Charge injection device boosts chances of detecting Earth-like planets near bright stars

A new camera device may help find small, Earth-like exoplanets by making them easier to see in the glare of their companion star(Credit: Florida Institute of Technology)

The quest to find small, Earth-like exoplanets isn't just a matter of pointing an exceptionally powerful telescope towards a star, as one may do to observe moons orbiting a planet. Apart from resolving images adequately in relation to the enormous distances involved, the glare from a distant sun often washes out the image of anything but the largest of planetary bodies in its vicinity. To help combat this problem, researchers at the Florida Institute of Technology (FIT) have developed a new type of astronomical camera that can detect the faint reflections of distant worlds near bright stars many millions of times better than that possible with an ordinary telescope.

Among the most coveted of exoplanet discoveries are those that uncover Earth-like planets that could potentially support life. Unfortunately, these types of planets are usually very much smaller and harder to detect than the giant gas exoplanets more often found, particularly when hunting for them with an optical telescope. This is because the reflected light of a planet may be millions of times fainter than that of its parent star, and direct optical observation is very rarely able to detect, let alone distinguish, distant planetary bodies.

For this reason, the vast majority of the more than 2000 confirmed exoplanet observations thus far have been through indirect methods. These include observing the change in visual brightness as a planet transits across the face of its sun, gravitational microlensing (where the increase in gravity near two passing stars warps spacetime sufficiently to magnify distant light), and detecting perturbations in the radial velocity of a star that may indicate it is being influenced by masses rotating around it.

The downside of such observation methods – particularly the radial velocity method – is that the exoplanets being detected need to be very large and the spin velocity of the observed star relatively small for the processes and equipment used to be effective. Observing exoplanets directly would negate these problems because it is not dependent on interpretations or the need for very specific types of stars or planets to carry out. Despite the direct observational accuracy, however, it means that astronomers have to overcome the problem of very faint objects adjacent to bright stars, often described as the candle-next-to-the-lighthouse problem (and we're talking about a very big lighthouse and a very small candle).

"Current instrument technology is very complex and expensive and still a ways off from achieving direct images of Earth-like planets," said FIT astrophysicist, and leader of the research, Daniel Batcheldor.

This is where FIT's charge injection device, or CID, comes in. A CID is basically a photosensitive semiconductor chip able to convert optical signals in the 185 nm to 1100 nm range to electronic charges that can be displayed as a video signal or further processed and enhanced via computer. To do this, every capacitive pixel in the CID array is able to be individually addressed using electronic indexing of the electrodes connected to the rows and columns. As such, when fitted to an optical telescope, the pixels in the CID that receive large amounts of light are acknowledged very quickly and idled down, while the pixels receiving faint amounts of light are allowed to continue gathering light for a longer time.

In other words, the various pixels in the CID array act in the opposite way expected of an optical detection array by responding least to bright incoming light and most to faint light. This, then, effectively means the device acts like it has semi-transparent masked areas that prevent the entire observed area being washed out.

"Personally, I like very simple, straightforward solutions, especially when there is a complex problem," he said. "The CID is able to look at a very bright source next to a very faint source and not experience much of the image degradation you would normally experience with a typical camera."

Different to Charge Coupled Device (CCD) cameras often used in astronomical telescopes – such as in the Pan-STARRS asteroid and comet tracker – that move collected charge out of the pixel when it is read (which then erases the image stored on the sensor), the charge stays within each pixel in the CID array. Converting the images captured in a CID involves detecting the displacement current between individual pixels, which is then amplified and converted to a voltage, and then forms part of a composite video or digitized signal. To clear the array for capturing the next image, the row and column electrodes connecting the pixels are briefly switched to ground thereby dissipating, or "injecting" the charge into the underlying semiconductor substrate.

"If this technology can be added to future space missions, it may help us make some profound discoveries regarding our place in the universe," said Batcheldor.

Batcheldor and a group of graduate students in the Physics and Space Sciences Department at FIT conducted their research using a CID attached to the university's 0.8-meter Ortega telescope. As a result, there were able to discern objects some 70 million times less luminous than Sirius, the most dazzling star in the night sky. Added to the fact that these faint objects were also accurately discerned from the Earth and through the dense Florida atmosphere, and the performance of the CID is very impressive.

Tests for the new CID are planned using an astronomical telescope located on the Canary Islands, and a more advanced prototype for a newer type of CID is scheduled for testing on the International Space Station, with both events planned for later some time this year.

Results of this research were recently published in the Publications of the Astronomical Society of the Pacific.

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