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Organic Molecule in Exoplanet Atmosphere

Well-studied HD 189733b is a Jupiter-sized planet again in the news. Studying this transiting world, scientists using Hubble Space Telescope data have made the first identification of an organic molecule — methane — in the atmosphere of an exoplanet. What’s particularly significant here is the growing sophistication of our use of spectroscopy, splitting light into its components to tease out the constituents of the atmosphere under study. This new finding shows that we’re on target in planning to use space-based observatories to make far more challenging detections.

Region of HD 189733b

Image: A wide field image of the region of sky in which HD 189733b is located. In this image we can see the asterism of the “Summer Triangle” a giant triangle in the sky composed of the three bright stars Vega (top left), Altair (lower middle) and Deneb (far left). HD 189733b is orbiting a star very close to the centre of the triangle. Credit: A. Fujii.

Methane in the atmosphere of a gas giant wouldn’t rank as a surprise — it’s found in abundance in our own Solar System, and can be created by processes that do not require life. Nor does this work in any way diminish the difficulty awaiting us as we move toward studying much smaller worlds. But if Hubble, using its Near Infrared Camera and Multi-Object Spectrometer (NICMOS) can make this catch, platforms like the James Webb Space Telescope should be able to extend our range dramatically. One day the hope is to use a Terrestrial Planet Finder or Darwin-grade mission to break down the light from an Earth-like world, possibly identifying the chemical signature of life.

The Jet Propulsion Laboratory’s Mark Swain notes the significance of the new result:

“These measurements are an important step to our ultimate goal of determining the conditions, such as temperature, pressure, winds, clouds, etc., and the chemistry on planets where life could exist. Infrared spectroscopy is really the key to these studies because it is best matched to detecting molecules.”

Getting the right data in this case meant observing HD 189733b as it made a transit, the light from the primary passing through the planet’s atmosphere, where the constituent gases imprinted their signature on the starlight. The observing team used the NICMOS camera over five contiguous spacecraft orbits. In addition to methane they were able to confirm the earlier identification of water vapor in the planetary atmosphere made by the Spitzer Space Telescope. And HD 189733b did have a bit of a surprise to offer, in the form of more methane than would have been expected from our current model of ‘hot Jupiters.’ Says Giovanna Tinetti (University College, London):

“A sensible explanation is that the Hubble observations were more sensitive to the dark night side of this planet where the atmosphere is slightly colder and the photochemical mechanisms responsible for methane destruction are less efficient than on the day side.”

But where is the carbon monoxide researchers expected to find? HD189733b is some 63 light years away in the constellation Vulpecula, roughly twenty percent more massive than Jupiter and orbiting its star at a tenth of the distance of Mercury from our Sun. Orbiting once every 2.2 Earth days, the planet diminishes the apparent brightness of its star by a scant 2.4 percent, an indication of the precision required in this work. The paper is Swain et al., “Methane present in an extrasolar planet atmosphere,” Nature 452 (March 20, 2008), pp. 296-297 (abstract)

A final thought: Life’s chemistry may still conceal its secrets even when we’re taking reliable spectrographic data from seemingly Earth-like worlds around other stars. Listen to what Marc Kuchner (NASA GSFC) has to say on this point:

“How can you take a spectrum of an extrasolar planet’s atmosphere and know for certain whether there is life or not? There is probably a huge range of atmospheres on extrasolar planets. Some probably have biomarkers of life, but those biosignatures are not a ‘slam-dunk’ for life on those planets. I think life on another planet may not be confirmed unless we go there.”

An interesting thought, given that the time may not be that far off (a few decades, perhaps) when we’ve found a terrestrial-size rocky world around a nearby star — Centauri B comes to mind — and have analyzed its atmosphere. Plants, animals and bacteria produce oxygen, methane and nitrous oxide in abundance on Earth. If we find these out of thermodynamic equilibrium with each other on a Centauri B planet, the probability of life will clearly soar, but what if the biomarkers are harder to interpret? Ambiguity is often the result in science, which is where propulsion engineers may find their opening.

Comments on this entry are closed.

  • andy March 20, 2008, 8:27

    If you don’t have a Nature subscription, the paper in question is on the arXiv here.

    One interesting possibility mentioned at the end of the paper is that the unusual levels of methane may have something to do with the reduced ultraviolet flux from the class K host star… atmospheric chemistry may be significantly different around stars of different spectral types, even for planets which receive the same amount of energy.

  • James M. Essig March 20, 2008, 12:19

    Hi Folks;

    This discovery of methane in HD189733b is really neat.

    It brings to mind that the chemical signature of our planetary atmosphere has changed with in introduction of a plethora of trace amounts of un-natural compounds. This sudden flux of artificial compounds, especially over the past 60 years of post war global economic-industrial development may be a strong signal to ETI civilizations in posession of very sensitive spectroscopy methods that we are here.

    If we were to find such dramatic atmospheric or hydrospheric chemisty changes on an extra-solar planet within say a human lifetime’s distance away from Earth by light, I can imagine a very extensive program would be developed to send radio and/or optical signals to the planet as well as the development and launching of an armada of unmanned space probes followed by long duration manned missions. This would be a planetary scientist’s field day.



  • dad2059 March 20, 2008, 17:06

    Quite fascinating. Methane in a hot Jupiter’s atmosphere is remarkable to say the least. Using spectroscopy to it’s ultimate ability is a very inventive way to study objects at great distances without actually going there.

    Eventually, probes will have to go to these places to get solid answers, like the current interplanetary explorers. What was once thought to be sure things turned out to be quite the opposite.


  • yeti March 21, 2008, 8:55

    sure detecting an earth like atmopshere is not direct proof of life- but come on, what else could cause a planet to sustain oxygen in its atmosphere in that abundance for hundreds of millions of years?

    we know of no process other than life which would do this. But then what are the chances of detecting an earth like atmosphere? will be fun to find out over the next couple of decades.

  • ljk March 26, 2008, 11:40

    Where is HD 189733?

    Image Credit: NASA, ESA, A. Fujii, and Z. Levay (STScI)

    Explanation: The star cataloged as HD 189733 is a mere 63 light
    years away. Its location is indicated in this deep, wide-angle image
    of the sky centered on the northern constellation of Cygnus.

    Considering the many bright stars, nebulae, and star clusters in
    the region more familiar to skygazers, HD 189733 may not seem
    to be remarkable, but it is known to have at least one hot, jupiter-
    sized planet orbiting very close, with an impressively short period
    of 2.2 days.

    Because the planet regularly eclipses its parent star, astronomers
    can study starlight that passes directly through the planet’s
    atmosphere and identify molecules through spectroscopy.
    Following the discovery of water vapor in the planetary
    atmosphere, astronomers now report that Hubble Space
    Telescope data also indicates the signature of methane (CH4).

    The exciting result is the first detection of an organic molecule
    on a planet orbiting another star. Although HD 189733’s planet
    is considered too hot and inhospitable to support life, the work
    is a step toward measuring conditions and chemistry on other
    extrasolar planets where life could exist.


  • ljk March 26, 2008, 14:03

    New Organic Molecule in Space

    Scientists detect amino acetonitrile near the centre of our Milky Way

    March 26, 2008
    Press Release PRI (MPIfR) 03/2008 (4)
    Max-Planck-Institut for Radio Astronomy
    & Max-Planck-Society.

    Researchers from the Max Planck Institute for Radio Astronomy (MPIfR) in
    Bonn have detected for the first time a molecule closely related to an
    amino acid: amino acetonitrile. The organic molecule was found with a 30
    metre radiotelescope in Spain and two radio interferometers in France
    and Australia in the “Large Molecule Heimat”, a giant gas cloud near the
    galactic centre in the constellation Sagittarius (Astronomy &
    Astrophysics, in press).

    *Figure 1: * /Amino acetonitrile (NH_2 CH_2 CN)
    Image: Sven Thorwirth, MPIfR /

    The ” Large Molecule Heimat ” is a very dense, hot gas clump within the
    star forming region Sagittarius B2. In this source of only 0,3
    light-year diameter, which is heated by a deeply embedded newly formed
    star, most of the interstellar molecules known to date have been found,
    including the most complex ones such as ethyl alcohol, formaldehyde,
    formic acid, acetic acid, glycol aldehyde (a basic sugar), and ethylene

    Starting from 1965, more than 140 molecular species have been detected
    in space, in interstellar clouds as well as in circumstellar envelopes.
    A large fraction of these molecules is organic or carbon-based. A lot of
    attention is given to the quest for so-called “bio”-molecules,
    especially interstellar amino acids. Amino acids, the building blocks of
    proteins and therefore key ingredients for the origin of life, have been
    found in meteorites on Earth, but not yet in interstellar space.

    The simplest amino acid, glycine (NH_2 CH_2 COOH), has long been
    searched for in the interstellar medium but has so far not been
    unambiguously detected. Since the search for glycine has turned out to
    be extremely difficult, a chemically related molecule was searched for,
    amino acetonitrile (NH_2 CH_2 CN), probably a direct precursor of glycine.

    The scientists from the Max Planck Institute for Radioastronomy in Bonn
    selected the “Large Molecule Heimat”, as the source has been named by
    experts, and investigated a dense forest of 3700 spectral lines from
    complex molecules with the IRAM 30-metre telescope in Spain. Atoms and
    molecules emit light at very specific frequencies, which appear as
    characteristic lines in the radiation spectrum. By analyzing these
    spectral lines, astronomers can determine the chemical composition of
    cosmic clouds. The more complex a molecule is, the more possibilities it
    has to radiate its internal energy. This is the reason why complex
    molecules emit many spectral lines, which are very weak and therefore
    difficult to identify in the “line jungle”.

    *Figure 2:* / Radio telescopes used for the detection of amino
    acetonitrile: the IRAM 30m Telescope (left), the IRAM Plateau de Bure
    interferometer (center) and the Australia Telescope Compact Array (right).
    Images: IRAM, ATNF. /

    “Still, we were finally able to assign 51 very weak lines to the
    molecule amino acetonitrile” says Arnaud Belloche, scientist at the Max
    Planck institute and first author of the research paper. This result was
    confirmed at 10 times higher spatial resolution with two radiotelescope
    arrays, the IRAM Plateau de Bure interferometer in France and the
    Australia Telescope Compact Array. These observations showed that all
    the candidate lines were emitted from the same position in the “Large
    Molecule Heimat”, “a strong proof of the reliability of our

    “Finding amino acetonitrile has greatly extended our insight into the
    chemistry of dense, hot star-forming regions. I am sure we will be able
    to identify in the future many new, even more complex organic molecules
    in the interstellar gas. We already have several candidates!”, says Karl
    Menten, director at the Max Planck Institute for Radioastronomy and head
    of the “Millimeter and Submillimeter” research group.


    IRAM, the “Institute for Radio Astronomy at Millimeter wavelengths”, is
    a joint German-French-Spanish radio astronomy venture which runs the 30m
    radio telescope on Pico Veleta in the Sierra Nevada mountains in
    southern Spain and also the Plateau de Bure interferometer in the French
    alps near Grenoble. Both facilities were utilized for the first
    detection of amino acetonitrile described here.

    ATCA, the “Australia Telescope Compact Array”, is an array of six 22-m
    antennas located about 25 km west of the town of Narrabri, about 500 km
    north-west of Sydney, Australia. It is operated by the Australia
    Telescope National Facility (ATNF).

    Weblink with Figures and additional references:


    Original Paper:

    *Detection of amino acetonitrile in Sgr B2(N)*
    , A. Belloche, K.
    M. Menten, C. Comito, H. S. P. Muller, P. Schilke, J. Ott, S. Thorwirth,
    C. Hieret, 2008, Astronomy & Astrophysics (in press).
    [DOI 10.1051/0004-6361: 20079203].


    * Dr. Arnaud Belloche*,
    Max-Planck-Institut fur Radioastronomie, Bonn.
    Fon: +49-228-525-376
    E-mail: belloche mpifr-bonn.mpg.de

    * Prof. Dr. Karl M. Menten *,
    Executive Director,
    Max-Planck-Institut fur Radioastronomie, Bonn.
    Fon: +49-228-525-279
    E-mail: kmenten mpifr-bonn.mpg.de

    * Dr. Norbert Junkes *,
    Public Outreach,
    Max-Planck-Institut fur Radioastronomie, Bonn.
    Fon: +49-228-525-399
    E-mail: njunkes mpifr-bonn.mpg.de

  • ljk May 9, 2008, 10:51

    Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity

    Authors: Jonathan J. Fortney, Mark S. Marley, Didier Saumon, Katharina Lodders

    (Submitted on 7 May 2008)

    Abstract: We examine the spectra and infrared colors of the cool methane-dominated atmospheres at Teff < 1400 K expected for young gas giant planets. We couple these spectral calculations to an updated version of the Marley et al. (2007) giant planet thermal evolution models that include formation by core accretion-gas capture. These relatively cool “young Jupiters” can be 1-6 magnitudes fainter than predicted by standard cooling tracks that include a traditional initial condition, which may provide a diagnostic of formation. If correct, this would make true Jupiter-like planets much more difficult to detect at young ages than previously thought.

    Since Jupiter and Saturn are of distinctly super-solar composition, we examine emitted spectra for model planets at both solar metallicity and a metallicity of 5 times solar. These metal-enhanced young Jupiters have lower pressure photospheres than field brown dwarfs of the same effective temperatures arising from both lower surface gravities and enhanced atmospheric opacity.

    We highlight several diagnostics for enhanced metallicity. A stronger CO absorption band at 4.5 $\mu$m for the warmest objects is predicted. At all temperatures, enhanced flux in $K$ band is expected due to reduced collisional induced absorption by H$_2$. This leads to correspondingly redder near infrared colors, which are redder than solar metallicity models with the same surface gravity by up to 0.7 in $J-K$ and 1.5 in $H-K$. Molecular absorption band depths increase as well, most significantly for the coolest objects. We also qualitatively assess the changes to emitted spectra due to nonequilibrium chemistry.

    Comments: Accepted to ApJ. Most figures in color

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.1066v1 [astro-ph]

    Submission history

    From: Jonathan J. Fortney [view email]

    [v1] Wed, 7 May 2008 20:39:34 GMT (220kb)


  • ljk July 3, 2008, 16:33

    A time-dependent radiative model for the atmosphere of the eccentric transiting planets

    Authors: N. Iro, D. Deming

    (Submitted on 2 Jul 2008)

    Abstract: We present a time-dependent radiative model for the atmosphere of the transiting planets that take into account the eccentricity of their orbit. We investigate the temporal temperature and flux variations due to the planet-star distance variability. We will also discuss observational aspects with Spitzer measurements.

    Comments: To appear in the Proceedings of the 253rd IAU Symposium: “Transiting Planets”, May 2008, Cambridge, MA. 4 pages, 3 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.0266v1 [astro-ph]

    Submission history

    From: Nicolas Iro [view email]

    [v1] Wed, 2 Jul 2008 02:58:25 GMT (249kb)


  • ljk July 30, 2008, 0:05

    Radiative Hydrodynamical Studies of Irradiated Atmospheres

    Authors: Ian Dobbs-Dixon (McGill University)

    (Submitted on 28 Jul 2008)

    Abstract: Transiting planets provide a unique opportunity to study the atmospheres of extra-solar planets. Radiative hydrodynamical models of the atmosphere provide a crucial link between the physical characteristics of the atmosphere and the observed properties.

    Here I present results from 3D simulations which solve the full Navier-Stokes equations coupled to a flux-limited diffusion treatment of radiation transfer for planets with 1, 3, and 7 day periods. Variations in opacity amongst models leads to a variation in the temperature differential across the planet, while atmospheric dynamics becomes much more variable at longer orbital periods.
    I also present 3D radiative simulations illustrating the importance of distinguishing between optical and infrared opacities.

    Comments: To appear in the Proceedings of IAU Symposium 253, “Transiting Planets”, May 2008, Cambridge, MA

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.4541v1 [astro-ph]

    Submission history

    From: Ian Dobbs-Dixon [view email]

    [v1] Mon, 28 Jul 2008 20:39:36 GMT (1023kb)


  • ljk August 14, 2008, 23:25

    Emergent Exoplanet Flux: Review of the Spitzer Results

    Authors: Drake Deming

    (Submitted on 8 Aug 2008)

    Abstract: Observations using the Spitzer Space Telescope provided the first detections of photons from extrasolar planets. Spitzer observations are allowing us to infer the temperature structure, composition, and dynamics of exoplanet atmospheres. The Spitzer studies extend from many hot Jupiters, to the hot Neptune orbiting GJ436.

    Here I review the current status of Spitzer secondary eclipse observations, and summarize the results from the viewpoint of what is robust, what needs more work, and what the observations are telling us about the physical nature of exoplanet atmospheres.

    Comments: 11 pages, 8 figures, to appear in Proceedings of IAU Symposium 253

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0808.1289v1 [astro-ph]

    Submission history

    From: Drake Deming [view email]

    [v1] Fri, 8 Aug 2008 20:09:51 GMT (1032kb)


  • ljk October 6, 2008, 9:23

    ESA White paper: Atmospheric modeling: Setting Biomarkers in context

    Authors: L. Kaltenegger, F. Selsis

    (Submitted on 23 Sep 2008)

    Abstract: Motivation: ESAs goal to detect biomarkers in Earth-like exoplanets in the Habitable Zone requires theoretical groundwork that needs to be done to model the influence of different parameters on the detectable biomarkers.

    We need to model a wide parameter space (chemical composition, pressure, evolution, interior structure and outgassing, clouds) to generate a grid of models that inform our detection strategy as well as can help characterize the spectra of the small rocky planets detected.

    Comments: Input to the Call for White Papers for Exo-Planet Roadmap Advisory Team May 2008. this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0809.4042v1 [astro-ph]

    Submission history

    From: Lisa Kaltenegger [view email]

    [v1] Tue, 23 Sep 2008 22:23:32 GMT (143kb)


  • ljk December 10, 2008, 1:29

    Carbon dioxide found in exoplanet atmosphere using the Hubble.

    Details are here: