Gliese 581 continues to draw the eye, whether or not the putative world Gl 581 g is there or not. The latter, whose existence has been the subject of controversy, would occupy a tantalizing place in its star’s habitable zone, though in some models the planet Gl 581 d might also skirt the outer edge of the HZ. Now we have interesting new work from the European Space Agency’s Herschel space observatory announcing that Gl 581, along with the G-class star 61 Vir, another nearby planetary system, shows the the signature of cold dust at -200 degrees Celsius.

It’s an abundant signature, too, meaning that both these systems must have ten times the number of comets found in our own Solar System’s Kuiper Belt. The two papers on this work grow out of a program called, fittingly, DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Sub-mm). What the researchers working these data are suggesting is that the lack of a large gas giant in the two systems may relate to the dense debris cloud. Instead of an era of heavy bombardment triggered by gas giants disrupting the Kuiper Belt, as occurred in our system, these stars may have experienced a much gentler inflow of volatiles. Thus Mark Wyatt (University of Cambridge), lead author of the paper on 61 Vir:

“The new observations are giving us a clue: they’re saying that in the Solar System we have giant planets and a relatively sparse Kuiper Belt, but systems with only low-mass planets often have much denser Kuiper belts… We think that may be because the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event, and instead experience a gradual rain of comets over billions of years.”

Have a look at this Herschel image showing the situation around Gl 581:

Image: An expanded diagram of the debris disc and planets around the star known as Gliese 581, superimposed on a composite Herschel image assembled from separate observations made with its PhotoArray Camera and Spectrometer (PACS) at 70, 100 and 160 micrometre wavelengths. The white region in the lower centre of the image is the emission that originates almost entirely from the disc, with only a small contribution from the unseen Gliese 581. The line drawing superimposed on the Herschel image gives a schematic representation of the location and orientation of the star, planets and disc, albeit not to scale. The black oval outline sketched onto the Herschel data represents the innermost boundary of the debris disc; the approximate location of the outermost boundary is represented by the outer set of dashed lines. Credit: ESA/AOES.

An older star like Gl 581 would have had two billion years or so for a substantial amount of water to be delivered to the inner system and, of course, to any potentially habitable worlds that reside there. What we now believe about the planets circling Gl 581 is that they have masses between 2 and 15 times that of the Earth, all located within 0.22 AU of the star, while the debris disk extends from 25 AU to 60 AU. A Neptune-class world further out, however, is a possibility. The researchers believe the large amount of dust Herschel has detected must be the result of cometary collisions, which could be triggered by a planet — perhaps about as large as the close-in planets — orbiting near the debris disk.

So we are looking at larger debris disks around systems where there is no Jupiter-class planet, and far less dense disks around stars where large gas giants are found. The paper on Gl 581 adds:

It is intriguing that, in our current analysis of the DEBRIS sample of 89 M-stars, the only debris disk confidently detected around a mature M-star also happens to be around the only star known to have low mass planets. This could mean that the correlation between low-mass planets and debris disks recently found for G-stars by Wyatt et al. (2012) also applies to M-stars. Then, the high fraction (∼ 25%) of M-stars known to host low mass planets in the radial velocity and Kepler observations should make debris disks relatively common around them. If these disks have not been detected yet, it may be because searches have simply not been deep enough, or because the disk around GJ 581 is the brightest owing to some intrinsic properties ; for example hosting a multiple planetary system.

Modeling of the debris disk around the G-class star 61 Vir shows that the dust extends from 30 AU to at least 100 AU. The two planets known to orbit the star have masses between 5 and 18 times that of the Earth, and are located within 0.22 AU. The paper on 61 Vir summarizes the situation with regard to G-class stars:

There is likely little interaction between the disk and the known planets, which are at < 0.5AU. The lack of planetesimals in the < 30AU region could be explained by the existence of planets in this region, but the depletion can also be explained by collisional erosion... Considering a sample of the nearest 60 G stars there is an emerging trend that stars which, like 61 Vir, only have low-mass planets, are more likely to have detectable debris... We attribute this trend to the fact that the formation processes that make low-mass planets are likely to also result in large quantities of distant debris.

The papers are Wyatt et al., “Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems,” in press at Monthly Notices of the Royal Astronomical Society (preprint), and Lestrade et al., “A DEBRIS Disk Around The Planet Hosting M-star GJ581 Spatially Resolved with Herschel,” accepted by Astronomy and Astrophysics (preprint).


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  • ljk November 29, 2012, 10:01

    28 November 2012

    Text & Image:



    Think back to the last time you saw the Milky Way — that faint stripe of stars that thickens and brightens as you get farther from city lights. At least 200 billion stars fill the Milky Way, our galaxy. How many planets might orbit those stars? What would those worlds be like? Twenty years ago, it was anybody’s guess.

    In the 1990s, astronomers began to discover planets around other stars — so-called exoplanets. Since then, the confirmed count of exoplanets has skyrocketed to more than 850, with thousands of candidates awaiting follow-up. Astronomers now estimate that the stars in our Milky Way have an average of at least one planet each. (The next time you look up into the night sky, think about that.)

    The sudden prospect of characterizing so many solar systems in our own galaxy has brought together two once-isolated camps: planetary scientists, who generally focus on the inside of our solar system, and astronomers, who mostly look beyond it. Planetary scientists see an opportunity to learn about our solar system and its origins by putting it into the context of a huge ensemble of other solar systems, and astronomers have a keen interest in what planetary scientists might help them discover about planet formation on a galactic or even larger scale.

    To those ends, nine Caltech astronomers and planetary scientists are forming a Center for Planetary Astronomy. Joining together in a single research center will help them maintain fruitful collaborations, collectively attract research funding and fellowships for young scholars, and recruit top students and postdoctoral scholars.

    The nascent center’s members bring complementary perspectives to the characterization of our newly discovered neighbors. Planetary science professor Geoff Blake, astronomy professor Lynn Hillenbrand, and senior research associate John Carpenter study planet-forming disks of gas and dust around young stars. Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, and Caltech’s infamous Pluto-killer, studies fossil rubble from just such a disk — a fantastic array of thousands of planetesimals and chunks of rock and ice on the fringes of our solar system, known as the Kuiper belt, that yields clues to the primordial solar system. The remaining scientists are focused more on the planets themselves. John Johnson, an assistant professor of planetary astronomy, focuses on the detection and characterization of exoplanets, searches for worlds like Earth, and investigates how stars’ masses affect planet formation by studying the relationships between exoplanets and the very different types of stars that they orbit. Heather Knutson, an assistant professor of planetary science, characterizes exoplanets’ compositions, temperatures, atmospheres, and even their weather. Yuk Yung, the Smits Family Professor of Planetary Science, studies the atmospheres of planets, and Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, studies planetary interiors and how they evolve. Greg Hallinan, an assistant professor of astronomy, is trying to detect radio signals from exoplanets, which would indicate the presence of magnetic fields that could be a signature of habitability.

    The center’s members are excited about its potential contribution to the major discoveries that are sure to come in this field. “The unique combination of Caltech’s top-ranked astronomical facilities, astronomy program, and planetary science program will allow us to access the deep and broad knowledge about planets and planetary systems that only comes from such a joint endeavor,” says Brown.

    Says Knutson, “I was trained as an astronomer, but what I do is planetary science. Caltech is one of the few places where we have great conversations between the two groups. And Caltech’s resources, in terms of telescopes, give us the opportunity to move quickly and think big.”


    Deborah Williams-Hedges
    Senior Media Relations Representative
    California Institute of Technology (Caltech)
    +1 (626) 395-3227

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  • Eniac November 30, 2012, 21:33

    There is likely little interaction between the disk and the known planets, which are at < 0.5AU. The lack of planetesimals in the < 30AU region could be explained by the existence of planets in this region, but the depletion can also be explained by collisional erosion…

    I like the former explanation. There could be hundreds of Earth sized planets there, with only the few outliers very close to the star visible to us. You could also imagine a much brighter star with a similar configuration, it could have a whole bunch of planets in the habitable zone.

    We should never forget that we have only scratched the surface of the body of exoplanets, and that many a surprise is sure to be in store.

  • ljk December 3, 2012, 15:16

    3 December 2012

    ** Contacts are listed below. **

    Text & Images:



    Scattered around the Milky Way are stars that resemble our own Sun — but a new study is finding that any planets orbiting those stars may very well be hotter and more dynamic than Earth.

    That’s because the interiors of any terrestrial planets in these systems are likely warmer than Earth — up to 25 percent warmer, which would make them more geologically active and more likely to retain enough liquid water to support life, at least in its microbial form.

    The preliminary finding comes from geologists and astronomers at Ohio State University who have teamed up to search for alien life in a new way.

    They studied eight “solar twins” of our Sun — stars that very closely match the Sun in size, age, and overall composition — in order to measure the amounts of radioactive elements they contain. Those stars came from a dataset recorded by the High Accuracy Radial Velocity Planet Searcher spectrometer at the European Southern Observatory in Chile.

    They searched the solar twins for elements such as thorium and uranium, which are essential to Earth’s plate tectonics because they warm our planet’s interior. Plate tectonics helps maintain water on the surface of the Earth, so the existence of plate tectonics is sometimes taken as an indicator of a planet’s hospitality to life.

    Of the eight solar twins they’ve studied so far, seven appear to contain much more thorium than our Sun — which suggests that any planets orbiting those stars probably contain more thorium, too. That, in turn, means that the interior of the planets are probably warmer than ours.

    For example, one star in the survey contains 2.5 times more thorium than our Sun, said Ohio State doctoral student Cayman Unterborn. According to his measurements, terrestrial planets that formed around that star probably generate 25 percent more internal heat than Earth does, allowing for plate tectonics to persist longer through a planet’s history, giving more time for life to arise.

    “If it turns out that these planets are warmer than we previously thought, then we can effectively increase the size of the habitable zone around these stars by pushing the habitable zone farther from the host star, and consider more of those planets hospitable to microbial life,” said Unterborn, who presented the results at the American Geophysical Union meeting in San Francisco this week.

    “At this point, all we can say for sure is that there is some natural variation in the amount of radioactive elements inside stars like ours,” he added. “With only nine samples including the Sun, we can’t say much about the full extent of that variation throughout the galaxy. But from what we know about planet formation, we do know that the planets around those stars probably exhibit the same variation, which has implications for the possibility of life.”

    His advisor, Wendy Panero, associate professor in the School of Earth Sciences at Ohio State, explained that radioactive elements such as thorium, uranium, and potassium are present within Earth’s mantle. These elements heat the planet from the inside, in a way that is completely separate from the heat emanating from Earth’s core.

    “The core is hot because it started out hot,” Panero said. “But the core isn’t our only heat source. A comparable contributor is the slow radioactive decay of elements that were here when the Earth formed. Without radioactivity, there wouldn’t be enough heat to drive the plate tectonics that maintains surface oceans on Earth.”

    The relationship between plate tectonics and surface water is complex and not completely understood. Panero called it “one of the great mysteries in the geosciences.” But researchers are beginning to suspect that the same forces of heat convection in the mantle that move Earth’s crust somehow regulate the amount of water in the oceans, too.

    “It seems that if a planet is to retain an ocean over geologic timescales, it needs some kind of crust ‘recycling system,’ and for us that’s mantle convection,” Unterborn said.

    In particular, microbial life on Earth benefits from subsurface heat. Scores of microbes known as archaea do not rely on the Sun for energy, but instead live directly off of heat arising from deep inside the Earth.

    On Earth, most of the heat from radioactive decay comes from uranium. Planets rich in thorium, which is more energetic than uranium and has a longer half-life, would “run” hotter and remain hot longer, he said, which gives them more time to develop life.

    As to why our solar system has less thorium, Unterborn said it’s likely the luck of the draw.

    “It all starts with supernovae. The elements created in a supernova determine the materials that are available for new stars and planets to form. The solar twins we studied are scattered around the galaxy, so they all formed from different supernovae. It just so happens that they had more thorium available when they formed than we did.”

    Jennifer Johnson, associate professor of astronomy at Ohio State and co-author of the study, cautioned that the results are preliminary. “All signs are pointing to yes — that there is a difference in the abundance of radioactive elements in these stars, but we need to see how robust the result is,” she said.

    Next, Unterborn wants to do a detailed statistical analysis of noise in the HARPS data to improve the accuracy of his computer models. Then he will seek telescope time to look for more solar twins.

    PIO Contact:

    Pam Frost Gorder
    +1 (614) 292-9475

    Science Contacts:

    Wendy Panero
    +1 (614) 292-6290

    Cayman Unterborn

    Editor’s note: Panero is not attending AGU and will best be reached in her office. Unterborn is best reached by email or through Pam Frost Gorder.

    Poster P11B-1816, “The Distribution of Radiogenic Elements in Stars with and without Planetary Systems: Implications for Dynamics and Habitability,” will be presented from 8:00 a.m. to 12:20 p.m. PST on Monday, Dec. 3, 2012, in Moscone South Hall A-C.

    This research was funded by Panero’s CAREER award from the National Science Foundation.

  • Ronald December 3, 2012, 18:37

    “the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event, and instead experience a gradual rain of comets over billions of years”

    Very fascinating with regard to diagnostiscs, i.e. the correlation between a large debris disk and the presence of only small planets, but I am still wondering whether this bodes well with regard to water.
    I mean, if there is such a large debris disk still present and hence so relatively few comets have been disrupted into an inward trajectory, the essential question is: will enough of them have been delivered to the inner system or will the small planets in the HZ be water-poor as a result?

  • Ronald December 4, 2012, 20:52

    ljk December 3, 2012 at 15:16 :

    Very interesting article, though a bit speculative.

    And more importantly, they may be partly betting on the wrong horse:
    though plate tectonics is without a doubt vital to the habitable lifespan of a terrestrial planet, this will be of little use if the star itself does not have a sufficiently long stable lifespan, i.e. if it grows brighter so rapidly that the planet moves out of the HZ (on the inside), as we have discussed several times before.

    See also the recent post “G-Class Outliers: Musings on Intelligent Life”
    https://centauri-dreams.org/?p=25359 ,
    with time windows for (higher) life, depending on spectral type.

    Our sun will become too hot for higher life (specialized organs) in about another 0.5 Gy, which is about 1 Gy after the beginning of the great Cambrian diversification of life, and hence solar twins (around G2V) with a similar history will have a window for higher life of about 1 Gy, maybe up to 1.5 or 2 Gy.

    A longer geological lifespan of a planet thanks to (radioactive decay) prolonged plate tectonics will do little then to extend the planet’s habitability.

    It seems that the optimal combination then is a somewhat dimmer solar type star (maybe around G5/G6 or even dimmer, G8/9, K0 ?) with a slightly larger mass planet and/or one with more thorium in the mantle.

  • Ronald December 5, 2012, 7:31

    ljk: “They studied eight “solar twins” of our Sun”

    I could not find the content of this poster presentation.
    Could somebody tell me which 8 solar twins?

  • ljk December 10, 2012, 9:58

    7 Exoplanets That Could Host Alien Life

    The Habitable Exoplanets Catalog lists seven alien planets that could prove habitable for life. Here’s a look at each.


    As of December 2012, the Habitable Exoplanets Catalog lists seven planets that have the best chance for life beyond our solar system. Not all of these planets are confirmed, and there’s still a lot to learn about their environments. But the catalog gives astrobiologists a great place to start when talking about life beyond Earth.

    Here, according to the University of Puerto Rico at Arecibo, are the seven planets we know of that are most likely to host alien life.