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Terrestrial Worlds May Be Common

We’re still arguing about how giant planets form around Sun-like stars, but terrestrial planets seem to be less controversial. Assuming the model is right, we start with a swarm of planetesimals in the range of one kilometer in size. As these objects grow, out to a range of at least 2 AU, the largest bodies at some point go through a runaway period of chaotic growth marked by collisions. Emerging from the debris should be terrestrial worlds, some in Earth-like orbits. Add to this the fact that gas and dust disks seem to be relatively routine outcomes of star formation and you have an indication that small rocky planets may be widespread.

The problem with all this is that theory has to be matched with observation. On that score, new work by Mike Meyer (University of Arizona) and colleagues Lynne Hillenbrand and John Carpenter (California Institute of Technology) is instructive. The researchers chose to look at mid-range infrared emissions at the 24 micron level, a range chosen because it originates between 1 and 10 AU from the parent star. As targets, they chose 328 Sun-like stars in spectral types F5-K3, with masses generally not dissimilar from the Sun (though in some cases ranging as high as 2.2 solar masses). A finding of excess emissions at 24 µm was taken to be evidence of dust debris from planetesimal collisions.

The conclusions from this work are absorbing indeed. From the paper (internal references omitted for brevity):

We suggest that SST observations at 24 µm can be interpreted as evidence for terrestrial planet formation occurring around many (19–32 %), if not most (62 %), sun–like stars. This range is higher than the observed frequency of gas giant planets (6.6–12 %) within 5–20 AU…but comparable to the inference that cool dust debris beyond 10 AU might be very common…Radial velocity monitoring of low mass stars, micro-lensing surveys, as well as transit surveys such as COROT and Kepler, will provide critical tests of our interpretation.

And so we test theory with observation (and note the reference to the doughty COROT mission, gathering key data at unprecedented rates as its work continues, and doubtless setting us up for its share of surprises). But the broader picture growing out of the work of Meyer’s team is that terrestrial planets may be common around Sun-like stars, an assumption most everyone connected with the exoplanet hunt would be delighted to see confirmed. Not only would it be striking evidence that the formation mechanisms for Earth-like planets are becoming better understood, but it would strengthen the hope for living worlds around stars for which the conditions of life may not be so rare after all.

The paper is Meyer et al., “Evolution of Mid‐Infrared Excess around Sun‐like Stars: Constraints on Models of Terrestrial Planet Formation,” Astrophysical Journal Letters 673 (February 1, 2008), pp. L-181-L184 (abstract); also available in full text here. I notice that Science News has picked up on this team’s work as well, with a story quoting Caltech astronomer Charles Beichman (not a member of Meyer’s group):

“Meyer’s result is exciting confirmation that around many other stars like our sun, the region analogous to our own asteroid belt is full of solid material, possibly related to past or present planet formation.”

Note the ‘possibly’ in that sentence, a reminder of how much work remains to be done. Beichman goes on to call the work “…a good sign that the basic stuff of planetary systems is widespread.” All of which gibes with current thinking, but there is no substitute for getting the right hardware into space (think Kepler and beyond) to verify the existence of those tantalizing worlds. Kepler is scheduled for launch next February.

Comments on this entry are closed.

  • ljk February 4, 2008, 10:06

    Not-so-rare Earth?

    Several years ago, the authors of the book “Rare Earth”
    argued that “complex” life in the universe was likely very
    uncommon. Taylor Dinerman argues that recent discoveries,
    particularly of extrasolar planets, put that hypothesis in


  • ljk February 4, 2008, 12:12

    H2O and OH gas in the terrestrial planet-forming zones of protoplanetary disks

    Authors: C. Salyk, K.M. Pontoppidan, G.A. Blake, F. Lahuis, E.F. van Dishoeck, N.J. Evans II

    (Submitted on 1 Feb 2008)

    Abstract: We present detections of numerous 10-20 micron H2O emission lines from two protoplanetary disks around the T Tauri stars AS 205A and DR Tau, obtained using the InfraRed Spectrograph on the Spitzer Space Telescope. Follow-up 3-5 micron Keck-NIRSPEC data confirm the presence of abundant water and spectrally resolve the lines. We also detect the P4.5 (2.934 micron) and P9.5 (3.179 micron) doublets of OH and 12CO/13CO v=1-0 emission in both sources. Line shapes and LTE models suggest that the emission from all three molecules originates between ~0.5 and 5 AU, and so will provide a new window for understanding the chemical environment during terrestrial planet formation. LTE models also imply significant columns of H2O and OH in the inner disk atmospheres, suggesting physical transport of volatile ices either vertically or radially; while the significant radial extent of the emission stresses the importance of a more complete understanding of non-thermal excitation processes.

    Comments: 9 pages, 3 figures, 1 table, aastex, to appear in the Astrophysical Journal

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.0037v1 [astro-ph]

    Submission history

    From: Colette Salyk [view email]

    [v1] Fri, 1 Feb 2008 00:01:00 GMT (183kb)


  • James M. Essig February 5, 2008, 4:31

    Hi ljk;

    I hope very soon we start to discover some of these Earth mass class terrestrial worlds. The the race to send manned missions to investigate would heat up. George mentioned to me that a recent issue of Scientific American states that if enough money were provided, we could design, assemble, and launch a manned fusion rocket to reach 0.12 C for manned excursions to our stellar niehboors within 10 years from the beginning of the project to the launch date. If the large ship rotated to provide artificial gravity, bone loss would not be a problem. The ship’s crew could contain highly trained professionals in there mid twenties which I am sure could be more than capable of carrying out their mission especially in light of the fact that many of the U.S. armed forces best fighter pilots are in their early to late 20s as well as are many of the crew members serving aboard nuclear powered, nuclear armed, submarines and aircraft carriers. They might be senior citizens by the time they arrived at their destination but they might still be as healthy well seasoned military generals by the time they reached their destinations. They would probably stay and study the system in their remaining years while new missions with craft that can achieve even higher gamma factors could be launched from Earth. The whole process could bootstrap up from there.



  • James M. Essig February 5, 2008, 13:50

    Hi Folks;

    Just a quick error to correct in my last posting above. The magazine article that quoted 0.12 C was not stated as being Scientific American, but rather another earlier publication several years ago. And there was no mention of the specific propulsion mechanism by George.

    As a result George, I apologize for the mis-representation. I will make every effort to be more careful in quoting others in the future. It is my policy to self-report any such errors that I make as soon as I realize what has been said. Still, 0.12 C is not bad.


    Your Friend Jim

  • MaDeR February 5, 2008, 18:45

    I do not think that is practical with current technology. Undoubtedly this kind of discovery would put pressute to develop and launch true interstellar probe, but frankly saying, we should not talking about manned interstellar missions when we even didn’t ever left Earth’s gravity influence. Yes, even moon missions was like that: if we decided by whatever reason not to orbit and land on moon, rocked would “return” to earth almost automatically thanks to law of physics.

    Baby steps before, WAY before running. And no, Mars will not suffice. I think that we would qualify to dream about interstellar manned mission only when we will control solar system. (control defined as capability to get into any place in solar system in reasonable amount of time with reasonable cost)

  • Ronald February 6, 2008, 4:27

    As I mentioned before, one step at a time: a ‘race to send manned missions’ seems vastly premature.
    First discovery and increasingly detailed (telescopic) description of terrestrial planets, then robot probes, only then, maybe in the more distant future, human missions. For scientific and exploratory purposes, telescopes and probes will (nearly) always be more realistic (and a lot cheaper) than humans.

    I see human missions as the ultimate goal of space colonization, rather than a means of exploration. And even then, some kind of suspended animation (hibernation) is probably a much safer and economic mode than constant activity, when travelling for decades. And colonization (and maybe terraforming) of Mars also seems a more realistic initial step.

  • george scaglione February 6, 2008, 10:05

    jim dont worry about it for a moment i myself find it difficult to keep on top of all i try to do.i think you are refering to the great article from discover magazine i quoted from aug of 03!!! the latest sci am was really good too as i think i had mentined. all the best, george

  • ljk February 6, 2008, 23:45

    Dynamical Shakeup of Planetary Systems II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping

    Authors: E. W. Thommes, M. Nagasawa, D. N. C. Lin

    (Submitted on 5 Feb 2008)

    Abstract: We revisit the “dynamical shakeup” model of Solar System terrestrial planet formation, wherein the whole process is driven by the sweeping of Jupiter’s secular resonance as the gas disk is removed. Using a large number of 0.5 Gyr-long N-body simulations, we investigate the different outcomes produced by such a scenario. We confirm that in contrast to existing models, secular resonance sweeping combined with tidal damping by the disk gas can reproduce the low eccentricities and inclinations, and high radial mass concentration, of the Solar System terrestrial planets. At the same time, this also drives the final assemblage of the planets on a timescale of several tens of millions of years, an order of magnitude faster than inferred from previous numerical simulations which neglected these effects, but possibly in better agreement with timescales inferred from cosmochemical data. In addition, we find that significant delivery of water-rich material from the outer Asteroid Belt is a natural byproduct.

    Comments: To appear in ApJ

    Subjects: Astrophysics (astro-ph)

    Journal reference: ApJ 675, March 1, 2008

    Cite as: arXiv:0802.0541v1 [astro-ph]

    Submission history

    From: E. W. Thommes [view email]

    [v1] Tue, 5 Feb 2008 03:32:13 GMT (138kb)


  • ljk February 8, 2008, 23:46

    Dusty Disk Around Nearby Star May Hide Earth-like Planet


    “A recent survey by a team of Japanese astronomers may
    have found an Earth-like planet hidden in the dust around a
    nearby star.

    Using the Coronagraphic Imager with Adaptive Optics (CIAO)
    at the Subaru Telescope, researchers recently resolved a
    circumstellar disk around the young lightweight star FN Tau.”

  • ljk February 17, 2008, 11:52

    Many, Perhaps Most, Nearby Sun-Like Stars May Form Rocky Planets

    Astronomers have discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy. These new results suggest that worlds with potential for life might be more common than we thought.

    University of Arizona, Tucson, astronomer Michael Meyer and his colleagues used NASA’s Spitzer Space Telescope to determine whether planetary systems like ours are common or rare in our Milky Way galaxy. They found that at least 20 percent, and possibly as many as 60 percent, of stars similar to the sun are candidates for forming rocky planets.

    Meyer is presenting the findings at the annual meeting of the American Association for the Advancement of Science in Boston. The results appear in the Feb. 1 issue of Astrophysical Journal Letters.

    The astronomers used Spitzer to survey six sets of stars, grouped depending on their age, with masses comparable to our sun. The sun is about 4.6 billion years old. “We wanted to study the evolution of the gas and dust around stars similar to the sun and compare the results with what we think the solar system looked like at earlier stages during its evolution,” Meyer said.

    The Spitzer telescope does not detect planets directly. Instead it detects dust — the rubble left over from collisions as planets form — at a range of infrared wavelengths. The hottest dust is detected at the shortest wavelengths, between 3.6 microns and 8 microns. Cool dust is detected at the longest wavelengths, between 70 microns and 160 microns. Warm dust can be traced at 24-micron wavelengths. Because dust closer to the star is hotter than dust farther from the star, the “warm” dust likely traces material orbiting the star at distances comparable to the distance between Earth and Jupiter.

    “We found that about 10 to 20 percent of the stars in each of the four youngest age groups shows 24-micron emission due to dust,” Meyer said. “But we don’t often see warm dust around stars older than 300 million years. The frequency just drops off.

    “That’s comparable to the time scales thought to span the formation and dynamical evolution of our own solar system,” he added. “Theoretical models and meteoritic data suggest that Earth formed over 10 to 50 million years from collisions between smaller bodies.”

    In a separate study, Thayne Currie and Scott Kenyon of the Smithsonian Astrophysical Observatory, Cambridge, Mass., and their colleagues also found evidence of dust from terrestrial planet formation around stars from 10 to 30 million years old. “These observations suggest that whatever led to the formation of Earth could be occurring around many stars between three million and 300 million years old,” Meyer said.

    Kenyon and Ben Bromley of the University of Utah, Salt Lake City, have developed planet formation models that provide a plausible scenario. Their models predict warm dust would be detected at 24-micron wavelengths as small rocky bodies collide and merge. “Our work suggests that the warm dust Meyer and colleagues detect is a natural outcome of rocky planet formation. We predict a higher frequency of dust emission for the younger stars, just as Spitzer observes,” said Kenyon.

    The numbers on how many stars form planets are ambiguous because there’s more than one way to interpret the Spitzer data, Meyer said. The warm-dust emission that Spitzer observed around 20 percent of the youngest cohort of stars could persist as the stars age. That is, the warm dust generated by collisions around stars three to 10 million years old could carry over and show up as warm dust emission seen around stars in the 10- to 30- million-year-old range and so on. Interpreting the data this way, about one out of five sun-like stars is potentially planet-forming, Meyer said.

    There’s another way to interpret the data. “An optimistic scenario would suggest that the biggest, most massive disks would undergo the runaway collision process first and assemble their planets quickly. That’s what we could be seeing in the youngest stars. Their disks live hard and die young, shining brightly early on, then fading,” Meyer said.

    “However, smaller, less massive disks will light up later. Planet formation in this case is delayed because there are fewer particles to collide with each other.”

    If this is correct and the most massive disks form their planets first and the wimpiest disks take 10 to 100 times longer, then up to 62 percent of the surveyed stars have formed, or may be forming, planets. “The correct answer probably lies somewhere between the pessimistic case of less than 20 percent and optimistic case of more than 60 percent,” Meyer said.

    The next critical test of the assertion that terrestrial planets like Earth could be common around stars like the sun will come next year with the launch of NASA’s Kepler mission.

    Meyer’s 13 co-authors include John Carpenter of the California Institute of Technology in Pasadena. NASA’s Jet Propulsion Laboratory in Pasadena manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA. More information about Spitzer is at http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .