≡ Menu

Direct Imaging of Nearby Planets

Depressing economic times inevitably cast a pall over our space plans. That makes it important to keep our eyes on the big picture — what we hope to accomplish — rather than succumbing to the fatigue induced by seeing good science pushed back on the calendar year after year. Will we get a terrestrial planet finder off in the next fifteen years? Will we get back to the Jupiter system some time before 2030? I don’t know, but times like these require persistence, patience, and continued hard thinking.

I was musing about this while looking through a paper Dave Moore passed along recently. It’s a discussion of where we need to go now that we’ve got missions like CoRoT and Kepler in space and the James Webb Space Telescope in the picture for 2014. Tom Greene (NASA Ames) and colleagues from various institutions are looking at a space telescope with relatively modest aperture in the 1.4-meter range, one that would use a coronagraph to block the light of central stars to allow direct imaging of planets all the way down to the habitable zones.

Why do we need such a mission? The JWST, able to take high quality spectra on transiting gas giants, is going to have trouble with low mass dust disks near habitable zones, and the same problem extends to small planets in those zones and planets that do not transit. An instrument like the one detailed here would be able to detect planets down to one to two Earth radii in the habitable zones of about two dozen of the nearest stars, looking for spectral features like H20, O2 or other molecules we believe necessary for life.

From the paper:

Our simulations show that any small planets in or near habitable zones of 20 of the nearest stars would have a 20% chance of detection in 6 – 12 hours of integration time with a moderate aperture space coronagraph. Thus there would be a 90% chance of detecting each one in a total of 10 uncorrelated visits. Therefore a complete survey and repeat followup characterization could be completed in about a year of real time. These observations will likely include short term monitoring for variation with rotation and longer term monitoring for seasonal effects (perhaps snow), phase effects in atmospheric scattering, and constraining orbits.

Add in what would be learned about giant planets and circumstellar debris disks over a wider range of target stars and the mission stacks up as a prudent one for our times, relatively modest in scope but capable of extending our knowledge significantly while affecting the design of future, more expansive projects. Kepler’s field of view doesn’t include the nearby F, G and K-class stars envisioned as targets for this hunt and ground-based radial velocity studies won’t be able to produce the detail of this mission, which would be capable of directly imaging many planets that are so far only inferred from our data.

The paper is Greene et al., “Discovering and Characterizing the Planetary Systems of Nearby Stars,” prepared for the Planetary Systems and Star Formation science frontier panel of the Astro2010 Subcommittee on Science (abstract).

Comments on this entry are closed.

  • philw1776 April 2, 2009, 15:23

    Such a modest telescope mission likely several hundred million dollars, could be privately funded by an individual. It would be quite a memorial to have habitable life bearing planets named after you and yours.

  • ljk April 2, 2009, 16:21

    Individuals are the ones who have been preserving the
    original Moon images taken by the five Lunar Orbiter probes
    in 1966 and 1967 when NASA balked at spending what for
    them would have been pocket change:


    There are plenty of individuals on this planet who could easily
    spend their excess cash to make several such space missions
    a reality. Perhaps the promise of near-immortality plus some
    corporate sponsors getting publicity might do the trick.

    For those who fear the damage of our current economic crisis
    to science and more, keep in mind that during the Great
    Depression, numerous rocket societies came into existence.

    Just three decades later, humanity went from a few simple
    rocket launches to the first men on the Moon with Apollo.

    Human issues tend to be temporary: What we are planning
    here and elsewhere in regards to space will be what matters
    to future generations.

  • Administrator April 2, 2009, 18:11

    Well spoken, Larry!

  • Ronald April 3, 2009, 4:47

    Larry, yes, well spoken indeed!

    We need advocacy like that.

    As HG Wells said: our choice is the universe or nothing.

  • Ronald April 3, 2009, 5:00

    Question: could the JWST be modified with relative ease and low expense (e.g. be fitted with a coronagraph, etc.) to do exactly that: directly image terrestrial planets around nearby stars?

    Since JWST will have a 6.5 meter mirror, if it could be adapted that way, it would make a terrific terrestrial planet finder.

    If I remember well, one of the alternative options considered for TPF was a single mirror telescope (large, 6-8 meters diam.) with a coronagraph.

  • Astronomy Link List April 3, 2009, 12:05

    This article has been added to the Astronomy Link List.

  • Dave Moore April 3, 2009, 17:46

    Talking about individual efforts and characterizing nearby planets.

    I got a note from the Planetary Society today. They are fund raising (they need $72,000) to get some test equipment for Marcy and Fischer at the Licke observatory. When the full system is installed at the Keck observatory, they will be able to measure stellar doppler shifts down to half a meter/sec. The fundraising request describes that the system uses a combination of adaptive optics and fiber optics to make the cone of light entering the spectrometer more uniform, improving the signal to noise ratio. (This description was aimed at the general public.) At 0.5 meters/sec they will be able to pick up Earth-sized planets.

    They hope to have this technique up and running in time to complement Kepler’s results.

    It was only a few years ago that this level of accuracy was dismissed as impossible.

  • Administrator April 3, 2009, 17:57

    Yes, they’ll need this for Keck to do its job confirming the Kepler findings. Let’s hope there are many to confirm in the terrestrial-mass range!