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Optical Detectors for the Discovery of Transiting Exoplanets

Challenge Background

The transit method is currently one of the most successful indirect techniques to detect extrasolar planets. To date, over 3800 exoplanets were discovered, from which the majority was found using the transit method. This technique is based on observations of a planet passing in front of its host star (= transit), as seen from an observer on Earth. The planet in transit blocks a fraction of the stellar light and thus causes the stellar brightness to decrease periodically. This periodic dimming of the stellar light is measured in terms of a transit light curve.

The first discovery of an exoplanet through ground-based transit observations, namely the hot-Jupiter ‘HD 209458 b’, was made in 2000. In the following years, the number of detected transiting exoplanets increased rapidly, revealing a large diversity of planetary types. These discoveries were made with the help of ground-based as well as space-based observing facilities.

One of the future goals of exoplanet science is to discover planets with properties similar (e.g., size, composition) to Earth. Using the latest technology, planets in this size domain can be most easily discovered around M-dwarf stars. M-dwarfs are smaller and cooler than, e.g., Sun-like stars. Thus, an Earth-sized exoplanet transiting an M-dwarf star causes a more significant decrease in stellar brightness than the same planet orbiting a Sun-like star. In general, however, detecting transit signatures of planets of this kind remains challenging due to the planet’s small relative sizes and/or faint host stars, combined with limited sensitivities of existing instruments.

Technology Solution

Andor’s large area iKon-XL and -L back-illuminated CCD cameras represent ideal solutions for exoplanet science, due to the sensor’s high sensitivity and high QE coverage (>90% peak QE) in combination with a low noise floor supporting high precision photometry. Sensor options for extended NIR performance (‘BEX2-DD’ and ‘BR-DD’) allow for the detection of small planets orbiting cool stars (see Fig. 1). Furthermore, the iKon-XL and -L are perfectly suited for usage in remote observing sites, thanks to the unique Andor UltraVac™ vacuum thermoelectric cooling technology, enabling superb sensor longevity and avoiding the need for vacuum re-pumping.

Andor strongly recommends the Andor iKon-XL and -L back-illuminated CCD Cameras for the detection of transiting extrasolar planets:

Exoplanet Discovery Requirement Exoplanet Discovery Solution: iKon-XL and iKon-L
Probe stellar photometric variability in a large area of sky iKon-XL is a 16.8 Megapixel camera platform, integrating the e2v CCD231-84 and CCD230-84 devices. iKon-L is a 4.2 Megapixel camera platform, integrating the e2v CCD42-40 device. Both sensors can support large field of views, allowing to search for exoplanet hosting stars in a greater area of sky.
High precision photometry Both iKon-XL and iKon-L combine 95% peak QE with low read noise (slow-scan) and -100 °C vacuum TE cooling for dark noise minimization, even over long acquisition periods.
Observe faint and bright objects The unique Extended Dynamic Range technology (18-bit) of the iKon-XL allows measuring signals of faint and bright stars simultaneously.
Detect planets orbiting cool stars Due to the extended NIR sensitivity options available for both the iKon-XL and -L, cool stars, such as M-dwarfs, can be probed, which would otherwise be too faint in the visible part of the spectrum (see Fig. 1).
Low maintenance and minimal downtime of cameras iKon-XL and iKon-L are perfectly architected for sustained usage in remote observing locations. The proprietary UltraVac™ permanent vacuum sensor enclosure (no re-pumping!) affords the ultimate in performance longevity. The thermoelectric cooling capability avoids need for LN2 or unreliable cryo-coolers. iKon-XL also comes with a field replaceable shutter unit.

Fig. 1 Quantum efficiency curves of the standard Silicon (‘BV’) and deep depletion (‘BR-DD’ and ‘BEX2-DD’) iKon-L sensor options. The figure additionally shows the peak wavelengths λmax of the Sun and the exoplanet hosting M-dwarf GJ 1132, assuming effective temperatures of 5778 K and 3270 K, respectively. It highlights the higher quantum efficiency in the NIR using deep depletion sensors, which is of great importance for observations of cool stars, such as M-dwarfs.

Date: January 2019

Author: Dr Ines Juvan-Beaulieu

Category: Solution Note

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