First Multi-wavelength Images of an Exoplanet

Colors are important in astronomy. They can be used to get a quick feel for the temperature of stars, map out hydrogen alpha, or even find oxygen when it gives off a distinctive green glow from the forbidden transistion. Yet thus far, all images of exoplanets have only been taken in a single color filter leaving astronomers with a flat picture and no understanding of the color of a planet. A new paper corrects this oversight while analyzing the polarization of reflected starlight to develop an understanding of the characteristics of the planet’s atmosphere.

One of the properties of light is that it often becomes polarized upon reflection. This allows for polarized sunglasses to effectively reduce glare from road surfaces because the reflection tends to polarize the light in a preferred direction. Similarly, light striking a planet’s atmosphere will have a preferred axis of polarization. The degree of polarization will depend on many factors including, the angle of incidence (corresponding to the planetary phase), the types of molecules in the atmosphere, and the color, or wavelength, of light through which the planet is observed.

The object of interest was HD189733b and observations were taken in using the UBV filters system which uses filters in the ultra-violet, blue, and green (or “visible”) portions of the spectra. They were conduced at the Nordic Optical Telescope in Spain.

To control for the variations, astronomers would need to observe the planet at several wavelengths to understand how the color was affecting the results, as well as to watch the planet for several orbits to trace how the phase impacted the observations. Presently, the authors have not gone so far as to compare various composition models against these observations as this study was largely intended to be a feasibility study at multi-wavelength polarization detection.

Results have shown that the planet is brightest in the blue portion of the spectra, a result that confirms earlier, theoretical predictions for hot Jupiters as well as tentative observational findings based on single color studies done last year. This supports the notion that the dominant mechanism of polarization is Rayleigh scattering in the atmosphere. The result of this is that the planet would likely appear to be a deep blue to the naked eye, much the same way our sky appears blue, but a much more vivid color due to the increased depth to which we would look. The observations also confirmed that polarization was greatest when the planet was near greatest elongation (as far to either side of the star as possible instead of near in front or behind when viewed from Earth) which supports that the polarization is due to scattering in the atmosphere as opposed to the starlight being initially polarized from large starspots.

Certainly, this study has demonstrated the potential for astronomers to begin exploring planetary characteristics with polarization. However, it may be some time before it becomes accepted in general use. While the findings were certainly above the background noise, there existed a significant degree of uncertainty in the measurements resulting from the faint nature of planets. Being a large, hot Jupiter, HD189733b is a strong candidate since it is close to its parent star and thus, receives a large amount of light. Using such methods for other exoplanets, more distant from their parent stars will likely prove an even more daunting task, requiring careful preparation and observations.

Gemini’s New Filters Reveal the Beauty of Star Birth

Sharpless 2-106 (Gemini Observatory/AURA, right; left: copyright Subaru Telescope, National Astronomical Observatory of Japan; All rights reserved)

About 2,000 light-years away, in the constellation of Cygnus (the Swan), lies Sharpless 2-106 (after Stewart Sharpless who put the catalog together in 1959), the birth-place of a star cluster-to-be.

Two recent image releases – by Subaru and Gemini – showcase their new filter sets and image capabilities; they also reveal the stunning beauty of the million-year-long process of the birth of a star.

Sharpless 2-106 (Gemini Observatory/AURA)

The filter set is part of the Gemini Multi-Object Spectrograph (GMOS) toolkit, and includes ones centered on the nebular lines of doubly ionized oxygen ([OIII] 499 nm), singly ionized sulfur ([SII] 672 nm), singly ionized helium (HeII 468nm), and hydrogen alpha (Hα 656 nm). The filters are all narrowband, and are also used to study planetary nebulae and excited gas in other galaxies.

The hourglass-shaped (bipolar) nebula in the new Gemini image is a stellar nursery made up of glowing gas, plasma, and light-scattering dust. The material shrouds a natal high-mass star thought to be mostly responsible for the hourglass shape of the nebula due to high-speed winds (more than 200 kilometers/second) which eject material from the forming star deep within. Research also indicates that many sub-stellar objects are forming within the cloud and may someday result in a cluster of 50 to 150 stars in this region.

The nebula’s physical dimensions are about 2 light-years long by 1/2 light-year across. It is thought that its central star could be up to 15 times the mass of our Sun. The star’s formation likely began no more than 100,000 years ago and eventually its light will break free of the enveloping cloud as it begins the relatively short life of a massive star.

For this Gemini image four colors were combined as follows: Violet – HeII filter; Blue – [SII] filter; Green – [OIII] filter; and Red – Hα filter.

Sharpless 2-106 (Copyright Subaru Telescope, National Astronomical Observatory of Japan. All rights reserved)

The Subaru Telescope image was made by combining images taken through three broadband near-infrared filters, J (1.25 micron), H (1.65 micron), and K’ (2.15 micron).

Sources: Gemini Observatory, NAOJ