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Probing Exoplanet Atmospheres in Texas

With Hubble’s Space Telescope Imaging Spectrograph now out of commission, the study of exoplanetary atmospheres becomes a bit more problematic. But Seth Redfield (University of Texas at Austin) has now used a ground-based instrument to detect the atmosphere of a planet orbiting the star HD189733, some 63 light years away in the constellation Vulpecula. Discovered in 2004, this transiting world is about twenty percent more massive than Jupiter, orbiting its parent ten times closer than Mercury orbits our Sun.

hd189733 transit

Working from the ground is tricky but the odds go up when you observe more than a single transit. Redfield worked with eleven transits observed over the course of a year, using the Hobby-Eberly Telescope (HET) at McDonald Observatory in Austin. Studying the chemical composition of a distant atmosphere involves taking a spectrum during a transit and another when no transit is occurring. Working with the difference and comparing results over multiple transits helps you put together the atmospheric spectrum.

Image: The dotted line shows the planet’s orbit around the star HD189733. The planet orbits the star once every 2.2 Earth days, crossing the face of the star well below its equator. The small circles indicate the planet’s location during each of Seth Redfield’s more than 200 HET observations over the course of one Earth year. The red circles indicate observations during transit; the rest of the circles denote out-of-transit observations. Credit: S. Redfield/T. Jones/McDonald Obs.

None of which is easy. The light blocked out by the planet amounts to only 2.5 percent of the star’s total light as seen from Earth, and Redfield figures another 0.3 percent for the planetary atmosphere. But gradually the pieces of the puzzle come together. Redfield says this about his method:

“Each time the planet passes in front of the star, the planet blocks some of the star’s light. If the planet has no atmosphere, it will block the same amount of light at all wavelengths. However, if the planet has an atmosphere, gases in its atmosphere will absorb some additional light.”

Which gets interesting indeed when you look at the planet at wavelengths corresponding to specific transitions of the sodium atom. The presence of sodium means that the planet absorbs more starlight at those wavelengths, making the planet appear larger by about six percent than at other wavelengths. With sodium now detected, the search can move to other atmospheric constituents. Hundreds of observations went into this result, along with the necessary task of filtering contamination to the data from Earth’s atmosphere.

So precise was the work that the transmission spectrum achieved from this distant transit via ground methods was higher in resolution than previous Hubble work on exoplanetary atmospheres. Honing this technique will get us into even more interesting territory as we start extending our methods to planets more amenable to life. It will also give us a head start as we wait for the next generation of space-based observatories to provide data at higher levels of precision.

The paper is Redfield, Endl et al., “Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum,” accepted for publication in Astrophysical Journal Letters (abstract).

Comments on this entry are closed.

  • david December 7, 2007, 0:06

    A large planet with a pretty fast orbit. I wonder how well the technique will apply to earth size planets that take a year, or a large part of a year, to orbit their star.

    I also wonder how well that will combine with such efforts as the kepler space telescope in its future hunt for earth size planets. If it detects hundreds, or thousands, of earth size planets it would tell us where and when to point telescopes.

    I also wonder, I do a lot of wondering, how many of the stars kepler will examine will be near by stars. If it detects earth size planets around a star only 9 or 10 light years away it could be interesting. Especially if we can examine their atmospheres and find unusual properties.

    In ten years time our catalogue of planets we believe to possess life might be a match for our current list of gas giants. Being able to fill in another variable of the fermi equation/paradox would be interesting.

  • andy December 7, 2007, 9:46

    Hot Jupiters have the advantage of having extended atmospheres which are escaping into space. Getting the atmospheric composition of a cooler planet with a thin atmosphere is likely to be a much more difficult task.

  • yeti December 10, 2007, 14:35

    hi David

    kepler will not be able to study G type stars within 10 light years im afraid. Most will be in the 500-1000 light year range

  • ljk December 10, 2007, 17:37

    But just how many G type stars are there within
    10 light years of us?

    And why won’t Kepler be able to study them?

  • andy December 10, 2007, 19:23

    ljk: how many G-type stars within 10 light years? Hmmm… that would be our Sun and Alpha Centauri A.

    If we want to be generous and allow stars of spectral types K and F as well, we can add Alpha Centauri B to that list (systems such as Epsilon Eridani, Tau Ceti and Procyon are beyond the 10 light year mark). The remaining stars within 10 light years are a few red dwarfs and the Sirius system. Perhaps a few brown dwarfs which haven’t been discovered yet as well.

    As far as I can tell, the plan is for Kepler to be aimed at a specific area of the sky (in Cygnus according to Wikipedia), rather than aiming for target stars. Since Alpha Centauri isn’t in Cygnus, it won’t get studied.

  • david December 11, 2007, 2:24

    Since kepler needs to be looking at the same part of the sky continuously knew it would not be able to look at many of the closer ones. Had been hoping 3 or 4 would be within its view though.

    There’s also the disappointing little fact that it will only be able to detect planets within its line of sight, which for earth like planets would be < 1 percent, I think. So it’s a one in a thousand chance. But such odds are beaten numerous times each day.

    Guess the best we can hope for is data that will tell us nearly all stars have planets, with a lot of them being in the habitable zone. *crosses fingers*

    It’s one thing to say there is a planet around a star 1000 light years away. It’s interesting but it’s not something we can even dream of sending a probe to yet. On the other hand if we see a planet around a star that is relatively near then it will spark more interest. If it seems to be earth like and we can get a glimpse of its atmosphere with that atmosphere having properties suggesting life then things get more interesting. It would be on the news more. It would become a main component of sci-fi plots. There would be debate. People would be curious about it and want to know more. Interest in it would be sparked enough that maybe space development would be funded more. That was my hope anyway.

  • ljk December 11, 2007, 11:10

    Detection of atmospheric haze on an extrasolar planet: The 0.55 – 1.05 micron transmission spectrum of HD189733b with the Hubble Space Telescope

    Authors: F. Pont, H. Knutson, R. L. Gilliland, C. Moutou, D. Charbonneau

    (Submitted on 9 Dec 2007)

    Abstract: The nearby transiting planet HD 189733b was observed during three transits with the ACS camera of the Hubble Space Telescope in spectroscopic mode. The resulting time series of 675 spectra covers the 550-1050 nm range, with a resolution element of ~8 nm, at extremely high accuracy (signal-to-noise ratio up to 10,000 in 50 nm intervals in each individual spectrum). Using these data, we disentangle the effects of limb darkening, measurement systematics, and spots on the surface of the host star, to calculate the wavelength dependence of the effective transit radius to an accuracy of ~50 km.

    This constitutes the “transmission spectrum” of the planetary atmosphere. It indicates at each wavelength at what height the planetary atmosphere becomes opaque to the grazing stellar light during the transit. In this wavelength range, strong features due to sodium, potassium and water are predicted by atmosphere models for a planet like HD 189733b, but they can be hidden by broad absorption from clouds or hazes higher up in the atmosphere.

    We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid-infrared (2-8 microns), and the slope of the spectrum, suggest the presence of a haze of sub-micron particles in the upper atmosphere of the planet.

    Comments: 11 pages, to appear in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.1374v1 [astro-ph]

    Submission history

    From: Fr\’ed\’eric Pont [view email]

    [v1] Sun, 9 Dec 2007 22:31:24 GMT (122kb)

    http://arxiv.org/abs/0712.1374

  • ljk December 17, 2007, 11:50

    A HET search for planets around evolved stars

    Authors: Andrzej Niedzielski, Alex Wolszczan

    (Submitted on 14 Dec 2007)

    Abstract: We present our ongoing survey of ~1000 GK-giants with the 9.2-m Hobby-Eberly Telescope in search for planets around evolved stars. The stars selected for this survey are brighter than 11 mag and are located in the section of the HR-diagram, which is approximately delimited by the main sequence, the instability strip, and the coronal dividing line. We use the High Resolution Spectrograph to obtain stellar spectra for radial velocity measurements with a 4-6 m/s precision.

    So far, the survey has discovered a planetary-mass companion to the K0-giant HD 17092, and it has produced a number of plausible planet candidates around other stars. Together with other similar efforts, our program provides information on planet formation around intermediate mass main sequence-progenitors and it will create the experimental basis with which to study dynamics of planetary systems around evolving stars.

    Comments: 6 pages, to appear in ,,Exoplanets. Detection, Formation & Dynamics” IAU Symposium 249, Cambridge University Press, 2007

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.2297v1 [astro-ph]

    Submission history

    From: Andrzej Niedzielski [view email]

    [v1] Fri, 14 Dec 2007 07:24:30 GMT (149kb)

    http://arxiv.org/abs/0712.2297

  • ljk December 17, 2007, 11:52

    On Signatures of Atmospheric Features in Thermal Phase Curves of Hot Jupiters

    Authors: Emily Rauscher, Kristen Menou (Columbia), James Y-K. Cho (QM Univ. London), Sara Seager (MIT), Brad Hansen (UCLA)

    (Submitted on 13 Dec 2007)

    Abstract: Turbulence is ubiquitous in Solar System planetary atmospheres. In hot Jupiter atmospheres, the combination of moderately slow rotation and thick pressure scale height may result in dynamical weather structures with unusually large, planetary-size scales. Using equivalent-barotropic, turbulent circulation models, we illustrate how such structures can generate a variety of features in the thermal phase curves of hot Jupiters, including phase shifts and deviations from periodicity. Such features may have been spotted in the recent infrared phase curve of HD 189733b. Despite inherent difficulties with the interpretation of disk-integrated quantities, phase curves promise to offer unique constraints on the nature of the circulation regime present on hot Jupiters.

    Comments: 21 pages, 6 figures, 1 table, submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.2242v1 [astro-ph]

    Submission history

    From: Kristen Menou [view email]

    [v1] Thu, 13 Dec 2007 21:04:03 GMT (378kb)

    http://arxiv.org/abs/0712.2242