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A Terrestrial Planet Finder That Works

Yesterday we looked at ESA’s Darwin mission, and the plan to use a fleet of space telescopes to see planets around other stars. How else could you accomplish this goal? One option is a starshade like New Worlds, working with a distant space telescope to null out glare from the star. Another is an internal coronagraph, a device within the telescope itself that masks the glare. Centauri Dreams has backed the starshade idea, looking at its practicality and advantages over existing coronagraph designs (click here to see a breakdown of the pros and cons of each).

JPL coronagraph test results

But what if the coronagraph were dramatically improved? Scientists at the Jet Propulsion Laboratory believe they have done just that. In fact, John Trauger, lead author on a paper on this work that has just appeared in Nature, has this to say: “Our experiment demonstrates the suppression of glare extremely close to a star, clearing a field dark enough to allow us to see an Earth twin. This is at least a thousand times better than anything demonstrated previously.” That’s a bold claim, but one rigorously developed in JPL’s High Contrast Imaging Testbed, which allows coronagraph technologies to be examined in a space-like environment.

Image: Three simulated planets — one as bright as Jupiter, one half as bright as Jupiter and one as faint as Earth — stand out plainly in this image created from a sequence of 480 images captured by the High Contrast Imaging Testbed at JPL. A roll-subtraction technique, borrowed from space astronomy, was used to distinguish planets from background light. The asterisk marks the location of the system’s simulated star. Credit: NASA/JPL-Caltech

So how do you get past the major coronagraph obstacles? Diffracted light can create a spike or ring pattern around stars in a telescopic image, blocking the faint light of planets. Trauger’s team uses a set of masks that clear the diffraction artifacts while blocking much of the starlight. A second problem is scattered light, the speckling caused by ripples in the telescope mirror. It is addressed by using a deformable mirror whose surface can be tweaked by computer-controlled actuators. Up next for the team is an improvement in speckle suppression as the technology battles for a place on a future terrestrial planet finding mission.

What lies ahead for this latest coronagraph concept? Ian Jordan (CSC/STScI) described Trauger’s work as ‘significant progress’ toward the goal of finding extrasolar terrestrial planets. And he agreed with Trauger and collaborator Wesley Traub that demonstrating the instrument’s sensitivity in broadband optical light — and not just in the coherent laser light thus far used — will further test and extend the idea. Says Jordan:

If the passband for adequate suppression is too small, then the throughput of the system will remain ‘slow’. Slow is problematic for several reasons: 1) exoterrestrial analogues are inherently faint and their photons are few and far between anyway, 2) spectroscopy of the planetary signatures is important for determining what the faint objects are and may demand relatively high system throughput.

Throughput is critical for finding and studying the faint objects the coronagraph is designed to hunt. “Telescope time is precious,” adds Jordan, “and preserving as many of the planetary photons making their way to the aperture from as much of the field of view as possible is a goal for TPF.” Another coronagraph advantage: ‘slew’ times — the time needed to move the telescope to a new target — are much shorter for internal coronagraph designs than for external occulters.

Centauri Dreams‘ take: This impressive work keeps the coronagraph in serious competition with starshade concepts as we work out the technologies for future planet finder missions. A major constraint will be cost — let’s see how the numbers match up as both technologies continue to be refined within budgets we can realistically hope to meet.

The paper is Trauger, John and Wesley Traub, “A laboratory demonstration of the capability to image an Earth-like extrasolar planet,” Nature 446 (12 April 2007), pp. 771-773 (abstract available).

Comments on this entry are closed.

  • Adam April 14, 2007, 21:20

    Hi Paul

    Imagine a trick coronagraph and a HUGE “inflatable” space mirror telescope. Or a linked network of them. If we ever get out into space in a big way then we can expect some amazing telescopic work – any interstellar destination within hundreds of light-years will be known to have planets or otherwise, and nearby systems will probably have ~ 100 pixel maps of the target planets available. Habitable planets will probably be priority targets for Solar Gravity Focus telescopes to map in detail – starships will have a pretty good idea of where they’re going and what they’ll find before they set out.

    Depending on the drive system I can also imagine starships being tracked directly – mag-sails are especially detectable in radio frequencies, while high-powered fusion/m-am drives can be spotted visually from a few parsecs or so. Laser beacons will make tracking even easier, so potentially we won’t be left in the dark if a star-probe or ship suffers catastrophic failure – an important consideration if we’re going to the stars in large numbers eventually.

  • Administrator April 15, 2007, 12:08

    This is one of the biggest changes in concept I’ve seen over the past fifteen years or so of interstellar thinking. Because now, instead of talking about sending a robotic probe to see what’s around another star, we’re looking at technologies that will tell us from near-Earth space (at least, within our own Solar System) what we’re going to be finding if and when we do send such probes. That makes Dan Goldin’s goal of having images of extraterrestrial worlds down to continent-level in the average classroom a real possibility in this century. Hey, is this a great time to be alive or what?

  • Adam April 16, 2007, 0:45

    Absolutely. I like the idea of showing kids new planets in my classrooms – once I make it through. Still have that rubicon to cross.

    It’s a tragedy that so many in our time are hooked on the idea of a catastrophic world-change – either Christian, Jewish, Muslim or New Age – without seeing the potential for spiritual “earth-changes” from our looking hard at the stars. There’s more things in heaven and earth, than in that philosophy.


  • Ronald April 16, 2007, 3:26

    Adam, Admin.
    How true and exciting! And how sad to note the short-sightedness of so many politicians (and other people) that keep cutting budgets of (relatively low-priced) important scientific projects to spend it on all sorts of politically fashionable hobbies (or wars).
    Slightly off-topic, but I didn’t know where else to put it: one of the recently discovered planets, HD 125612 b (see Extrasolar Planets Encyclopedia), is probably situated within the habitable zone (1.2 AU) of its (G3V) mother star. However, it is of giant Jupiter mass (3.2 MJ). There has been some theoretical research in recent years considering the habitability of large moons of this giant gas planet and, if I remember well, there are good possibilities for habitable moons near these planets in a habitable zone. Kind of ‘Moon of Endor’ (sorry, but I just can’t help loving that image).
    Question: could an advanced interferometer/coronagraph as described offer any hope of detecting a ‘life-bearing’ moon near a gas giant, or would the image of such a moon always simply be overwhelmed by the reflection of its giant mother???

  • Chris Wren April 16, 2007, 15:10

    Even Canada could fund a terrestrial planet finding mission from the money our government currently invests in promoting obscure arts projects ostensibly meant to inspire the Canadian public. While I don’t want to get into an argument about the merits of various art forms, I can’t personally think of anything that would be more inspiring to people than participating in a search for other earths.

  • ljk April 16, 2007, 15:15

    But the public, Canadian and everywhere else, has to have
    some understanding and appreciation of the science and
    astronomy in particular for this to happen.

    Why do you think most forms of entertainment are usually
    the lowest common denominator?

    I have met far more adults than there ever should be who
    know virtually nothing about astronomy and general science,
    and who also perceive it as boring and the realm of the nerd.

    So before we go expecting the public and the government to
    fund a terrestrial planet finder or any other type of space
    mission, we need to make people at least realize and have
    some understanding of the Universe they live in. Otherwise
    it will remain remote and unrelated to their lives. Or worse –
    not cool.

  • ljk April 20, 2007, 14:43

    Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood

    Authors: I. Neill Reid (STScI), Edwin L. Turner (Princeton University), Margaret C. Turnbull, M. Mountain, Jeff A. Valenti (STScI)

    (Submitted on 18 Apr 2007)

    Abstract: The chemical composition of Earth’s atmosphere has undergone substantial evolution over the course of its history. It is possible, even likely, that terrestrial planets in other planetary systems have undergone similar changes; consequently, the age distribution of nearby stars is an important consideration in designing surveys for Earth-analogues. Valenti & Fischer (2005) provide age and metallicity estimates for 1039 FGK dwarfs in the Solar Neighbourhood. Using the Hipparcos catalogue as a reference to calibrate potential biases, we have extracted volume-limited samples of nearby stars from the Valenti-Fischer dataset. Unlike other recent investigations, our analysis shows clear evidence for an age-metallicity relation in the local disk, albeit with substantial dispersion at any epoch. The mean metallicity increases from -0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day. Supplementing the Valenti-Fischer measurements with literature data to give a complete volume-limited sample, the age distribution of nearby FGK dwarfs is broadly consistent with a uniform star-formation rate over the history of the Galactic disk. In striking contrast, most stars known to have planetary companions are younger than 5 Gyrs; however, stars with planetary companions within 0.4 AU have a significantly flatter age distribution, indicating that those systems are stable on timescales of many Gyrs. Several of the older, lower metallicity host stars have enhanced [alpha/Fe] ratios, implying membership of the thick disk. If the frequency of terrestrial planets is also correlated with stellar metallicity, then the median age of such planetary system is likely to be ~3 Gyrs. We discuss the implications of this hypothesis in designing searches for Earth analogues among the nearby stars.


    Accepted for publication in ApJ


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0704.2420v1 [astro-ph]

    Submission history

    From: I. Neill Reid [view email]

    [v1] Wed, 18 Apr 2007 20:11:57 GMT (468kb)


  • ljk April 27, 2007, 9:56

    Optimal Occulter Design for Finding Extrasolar Planets

    Authors: R.J. Vanderbei, N.J. Kasdin, E. Cady

    (Submitted on 26 Apr 2007)

    Abstract: One proposed method for finding terrestrial planets around nearby stars is to use two spacecraft–a telescope and a specially shaped occulter that is specifically designed to prevent all but a tiny fraction of the starlight from diffracting into the telescope. As the cost and observing cadence for such a mission will be driven largely by the separation between the two spacecraft, it is critically important to design an occulter that can meet the observing goals while flying as close to the telescope as possible. In this paper, we explore this tradeoff between separation and occulter diameter. More specifically, we present a method for designing the shape of the outer edge of an occulter that is as small as possible and gives a shadow that is deep enough and large enough for a 4m telescope to survey the habitable zones of many stars for Earth-like planets. In particular, we show that in order for a 4m telescope to detect in broadband visible light a planet 0.06 arcseconds from a star shining $10^{10}$ times brighter than the planet requires a specially-shaped occulter 50m in diameter positioned about $72,000$ km in front of the telescope.


    14 pages, 4 figures, 15 subfigures


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0704.3488v1 [astro-ph]

    Submission history

    From: R.J. Vanderbei [view email]

    [v1] Thu, 26 Apr 2007 07:49:09 GMT (209kb,D)


  • ljk May 3, 2007, 11:30

    An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with SDI at the VLT and MMT

    Authors: Beth A. Biller, Laird M. Close, Elena Masciadri, Eric Nielsen, Rainer Lenzen, Wolfgang Brandner, Donald McCarthy, Markus Hartung, Stephan Kellner, Eric Mamajek, Thomas Henning, Douglas Miller, Matthew Kenworthy, Craig Kulesa

    (Submitted on 1 May 2007)

    Abstract: We present the results of a survey of 45 young ( 10 mag (5 sigma) at a separation of 0.5″ from the primary star on 45% of our targets and H band contrasts of > 9 mag at a separation of 0.5” on 80% of our targets. With this degree of attenuation, we should be able to image (5sigma detection) a 5 M_{Jup} planet 15 AU from a 70 Myr K1 star at 15 pc or a 5 M_{Jup} planet at 2 AU from a 12 Myr M star at 10 pc. We believe that our SDI images are the highest contrast astronomical images ever made from ground or space for methane rich companions

  • ljk July 13, 2007, 15:11

    Technology challenges for space interferometry: the option of mid-infrared integrated optics

    Authors: L. Labadie (MPIA/Laog), P. Kern (LAOG), P. Labeye (CEA Leti), E. Le Coarer (LAOG), C. Vigreux-Bercovici (LPMC), A. Pradel (LPMC), J.-E. Broquin (IMEP), V. Kirschner (ESA/Estec)

    (Submitted on 11 Jul 2007)

    Abstract: Nulling interferometry is a technique providing high angular resolution which is the core of the space missions Darwin and the Terrestrail Planet Finder. The first objective is to reach a deep degree of starlight cancelation in the range 6 — 20 microns, in order to observe and to characterize the signal from an Earth-like planet. Among the numerous technological challenges involved in these missions, the question of the beam combination and wavefront filtering has an important place. A single-mode integrated optics (IO) beam combiner could support both the functions of filtering and the interferometric combination, simplifying the instrumental design.

    Such a perspective has been explored in this work within the project Integrated Optics for Darwin (IODA), which aims at developing a first IO combiner in the mid-infrared. The solutions reviewed here to manufacture the combiner are based on infrared dielectric materials on one side, and on metallic conductive waveguides on the other side. With this work, additional inputs are offered to pursue the investigation on mid-infrared photonics devices.

    Comments: Accepted in Adv. in Space Research

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.1701v1 [astro-ph]

    Submission history

    From: Lucas Labadie [view email]

    [v1] Wed, 11 Jul 2007 20:53:12 GMT (894kb)


  • ljk February 22, 2008, 9:11


    Feb. 21, 2008

    NASA has awarded the University of Colorado at Boulder $1 million to
    lead the study of a space observatory to find Earth-like planets in
    distant solar systems and open the search for life outside our solar

    A second proposal from the Naval Research Laboratory involving CU-
    Boulder also was selected for $500,000 in NASA funding. The proposal
    would place a low-frequency radio telescope on the far side of the
    moon to probe the first structures that formed in the early universe.

    The CU-Boulder planetary proposal, called the New Worlds Observer,
    was one of 19 proposals for major new observatories in the coming
    decade selected for further study. The New Worlds Observer proposal
    features a giant, daisy-shaped plastic “starshade” to block starlight
    and allow a telescope to image the faint light from distant planets
    circling other stars, said Professor Webster Cash, chief scientist on
    the effort.

    Astronomers will, for the first time, be able to identify planetary
    features like oceans, continents, polar caps and cloud banks and even
    detect biomarkers like methane, oxygen and water if they exist, said
    Cash, who is chair of the astrophysical and planetary sciences
    department. The 4-meter telescope planned for the project will be
    larger and more powerful than the 2.4-meter mirror on the Hubble
    Space Telescope, allowing researchers to record the light from rocky
    planets tens of trillions of miles away.

    The telescope and its 50-yard-diameter starshade would launch into an
    orbit roughly 1 million miles from Earth, with the parasol unfurling
    and moving via thrusters into the lines of sight of nearby stars
    thought to harbor planets, said Cash. About 80 percent of the cost of
    the New World’s Observer would go toward telescope development with
    20 percent for the development of the starshade. “This observatory
    can be built today with existing technology,” said Cash.

    The Naval Research Laboratory-led proposal, known as the Dark Ages
    Lunar Interferometer, is for a low-frequency lunar radio telescope
    that would search for the faint hiss generated by pristine,
    primordial material as it formed the first stars and galaxies when
    the universe literally was still dark. CASA Professor Jack Burns and
    his colleagues will be developing a novel kind of radio telescope
    with elements embedded within plastic sheets. Astronauts could
    unroll the sheets on the far side of the moon, shielded from the
    interference of Earth-generated radio noise and with an unprecedented
    view of the sky.

    Proposals by the 19 teams selected by NASA Feb. 15 will help guide
    decisions made during the Astronomy and Astrophysics Decadal Survey
    in 2010, led by the National Academy of Sciences to identify the most
    promising space observatory proposals. The 2008 NASA awards for the
    next-generation of astronomy missions ranged from $250,000 to $1
    million each.

    “New World Observer is a clean win for us because we got full funding
    and high ratings from NASA,” said Cash. “This puts us on firm
    footing to compete with the other mission concepts for the right to
    build the next major observatory in space.” The estimated cost to
    design and build the New Worlds Observer mission would be roughly
    $3.3 billion, said Cash.

    The New Worlds Observer team also includes researchers from NASA’s
    Goddard Space Flight Center, Ball Aerospace of Boulder, Northrop
    Grumman Corp. and other research institutions around the world.

    In addition, CASA researchers were members of several of the other
    winning NASA proposals for new observatories. Co-investigators on
    other efforts included Cash, Professor James Green, Professor Michael
    Shull and Research Associate Matthew Beasley. Green also is the
    Principal Investigator on a $70 million instrument known as the
    Cosmic Origins Spectrograph, which will be inserted on the Hubble
    Space Telescope during its NASA space shuttle servicing mission this

  • ljk June 13, 2008, 23:08

    Precise Wavefront Correction with an Unbalanced Nulling Interferometer for Exo-Planet Imaging Coronagraphs

    Authors: J. Nishikawa, L. Abe, N. Murakami, T. Kotani

    (Submitted on 12 Jun 2008)

    Abstract: Very high dynamical range coronagraphs targeting direct exo-planet detection (10^9 – 10^10 contrast) at small angular separation (few lambda/D units) usually require an input wavefront quality on the order of ten thousandths of wavelength RMS.

    We propose a novel method based on a pre-optics setup that behaves partly as a low-efficiency coronagraph, and partly as a high-sensitivity wavefront aberration compensator (phase and amplitude). The combination of the two effects results in a highly accurate corrected wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI) performs a rejection of part of the wavefront electric field. Then the recombined output wavefront has its input aberrations magnified. Because of the unbalanced recombination scheme, aberrations can be free of phase singular points (zeros) and can therefore be compensated by a downstream phase and amplitude correction (PAC) adaptive optics system, using two deformable mirrors.

    In the image plane, the central star’s peak intensity and the noise level of its speckled halo are reduced by the UNI-PAC combination: the output-corrected wavefront aberrations can be interpreted as an improved compensation of the initial (eventually already corrected) incident wavefront aberrations. The important conclusion is that not all the elements in the optical setup using UNI-PAC need to reach the lambda/10000 rms surface error quality.

    Comments: Accepted for publication in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.2026v1 [astro-ph]

    Submission history

    From: Jun Nishikawa [view email]

    [v1] Thu, 12 Jun 2008 19:14:00 GMT (268kb)