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Planetary Debris and Its Effects

Since we’ve just been looking at stellar metallicity and planet formation, news from the European Southern Observatory catches my attention. A new paper from ESO astronomers discusses the question of planetary debris falling onto the surface of stars, and its effects on what we observe. Evidence has been accumulating that planets tend to be found around stars that are enriched in iron. On average, stars with planets are almost twice as rich in metals as stars with no known planetary system.

But what exactly does this result mean? On the one hand, it’s possible that stars that are rich in metals naturally enhance planet formation. But the reverse is also possible: It could be that debris from the planetary system could have polluted the star itself, so that the metals we see aren’t intrinsic to the star. Bear in mind that a stellar spectrum shows only the star’s outer layers, so we can’t be sure what’s at the core. And in-falling planetary debris would stay in the star’s outer regions.

In other words, observed metallicity could actually be caused by the planetary system itself, and not by the star. The ESO team, led by Luca Pasquini, approached the question by studying red giant stars that have exhausted hydrogen in their core. And in the fourteen planet-hosting red giants under investigation, a clear difference appeared between these and normal planet-hosting stars. “We find that evolved stars are not enriched in metals, even when hosting planets,” says Pasquini. “Thus, the anomalies found in planet-hosting stars seem to disappear when they get older and puff up!”

Now it gets interesting, because we’re gaining insights that could affect the evolution of planet formation theories. The ESO astronomers think the difference between red giants and stars like our own in terms of metallicity studies is the size of the convective zone (see image and caption below). In a star like the Sun, this region is about two percent of the star’s mass, but the convective zone in red giants is 35 times larger. Any polluting metals would thus be 35 times more diluted in a red giant, and correspondingly that much more difficult to observe.

Stellar structure

Image (click to enlarge): Artist’s impression of the structure of a solar-like star and a red giant. The two images are not to scale – the scale is given in the lower right corner. It is common to divide the Sun’s (and solar-like stars’) interior into three distinct zones. Here, note the uppermost, called the Convective Zone. It extends downwards from the bottom of the photosphere to a depth of about 15% of the radius of the Sun. The energy in the Convective Zone is mainly transported upwards by (convection) streams of gas. In red giants, the convection zone is much larger, encompassing more than 35 times more mass than in the Sun. Credit: ESO.

Artie Hatzes (Thüringer Landessternwarte Tautenburg) puts it starkly: “Although the interpretation of the data is not straightforward, the simplest explanation is that solar-like stars appear metal-rich because of the pollution of their atmospheres.” A co-author of the paper, Hatzes illustrates the tricky nature of metallicity studies. We may be seeing metal excesses resulting from heavy elements falling onto the star from its proto-planetary disk, making the case that how metals function in planet formation is something our theories are a long way from explaining.

And note this: The core accretion model of planet formation seems to be challenged by this finding. Core accretion assumes that planets ‘grow’ as protoplanetary materials bang together and accumulate until, gaining enough mass and forming a solid core, they are able to capture a gas atmosphere. The model depends on dust content to function, and implies that the host stars should show high metallicity down to their core.

The gravitational instability model is different. Let me quote from the Pasquini paper on this mechanism: “…a gravitationally unstable region in a protoplanetary disk forms self-gravitating clumps of gas and dust within which the dust grains coagulate and sediment to form a central core.” Alan Boss (Carnegie Institution of Washington), the major proponent of this theory, has argued that gravitational instability has little dependence on metallicity. The current ESO work seems to be a point in gravitational instability’s favor. The situation is still in flux, however, with more results being gathered from sub-giant star planet searches in other venues. The metallicity findings of these surveys should provide valuable new clues.

The paper is Pasquini et al., “Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars,” to be published in Astronomy & Astrophysics, with preprint available.

Comments on this entry are closed.

  • andy July 11, 2007, 20:25

    However one thing to bear in mind here is that the giant stars are all more massive than the main sequence stars in which the metallicity trend was detected. Whether the environment for forming planets around stars more massive than the Sun is significantly different to that around solar-mass stars, I don’t know.

  • Shaun July 14, 2007, 12:56

    This anomaly may not be just limited to Red Giant stars but also to these low Fe/H M dwarf stars here > https://www.centauri-dreams.org/?p=1341

  • ljk October 30, 2007, 12:15

    Debris disks around Sun-like stars

    Authors: D. E. Trilling, G. Bryden, C. A. Beichman, G. H. Rieke, K. Y. L. Su, J. A. Stansberry, M. Blaylock, K. R. Stapelfeldt, J. W. Beeman, E. E. Haller

    (Submitted on 29 Oct 2007)

    Abstract: We have observed nearly 200 FGK stars at 24 and 70 microns with the Spitzer Space Telescope. We identify excess infrared emission, including a number of cases where the observed flux is more than 10 times brighter than the predicted photospheric flux, and interpret these signatures as evidence of debris disks in those systems. We combine this sample of FGK stars with similar published results to produce a sample of more than 350 main sequence AFGKM stars. The incidence of debris disks is 4.2% (+2.0/-1.1) at 24 microns for a sample of 213 Sun-like (FG) stars and 16.4% (+2.8/-2.9) at 70 microns for 225 Sun-like (FG) stars. We find that the excess rates for A, F, G, and K stars are statistically indistinguishable, but with a suggestion of decreasing excess rate toward the later spectral types; this may be an age effect. The lack of strong trend among FGK stars of comparable ages is surprising, given the factor of 50 change in stellar luminosity across this spectral range. We also find that the incidence of debris disks declines very slowly beyond ages of 1 billion years.

    Comments: ApJ, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.5498v1 [astro-ph]

    Submission history

    From: David E. Trilling [view email]

    [v1] Mon, 29 Oct 2007 18:49:19 GMT (116kb)

    http://arxiv.org/abs/0710.5498

  • ljk October 31, 2007, 10:23

    Giant Planet Formation by Core Accretion

    Authors: Christoph Mordasini, Yann Alibert, Willy Benz, Dominique Naef

    (Submitted on 30 Oct 2007)

    Abstract: We present a review of the standard paradigm for giant planet formation, the core accretion theory. After an overview of the basic concepts of this model, results of the original implementation are discussed. Then, recent improvements and extensions, like the inclusion of planetary migration and the resulting effects are discussed. It is shown that these improvement solve the timescale problem. Finally, it is shown that by means of generating synthetic populations of (extrasolar) planets, core accretion models are able to reproduce in a statistically significant way the actually observed planetary population.

    Comments: 8 pages, 3 figures, invited review, to appear in “Extreme Solar Systems” ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.5667v1 [astro-ph]

    Submission history

    From: Christoph Mordasini [view email]

    [v1] Tue, 30 Oct 2007 14:59:43 GMT (93kb)

    http://arxiv.org/abs/0710.5667

  • ljk November 15, 2007, 0:40

    http://www.newsroom.ucla.edu/portal/ucla/rocky-planets-are-forming-in-the-40289.aspx

    Planets forming in Pleiades star cluster, astronomers report
    By Stuart Wolpert| 11/14/2007 11:45:00 AM
    Rocky terrestrial planets, perhaps like Earth, Mars or Venus, appear to be forming or to have recently formed around a star in the Pleiades (“seven sisters”) star cluster, the result of “monster collisions” of planets or planetary embryos.

    Astronomers using the Gemini Observatory in Hawaii and the Spitzer Space Telescope report their findings in an upcoming issue of the Astrophysical Journal, the premier journal in astronomy.

    “This is the first clear evidence for planet formation in the Pleiades, and the results we are presenting may well be the first observational evidence that terrestrial planets like those in our solar system are quite common,” said Joseph Rhee, a UCLA postdoctoral scholar in astronomy and lead author of the research.

    The Pleiades star cluster, in the constellation Taurus, is well-known in many cultures. It is named for the seven daughters of Atlas and Pleione, who were placed by Zeus among the stars in Greek mythology and is cited in the Bible — “Can you bind the beautiful Pleiades? Can you loose the cords of Orion?” (Job 38:31). The automaker Subaru’s name is the Japanese word for the Pleiades, Rhee said.

    The Pleiades is probably the best known star cluster and the most striking to the naked eye. “You’ve seen it many times, and it’s now easily visible in the evening sky,” said research co-author Benjamin Zuckerman, UCLA professor of physics and astronomy.

    Although referred to as the “seven sisters,” “the cluster actually contains some 1,400 stars,” said co-author Inseok Song, a staff scientist at NASA’s Spitzer Science Center at the California Institute of Technology and a former astronomer with the Gemini Observatory.

    Located about 400 light-years away, the Pleiades is one of the closest star clusters to Earth. One of the cluster’s stars, known as HD 23514, which has a mass and luminosity a bit greater than those of the sun, is surrounded by an extraordinary number of hot dust particles — “hundreds of thousands of times as much dust as around our sun,” Zuckerman said. “The dust must be the debris from a monster collision, a cosmic catastrophe.”

    The astronomers analyzed emissions from countless microscopic dust particles and concluded that the most likely explanation is that the particles are debris from the violent collision of planets or planetary embryos.

    Song calls the dust particles the “building blocks of planets,” which can accumulate into comets and small asteroid-size bodies and then clump together to form planetary embryos, eventually becoming full-fledged planets.

    “In the process of creating rocky, terrestrial planets, some objects collide and grow into planets, while others shatter into dust,” Song said. “We are seeing that dust.”

    HD 23514 is the second star around which Song and Zuckerman recently have found evidence of terrestrial planet formation. They and their colleagues reported in the journal Nature in July 2005 that a sun-like star known as BD +20 307, located 300 light-years from Earth in the constellation Aries, is surrounded by one million times more dust than is orbiting our sun.

    In an effort to uncover comparably dusty stars after their 2005 research, Rhee, Song and Zuckerman began looking through thousands of publicly accessible, deep-infrared images obtained by the Spitzer Space Telescope and soon discovered HD 23514. The astronomers then used the Gemini North telescope, located on Hawaii’s dormant volcano Mauna Kea, to measure the heat radiation coming from the dust; the heat emerges at infrared wavelengths, just as the heat from our bodies does, Song said.

    “The Gemini and Spitzer data were crucial in identifying and establishing the amount and location of dust around the star,” Song said.

    While our sun is 4.5 billion years old, the Pleiades Aries stars are “adolescents,” about 100 million and 400 million years old, respectively, Rhee said. Based on the age of the two stars and the dynamics of the orbiting dust particles, the astronomers deduce that most adolescent sun-like stars are likely to be building terrestrial-like planets through recurring violent collisions of massive objects. The cosmic debris from only a small percentage of such collisions can be seen at any one time — currently, only HD 23514 and BD +20 307 have visible debris.

    “Our observations indicate that terrestrial planets similar to those in our solar system are probably quite common,” Zuckerman said.

    The astronomers calculate that terrestrial planets or planetary embryos in the Pleiades collided within the last few hundred thousand years — or perhaps much more recently — but they cannot rule out the possibility that multiple, somewhat smaller collisions occurred.

    Many astronomers believe our moon was formed through the collision of two planetary embryos — the young Earth and a body about the size of Mars. That crash created tremendous debris, some of which condensed to form the moon and some of which went into orbit around the young sun, Zuckerman said.

    By contrast, the collision of an asteroid with Earth 65 million years ago, the most favored explanation for the final demise of the dinosaurs, was a mere pipsqueak, he said.

    “Collisions between comets or asteroids wouldn’t produce anywhere near the amount of dust we are seeing,” Song said.

    HD 23514 and BD +20 307 are by far the dustiest not-so-young stars in the sky. “Nothing else is even close,” Song said.

    Very young stars — those 10 million years old or younger — may have a similar amount of dust around them as a result of the star-formation process. However, by the time a star is 100 million years old, this “primordial” dust has dissipated because the dust particles get blown away or dragged onto the star, or the particles clump together to form much larger objects.

    “Unusually massive amounts of dust, as seen at the Pleiades and Aries stars, cannot be primordial but rather must be the second-generation debris generated by collisions of large objects,” Song said.

    The Pleiades have been considered important by many cultures throughout history.

    “To the Vikings, the Pleiades was Freyja’s hens,” Rhee said. In Bronze Age Europe, the Celts and others associated the Pleiades with mourning and funerals because the cluster rose in the eastern night sky between the autumnal equinox and the winter solstice, which was a festival devoted to the remembrance of the dead. The ancient Aztecs of Mexico and Central America based their calendar on the Pleiades.

    The astronomers’ research results are based on mid- and far- infrared observations made with the Gemini 8-meter Frederick C. Gillett Telescope at Gemini North and the space-based infrared observatories Infrared Astronomical Satellite (IRAS), Infrared Space Observatory (ISO) and NASA’s Spitzer Space Telescope.

    The Gemini Observatory is an international collaboration utilizing two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawaii (Gemini North); the other is at Cerro Pachón in central Chile (Gemini South). Together they provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

    UCLA is California’s largest university, with an enrollment of nearly 37,000 undergraduate and graduate students. The UCLA College of Letters and Science and the university’s 11 professional schools feature renowned faculty and offer more than 300 degree programs and majors. UCLA is a national and international leader in the breadth and quality of its academic, research, health care, cultural, continuing education and athletic programs. Four alumni and five faculty have been awarded the Nobel Prize.

  • ljk November 15, 2007, 13:15

    Warm dust in the terrestrial planet zone of a sun-like Pleiad: collisions between planetary embryos?

    Authors: Joseph H. Rhee, Inseok Song, B. Zuckerman

    (Submitted on 14 Nov 2007)

    Abstract: Only a few solar-type main sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD+20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades cluster. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the non-standard wavelength of ~9 micron.

    The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (greater than ~1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust.

    The observed frequency of the BD+20 307/HD 23514 phenomenon indicates that the mass equivalent of Earth’s Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) sun-like stars.

    Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.

    Comments: Accepted to ApJ, 19 pages including 3 figures and 2 tables

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.2111v1 [astro-ph]

    Submission history

    From: Joseph Rhee [view email]

    [v1] Wed, 14 Nov 2007 05:58:52 GMT (68kb)

    http://arxiv.org/abs/0711.2111

  • ljk November 16, 2007, 19:53

    Building Giant-Planet Cores at a Planet Trap

    Authors: Alessandro Morbidelli (OCA), Aurelien Crida, Frederic Masset, Richard P. Nelson

    (Submitted on 15 Nov 2007)

    Abstract: A well-known bottleneck for the core-accretion model of giant-planet formation is the loss of the cores into the star by Type-I migration, due to the tidal interactions with the gas disk. It has been shown that a steep surface-density gradient in the disk, such as the one expected at the boundary between an active and a dead zone, acts as a planet trap and prevents isolated cores from migrating down to the central star. We study the relevance of the planet trap concept for the accretion and evolution of systems of multiple planetary embryos/cores. We performed hydrodynamical simulations of the evolution of systems of multiple massive objects in the vicinity of a planet trap. The planetary embryos evolve in 3 dimensions, whereas the disk is modeled with a 2D grid. Synthetic forces are applied onto the embryos to mimic the damping effect that the disk has on their inclinations. Systems with two embryos tend to acquire stable, separated and non-migrating orbits, with the more massive embryo placed at the planet trap and the lighter one farther out in the disk. Systems of multiple embryos are intrinsically unstable. Consequently, a long phase of mutual scattering can lead to accreting collisions among embryos; some embryos are injected into the inner part of the disk, where they can be evacuated into the star by Type I migration. The system can resume a stable, non-migrating configuration only when the number of surviving embryos decreases to a small value (~2-4). This can explain the limited number of giant planets in our solar system. These results should apply in general to any case in which the Type-I migration of the inner embryo is prevented by some mechanism, and not solely to the planet trap scenario.

    Comments: in press in Astronomy and Astrophysics

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.2344v1 [astro-ph]

    Submission history

    From: Morbidelli Alessandro [view email] [via CCSD proxy]

    [v1] Thu, 15 Nov 2007 06:10:00 GMT (231kb)

    http://arxiv.org/abs/0711.2344

  • ljk December 5, 2007, 14:16

    Discovery of an extended debris disk around the F2V star HD 15745

    Authors: Paul Kalas, Gaspard Duchene, Michael P. Fitzgerald, James R. Graham

    (Submitted on 3 Dec 2007)

    Abstract: Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we have discovered dust-scattered light from the debris disk surrounding the F2V star HD 15745. The circumstellar disk is detected between 2.0″ and 7.5″ radius, corresponding to 128 – 480 AU radius. The circumstellar disk morphology is asymmetric about the star, resembling a fan, and consistent with forward scattering grains in an optically thin disk with an inclination of ~67 degrees to our line of sight. The spectral energy distribution and scattered light morphology can be approximated with a model disk composed of silicate grains between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03 M_earth) representing a narrow grain size distribution (1 – 10 micron). Galactic space motions are similar to the Castor Moving Group with an age of ~10E+8 yr, although future work is required to determine the age of HD 15745 using other indicators.

    Comments: 7 pages, 4 figures, ApJ Letters, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.0378v1 [astro-ph]

    Submission history

    From: Paul Kalas [view email]

    [v1] Mon, 3 Dec 2007 20:48:51 GMT (990kb)

    http://arxiv.org/abs/0712.0378

  • ljk January 3, 2008, 16:40

    Red Dust in Planet-Forming Disk May Harbor Precursors to Life

    Washington, DC – Astronomers at the Carnegie Institution have found the first
    indications of highly complex organic molecules in the disk of red dust
    surrounding a distant star. The eight-million-year-old star, known as HR 4796A,
    is inferred to be in the late stages of planet formation, suggesting that the
    basic building blocks of life may be common in planetary systems.

    In a study published in the current Astrophysical Journal Letters, John Debes
    and Alycia Weinberger of the Carnegie Institution’s Department of Terrestrial
    Magnetism with Glenn Schneider of the University of Arizona report observations
    of infrared light from HR 4796A using the Near-Infrared Multi-Object
    Spectrometer aboard the Hubble Space Telescope. The researchers found that the
    spectrum of visible and infrared light scattered by the star’s dust disk looks
    very red, the color produced by large organic carbon molecules called tholins.
    The spectrum does not match those of other red substances, such as iron oxide.

    Tholins do not form naturally on present-day Earth because oxygen in the
    atmosphere would quickly destroy them, but they are hypothesized to have existed
    on the primitive Earth billions of years ago and may have been precursors to the
    biomolecules that make up living organisms. Tholins have been detected elsewhere
    in the solar system, such as in comets and on Saturn’s moon Titan, where they
    give the atmosphere a red tinge. This study is the first report of tholins
    outside the solar system.

    “Until recently it’s been hard to know what makes up the dust in a disk from
    scattered light, so to find tholins this way represents a great leap in our
    understanding,” says Debes.

    HR 4796A is located in the constellation Centaurus, visible primarily form the
    southern hemisphere. It is about 220 light years from Earth. The discovery of
    its dust disk in 1991 generated excitement among astronomers, who consider it a
    prime example of a planetary system caught in the act of formation. The dust is
    generated by collisions of small bodies, perhaps similar to the comets or
    asteroids in our solar system, and which may be coated by the organics. These
    planetesimals can deliver these building blocks for life to any planets that may
    also be circling the star.

    “Astronomers are just beginning to look for planets around stars much different
    from the Sun. HR 4796A is twice as massive, nearly twice as hot as the sun, and
    twenty times more luminous than the Sun,” says Debes. “Studying this system
    provides new clues to understanding the different conditions under which planets
    form and, perhaps, life can evolve.”

    This research is based on observations with the NASA/ESA Hubble Space Telescope
    and was supported by NASA and the NASA Astrobiology Institute.

    The Carnegie Institution (www.CIW.edu) has been a pioneering force in basic
    scientific research since 1902. It is a private, nonprofit organization with six
    research departments throughout the U.S. Carnegie scientists are leaders in
    plant biology, developmental biology, astronomy, materials science, global
    ecology, and Earth and planetary science.

    For images and captions, see

    http://www.ciw.edu/news/red_dust_planet_forming_disk_may_harbor_precursors_life

    For Immediate Release January 3, 2008
    Contact:
    John Debes
    1-202-478-8862, debes@dtm.ciw.edu

    PIO Source:
    Alan Cutler
    1-202-939-1142
    acutler@ciw.edu

    For copies of the paper go to http://arxiv.org/abs/0712.3283