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An Early Mission for Exoplanet Imaging

With the European Space Agency’s DARWIN at least a decade off and funding for the Terrestrial Planet Finder as problematic as ever, what would be a suitable interim mission to extend our exoplanet exploration program? COROT is already flying, with the possibility of detecting large terrestrial planets in close orbits, while Kepler should be able to detect Earth-mass planets in Earth-like orbits by 2013 or earlier. All of which is promising, but we’re still missing key elements of the puzzle.

Take those transiting exoplanets COROT and Kepler track. We’ll retrieve a wealth of data, but we probably won’t be able to get the kind of spectroscopic information we’d like to see from a more advanced mission. Similarly, ground-based telescopes using adaptive optics, and future space missions like the James Webb Space Telescope should show us hot gas giants, but we’ll be unlikely to see planets like our own Jupiter and Saturn, cooler worlds in more distant orbits.

The solution? A team of NASA researchers including Karl Stapelfeldt and Wesley Traub at the Jet Propulsion Laboratory make a strong case for a 2-meter optical space telescope equipped with the latest in coronagraph technology to reveal details of planetary systems that would otherwise be lost in the central star’s glare. A smaller space telescope could be flown at a cost much reduced over the kind of budgets the full-scale Terrestrial Planet Finder would have required.

Among its benefits:

  • Measuring the color, taking the spectra and providing astrometric measurements of outer gas giant planets in nine nearby systems known to host planets. We should be able to analyze their atmospheres and measure the depth of their uppermost cloud decks. Factors such as the planets’ albedo and the presence of ring systems should be accessible, as will be planetary size.
  • The discovery of new gas giants in the kind of five to ten AU orbit Jupiter and Saturn occupy in our own system. Thirty stars within 25 parsecs that are already known to host close-in planets (detected through radial velocity methods) would be prime candidates for study. Ultimately, such an instrument has the potential to find outer Jovian worlds around as many as 200 nearby stars.
  • A 2-meter class instrument in space should be able to resolve rings, warps and asymmetries caused by planetary movement within circumstellar dust disks. With a contrast a thousand times sharper than Hubble’s, such an instrument could be sensitive enough to detect exosystem analogues to our own tenuous Kuiper Belt.

I save the best for last. The researchers believe their instrument may have the ability to detect Earth-sized planets in the habitable zones of five to ten of the brightest, nearest stars. Studying their photons would allow a basic spectral characterization, doubtless motivating intense studies by later, more powerful instruments (and, of course, tuning their target lists).

A mission like this is currently missing from the catalog of upcoming observatories, doubtless because of over-optimism in early assessments about both the growing power of adaptive optics on ground-based telescopes and the possibility of refitting the Hubble instrument with an advanced coronagraph, an idea since abandoned. “Our community can either resign itself to waiting out JWST,” the authors write, “or look for ways to achieve significant new exoplanet science, sooner, through more modest projects.”

The paper is Stapelfeldt, “First Steps in Direct Imaging of Planetary Systems Like our Own: The Science Potential of 2-m Class Optical Space Telescopes,” a white paper submitted to the AAAC Exoplanet Task Force (Spring, 2007), available online.

Comments on this entry are closed.

  • mz August 13, 2007, 10:14

    Spectrography seems interesting to me too – but it’s possibe Corot and Kepler will yield some surprises, no?

  • Michael Devirian August 13, 2007, 11:03

    Re MZ question: actually, Corot and Kepler will yield surprises in the area of statistics of planet sizes/orbits among a set of very distant stars; whereas a coronagraph mission such as that discussed by Traub/Stapelfeldt will look at “nearby” exoplanets and obtain spectral information that alone can begin to tell the tale of the exoecosystem.

  • mz August 13, 2007, 14:17

    Yes, but what what I meant is, don’t the Corot / Kepler datasets actually form input to any planet observation mission under study? Or is the necessary info already there so a coronagraph spectrography mission can be safely specified?

    Though I guess there are many large scale automatic distributed terrestrial planet survey methods underway as well.

  • yetu August 14, 2007, 12:26

    well i really hope they do a coronagraph mission- they proved the tech is ready just recently.

    corot/kepler still have time to contribute their data to this mission- in any case the coronagraph will be looking at closer star systems and wont be going after any corot/kepler targets. Although even statistical info from those missions would help

  • yetu August 14, 2007, 12:33

    if you read the paper they plan to launch in 2015- 2 years after keplers mission is finished. So they will be able to use the corot/kepler data on their mission.

  • ljk October 9, 2007, 0:34

    Detecting Life-bearing Extra-solar Planets with Space Telescopes

    Authors: Steven V. W. Beckwith

    (Submitted on 7 Oct 2007)

    Abstract: One of the promising methods to search for life on extra-solar planets (exoplanets) is to detect life’s signatures in their atmospheres. Spectra of exoplanet atmospheres at the modest resolution needed to search for oxygen, carbon dioxide, water, and methane will demand large collecting areas and large diameters to capture and isolate the light from planets in the habitable zones around the stars. For telescopes using coronagraphs to isolate the light from the planet, each doubling of telescope diameter will increase the available sample of stars by an order of magnitude, indicating a high scientific return if the technical difficulties of constructing very large space telescopes can be overcome. For telescopes detecting atmospheric signatures of transiting planets, the sample size increases only linearly with diameter, and the available samples are probably too small to guarantee detection of life-bearing planets. Using samples of nearby stars suitable for exoplanet searches, this paper shows that the demands of searching for life with either technique will require large telescopes, with diameters of order 10m or larger in space.

    Comments: 15 pages, 6 figures, submitted to Ap. J

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.1444v1 [astro-ph]

    Submission history

    From: Steven V. W. Beckwith [view email]

    [v1] Sun, 7 Oct 2007 21:34:58 GMT (122kb,D)

    http://arxiv.org/abs/0710.1444

  • ljk January 18, 2008, 12:39

    Impact of Orbital Eccentricity on the Detection of Transiting Extrasolar Planets

    Authors: Christopher J. Burke

    (Submitted on 16 Jan 2008)

    Abstract: For extrasolar planets with orbital periods, P greater than 10 days, radial velocity surveys find non-circular orbital eccentricities are common, e~0.3. Future surveys for extrasolar planets using the transit technique will also have sensitivity to detect these longer period planets. Orbital eccentricity affects the detection of extrasolar planets using the transit technique in two opposing ways: an enhancement in the probability for the planet to transit near pericenter and a reduction in the detectability of the transit due to a shorter transit duration.

    For an eccentricity distribution matching the currently known extrasolar planets with P greater than 10 day, the probability for the planet to transit is ~1.25 times higher than the equivalent circular orbit and the average transit duration is ~0.88 times shorter than the equivalent circular orbit. These two opposing effects nearly cancel for an idealized field transit survey with independent photometric measurements that are dominated by Poisson noise.

    The net effect is a modest ~4% increase in the transiting planet yield compared to assuming all planets have circular orbits. When intrinsic variability of the star or correlated photometric measurements are the dominant source of noise, the transit detectability is independent of the transit duration. In this case the transit yield is ~25% higher than that predicted under the assumption of circular orbits.

    Since the Kepler search for Earth-sized planets in the habitable zone of a Solar-type star is limited by intrinsic variability, the Kepler mission is expected to have a ~25% higher planet yield than that predicted for circular orbits if the Earth-sized planets have an orbital eccentricity distribution similar to the currently known Jupiter-mass planets.

    Comments: 8 pages, 6 Figures, Submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.2579v1 [astro-ph]

    Submission history

    From: Christopher J. Burke [view email]

    [v1] Wed, 16 Jan 2008 22:14:52 GMT (64kb)

    http://arxiv.org/abs/0801.2579