We have heard a lot about exoplanets in the past year. But for all the talk about these planets, which orbit a star other than our sun, we still haven’t actually seen one.
One tool that could change that, giving us our first look at a distant planet that could be the next best thing to Earth.
Currently, scientists detect an extra-solar planet by measuring the dimming of the star as the planet passes between it and our line of sight (this is known as the Transit Method). By observing the way the star’s light shines around the planet, it’s possible to gather spectroscopic data to determine some information about its composition. But this method, used in NASA’s Kepler project, only works for extremely large planets with gravity strong enough to crush a human. To find Earth-sized planets (the ones Earthlings are really interested in) a significantly more sensitive telescope is needed.
The man in charge of developing that new instrument is Dr. Rus Belikov, wearing the equipment-shielding shower cap in the video above. Belikov is the Technical Lead for the Ames Coronagraph experiment, a project intended to directly image, or see, distant planets. But first, Belikov and his team have to overcome three problems.
1. Relative Brightness
On average, stars are one billion times brighter than the Earth-like planets orbiting them. It’s hard to see something very dim next to something very bright. When looking from such a great distance, trying to see an Earth-sized planet by a star is like trying to see a lightning bug sitting on the lamp of a lighthouse while it’s shining in your eyes and you’re 10 miles away.
When a telescope sensitive enough to see a planet points at a star, the image comes back completely white. Dim the scope, and the image takes on concentric rings caused by diffraction. When light waves encounter a solid object (in this case, the opening of a telescope), they produce ripples. Those ripples make it impossible to see something as tiny as a planet.
To overcome the problems, Ames is working on phase-induced amplitude apodization (PIAA). This uses a series of mirrors to refocus a light beam and soften its edges (where diffraction occurs) without diminishing the amount of light the telescope receives. If it works perfectly, it would produce a sharp image of a star. If there are planets near the star, a scientist would be able to see them clearly. Direct visual confirmation of an exoplanet would happen for the first time. (Note: The glass lens seen in the video are from PIAA's successful proof-of-concept prototype, and the mirrors found in the current PIAA are warped in a similar fashion.
2. The Limits of Optics
But there's another fundamental engineering problem. When dealing with objects this far away, the optical tools like the lenses and the PIAA mirrors must be absolutely perfect. Like, perfect in the literal sense. Even the very best lenses ever created have small abnormalities, known optical aberrations, which compromise the integrity of the image. These aberrations would obscure exoplanets, making them impossible to see. Manufacturers can't make glass that flawless -- the moment it is touched or moved, aberrations appear. But the Ames Coronagraph scientists have found a way to solve the problem of aberrations: deformable mirrors.
The deformable mirror is placed between the PIAA mirrors and the imaging sensor. It's just one centimeter by one centimeter, but under its shiny gold sheet is a grid of 32 by 32 actuators which can go up or down. In other words, there are 1,024 points of adjustment within that tiny space. Not only is it accurate enough to compensate for the tiny aberrations in the optics, but it's so accurate that we don't know how accurate it is because we don't yet have instruments accurate enough to measure the level of its accuracy. The point is it's pretty accurate.
But breakthrough mirrors can only do so much.
3. Temperature Fluctuations
Heat changes in space (and on Earth) cause metal and glass to expand and contract. To protect the system from these variables, the Ames crew has built an enclosure that stays just cool enough to be perfectly stable.
The enclosure is built in two layers. The outer layer is composed of insulating panels. The inner enclosure contains a network of water circuits, with 14 hoses coming in and 14 hoses coming out. Water flows though the panels at a steady heat, keeping the inner enclosure at a constant temperature. The heat remains within a 1 millikelvin range -- that's a change of less than 1/1000th of a degree (Celsius), regardless of the massive temperature fluctuations outside the box. It's necessary, as the tiniest temperature variation will cause enough misalignment to ruin everything.
Testing the Solutions
To test the system, Ames has been collaborating with NASA JPL in Pasadena, California, using a High Contrast Imaging Testbed (HCIT). The HCIT shoots a tiny laser though a piece of fibre to create a stand-in for a star. As of now, the researchers can resolve an image at a 10-8 contrast, which is enough to see large exoplanets. Once they get into the 10-10 and 10-12 ranges, they will be able to generate raw images of Earth-like planets. If the work can continue to progress at the rate it has been going, the team could achieve this goal within the next few years.
If the tests succeed, it's likely that NASA will first fly a smaller version of this system in the next five to 10 years. This round of testing will show how well the machine will hold up and perform in space. If all goes well, we could see the finished coronagraph fly in 10 to 20 years, depending on funding and other factors. Yes, that seems like forever. Everybody wants to see the first real pictures of another Earth right now. But considering that it would likely take 10,000 years to physically travel to one of the closer planets, a couple of decades of development isn't really that much of a wait.
Space Camp is all about the under-explored side of NASA. From robotics to medicine to deep-space telescopes to art. For these couple of weeks we'll be coming at you direct from NASA JPL and NASA Ames, shedding a light on this amazing world. You can follow the whole series here.
Video shot by Bill Bowles, edited by Michael Hessian.
Special thanks to Mark Rober, Jessica Culler, Dan Goods, Val Bunnell, and everybody at NASA JPL and NASA Ames for making this happen. The list of thank yous would take up pages, but for giving us access, and for being so generous with their time, we are extremely grateful to everyone there.
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