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.
Staring up at the night sky is one of the most humbling experiences I’ve ever felt. Each of those tiny points of light is a sun burning furiously in a runaway fusion reaction. By comparison I, a mere human, am no more than a tiny fleck in comparison to one of those stars and barely even an atom when compared to the teaming masses of stars that make up that beautiful nightscape. Even more daunting then is the possibility that each of those twinkling stars plays host to a solar system like our own with dozens of planets just waiting for discovery. Our hunt for these planets has brought us hundreds of large gas giants who by the nature have been very easy to detect. Direct imaging of these planets has been nigh on impossible with the precious few we’ve managed to glimpse being extraordinary examples, rather than the rule. That is set to change, however.
Light, you see, is a funny thing. For centuries scientists pondered over the modelling of it, with the two dominant theories describing it as either as a particle or a wave phenomena. Problem is that light didn’t fit neatly into either of the models, requiring complex modelling in order to fit its behaviour into either the particle or wave category. Today many of the properties of light are now explained thanks to Einstein’s theory of wave-particle duality but for a long time one of the most confounding properties of light was that light can interfere with itself. You’ve probably seen this demonstrated to you back in college via the double slit experiment where you get a pattern of light and dark from a single source of light. At the time I didn’t think much of it past the initial intrigue but my discovery of my passion for space many years later had me thinking about how this might be used.
I had been reading about the hundreds of exoplanet discoveries for a while when I heard of 2M1207b which is thought to be the first directly imaged planet outside our solar system. It’s an exceptional planet being an extremely hot gas giant orbiting a very dim companion star. For systems like our own there would be no chance of seeing any planets from the outside thanks to our extremely bright sun and our relative proximity to it. Still knowing that light had the novel ability to cancel itself out I had wondered if we could say build an apparatus that forced light from a parent star to cancel itself out, letting us peer behind the blazing might to see what lie beneath.
It wasn’t until a few years later when I stumbled across the idea of a StarShade which had been proposed many years previously. In essence it would function as an augmentation to any space based telescope positioning itself perfectly in front of the parent star and reducing its brightness by a whopping 10 billion times. In comparison then the tiny planets which were once outshone would glow bright enough for the telescopes to be able to see them directly, hopefully leading to direct detection of many planets orbiting the star. Unfortunately it appears that this project is now defunct but that doesn’t mean the idea doesn’t live on in other forms.
Most recently an international collaboration of scientists developed a Apodizing Phase Plate coronagraph which is in essence a scaled down version of a starshade that can be installed in current telescopes:
Installed on the European Southern Observatory’s Very Large Telescope, or VLT, atop Paranal Mountain in Chile, the new technology enabled an international team of astronomers to confirm the existence and orbital movement of Beta Pictoris b, a planet about seven to 10 times the mass of Jupiter, around its parent star, Beta Pictoris, 63 light years away.
At the core of the system is a small piece of glass with a highly complex pattern inscribed into its surface. Called an Apodizing Phase Plate, or APP, the device blocks out the starlight in a very defined way, allowing planets to show up in the image whose signals were previously drowned out by the star’s glare.
It’s not just planets that this device helps discover either, it can also help detect distant objects that are hidden behind brighter ones. This enables telescopes to become even more powerful than they once were with minimal modifications. Probably the best part about this is that they’re already using them on the Very Large Telescope in Chile, proving that technology is much more than just a theory.
There’s so much to discover in our universe and it always gets me excited to see these pieces of technology that allow us to pull back the veil and peer ever further into the deepest parts of space. It’s so humbling to know that you’re just a tiny piece of a seemingly infinite universe yet it’s so enthralling that I lose myself for hours just staring up at the night sky. I feel so privileged to be living in a time were our knowledge of this universe is increasing at an ever accelerating rate yet we’re still left wondering at the awesome beauty that’s put before us.