For decades, scientists have been on the hunt for planets outside our solar system. Finding them is key to continuing the search for life outside our planet, as well as learning about how other star systems are formed.
Detecting exoplanets is no easy business, however, since we lack the long-range sensor technology of the Star Trek Universe. But astronomers are ingenious inventors of new ideas, and over the years have come up with different methods for detecting these alien worlds.
The first method that worked was the Radial Velocity Method (aka Doppler Spectroscopy). Relying on the fact that stars are affected by the gravitational tugs from their orbiting planets, Radial Velocity is able to measure changes in the light spectrum of the star being monitored.
This works because when the star is moving closer to the observer, the light appears slightly shifted toward the blue spectrum. If the star is being pulled away, the spectrum shift will be slightly red.
For finding Earth-like planets, Transit Photometry is used. This method measures minute changes in brightness as a planet passes between the observer and the host star. If this change lasts for a fixed amount of time and occurs at regular intervals, that increases the likelihood that a planet is passing in front of the star during its orbital period.
Measuring how much the brightness of the host star changes gives scientists an idea of the actual size of the planet. When using this method in conjunction with the Radial Velocity method, astronomers are able to calculate the planet’s density.
Microlensing is the method used to detect planets that are not in our cosmic neighborhood. This method, based on Einstein’s Theory of Relativity, is a bit more complicated. Here’s how it works: let’s say you have a far away star, we’ll call him Roger. Roger has a large, overweight neighbor named Big Blue.
When Roger and Big Blue are very close to each other, say talking near the fence line that divides their yards about the latest football game, the lensing affect will cause Roger and Big Blue to appear further apart than they actually are. Now imagine that on another day, Roger is standing in line at a drugstore counter directly behind Big Blue. Roger would now appear to be surrounding Big Blue on all sides.
This affect is known as the Einstein Ring, and happens when the ‘lensing star’ (Big Blue) bends the light of the source star (Roger) all around it. Now, picture Roger and Big Blue are at a barbeque and Roger’s young son, Ivan, is standing closer to Big Blue than he is to Roger.
According to Einstein, this would cause there to appear to be a third Roger. When observers from Earth measure this, it appears as a temporary spike in brightness that can last from several hours to several days. When hunting for planets that are very far away, these spikes are telltale signs of a planet. And by measuring the characteristics of the light curve (intensity and length), astronomers can learn a lot about the planet’s mass and orbit.
Directly observing exoplanets is very difficult to do, but not impossible. In 2008, scientists were able, for the first time, to directly image three planets orbiting the star HR8799 thanks to the Keck and Gemini telescopes. In 2010, astronomers were able to image a fourth planet in this system. But this year, the focus has been on one planet in particular, HR8799c. At seven times the mass of Jupiter it’s a rather large target.
Using a combination of the two telescope’s technologies, scientists were able to confirm the presence of water in its atmosphere. Adaptive optics on one telescope were used to counteract the blurring affects of the Earth’s atmosphere. The spectrometer on the Keck 2 called NIRSPEC (Near-Infrared Cryogenic Echelle Spectrograph), is a high-resolution spectrometer that works in the L-Band.
“The L-Band has gone largely overlooked before because the sky is brighter at this wavelength,” says Dimitri Mawet, an associate professor of astronomy at Caltech and research scientist at JPL. “If you were an alien with eyes tuned to the L-Band, you’d see an extremely bright sky. It’s hard to see exoplanets through this veil.”
But, when astronomers combined L-Band spectrography with the adaptive optics, they were able to overcome these difficulties. Instead, they were able to precisely measure the chemical signature of the atmosphere of HR8799c, which confirmed not only the presence of water but the absence of methane.
“We are now more certain about the lack of methane in this planet,” said Ji Wang, former postdoctoral scholar at Caltech and Assistant Professor at Ohio State University. “This may be due to mixing in the planet’s atmosphere. The methane, which we would expect to be there on the surface, could be diluted if the process of convection is bringing up deeper layers of the planet that don’t have methane.”
With technologies like adaptive optics and the spectroscopy of NIRSPEC being applied to future telescopes such as KPIC (Keck Planet Imager and Characterizer), direct planet imaging will be able to detect alien worlds that are fainter and closer to their host star than ever before. In the meantime, astronomers are not only learning a great deal about the ways planets in our universe form, but they are finally able to see these worlds with their own eyes.