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Playing the Percentages: Terrestrial Planets

About two weeks ago we looked at the work of Michael Meyer (University of Arizona), whose team examined over 300 Sun-like stars (spectral types F5-K3) at mid-range infrared wavelengths. A wavelength of 24 microns detects warm dust, material at temperatures likely to be found between 1 and 5 AU from the parent star. The headline that day was Meyer’s contention that many if not most such stars produce terrestrial planets. Now Meyer is presenting these findings at the annual meeting of the American Association for the Advancement of Science, doubtless putting the exoplanet hunt back in the daily papers, at least for a day.

Terrestrial worlds in profusion

Bear in mind that in using the term ‘terrestrial’ we’re talking about small, rocky worlds like the inner planets of our Solar System. That could include worlds like our own, of course, but could also include hellish places like Mercury and Venus and their analogues around other stars. Nonetheless, it’s exciting to think that the chances of rocky planet formation are high enough to make Earth-like worlds a likely outcome around a large number of stars.

Image: This artist’s concept illustrates the idea that rocky, terrestrial worlds like the inner planets in our solar system may be plentiful, and diverse, in the universe. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech).

The stars surveyed by Meyer’s team were grouped by age. But does the warm dust found around ten to twenty percent of the stars in the four youngest age groups really indicate Earth-like planets? What we know is that stars older than 300 million years don’t show the kind of warm dust Meyer’s younger stars do, a possible indication that planet formation is complete. Now Thayne Currie and Scott Kenyon (Smithsonian Astrophysical Observatory) have performed a separate study on dust around ten to thirty-million year old stars, with results suggesting similar processes.

Here is the scenario: Warm dust should be detectable during planet formation because the collision of small rocky bodies in the circumstellar debris disk is building larger and larger objects, creating warm dust as the result of the activity. Says Kenyon: “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.”

So rocky planets may indeed be forming, but around what percentage of stars? The Spitzer data are susceptible to various interpretations, but if you want to shoot for the most favorable outcome for terrestrial worlds, you can look at them this way (the words are Meyer’s):

“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. 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.”

Now we’re in the sixty-percent plus zone Meyer flagged in his original paper. But we won’t know whether that or the conservative twenty percent figure is correct without the kinds of observation the Kepler mission will shortly commence. And then there’s the Giant Magellan Telescope, scheduled for completion in 2016 at its site at Las Campanas, Chile. Remarkably, astronomers are already talking about using its seven 8.4-meter primary mirrors to image Earth-like planets from the ground. The GMT will produce images ten times sharper than the best the Hubble Space Telescope can offer, another exceptional tool that should help us snare a terrestrial world by 2020 (although I bet an M-dwarf transit bags one much sooner).

The complete Meyer reference is in our previous coverage. The Currie/Kenyon study is Currie et al., “The Rise and Fall of Debris Disks: MIPS Observations of h and chi Persei and the Evolution of Mid-IR Emission from Planet Formation,” accepted by The Astrophysical Journal and available online. This Spitzer Space Telescope news release summarizes both studies.

Comments on this entry are closed.

  • Lubo February 19, 2008, 4:58

    Hi Paul!

    According to Seth Shostak, it’s only a matter of time before we find Earth-like world.

    Now what remain is CERN to prove that hyperspace travel is possible, and FTL won’t be a sci-fi any longer. We could reach these terrestrial planets in reasonable time 8-)

  • andy February 19, 2008, 10:32

    Well, if the discovery of truly Earth-like planets is getting ever closer, it might be wise to start considering protocols for pre-emptive transmissions now, rather than waiting for the discovery and having every wannabe ambassador for Earth who can get to a transmitter decide to act out their fantasy of being the one who speaks for all humanity.

  • Ronald February 19, 2008, 10:34

    “Remarkably, astronomers are already talking about using its seven 8.4-meter primary mirrors to image Earth-like planets from the ground”.

    This follows up on a question I asked recently: to what extent can we expect to image eartlike planets using ground-based telescopes (and thereby avoid the need for space-based interferometers)? Same for other planned giant telescopes, such as the European ELT and the American 30-meter telescope.

    Is there more information available particularly on this ambition of the GMT? Will it be possible to do all imaging and spectroscopic analysis from ground level (also in view of atmospheric background radiation)?

  • Administrator February 19, 2008, 14:41

    Ronald, I don’t know the answer re spectroscopic analysis — hard to see that working for terrestrial targets around other stars without a space platform, but if I can find anything further re GMT (and other giant telescopes) and exoplanet ambitions, I’ll post it right away.

  • ljk February 20, 2008, 0:52


    MOFFETT FIELD, Calif. – Sensitive detectors that may help find habitable planets orbiting distant stars as part of NASA’s Kepler Mission are undergoing tests at Ames Research Center, Moffett Field, Calif.

    Scheduled to launch in February 2009, the Kepler Mission will measure tiny variations in the brightness of stars to find planets that pass in front of them during their orbits. During these passes or “transits” the planets will slightly decrease the star’s brightness. The detectors are similar to the image detectors found in a digital camera, but much more sensitive.

    “This is a major milestone for the Kepler mission,” said David Koch, deputy principal investigator for the Kepler Mission. “We will use hardware identical to what we will be flying on Kepler in the test bed at Ames. We will have the ability to create transits of a star so that we can see the change in the star’s brightness. By simulating transits, we will be able to demonstrate that the flight hardware will work,” Koch explained.

    Kepler mission scientists will determine the frequency of Earth-size and larger planets in or near the habitable zone around other stars. Although hundreds of larger, Jupiter-like planets composed of gas already have been detected, Kepler mission scientists are seeking smaller planets where water, and perhaps, life, could exist.

    “We expect to find dozens of planets in the habitable zone of solar-like stars that are terrestrial size, rocky planets, similar to Earth,” said William Borucki, Kepler’s science principal investigator. “We will learn whether Earths are common or rare in our galaxy.”

    There will be 42 charge coupled devices (CCDs) used in the focal plane of the telescope during the actual mission. Together, the 42 CCDs make up a large array measuring about a foot square in Kepler’s telescope. This is the largest array of CCD detectors ever flown in space, Koch said.

    In this month’s Single String Transit Verification Test at Ames, scientists will be testing only one CCD, measuring approximately one-inch by two inches. Scientists will use a Kepler Technology Demonstration test bed to generate a star field, a pattern of stars, to represent that part of the sky where mission scientists will search for transits. The tests will verify the detectors’ ability to measure the tiny light intensity variations.

    In space, the array of detectors will be covered with sapphire field-flattener lenses and use a telescope, which Borucki said will search a region of sky 30,000 times larger that the Hubble Space Telescope is able to observe.

    Kepler is a NASA Discovery mission. NASA Ames is the home organization of the science principal investigator and is responsible for the ground system development, mission operations and science data analysis. Kepler mission development is managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Ball Aerospace & Technologies Corp., Boulder, Colo., is responsible for developing the Kepler flight system.

    For more information about the Kepler mission, visit:


  • Adam February 20, 2008, 3:57

    Hi All

    Dust disks are one thing, but do they lead inexorably to planets? I’m yet to be convinced we’re guaranteed a bunch of terrestrial planets whenever a star sports a dust-disk, though the modelling gets better and better all the time. When we actually spot terrestrials around stars without Jovians, then the case will be stronger. If we don’t, then things get really interesting for cosmogony.

  • Administrator February 20, 2008, 10:07

    The difference in outcomes cited in the Meyer paper reflects the uncertainty about how often dust disks lead to planets — hard to tell when looking at a specific star how long its disk has been there, or whether that disk has just begun to get active at a certain solar age. So you’re right, Adam, that we have a long way to go in understand these disks and the planets they spawn.

  • ljk February 22, 2008, 0:13

    Modelling solar-like variability for the detection of Earth-like planetary transits. I. Performance of the three-spot modelling and harmonic function fitting

    Authors: A. S. Bonomo, A. F. Lanza

    (Submitted on 21 Feb 2008)

    Abstract: We present a comparison of two methods of fitting solar-like variability to increase the efficiency of detection of Earth-like planetary transits across the disk of a Sun-like star. One of them is the harmonic fitting method that coupled with the BLS detection algorithm demonstrated the best performance during the first CoRoT blind test. We apply a Monte Carlo approach by simulating a large number of light curves of duration 150 days for different values of planetary radius, orbital period, epoch of the first transit, and standard deviation of the photon shot noise. Stellar variability is assumed in all the cases to be given by the Total Solar Irradiance variations as observed close to the maximum of solar cycle 23. After fitting solar variability, transits are searched for by means of the BLS algorithm.

    We find that a model based on three point-like active regions is better suited than a best fit with a linear combination of 200 harmonic functions to reduce the impact of stellar microvariability provided that the standard deviation of the noise is 2-4 times larger than the central depth of the transits. On the other hand, the 200-harmonic fit is better when the standard deviation of the noise is comparable to the transit depth.

    Our results show the advantage of a model including a simple but physically motivated treatment of stellar microvariability for the detection of planetary transits when the standard deviation of the photon shot noise is greater than the transit depth and stellar variability is analogous to solar irradiance variations.

    Comments: 8 pages, 6 figures, accepted by Astronomy & Astrophysics

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2990v1 [astro-ph]

    Submission history

    From: Antonino Francesco Lanza [view email]

    [v1] Thu, 21 Feb 2008 08:32:55 GMT (210kb)


  • Tom February 24, 2008, 3:54

    I disagree with Adam’s skepticism and here’s why: The 60% figure is probably the correct one. Radial velocity planet searches already tell us, and mind you they are far from complete, that greater than or equal to 12 percent of all FGK type stars have gas giants between 0 and 4 A.U (Marcy et al 2008). Microlensing searches tell us, albeit with larger error bars, that roughly 35 percent of M-dwarfs have super-earth planets between 1 and 4 A.U. (Gould et al 2006). So, even with our limited ability to detect planets, the preliminary figures are suggesting that a large fraction of stars have planets. I’d be willing to bet that the true fraction of stars with planets down to Mercury or Pluto size is at least 50%. Once you get debris swirling around a star it seems very hard NOT for a spherical body to coalesce out of it. All of the evidence to date coheres with the notion that planets are not just common but very common.

  • Adam February 25, 2008, 4:44

    Hi All

    Tom, there’s a long way from the proven presence of Jovians to the presence of terrestrials. And you’ve stacked the deck if you include planetoids like Pluto – anything up from Mercury/Mars-size is the limit of planethood, at least in Sol Space. Everything Eris-size and downwards is dwarf planet material – the building blocks of planets, not the finished product.

    Dust proves dust, planetesimals, comets, maybe Ceres-class dwarf planets, but the planet link is still hypothetical, based on cosmogonic theories still on trial in the court of observational planetology.

  • andy February 25, 2008, 13:13

    I’d guess the observed rarity of warm dust around older stars suggests that systems don’t typically end up with extensive asteroid belts (i.e. systems don’t end up as a bunch of “dwarf planets” in a belt of smaller objects)

    Whether that means that the majority of systems go all the way to planets, or whether the majority go to nothing (all the dust gets cleared out before it condenses to planets), we don’t know yet.

    Whether the lack of terrestrial planets (with the exception of PSR B1257+12) around millisecond pulsars implies rarity in general, I don’t know – such systems are likely to have had a rather different formation history to planets around a sunlike star.

  • Tom February 26, 2008, 1:10

    Actually, it is the coherence of the available evidence that makes me disagree with Adam’s skepticism. Adam mentioned the “there’s a long way from the proven presence of Jovians to the presence of terrestrials”, but microlensing searches are already telling us, and I’ll quote Andrew Gould of OSU here, that “These icy super-earths are pretty common,” Gould said. “Roughly 35 percent of all stars have them.” In other words, about a third of stars have already been shown to have large terrestrial or ice giant planets– a greater fraction than have gas giants! This is a direct refutation of Adam’s above statement which I just quoted. These icy super-earths almost certainly formed via core accretion which is precisely the mechanism that makes the most sense physically and is the one the Spitzer scientists are observing the consequences of. Also, the title of the press release said “may” not “proved.” I also never said “proven”– I am merely saying that it is rediculous to be skeptical about the existence of terrestrials with all of the accumulating, mutually reinforcing evidence for their commonality.

    I think such exoplanet skepticism will turn out to be wrong, as much of the skepticism regarding even the very existence of planets around other stars has turned out to be unfounded. Remember, not all that long ago the leading model for the formation of the solar system was that another star collided with or at least came very near to our Sun and the strewn matter from the collison turned into our solar system.

    The absence of terrestrial planets around millisecond pulsars should not be a reason for pessimism regarding the existence of terrestrial in general. Remember, scientists were surprised to find ANY planets around these end states of stellar evolution!

  • Adam February 26, 2008, 4:40

    Hi andy

    That’s what I really meant – it’s as far as present inferences can determine.

    OTOH there’s nothing in the evidence against disks becoming terrestrial planets in most cases – simulations form planets readily once planetoids are available, though there are subtleties yet to be fully understood in oligarchic growth. Eccentricities of the final planets still don’t match the Solar System for example, though simulations are getting closer all the time.

  • Adam February 28, 2008, 6:15

    Hi Tom

    Another point worth mentioning is that we still don’t have a good grasp on what such “icy super-earths” are predominantly made of. Are they really sub-Neptunes formed from gravitational instabilities, or terrestrials accreted out of a dust disk? We need more transit data, more definite radii of the “super-earths” before we can say they’re really “Earths” or “Neptunes”.

    And, in the end, there may be no difference at all, and this argument is just hair-splitting.

  • ljk March 22, 2008, 17:18

    Google Joins MIT in Search for Earth-like Planets

    “When starships transporting colonists first depart the solar system, they may well be headed toward a TESS-discovered planet as their new home.”

    George R. Ricker, senior research scientist at the Kavli Institute for Astrophysics and Space Research at MIT

    Google has joined MIT scientists who are designing a satellite-based observatory -the Transiting Exoplanet Survey Satellite (TESS)- that they say could for the first time provide a sensitive survey of the entire sky to search for earth-like planets outside the solar system that appear to cross in front of bright stars. Google will fund development of the wide-field digital cameras needed for the satellite.

    “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,” says George Ricker, leader of the project.

    Most of the more than 200 extrasolar planets discovered so far have been much larger than Earth, similar in size to the solar system’s giant planets (ranging from Jupiter to Neptune), or even larger. But to search for planets where there’s a possibility of finding signs of living organisms, astronomers are much more interested in those that are similar to our own world.

    Most searches so far depend on the gravitational attraction that planets exert on their stars in order to detect them, and therefore are best at finding large planets that orbit close to their stars. TESS, however, would search for stars whose orbits as seen from Earth carry them directly in front of the star, obscuring a tiny amount of starlight. Some ground-based searches have used this method and found about 20 planets so far, but a space-based search could detect much smaller, Earth-sized planets, as well as those with larger orbits.

    This transit-detection method, by measuring the exact amount of light obscured by the planet, can pinpoint the planet’s size. When combined with spectroscopic follow-up observations, it can determine the planet’s temperature, probe the chemistry of its atmosphere, and perhaps even find signs of life, such as the presence of oxygen in the air.

    The satellite will be equipped with six high-resolution, wide-field digital cameras, which are now under development. Two years after launch, the cameras–which have a total resolution of 192 megapixels–will cover the whole sky, getting precise brightness measurements of about two million stars in total.

    Statistically, since the orientation of orbits is random, about one star out of a thousand will have its planets’ orbits oriented perpendicular to Earth so that the planets will regularly cross in front of it, which is called a planetary transit. So, out of the two million stars observed, the new observatory should be able to find more than a thousand planetary systems within two years.

    In fact, if a new estimate based on recent observations of dusty disks is confirmed, there might even be up to 10 times as many such planets.

    Because the satellite will be repeatedly taking detailed pictures of the entire sky, the amount of data collected will be enormous. As a result, only selected portions will actually be transmitted back to Earth. But the remaining data will be stored on the satellite for about three months, so if astronomers want to check images in response to an unexpected event, such as a gamma-ray burst or supernova explosion, “they can send us the coordinates [of that event] and we could send them the information,” Ricker says.

    Because of the huge amount of data that will be generated by the satellite, which could launched as early as 2012, Google has an interest in working on the development of ways of process that data to find useful information.

    Regardless of the funding for the satellite, the same wide-field cameras being developed for TESS could also be used for a planned ground-based search for dark matter in the universe–the invisible, unknown material that astronomers believe is more prevalent in space than the ordinary matter that we can see. Some of the unknown dark-matter particles must constantly be striking the Earth, and the plan is to train a bank of cameras inside tanks of fluid deep underground, to detect flashes of light produced by the impacts of these dark particles. Ricker’s Kavli group is participating with MIT physics professor Peter Fisher’s team in this new physics research initiative.

    The electronic detectors for the new cameras are being developed in collaboration with MIT’s Lincoln Laboratory. The lab’s expertise in building large, highly sensitive detectors is a significant factor in making possible these unique cameras, which have no moving parts at all. If all goes well and funding is secured, the satellite could be launched in 2012 with NASA support, or even earlier with a private sponsor.

    Posted by Casey Kazan. Adapted from an MIT release.


  • ljk April 4, 2008, 10:33

    How to Find Other “Earths”

    Technology Review April 3, 2008


    Harvard-Smithsonian Center for
    Astrophysics researchers have
    adapted a laser technology to
    discern the faint gravitational
    influence that Earth-like planet
    revolving around a Sun-like star
    exert on their home stars’ light
    output, based on the induced wobble.
    (Chih-Hao Li, Center for
    Astrophysics) Their system increases
    the precision of…