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Terrestrial Planet Forming?

Are we seeing an Earth — or at least a Mars-sized world — in the making? Look no further than HD 113766, a binary system perhaps ten million years old some 424 light years away, for the story. One of its stars contains a warm dust belt that may be undergoing planetary formation. If that’s the case, the emerging planet will orbit in the classical habitable zone, defined as that region where liquid water can exist on the surface.

Formation of an Earth-like planet?

What counts here is the composition of the dusty materials making up its interesting disk. The Spitzer Space Telescope performs its usual yeoman service at this task, its infrared spectrometer flagging the material as a step up from the pristine building blocks of comets. The latter contain interesting organic materials like polycyclic aromatic hydrocarbons (PAHs), along with their water ice and carbonates. But HD 113766’s disk contains no water ice, carbonates or fragile organic materials.

Image: This artist’s conception shows a binary-, or two-star, system, called HD 113766, where astronomers suspect a rocky Earth-like planet is forming around one of the stars. The system is located approximately 424 light-years away. At approximately 10 to 16 million years old, the star is also at just the right age for forming rocky planets. Credit: NASA/JPL-Caltech/JHUAPL.

On the other hand, the disk materials have not yet reached reached the stage of differentiation, where heavy metals separate from rocks early in the planet forming process. The metals around HD 113766 have not totally separated from the rocky material, implying that whatever rocky planets are forming are far from maturing. Carey Lisse (Johns Hopkins University Applied Physics Laboratory) likens the material in the belt to Earthly lava flows, containing plenty of raw rock and iron sulfides.

In fact, Lisse argues that the young star is caught in the act of building a rocky world, observed at a useful point in its evolution:

“The timing for this system to be building an Earth is very good. If the system was too young, its planet-forming disk would be full of gas, and it would be making gas-giant planets like Jupiter instead. If the system was too old, then dust aggregation or clumping would have already occurred and all the system’s rocky planets would have already formed.”

The right timing, the right material mix, all suggestive of interesting things to come as the infant world grows. Are planets also forming around Beta Pictoris, Fomalhaut and AU Microscopii? Possibly, but we need to tread carefully. The team from the University of Rochester that has been studying these three nearby stars has been using Hubble imagery to measure the thickness of their dust disks, drawing conclusions about the size of planets that may be forming within. You can see in the Beta Pictoris image below how interesting its disk turns out to be.

Disk around Beta Pictoris

Image: Beta Pictoris from the Hubble Space Telescope. Credit: NASA.

We need to study such things because we’re seeing the building blocks of planet formation and their dynamics on display. A ‘congealing’ planet ought to knock surrounding materials around, leaving visual clues that make the disk seem somewhat inflated. Rochester’s Alice Quillen is an expert on such interactions, which are only subject to scrutiny when the stellar disks appear edge-on as seen from Earth. The relevant stars also need to be near enough for useful Hubble imagery and young enough to be in the process of forming what can be termed embryonic planets.

Pulling all this material together for the three stars studied, Quillen and team think they’re looking at the effects of Pluto-sized objects (’embryos’), about 1000 kilometers in size. But note: We haven’t detected Pluto-sized objects. What we’ve done is to establish that such objects make sense as a plausible explanation for the action we observe in three planetary disks. Are there other explanations? Possibly so, and they may well emerge. This is a small sample, and as with HD 113766, we have no direct detections of the planets themselves, but only inferences subject to our limited knowledge of how protoplanetary disks work.

None of which is intended to cast cold water on some innovative work indeed, but merely to suggest that when we have advanced ground and space-based equipment in place to deliver far more detailed analysis, we are still likely to be surprised about the variety of systems we find. Having few explanations for some of our own outer system objects and knowing as little as we do about the Kuiper Belt, we may discover that even a Beta Pictoris or a AU Microscopii can still confound our best theories.

Lisse’s paper on the HD 113766 is scheduled for The Astrophysical Journal. You can read the Quillen paper “Planetary embryos and planetesimals residing in thin debris disks,” submitted to Monthly Notices of the Royal Astronomical Society, online.

Comments on this entry are closed.

  • andy October 3, 2007, 18:22

    I’m going to call out this press release for using blatantly recycled artwork. Compare the image with the one used on the press release about the quadruple system HD 98800.

  • Adam October 3, 2007, 20:24

    Hi andy

    Well spotted. Someone was lazy weren’t they?

    Or time-pressured, I suspect, so let’s cut the guy some slack.

    OTOH the journalism is kind of lazy, focusing on the “Possible Earth formation” angle. I’m sick to death of publicity-grabbing via bringing up terrestrial planets that maybe forming “somewhere” off in the galaxy. Big deal! Give me real terrestrial planet news when they find one around Alpha or Proxima Centauri.

    The research itself is vital and interesting – and Joe Public is probably as interested in cosmogony as they are in far away Earths, so I wish journos would give them a bit more credit.


  • andy October 4, 2007, 15:13

    Given the star in question is of spectral type F3, the lifetime of any habitable world that may eventually form is going to be severely limited by stellar evolution to a couple of billion years or so (and that’s an upper limit based on the main sequence lifetime: depending on location in the system, the habitable lifetime can easily be much less due to luminosity increase during the main sequence stage).

    The two most interesting results in extrasolar planetary science this year in my opinion are the transiting planets around Gliese 436 and HD 17156: these give much-needed information on “hot Neptunes” and intermediate-period gas giants. But I guess you can’t spin those into “alien Earths” for public consumption.

  • David October 5, 2007, 8:40


    Very good points. I sometimes hear people say that we need not have to worry about the survival of life on Earth until the Sun becomes a red giant in 4 or 5 billion years. I tell them that for the Earth to be suitable for complex life such as ourselves, we “only” have a few hundred million years. There is some evidence that biodiversity on this planet peaked sometime during the Mesozoic period.

    Agreed, the discovery of transits by GJ 436b was the most important result this year. I would say that the
    HD 17156b results is in a tie with the discovery of transits by that Super-Jovian mass planet HD 147506b. Confirming that the radius of the latter was exactly what was expected was relief. If it was significantly diffferent, it would really call into question our basic knowledge of planetary interiors.

  • Ronald October 5, 2007, 10:37

    Yes, I have read estimates from 500 – 900 million years, before the earth leaves the sun’s habitable zone on the inside, it is already rather close toward the inner edge.

    Personally, I think the Mesozoic was the best period anyway, after that meteorite things generally went downhill ;-)

  • Dr;. Carey Lisse October 6, 2007, 23:40

    Our paper can also be found online at the astro-ph web server now. I hope you will get a chance to read it, and see that what we write consists of a careful study using all the possible information we could glean out of the Spitzer spectrum concerning the nature of the system. And that what we wrote was conservative – while we state with certainty that there is a Mars mass worth of small dust, there could easily be Earth masses worth of material, and a large “oligarch”, or proto-planet in the making, or a very dense asteroid belt instead. We don’t know for sure if the terrestrial planet is in the beginning or end of its formation (although current models would say that it has tens of millions of years left to finish its assembly).
    A very good review paper on how terrestrial planets are thought to form, which I suggest for further reading, was written by J. Chambers in 2004.

    We hope that all the interest generated by our work will spur other groups into studying the system more completely, and look for the presence of large plaent-like objects. The relatively large distance (130 pc) to the system will make this somewhat tough, though, to carry out.

    BTW, I agree with the assessment that this system, unlike ours, has about 1-2 Byr to go before the star goes nova, which is unfortunatelythe time when things began to get interesting for mutli-cellular life on the early Earth, according to the fossil record. We will be long gone before anything like this occurs.

    What I think is just as interesting to think about is that this is one of the first clues as to how the Earth was formed. We need to find many more systems like this, of earlier and later ages, to see exactly how rocky planets are formed.

    We may also learn more about this by
    studying the planets in our own solar system more closely – the Messenger spacecraft will flyby Mercury for the first time this January. One of Messengers main science goals is to learn why Mercury has such a high density – is it akin to a “naked” planetary core? Or were all the light “volatiles” driven out of the solar nebula during its formation due to the turn-on of the infant Sun?

    As I suggested, stay tuned. Since I was young, we have come so far in learning all the myriad ways the Universe is put together. I am sure there is a lot more to come!

  • Adam October 7, 2007, 1:15

    Hi Dr. Carey

    Nice of you to drop by our little discussions. Seems a shame for a system to assemble and then go to waste as its star goes off the Main Sequence. I’d hope that our descendents might have a say in how star’s evolve in the billennia ahead.

    “As I suggested, stay tuned. Since I was young, we have come so far in learning all the myriad ways the Universe is put together. I am sure there is a lot more to come!”

    I whole-heartedly concur on that score.

  • Administrator October 7, 2007, 7:14

    We’re pleased indeed to have you here, Dr. Lisse, and thank you for the additional information. This is fascinating work indeed, extending our knowledge in significant directions. I’ll watch the Messenger data with great interest.

  • ljk October 12, 2007, 9:18

    UH Manoa researchers look to the horizon of future planet searches

    HONOLULU – In a paper published this week in the journal Science, three
    University of Hawaii at Manoa researchers and their colleagues review the
    prospects for discovering smaller planets more like Earth, some of which may
    even have conditions suitable for life. Astronomers reported the first planet
    around another Sun-like star in 1995 and since then have found more than 200
    such planets, all thought to be “gas giants” made mostly of hydrogen and
    helium like Jupiter and Saturn in our Solar System.

    “The most successful technique for discovering planets to date spreads light
    from the host star into its constitutive wavelengths (colors)” said lead
    author Eric Gaidos, who is an associate professor in the Department of Geology &
    Geophysics and the NASA Astrobiology Institute at UH Manoa. “A shift in
    wavelength, analogous to the change in pitch of the horn of a passing
    automobile, reveals any motion of the star along the line of sight. Monitoring
    of a star can detect periodic motion caused by the gravitational pull of any
    unseen, orbiting planet.”

    Improved techniques and the ability to monitor fainter stars now enable
    astronomers to discover smaller planets, particularly planets orbiting much
    closer to their host star than the Earth is to the Sun. New computer
    simulations such as those performed by Sean Raymond, co-author of the paper and
    NASA Postdoctoral Fellow at the University of Colorado Boulder, show how such
    planets could form further out and then “migrate” inwards to such orbits.

    Another method now used to find planets is to observe the slight decrease in
    light from the star as an orbiting planet passes in front of it. This happens
    only for those planets whose orbits by chance are seen edge-on. Jupiter-sized
    planets can be found this way using telescopes on the ground, but Earth-size
    planets might be detected by the European CoRoT spacecraft, now in orbit, and
    NASA’s Kepler spacecraft, scheduled to launch in 2009.

    “These methods can sometimes be combined to estimate the density of the
    planet, which will tell us whether the planet is composed mostly of rock and
    metal, like Earth, or something else such as water ice,” said Gaidos.

    Computational simulations by co-author Nader Haghighipour, a planetary
    dynamicist at the Institute for Astronomy and the Astrobiology Institute at UH
    Manoa, have shown that smaller Earth-sized planets can indeed exist in such
    tight planetary environments.

    According to the paper, planets orbiting much closer to a star like the Sun will
    be much hotter and, like Mercury and Venus in our Solar System, inhospitable to
    life. However, many stars are much less bright than the Sun, and planets close
    to them could still orbit within a “habitable zone” where surface
    temperatures could permit stable liquid water.

    “Explaining the formation of habitable planets in such environments is a
    challenging task. However, our simulations have been successful in determining
    condition under which planets similar to Earth can form in the habitable zones
    of less bright stars,” said Haghighipour.

    Future space observatories beginning with NASA’s James Webb Space Telescope
    have the potential to study such planets and determine whether they have
    atmospheres or oceans.

    Added Gaidos, “The discovery of another life-bearing planet would be a
    scientific triumph for humanity, but the study of many lifeless, un-Earthly
    worlds would nevertheless tell us about how planets form, and help us appreciate
    the Earth all that much more”.

    Other researchers contributing to the paper were John Rayner of the Institute
    for Astronomy at UH Manoa, Eric Agol of the University of Washington and David
    Latham of the Harvard-Smithsonian Center for Astrophysics.


    To obtain a copy of the Science paper and supplemental images, contact the AAAS
    Office of Public Programs. A list of staff contacts can be found at:


  • ljk October 22, 2007, 11:22

    Can Terrestrial Planets Form in Hot-Jupiter Systems?

    Authors: Martyn J. Fogg, Richard P. Nelson

    (Submitted on 19 Oct 2007)

    Abstract: Models of terrestrial planet formation in the presence of a migrating giant planet have challenged the notion that hot-Jupiter systems lack terrestrial planets. We briefly review this issue and suggest that hot-Jupiter systems should be prime targets for future observational missions designed to detect Earth-sized and potentially habitable worlds.

    Comments: 4 pages, 1 figure. 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:0710.3730v1 [astro-ph]

    Submission history

    From: Martyn Fogg [view email]

    [v1] Fri, 19 Oct 2007 16:12:56 GMT (85kb)


  • ljk October 26, 2007, 11:08


    Lonely Planets of the Cosmos

    October 23, 2007by Laura Kinoshita


    Scientists think free-floating planets — occasionally with moons — should exist in the cold depths of interstellar space, but without any sunlight they will be very difficult to find.

    A brief letter in Nature was John Debes’s inspiration. The 1999 piece, by David J. Stevenson (Caltech), proposed that planets with liquid water oceans — and even life — could exist in the cold, dark depths of interstellar space far from any star. Based on the knowledge that some fraction of planets must get gravitationally ejected from their systems during the systems’ formation, the paper theorized that some of these ejected planets could, with enough internal heat, keep their atmospheres and stay warm enough to support liquid water below a thick frozen crust.

    What might happen if such an outcast had a big moon? To find out, Debes (at the Carnegie Institution of Washington) ran 2,700 computer simulations based on an Earth-mass planet and a lunar-mass companion.

    “The ejection process can be very intense,” says Debes. “It wasn’t clear to us if any bound systems would actually survive.” But in 123 of the cases, or between 4 and 5 percent of the time, the “Earth-Moon” system did survive ejection from its solar system intact.

    “Anytime something happens in astronomy a few percent of the time, it is interesting to us because on the grand scale of things, it means it’s happening a lot and people should probably know about it,” says Debes.

    And these pairs have a better chance to harbor life, since the dissipation of tidal energy between the moon and the spinning planet causes the interior of the planet to warm. Debes’ models predict that this heating would match what happened in Earth’s case more than 4 billion years ago, when the young Moon was much closer and Earth was rotating faster.

    In the October 20th Astrophysical Journal Letters, Debes and Steinn Sigurdsson write that the heating would likely be localized in hot spots of volcanism or other geothermal processes. Biologists are finding many examples on Earth of life surviving on these energy sources, such as at mid-ocean ridges. Could such “extremophiles” be the most common form of life in the universe?

    The team’s best-case scenario found that an ejected Earth-Moon system can sustain its heat for up to 250 million years — long enough for life to arise. But is it long enough for that life to adapt to the eventual decreasing temperatures?

    And if dark, free-floating Earths exist, when will astronomers be able to detect one? It’s not as impossible as you might think. Debes estimates the next generation of space-based microlensing surveys will have around a 2% chance.

  • ljk December 10, 2007, 1:08

    Evolution of Mid-IR Excess Around Sun-like Stars: Constraints on Models of Terrestrial Planet Formation

    Authors: M.R. Meyer (The University of Arizona), J.M. Carpenter (Caltech), E.E. Mamajek (Harvard-Smithsonian CfA), L.A. Hillenbrand (Caltech), D. Hollenbach (NASA-Ames), A. Moro-Martin (Princetone), J.S. Kim (The University of Arizona), M.D. Silverstone (The University of Arizona), J. Najita (NOAO), D.C. Hines (Space Sciences Institute), I. Pascucci (The University of Arizona), J.R. Stauffer (Spitzer Science Center), J. Bouwman (Max-Planck Institut fuer Astronomie), D.E. Backman (SETI Institute)

    (Submitted on 6 Dec 2007)

    Abstract: We report observations from the Spitzer Space Telescope (SST) regarding the frequency of 24 micron excess emission toward sun-like stars. Our unbiased sample is comprised of 309 stars with masses 0.7-2.2 Msun and ages from less than 3 Myr to greater than 3 Gyr that lack excess emission at wavelengths less than =8 microns. We identify 30 stars that exhibit clear evidence of excess emission from the observed 24/8 micron flux ratio. The implied 24 micron excesses of these candidate debris disk systems range from 13 % (the minimum detectable) to more than 100 % compared to the expected photospheric emission. The frequency of systems with evidence for dust debris emitting at 24 micron ranges from 8.5-19 % at ages less than 300 Myr to less than 4 % for older stars.

    The results suggest that many, perhaps most, sun-like stars might form terrestrial planets.

    Comments: Accepted for publication in the Astrophysical Journal Letters

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.1057v1 [astro-ph]

    Submission history

    From: Michael R. Meyer [view email]

    [v1] Thu, 6 Dec 2007 23:09:43 GMT (34kb)


  • ljk January 3, 2008, 10:01

    The Physics of Bodily Tides in Terrestrial Planets, and the Appropriate Scales of Dynamical Evolution

    Authors: Michael Efroimsky, Valery Lainey

    (Submitted on 13 Sep 2007 (v1), last revised 29 Dec 2007 (this version, v2))

    Abstract: Any model of tides is based on a specific hypothesis of how lagging depends on the tidal-flexure frequency. For example, Gerstenkorn (1955), MacDonald (1964), and Kaula (1964) assumed constancy of the geometric lag angle, while Singer (1968) and Mignard (1979, 1980) asserted constancy of the time lag. Thus, each of these two models was based on a certain law of scaling of the geometric lag.

    The actual dependence of the geometric lag on the frequency is more complicated and is determined by the rheology of the planet. Besides, each particular functional form of this dependence will unambiguously fix the appropriate form of the frequency dependence of the tidal quality factor, Q. Since at present we know the shape of the dependence of Q upon the frequency, we can reverse our line of reasoning and single out the appropriate actual frequency-dependence of the angular lag. This dependence turns out to be different from those employed hitherto, and it entails considerable alterations in the time scales of the tide-generated dynamical evolution. Phobos’ fall on Mars is an example we consider.

    Subjects: Astrophysics (astro-ph)

    Journal reference: Journal of Geophysical Research – Planets, Vol. 112, p. E12003 (2007)

    DOI: 10.1029/2007JE002908

    Cite as: arXiv:0709.1995v2 [astro-ph]

    Submission history

    From: Michael Efroimsky [view email]

    [v1] Thu, 13 Sep 2007 07:03:57 GMT (228kb)

    [v2] Sat, 29 Dec 2007 06:40:22 GMT (202kb)


  • ljk January 17, 2008, 1:45

    Terrestrial Planet Formation in Extra-Solar Planetary Systems

    Authors: Sean N. Raymond (University of Colorado)

    (Submitted on 16 Jan 2008)

    Abstract: Terrestrial planets form in a series of dynamical steps from the solid component of circumstellar disks. First, km-sized planetesimals form likely via a combination of sticky collisions, turbulent concentration of solids, and gravitational collapse from micron-sized dust grains in the thin disk midplane. Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also called “planetary embryos”. Finally, full-sized terrestrial planets accrete from protoplanets and planetesimals. This final stage of accretion lasts about 10-100 Myr and is strongly affected by gravitational perturbations from any gas giant planets, which are constrained to form more quickly, during the 1-10 Myr lifetime of the gaseous component of the disk. It is during this final stage that the bulk compositions and volatile (e.g., water) contents of terrestrial planets are set, depending on their feeding zones and the amount of radial mixing that occurs. The main factors that influence terrestrial planet formation are the mass and surface density profile of the disk, and the perturbations from giant planets and binary companions if they exist. Simple accretion models predicts that low-mass stars should form small, dry planets in their habitable zones. The migration of a giant planet through a disk of rocky bodies does not completely impede terrestrial planet growth. Rather, “hot Jupiter” systems are likely to also contain exterior, very water-rich Earth-like planets, and also “hot Earths”, very close-in rocky planets. Roughly one third of the known systems of extra-solar (giant) planets could allow a terrestrial planet to form in the habitable zone.

    Comments: 19 pages, 5 figures. To appear in the proceedings of IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, held in Suzhou, China, Oct 22-26 2007

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.2560v1 [astro-ph]

    Submission history

    From: Sean Raymond [view email]

    [v1] Wed, 16 Jan 2008 20:03:05 GMT (297kb,D)


  • ljk March 13, 2008, 12:36

    Spitzer/MIPS Observations of Stars in the Beta Pictoris Moving Group

    Authors: L. M. Rebull (SSC), K. R. Stapelfeldt (JPL), M. W. Werner (JPL), V. G. Mannings (SSC), C. Chen (NOAO), J. R. Stauffer (SSC), P. S. Smith (U.Arizona), I. Song (SSC), D. Hines (SSI), F. J. Low (U. Arizona)

    (Submitted on 11 Mar 2008)

    Abstract: We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24 and 70 microns for 30 stars, and at 160 microns for a subset of 12 stars, in the nearby (~30 pc), young (~12 Myr) Beta Pictoris Moving Group (BPMG). In several cases, the new MIPS measurements resolve source confusion and background contamination issues in the IRAS data for this sample.

    We find that 7 members have 24 micron excesses, implying a debris disk fraction of 23%, and that at least 11 have 70 micron excesses (disk fraction of >=37%). Five disks are detected at 160 microns (out of a biased sample of 12 stars observed), with a range of 160/70 flux ratios. The disk fraction at 24 and 70 microns, and the size of the excesses measured at each wavelength, are both consistent with an “inside-out” infrared excess decrease with time, wherein the shorter-wavelength excesses disappear before longer-wavelength excesses, and consistent with the overall decrease of infrared excess frequency with stellar age, as seen in Spitzer studies of other young stellar groups.

    Assuming that the infrared excesses are entirely due to circumstellar disks, we characterize the disk properties using simple models and fractional infrared luminosities. Optically thick disks, seen in the younger TW Hya and eta Cha associations, are entirely absent in the BPMG.

    Additional flux density measurements at 24 and 70 microns are reported for nine Tucanae-Horologium Association member stars. Since this is <20% of the association membership, limited analysis on the complete disk fraction of this association is possible.

    Comments: Accepted for ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1674v1 [astro-ph]

    Submission history

    From: Luisa Rebull [view email]

    [v1] Tue, 11 Mar 2008 20:34:17 GMT (178kb)


  • ljk November 21, 2008, 8:23

    Beta Pictoris planet finally imaged? (ESO 42/08)‏

    A team of French astronomers using ESO’s Very Large Telescope have discovered an object located very close to the star Beta Pictoris, and which apparently lies inside its disc. With a projected distance from the star of only 8 times the Earth-Sun distance, this object is most likely the giant planet suspected from the peculiar shape of the disc and the previously observed infall of comets onto the star.

    It would then be the first image of a planet that is as close to its host star as Saturn is to the Sun.

    Read more in ESO 42/08 at


    Kind Regards, The ESO education and Public Outreach Dept.

    21 November 2008