51-Eridani-b, The Smallest Exoplanet Caught on Camera.

When it comes to exoplanets the question that I often hear asked is: why are they all largely the same? The answer lies in the methods that we use for detecting exoplanets, the most successful of which is observing the gravitational pull that planets have on their host stars. This method requires that planets make a full orbit around their parent start in order for us to detect them which means that many go unnoticed, requiring observation times far beyond what we’re currently capable of. However there are new methods which are beginning to bear fruit with one of the most recent discoveries being a planet called 51-Eridani-b.

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Unlike most other exoplanets, whose presence is inferred from the data we gather on their parent star, 51-Eridani-b is the smallest exoplanet that we’ve ever imaged directly. Whilst we didn’t get anything like the artist’s impression above it’s still quite an achievement as planets are usually many orders of magnitude dimmer than their parent stars. This makes directly imaging them incredibly difficult however this new method, which has been built into a device called the Gemini Planet Imager, allows us to directly image a certain type of exoplanet. The main advantage of this method is that it does not require a lengthy observation time to produce results although like other methods it also has some limitations.

The Gemini Planet Imager was built for the Gemini South Telescope in Chile, the sister telescope of the more famous Gemini North Telescope in Hawaii. Essentially it’s an extremely high contrast imager, one that’s able to detect a planet that’s one ten millionth as bright as its parent star. Whilst this kind of sensitivity is impressive even it can’t detect Earth-like planets around a star similar to our sun. Instead the planets that we’re likely to detect are young jupiter planets which are still hot from their formation being far more luminous than a planet typically is. This is exactly what 51-Eridani-b is, a fiery young planet that orbits a star that’s about 5 times as bright as our own.

Equally as impressive is the technology behind the Gemini Planet Imager which enables it to directly image planets like this. The first part is a coronagraph, a specially designed interference device which allows us to block out the majority of a parent star’s light. Behind that is a set of adaptive optics, essentially a set of tiny mirrors that can make micro-adjustments in order to counteract atmospheric distortions. It has to do this since, unlike space based telescopes, there’s a lot of turbulent air between us and the things we want to look at. These mirrors, which are deformable at the micro level using MEMS, are able to do this with incredible precision.

With the successful discovery of 51-Eridani-b I’m sure further discoveries won’t be far off. Whilst the Gemini Planet Imager might only be able to discover a certain type of planet it does prove that the technology platform works. This then means that improvements can be made, expanding its capabilities further. I have no doubt that future versions of this technology will be able to directly image smaller and smaller planets, one day culminating in a direct image of an Earth-like planet around a sun-like star. That, dear read, will be a day for the history books and it all began here with 51-Eridani-b.

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