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.