For all the abundance of newly discovered planets beyond our Solar System, we still haven’t directly seen any worlds that have the potential to host life. Now that may have changed. A new technique might allow us to see a small subset of the worlds we seek, and on its first outing has found a possible circling one of the Sun’s nearest neighbors.
Most planets around other stars (exoplanets) have been found through their effect on their parent star, either by blocking a little light or slightly changing its movements. Direct detection has been limited to rare cases, such as planets so young they are still hot enough to observe in the infrared. Even then, we’re still limited to large worlds far enough from their Sun that their light isn’t lost in the glare. Not the places we go to find life.
However, University of Arizona graduate student Kevin Wagner says that is partly because we have been looking in the wrong part of the spectrum. Efforts to spot exoplanets directly have been conducted in the near-infrared, with wavelengths less than 10 microns. Yet the planets we are looking for are likely to be brightest at slightly longer wavelengths.
As self-defeating as this may seem, Wagner noted in a statement; "There is a good reason for [these choices] because the Earth itself is shining at you at those wavelengths.” It makes sense, after all, that the Earth would be brightest at the same wavelengths as the Earth-like planets we are trying to find, but it doesn’t make things easy. "Infrared emissions from the sky, the camera and the telescope itself are essentially drowning out your signal,” Wagner added.
In Nature Communications Wagner describes using a combination of instruments to allow the Very Large Telescope (VLT) to observe the Alpha Centauri system at wavelengths of 10-20 µm, blocking out the radiation both from the Earth and the twin stars.
"We're moving one star on and one star off the coronagraph every tenth of a second," Wagner said. "That allows us to observe each star for half of the time, and, importantly, it also allows us to subtract one frame from the subsequent frame, which removes everything that is essentially just noise from the camera and the telescope."
Wagner and co-authors took more than 5 million images in almost 100 hours, stacking them on top of each other and removing unwanted contributions in a way he compares to noise-cancelling headphones.
In the process Wagner found a source of light he has called C1 that appears to lie in the habitable zone of Alpha Centauri A. More still needs to be done to rule out instrumental error or dust cloud, but C1 could also be the real thing.
"There is one point source that looks like what we would expect a planet to look like, that we can't explain with any of the systematic error corrections,” Wagner said.
The team hopes to verify C1’s existence, both with follow-up observations on the VLT, and through alternative planet hunting methods, and to apply the same method to other stars. Nevertheless, the approach has its limitations. Besides requiring an immense amount of time on one of the world’s most expensive telescopes, targets are limited to nearby stars. Moreover, as yet Wagner’s technique could not find an Earth-sized planet. The smallest object it would be likely to be around 3-5 times the radius of the Earth, making it almost certainly a Neptune-style gas planet than a super-Earth.