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.
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.
There’s no denying the fact that space based telescopes are by far the best instruments for us to observe the universe. They don’t suffer from atmospheric interference, observe targets for incredibly long periods of time and aren’t limited to observing a section of the sky. Of course they come with quite a lot of drawbacks as well often being incredibly expensive to build, launch and operate and unless you’re the Hubble you can forget ever being serviced or repaired, you’re more likely to be ditched in the pacific while your replacement is launched. Still it’s not like ground telescopes are useless by comparison and in the not too distant future our most powerful telescope might just be one located here on terra firma.
That, my friends is a rendering of the European Extremely Large Telescope (E-ELT) an upcoming ground telescope that has just received approval from the European Southern Observatory organisation to go ahead. The tiny car at the bottom of the picture gives you some clue into just how large this particular telescope will be, dwarfing nearly all those that have come before it. House inside that giant building is primary mirror array that is some 39.3 meters across, about half the length of a football field. To put this in perspective Hubble’s main mirror is about 8 meters across or almost 5 times smaller than that of the E-ELT.
Ah, I hear you say, but what about the fact that this one isn’t in space but Hubble is? Well traditionally that was quite a problem for telescopes as there wasn’t a good way to compensate for the changes in the atmosphere leading to blurred or low resolution images. There’s a couple ways to combat this and the usual method was to locate the telescope in a place that had favorable conditions for night time observations. This is usually in high up places so the amount of atmosphere is decreased but places like the Atacama desert, known as the driest place on earth, also provide excellent viewing opportunities almost all year round (320 out of 365 days are cloud free). There’s also a much more advanced way of dealing with atmospheric disturbances and that’s called adaptive optics.
Whilst I referred to the E-ELT as having a 39.3m mirror it is in fact more accurate to say it has a mirror array consisting of 800 individual elements that are all about 1m across. Each of these mirrors can be adjusted independently to compensate for any changes in the above atmosphere. They do this by using a laser to illuminate the sky above them providing a kind of artificial star with a controllable brightness that they can use to adjust the mirror array. Additionally the telescope has a secondary focusing mirror that has over 6000 actuators able to make adjustments 1000 times per second. Combining all of this together means that the E-ELT will have imaging capabilities far surpassing that of any telescope previously and will be the first telescope able to directly image rocky planets like our own orbiting other stars.
It’s that simple fact which has got me so excited about this new telescope. We’ve been able to directly image some planets around other stars in the past but we haven’t been able to get much detail out of them past a bright blob on a black background. The E-ELT will be able to resolve objects with astonishing levels of detail and I’m sure that our hunt for planets like our own will receive a massive boost at the hands of its giant mirror array. It’s projects like this, real envelope pushers, that keep me so excited about the wide vastness of space and how much of it we still have to explore.