In 1998, the Hubble Space Telescope was pointed at HR 8799, in hopes of seeing any potential planets that might be orbiting that nearby, Sun-like star. None were found... but in 2008 images using the Gemini telescope found several planets orbiting HR 8799. In fact, four planets were discovered there!
So why weren't they seen in the Hubble data? The star was too bright, and software techniques in 1998 couldn't sufficiently remove the star's light to reveal the much fainter planets. But things have changed in 13 years, and astronomers went back to the old HST data, using newly-developed methods to clean the images. And lo, they saw three of the four planets!
[Click to exoplanetate.]
On the left is the image of the star as seen by Hubble's infrared camera NICMOS. On the right, the star's light has been subtracted, and the three planets (HR 8799 c, d, and b, from left to right) can be clearly spotted.
In a sense, this makes the Hubble image to be the very first image of exoplanets ever taken! But I think that's not entirely fair, since they weren't seen, nor really even able to be seen back then.
The very cool thing is that this older image gives us a much longer baseline over which we can see the planets. Why is that cool? Because the planets have moved over those 10 years between images! By comparing the old image and the one from 2008, the motion of the planets as they orbit the star can be directly seen and measured. Calculations based on that observation show the planets d, c, and b take about 100, 200, and 400 years to circle HR 8799 once, respectively. Given more time, we'd've figured that out anyway, but getting this image from Hubble is like getting an extra 10 years for free. So these planets join one around the star Beta Pic and another around Fomalhaut as being directly seen to move over time. Pretty amazing.
I'll note that the fourth planet, e, is too close to the star to be seen even using the new method. It sits just on the edge of a piece of metal called an occulting disk, which is used to block the majority of the light from the star, allowing fainter stuff to be seen. Even with the disk masking the star, the planets are incredibly faint and difficult to see.
I worked on data very similar to this on Hubble around the same time this image was taken, and in fact wrote software to try to spot planets (as well as dust and other material) near their parent stars. It was incredibly finicky work, and obviously I wasn't able to see any planets then.
The new technique makes me a bit jealous. One thing it relies on is a database of images of 466 (!) stars observed in the same way as the target star. By adding them together digitally, a very accurate model of the target star can be made, and used to literally subtract the star's light. I tried this same method back in '98, but we didn't have the observations necessary for it. I used computer-generated star models, and they simply weren't accurate enough back then to do this kind of work.
But oh, how hard I worked on this project! To be the first to have actually seen a planet orbiting another star... but I probably would've had a coronary. Maybe it's all for the best.
Of course, that makes me wonder: how many more planets are lurking in images already taken, simply waiting to be dug up? We observed quite a few nearby stars, looking for exoplanets. I have a sneaky suspicion we may be seeing more of these kinds of images soon.
Image credit: NASA, ESA, and R. Soummer (STScI)
[Below is a gallery of exoplanets that have been directly imaged using telescopes on ground and in space. Click the thumbnail picture to get a bigger picture and more information, and scroll through the gallery using the left and right arrows.]
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