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Studying Habitable Planets with JWST

Spotting transiting planets is what missions like CoRoT and Kepler are all about. The next step, getting a read on what’s in the atmosphere of any transiting, terrestrial world, is going to be tricky. The biomarkers like ozone and methane, so crucial for determining whether there’s life on a distant planet, are beyond the range of existing spacecraft. But the the next generation James Webb Space Telescope is also in the works, scheduled for launch in 2013.


For nearby Earth-class worlds, JWST may be up to the task, at least for terrestrial planets that transit. In fact, if Alpha Centauri A turns out to have a transiting Earth-like planet (a major if!), it would take only a few transits to study the light filtering through its atmosphere to look for signs of life. Alpha Centauri is problematic in any case, but a recent study shows that the method — breaking down the star’s light during a transit to look for the characteristic markers — could be extended to other stars, provided enough transits can be measured.

Image: This artist’s conception shows a hypothetical twin Earth orbiting a Sun-like star. A new study shows that characterizing a distant Earth’s atmosphere will be difficult, even using next-generation technology like the James Webb Space Telescope. If an Earth-like world is nearby, though, then by adding observations of a number of transits, astronomers should be able to detect biomarkers like methane or ozone. Credit: David A. Aguilar (CfA).

Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and Wesley Traub (Jet Propulsion Laboratory) have been studying JWST in this context. Says Kaltenegger:

“We’ll have to be really lucky to decipher an Earth-like planet’s atmosphere during a transit event so that we can tell it is Earth-like. We will need to add up many transits to do so – hundreds of them, even for stars as close as 20 light-years away.”

If we’re looking at a planet in an Earth-like orbit around a G-class star, then a ten-hour transit once a year is the best we can expect, meaning that collecting a hundred hours of transit data would take ten years. But the same world orbiting in the habitable zone of a red dwarf would make many more transits in the same amount of time because of its proximity to the primary star, which is why Kaltenegger says nearby M-dwarfs “…offer the best possibility of detecting biomarkers in a transiting Earth’s atmosphere.”

The paper is Kaltenegger and Traub, “Transits of Earth-like Planets,” accepted by The Astrophysical Journal and available online.

Comments on this entry are closed.

  • Usman Saeed March 22, 2009, 5:01

    You are absolutely right, but I think interstellar probes should also be considered for near future. They are the only things that would settle this matter for sure, for the bio chemical markers of life may show that there are organic compounds in a exo planet’s atmosphere but they would not confirm presence of life. That one can only be sure of by actually going there or sending something there.

  • Enzo March 23, 2009, 19:07

    Usman, with something completely trivial in comparison to interstellar travel like sending a probe to Europa arriving there in 2029 (if we are lucky), I don’t think there will be any interstellar probe any time soon. Besides, a proper survey with telescopes etc. will be needed before anything is launched to at least decide where to send it to. Even that is likely 20 years away.
    That does not mean that investigations like Dedalus and now Icarus are useless, just that one should not expect them to happen anytime soon.

    Regarding transiting planets on Alpha Centauri A and B, assuming that such planets are coplanar to the two stars orbit, it should be quite easy to calculate the minimum distance these planets need to orbit to make a visible transit. From the Wikipedia page, the Alpha Centauri star system is not edge on, but close enough planets should still produce transits visible from earth.

  • Ronald March 24, 2009, 5:54

    @Enzo: “From the Wikipedia page, the Alpha Centauri star system is not edge on, but close enough planets should still produce transits visible from earth”.

    This sounds promising. How close is close enough? Would roughly earthsized planets in roughly earthlike orbits produce visible transits?

  • ljk March 24, 2009, 9:29

    Enzo said:

    “That does not mean that investigations like Daedalus and now Icarus are useless, just that one should not expect them to happen anytime soon.”

    You are unfortunately correct, but there is a plan for a probe to
    explore the interstellar medium beyond our Sol system that could
    be ready to go and return data in our lifetimes.

    Read more here:


  • Enzo March 24, 2009, 18:43

    Ronald, to know how close is close enough, I would need the exact plane equation for the system as seen form Earth and the positions of Alpha Centauri A and B. My geometry is a bit rusty, but maybe I could calculate it.
    Basically you need to write the equation of two generic ellipses (I would start with a circle for simplicity) that lay on the orbit plane and centered on A and B. Then, knowing the diameter of Alpha Centauri A and B, you can calculate the intersection of those generic ellipses with the disks of A and B (as seen from here).

    Someone else surely must have done this already, especially if they are searching for transits. Of course there might be non co-planar planets, but I’m not sure if they would be stable given the other star.

  • ljk April 7, 2009, 13:14

    Detectability of Terrestrial Planets in Multi-Planet Systems: Preliminary Report

    Authors: Wesley A. Traub, Charles Beichman, Andrew F. Boden, Alan P. Boss, Stefano Casertano, Joseph Catanzarite, Debra Fischer, Eric. B. Ford, Andrew Gould, Sam Halverson, Andrew Howard, Shigeru Ida, N. Jeremy Kasdin, Gregory P. Laughlin, Harold F. Levison, Douglas Lin, Valeri Makarov, James Marr, Matthew Muterspaugh, Sean N. Raymond, Dmitry Savransky, Michael Shao, Alessandro Sozzetti, Cengxing Zhai

    (Submitted on 5 Apr 2009)

    Abstract: We ask if Earth-like planets (terrestrial mass and habitable-zone orbit) can be detected in multi-planet systems, using astrometric and radial velocity observations. We report here the preliminary results of double-blind calculations designed to answer this question.

    Comments: 10 pages, 0 figures

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:0904.0822v1 [astro-ph.EP]

    Submission history

    From: Wesley Traub [view email]

    [v1] Sun, 5 Apr 2009 22:41:51 GMT (12kb)


  • ljk April 7, 2009, 13:17

    Photometric Variability in Earthshine Observations

    Authors: Sally V. Langford, J. Stuart B. Wyithe, Edwin L. Turner

    (Submitted on 6 Apr 2009)

    Abstract: The identification of an extrasolar planet as Earth-like will depend on the detection of atmospheric signatures or surface non-uniformities. In this paper we present spatially unresolved flux light curves of Earth for the purpose of studying a prototype extrasolar terrestrial planet.

    Our monitoring of the photometric variability of earthshine revealed changes of up to 23 % per hour in the brightness of Earth’s scattered light at around 600 nm, due to the removal of specular reflection from the view of the Moon. This variability is accompanied by reddening of the spectrum, and results from a change in surface properties across the continental boundary between the Indian Ocean and Africa’s east coast.

    Our results based on earthshine monitoring indicate that specular reflection should provide a useful tool in determining the presence of liquid water on extrasolar planets via photometric observations.

    Comments: To appear in Astrobiology 9(3). 17 pages, 3 figures, 1 table

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0904.0845v1 [astro-ph.EP]

    Submission history

    From: Sally Langford [view email]

    [v1] Mon, 6 Apr 2009 04:53:04 GMT (1389kb)


  • ljk April 7, 2009, 13:19

    Advanced Technology Large-Aperture Space Telescope (ATLAST): A Technology Roadmap for the Next Decade

    Authors: Marc Postman, et al

    (Submitted on 6 Apr 2009)

    Abstract: The Advanced Technology Large-Aperture Space Telescope (ATLAST) is a set of mission concepts for the next generation of UVOIR space observatory with a primary aperture diameter in the 8-m to 16-m range that will allow us to perform some of the most challenging observations to answer some of our most compelling questions, including “Is there life elsewhere in the Galaxy?”

    We have identified two different telescope architectures, but with similar optical designs, that span the range in viable technologies. The architectures are a telescope with a monolithic primary mirror and two variations of a telescope with a large segmented primary mirror. This approach provides us with several pathways to realizing the mission, which will be narrowed to one as our technology development progresses.

    The concepts invoke heritage from HST and JWST design, but also take significant departures from these designs to minimize complexity, mass, or both.

    Our report provides details on the mission concepts, shows the extraordinary scientific progress they would enable, and describes the most important technology development items. These are the mirrors, the detectors, and the high-contrast imaging technologies, whether internal to the observatory, or using an external occulter.

    Experience with JWST has shown that determined competitors, motivated by the development contracts and flight opportunities of the new observatory, are capable of achieving huge advances in technical and operational performance while keeping construction costs on the same scale as prior great observatories.

    Comments: 22 pages, RFI submitted to Astro2010 Decadal Committee

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Cosmology and Extragalactic Astrophysics (astro-ph.CO); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0904.0941v1 [astro-ph.IM]

    Submission history

    From: Marc Postman [view email]

    [v1] Mon, 6 Apr 2009 19:09:44 GMT (3619kb)


  • ljk April 7, 2009, 13:22

    Astrometric Detection of Earthlike Planets

    Authors: Michael Shao (Jet Propulsion Laboratory, California Institute of Technology), Geoff Marcy (UC Berkeley), Joseph H. Catanzarite, Stephen J. Edberg (Jet Propulsion Laboratory, California Institute of Technology), Alain Leger (Institut d’Astrophysique Spatiale), Fabien Malbet (Laboratoire d’Astrophysique de l’Observatoire de Grenoble), Didier Queloz (Observatoire de Geneve), Matthew W. Muterspaugh (UC Berkeley), Charles Beichman (NExScI), Debra A. Fischer (SFSU), Eric Ford (University of Florida), Robert Olling (University of Maryland), Shrinivas Kulkarni (Caltech), Stephen C. Unwin, Wesley Traub (Jet Propulsion Laboratory, California Institute of Technology)

    (Submitted on 6 Apr 2009)

    Abstract: Astrometry can detect rocky planets in a broad range of masses and orbital distances and measure their masses and three-dimensional orbital parameters, including eccentricity and inclination, to provide the properties of terrestrial planets. The masses of both the new planets and the known gas giants can be measured unambiguously, allowing a direct calculation of the gravitational interactions, both past and future. Such dynamical interactions inform theories of the formation and evolution of planetary systems, including Earth-like planets.

    Astrometry is the only technique technologically ready to detect planets of Earth mass in the habitable zone (HZ) around solar-type stars within 20 pc. These Earth analogs are close enough for follow-up observations to characterize the planets by infrared imaging and spectroscopy with planned future missions such as the James Webb Space Telescope (JWST) and the Terrestrial Planet Finder/Darwin.

    Employing a demonstrated astrometric precision of 1 microarcsecond and a noise floor under 0.1 micro-arcseconds, SIM Lite can make multiple astrometric measurements of the nearest 60 F-, G-, and K-type stars during a five-year mission. SIM Lite directly tests theories of rocky planet formation and evolution around Sun-like stars and identifies the nearest potentially habitable planets for later spaceborne imaging, e.g., with Terrestrial Planet Finder and Darwin.

    SIM was endorsed by the two recent Decadal Surveys and it meets the highest-priority goal of the 2008 AAAC Exoplanet Task Force.

    Comments: Astro2010 White Paper, 8 pages

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:0904.0965v1 [astro-ph.EP]

    Submission history

    From: Joseph Catanzarite [view email]

    [v1] Mon, 6 Apr 2009 16:30:05 GMT (358kb)