# First Reflected Light from an Exoplanet?

by on December 31, 2007

Apropos of Saturday’s article on the Subaru Telescope — and the optical improvements that may help it detect an exoplanet image — comes news that scattered light from a planet orbiting another star has been detected by an international team. The planet is our old friend HD189733b, some sixty light years away in the constellation Vulpecula. The transiting world is a ‘hot Jupiter,’ orbiting its star in 2.2 days at a blindingly close 0.03 AU.

Thanks to Hans Bausewein for passing along the link to a news release on this work. The scientists involved — Svetlana Berdyugina (ETH Zurich & Tuorla Observatory), Andrei Berdyugin and Vilppu Piirola (Tuorla Observatory), and Dominique Fluri (ETH Zurich) did their study using polarimetry, examining starlight that is scattered in the distant planet’s atmosphere. Polarimetry measures the angle of rotation of the plane of polarized light that occurs when it moves through particular materials. It’s a useful technique for exoplanetary work because such detections tell us about the chemistry and thermodynamics at play within that atmosphere.

Using the remotely controlled KVA telescope on La Palma (Spain), the team will report in its upcoming paper that the atmosphere under study consists of particles smaller than half a micron, scattering light in the blue, like Earth’s atmosphere. Such findings are consistent with water vapor or tiny dust grains. The work also allows us to see the shape of the planet’s orbit, a determination that can be made because as the planet moves around its star, the scattering angle changes. From the paper:

The observed polarization variability should thus, in principle, exhibit the orbital period of the planet and reveal the inclination, eccentricity, and orientation of the orbit, and, if detected with high enough polarimetric accuracy, also the nature of scattering particles in the planetary atmosphere.

The image below shows the result.

Image: The orbit of HD189733b as projected on the sky. Solid and dashed lines indicate parts of the orbit in front of and behind the sky plane, respectively. The orange circle in the center depicts the star and the blue circle on the orbit the planet. Credit: Svetlana Berdyugina. For more background, see Dr. Berdyugina’s description of this work.

Hot Jupiters, it turns out, are the best candidates for this kind of work because they are so close to their star that they develop extended hydrogen haloes. That makes for effective scattering of the star’s light, especially in the blue wavelengths. The authors are calling this the first direct detection of an extrasolar planet in visible light. But they argue that polarimetric techniques have significance for future planet studies not just of hot Jupiters but a wide variety of worlds:

Our ﬁndings open the door to new opportunities for direct detections of extrasolar planets, both hot Jupiters and Earth-like, to a large degree independently of their mass and gravitational effect on the host star. Furthermore, until now probing exoplanetary atmospheres was largely limited to systems with transits, which are relatively rare events. Polarimetry provides us with the new prospect of detecting directly the light from the planetary atmosphere outside transits. Thus, with polarimetric accuracy approaching the photon noise limit, direct studies of exoplanetary atmospheres in the visible at any orbital inclination become reality.

The paper is Berdyugina, S.V. et al., “First detection of polarized scattered light from an exoplanetary atmosphere,” in press at Astrophysical Journal Letters and available online.

dad2059 December 31, 2007 at 11:30

Okay, I have a few dumb questions to ask Paul, why are we finding so many “hot Jupiters”? Are the stars we’re finding these planets at relatively young? I realize the reason we’re learning to detect extrasolar planets at all is because the fast orbiting hot jupiters exert an obvious gravitational effect on it’s star. But our solar system doesn’t have such planets. Did our solar system have one or more during it’s formation?

Administrator December 31, 2007 at 11:43

Those are good questions, Dad2059, and you’re right about the primary thing: Radial velocity methods are such that a ‘hot Jupiter’ is going to show up a lot faster than a gas giant orbiting at 10 AU or so from its star. We can still use radial velocity to detect planets in much wider orbits, but gathering the data takes much more time. You can see that a planet buzzing around its star in a few days is having all kinds of useful effects that RV measurements can work with.

I’m aware of no evidence of a hot Jupiter during our own Solar System’s formation. One question we’ll want to answer is just how widespread these hot Jupiter systems are. Another crucial one: Is a hot Jupiter a necessary deterrent to the development of habitable worlds further out in the system? Opinion on that one is divided, but there is the possibility of living worlds even in these systems. Anyway, we shouldn’t be surprised we’re finding lots of hot Jupiters given our RV methods (although the initial discovery that such planets existed at all was a surprise to many). As to the relative age of the stars these planets orbit, I don’t have a broad answer on that. Maybe one of the readers can supply it — I’ll also dig around to see if I can find out.

dad2059 December 31, 2007 at 15:34

Thanks for the answers Paul. I should’ve realized given the search method, we’d find large planets like that. And yes, I guess the rapid orbital period of the bodies took people by surprise.

We’re still babes in the wilderness when it comes to this stuff.

Administrator December 31, 2007 at 16:03

One other comment on the ‘hot Jupiter’ question. Gregory Matloff told me that when he worked with Buzz Aldrin and John Barnes, who were then writing Encounter with Tiber, he like just about everyone had no idea that a star-hugging gas giant could exist. But when he ran the numbers back in the early 1990s, he realized that the possibility of such planets remaining stable over long periods was there. Hot Jupiters went from raw science fiction to demonstrated reality in a very short time indeed. As you say, we’re babes in the wilderness in many ways, and can expect many, many more surprises ahead.

David January 1, 2008 at 9:08

Happy New years to everyone.

Actually Otto Struve in 1952 was the first person to think of hot jupiters though he did not call them as such. He also pointed out that that the spectrometers of the time had sufficient precision to discover them and that they could also be discovered by transits.

He reasoned since that there are stellar binaries where one star can orbit the other with a period as short as a day, he points out that there was no reason why a planet could not be in such an orbit. The paper is quite readable and is only two pages long. It has now been cited 25 times with the first citation in 1988.

Had someone followed up on Struve’s work, hot jupiters could have been discovered decades ago.

Administrator January 1, 2008 at 9:38

That’s remarkable. I had no idea that Struve came up with this so early. Thanks for the link, David.

Steve January 1, 2008 at 13:12

Is it possible to save the moving GIF as a moving GIF – simple copy & paste just captures a single frame.

Administrator January 1, 2008 at 14:59

Steve, capturing a moving GIF like the one above seems to require software that recognizes the format. I work in Linux and what I do is to save the file with a right mouse-click; I then open it in an image editor like GIMP, which recognizes it as an animation and lets me edit it as such. I know there are Windows and Mac tools that do the same thing, but I have to ask readers to help out on this, because I don’t know the specifics. Can anyone suggest software for either of these platforms that can help Steve out?

Hans Bausewein January 1, 2008 at 16:27

If I understand it correctly, it opens a whole new window for exoplanet searches. Planetary systems where we look on a pole of the star should show a constant light curve, but with a small fraction of the light having a rotating polarisation.
Still most sensitive to big planets in close orbits, but with orbits in other planes than directed to us.

dad2059 January 2, 2008 at 7:23

The Struve paper answered my questions exactly, thanks for posting the link David!

ljk January 14, 2008 at 0:42

Improved parameters for extrasolar transiting planets

Authors: G. Torres (CfA), J. N. Winn (MIT), M. J. Holman (CfA)

(Submitted on 11 Jan 2008 (v1), last revised 11 Jan 2008 (this version, v2))

Abstract: We present refined values for the physical parameters of transiting exoplanets, based on a self-consistent and uniform analysis of transit light curves and the observable properties of the host stars. Previously it has been difficult to interpret the ensemble properties of transiting exoplanets, because of the widely different methodologies that have been applied in individual cases. Furthermore, previous studies often ignored an important constraint on the mean stellar density that can be derived directly from the light curve.

The main contributions of this work are 1) a critical compilation and error assessment of all reported values for the effective temperature and metallicity of the host stars; 2) the application of a consistent methodology and treatment of errors in modeling the transit light curves; and 3) more accurate estimates of the stellar mass and radius based on stellar evolution models, incorporating the photometric constraint on the stellar density.

We use our results to revisit some previously proposed patterns and correlations within the ensemble. We confirm the mass-period correlation, and we find evidence for a new pattern within the scatter about this correlation: planets around metal-poor stars are more massive than those around metal-rich stars at a given orbital period. Likewise, we confirm the proposed dichotomy of planets according to their Safronov number, and we find evidence that the systems with small Safronov numbers are more metal-rich on average. Finally, we confirm the trend that led to the suggestion that higher-metallicity stars harbor planets with a greater heavy-element content.

Comments: To appear in The Astrophysical Journal. 23 pages in emulateapj format, including figures and tables. Figures 7, 8, and 9 are low resolution; higher resolution versions will be available from the journal when published

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.1841v2 [astro-ph]

Submission history

From: Guillermo Torres [view email]

[v1] Fri, 11 Jan 2008 20:59:55 GMT (327kb)

[v2] Fri, 11 Jan 2008 21:06:38 GMT (327kb)

http://arxiv.org/abs/0801.1841

ljk February 7, 2008 at 11:18

The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b

Authors: David Charbonneau, Heather A. Knutson, Travis Barman, Lori E. Allen, Michel Mayor, S. Thomas Megeath, Didier Queloz, Stephane Udry

(Submitted on 6 Feb 2008)

Abstract: We present Spitzer Space Telescope time series photometry of the exoplanet system HD 189733 spanning two times of secondary eclipse, when the planet passes out of view behind the parent star. We estimate the relative eclipse depth in 5 distinct bands and find the planet-to-star flux ratio to be 0.256 +/- 0.014% (3.6 microns), 0.214 +/- 0.020% (4.5 microns), 0.310 +/- 0.034% (5.8 microns), 0.391 +/- 0.022% (8.0 microns), and 0.598 +/- 0.038% (24 microns). For consistency, we re-analyze a previously published time series to deduce a contrast ratio in an additional band, 0.519 +/- 0.020% (16 microns). Our data are strongly inconsistent with a Planck spectrum, and we clearly detect emission near 4 microns as predicted by published theoretical models in which this feature arises from a corresponding opacity window.

Unlike recent results for the exoplanet HD 209458b, we find that the emergent spectrum from HD 189733b is best matched by models that do not include an atmospheric temperature inversion. Taken together, these two studies provide initial observational support for the idea that hot Jupiter atmospheres diverge into two classes, in which a thermal inversion layer is present for the more strongly irradiated objects.

Comments: 19 pages, 3 figures, submitted to the Astrophysical Journal

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.0845v1 [astro-ph]

Submission history

From: David Charbonneau [view email]

[v1] Wed, 6 Feb 2008 19:19:04 GMT (62kb)

http://arxiv.org/abs/0802.0845

ljk February 7, 2008 at 11:20

On the Presence of Water and Global Circulation in the Transiting Planet HD 189733b

Authors: Travis S. Barman

(Submitted on 6 Feb 2008)

Abstract: Detailed models are compared to recent infrared observations of the nearby extrasolar planet, HD 189733b. It is demonstrated that atmospheric water is present and that the planet’s day side has a non-isothermal structure down to gas pressures of ~ 0.1 bars. Furthermore, model spectra with different amounts of CO are compared to the observations and an atmosphere absent of CO is excluded at roughly 2-sigma. Constraining the CO concentration beyond that is unfortunately not possible with the current Spitzer photometry. However, radically enhanced (or depleted) metal abundances are unlikely and the basic composition of this planet is probably similar to that of its host star. When combined with Spitzer observations, a recent ground-based upper limit for the K-band day side flux allows one to estimate the day-to-night energy redistribution efficiency to be ~ 43%.

Comments: accepted (2008 Feb. 5), ApJ Letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.0854v1 [astro-ph]

Submission history

From: Travis S. Barman [view email]

[v1] Wed, 6 Feb 2008 18:18:20 GMT (126kb)

http://arxiv.org/abs/0802.0854

ljk February 8, 2008 at 10:46

Methane present in an extrasolar planet atmosphere

Authors: Mark R. Swain, Gautam Vasisht, Giovanna Tinetti

(Submitted on 7 Feb 2008)

Abstract: Molecules present in exoplanetary atmospheres are expected to strongly influence the atmospheric radiation balance, trace dynamical and chemical processes, and indicate the presence of disequilibrium effects. Since molecules have the potential to reveal the exoplanet atmospheric conditions and chemistry, searching for them is a high priority. The rotational-vibrational transition bands of water, carbon monoxide, and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jovian planets. Since these bands overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy.

Here we report on a near-infrared transmission spectrum of the planet HD 189733b showing the presence of methane. Additionally, a resolved water-vapour band at 1.9 microns confirms the recent claim of water in this object. On thermochemical grounds, carbon-monoxide is expected to be abundant in the upper atmosphere of hot-Jovian exoplanets; thus the detection of methane rather than carbon-monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanisms that leads to an enhancement of methane.

Comments: accepted for publication in Nature

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.1030v1 [astro-ph]

Submission history

From: Mark Swain [view email]

[v1] Thu, 7 Feb 2008 18:00:38 GMT (442kb)

http://arxiv.org/abs/0802.1030

ljk February 21, 2008 at 11:58

MOST Spacebased Photometry of the Transiting Exoplanet System HD 189733: Precise Timing Measurements for Transits Across an Active Star

Authors: E. Miller-Ricci, J.F. Rowe, D. Sasselov, J.M. Matthews, R. Kuschnig, B. Croll, D.B. Guenther, A.F.J Moffat, S.M. Rucinski, G.A.H. Walker, W.W. Weiss

(Submitted on 19 Feb 2008)

Abstract: We have measured transit times for HD 189733b passing in front of its bright (V = 7.67) chromospherically active and spotted parent star. Nearly continuous broadband optical photometry of this system was obtained with the MOST (Microvariability & Oscillations of STars) space telescope during 21 days in August 2006, monitoring 10 consecutive transits. We have used these data to search for deviations from a constant orbital period which can indicate the presence of additional planets in the system that are as yet undetected by Doppler searches. There are no transit timing variations above the level of ${\pm}45$ s, ruling out super-Earths (of masses $1 – 4 M_{\earth}$) in the 1:2 and 2:3 inner resonances and planets of 20 $M_{\earth}$ in the 2:1 outer resonance of the known planet.

We also discuss complications in measuring transit times for a planet that transits an active star with large star spots, and how the transits can help constrain and test spot models. This has implications for the large number of such systems expected to be discovered by the CoRoT and Kepler missions.

Comments: 26 pages, 7 figures, accepted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.2722v1 [astro-ph]

Submission history

From: Eliza Miller-Ricci [view email]

[v1] Tue, 19 Feb 2008 21:41:22 GMT (323kb)

http://arxiv.org/abs/0802.2722

ljk February 25, 2008 at 10:51

Rayleigh scattering in the transit spectrum of HD 189733b

Authors: A. Lecavelier des Etangs, F. Pont, A.Vidal-Madjar, D. Sing

(Submitted on 21 Feb 2008)

Abstract: The transit spectrum of the exoplanet HD 189733b has recently been obtained between 0.55 and 1.05 microns. Here we present an analysis of this spectrum. We develop first-order equations to interpret absorption spectra. In the case of HD 189733b, we show that the observed slope of the absorption as a function of wavelength is characteristic of extinction proportional to the inverse of the fourth power of the wavelength (lambda^-4). Assuming an extinction dominated by Rayleigh scattering, we derive an atmospheric temperature of 1340+/-150 K. If molecular hydrogen is responsible for the Rayleigh scattering, the atmospheric pressure at the planetary characteristic radius of 0.1564 stellar radius must be 410+/-30 mbar. However the preferred scenario is scattering by condensate particles. Using the Mie approximation, we find that the particles must have a low value for the imaginary part of the refraction index. We identify MgSiO3 as a possible abundant condensate whose particle size must be between 0.01 and 0.1 microns. For this condensate, assuming solar abundance, the pressure at 0.1564 stellar radius is found to be between a few microbars and few millibars, and the temperature is found to be in the range 1340-1540 K, and both depend on the particle size.

Comments: Accepted for publication in A&A Letter

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.3228v1 [astro-ph]

Submission history

From: Alain Lecavelier des Etangs [view email]

[v1] Thu, 21 Feb 2008 22:08:53 GMT (28kb)

http://arxiv.org/abs/0802.3228

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